This paper is for staff use.The views expressed arethose of the authors andnot necessarily those ofthe World Bank.

WORLD BANK STUDY OF TEE SUBSTITUTION OF LABOR

AND EQUIPPENT IN CIVIL CONSTRUCTION

Technical Memorandum No. 24

The Use of Agricultural Tractor/Trailer Combinations

June 1976

This memorandum describes the use of agricultural tractors tohaul construction materials in trailers, explans the problansin their use and discusses ways of overcoming these problems.Various options are analyzed, such as the choice between two- and four-wheeled trailers, tipping trailers and ballasting of the tractor.A large portion of the memorandum is devoted to the question ofload transfer from trailer to tractor and to the manner in whichthis affects the traction available to the tractor. The papergives a numerical gaide to the selection of appropriate combi-nations of tractor and trailer size for a given haul route surfaceand gradient, followed by an explanation of how to calculate thenumbers of laborers and trailers reauired. Detailed instructionsfor calculating productivity and unit costs are given, and therelative merits, in cost terms, of truck and tractor/trailerhaulage are discussed. The memorandum ends with a review ofother implements available for use in conjunction with tractors,such as rippers, graders and rollers.

Transport Research DivisionTransportation Department

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Preface

This is one of a series of papers prepared in the co-irse ofthe Study of the Substitution of Labor and Equipment in Civil Construction.The paper is prepared with the objective of disseminating the results of thestudy as and when they become available and to generate discussion on thesubject. The conclusions given in this paper are therefore tentative and mayrequire revision when the results of further field work and analysis becomeavailable. Comments are solicited from all interested persons, particularlythose who plan and execute labor-intensive construction projects.

The paper is based on field work in India undertaken by ScottWilson Kirkpatrick and Partners (Consultants) in collaboration with theGovernment of India; P.D. Brown carried out the analysis under the overalldirection of P.A. Green. Until 31st December 1975 all work in the study was.supervised by Inder K. Sud of the World Bank; this supervision is now carriedout by Basil P. Coukis. Financial support for the study is being provided bythe World Bank and the Governments of Canada, Denmark, Federal Republic ofGermany, Finland, Japan, Norway, Sweden, United Kingdom and the United States.Further :financing has been provided by the Ministry of Overseas De-elo=nent .of the British Government.

During the course of the study technical help regarding equipment-has been given by a number of manufacturers in several countries of the World.Of particular help in the preparation of data for this publication have been -Messrs Massey-Ferguson Limited (UY) and TAE (India) who manufacture the former'sequipment in India.

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Contents

I. INTRODUCTION Page No.

II. TECHNICAL FACTORS AFFECTING TRACTOR/TRAILEROPERATION 3Operating Requirements 3Two-Wheel and Four-Wheel Trailers 3Hauling 5Loading and Unloading 6

Pallet Trailers 7Tandem Trailers 7Rear-Dump Trailers 7

III. PLANNING FACTORS 8

Planning Phases 8

One Tractor per Unit 11

IV. PLANING (PHASE A): SELECTION OF EQUIPMENT 12

Introduction 12

A. Relation of Speed, Load & Power to Unit Cost 14

B. Speedi Load & Their Relationship 14

C. Criteria for Selection of Tractor/Trailer 16Combination

D. Determinants of Maximum Pull 17

E. Determinants of Maximum Speed 19

F. Selection of Technically Feasible Tractor/ 20Trailer/Haul Route Conditions

G. Selection of Economically Optimal Tractor/ 28Trailer/Haul Route Combination

Tipping or Non-Tipping Trailers 30

V. PLANNING (PHASE B): ORGANISATION & BALANCING 32OF RESOURCES

Labor Organisation 32

Incentive Payments 32

Balancing of Resources : Separate Loading & 33Unloading Gangs

Balancing of Resources : Loaders Ride on Vehicles 35to Unload

Loading Arrangements, Coupling & Manoeuvring TimeL 36

Unloading Arrangements 38

Page No.

Vi. PRODUCTIVITY & UNIT COST CALCULATIONS 39Time Definitions & Conversion Factors 39Calculation Steps 41

VII. COMPARISON OF TRACTOR/TRAILERS WITH TRUCKS 49

Break-Even Haul Distance 49

Factors Affecting Break-Even Point 49

Influence of Costing Method 50Example Costing Exercise 51

VIII. OTEER TRACTOR IMPLEMENTS 54

Haulage Attachments 54Ploughs and Rippers 54

Earth Moving Attachments 56Rollers - 57

IX. CONCLUSIONS 58

Tables

1. Factors Affecting Productivity & Unit Costs 10

2. Tractive and Rolling Resistance Coefficients 22

3. Technically Feasible Tractor/Trailer Combinations: 24Two-Wheel Trailers, Average Conditions

4. Technically Feasible Tractor/Trailer Combinations: 25Four-Wheel Trailers, Good Conditions

5. Units for Productivity & Unit Cost Calculations 486. Data for Fig. 6 (Tractor Performance Characteristics) -

(included after Fig. 6 at end of main text)

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Figures

1. Tractors & Trailers

.(a) and (b) 35hp Agricultural Tractors Towing Two-WheelTrailers (India) (Photos)

(c) Four-Wheel Trailer (India) (Photo)

2. Special Trailers

(a) Tandem Trailer (12-tonne) (Catalogue) (Photo)

(b) Jackfoot for Two-Wheel Trailer (Catalogue) (Photo)

(c) Pallet Trailer (3-tonne) (Catalogue) (Photo)

3. Pressure Control Lift Device

4. Rear-Dump Trailer (Catalogue)

5. Anti-Flip Hitch (Catalogue )

6. Performance Characteristics of a 62hp Tractor

7. Flowchart for a Tractor with 3 Trailers

8. Loading Arrangements in Hillside Quarry (Theory)

(a) Without Reversing

(b) With Reversing

9. Loading Arrangement in Borrow Pit (Theory)

10. Loading Arrangements (Traditional Practice)

(a) Earthwork (India) (Photo)

(b) Murum Quarry (Kenya) (Photo)

(c') Murum Quarry (India) (Photo)

11. Loading Arrangements (Experimental)

(a) Murum Quarry (India) (Photo)

(b) Murum Quarry (India) (Photo)

(c) Stone Quarry (India) (Photo)

12. Unloading Arrangements

(a) Unloading Murum by Hand (India) (Photo)

(b) Tipping Soil for Earthwork (India) (Photo)

13. Special Tipping Trailers

(a) High Level Platform Tipper (Catalogue) (Photo)

(b) High Level Tipper (Catalogue) (Photo)

(c) Automatic Tailgate Opening (Catalogue) (Photo)

(d) Side opening Tailgate (Catalogue) (Photo)

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14. Other Transport Attachments

(a) Jib Crane (Catalogue) (Photo)(b) Transporter Platform (Catalogue) (Photo)(c) Water Trailer (India) (Photo)

15. Ploughing (Experimental)

(a) 200mm Cut in Stiff Clay Soil (India) (Photo)(b) 50mm Cut in Very Stiff Clay Soil (India) (Photo)

16. Subsoilers (Catalogue) (Photos)

17. Soil Scoops (Catalogue) (Photos)

18. Scrapers (Catalogue) (Photos)

19. More Scrapers (Africa) (Photos)

20. Dozer Attachments (Catalogue) (Photos)

21. Grader Attachments (Catalogue) (Photos)

22. Front-end Loaders (Catalogue) (Photos

25. Rollers

(a) Three Views of a Tractor-Mounted Roller(Catalogue) (Photo)

(b) Towed Sheepsfoot Roller (India) (Photo)

APPENDIX A: Selection of Appropriate Tractor/TrailerCombinations

APPENDIX 3: Productivity and Cost Data

APPENDIX C: Costing Exercise Tractor/Trailers vs Trucks

Note: Tables and figures in appendices are listed on thefront pages of the appendices themselves.

I. INTRODUCTION

1. The object of this memorandum is to describe the uses ofagricultural tractor/trailer combinations in civil construction and toreport on field observations taken dur*1ng experimental studies in India.

2. The word tractor is used to describe any land vehicle which ispurely a prime mover, that is, which must be used in conjunction with attach-ments or trailers. Such a vehicle may have two, four or six wheels, any ofwhich could be driven or steered; or it may have tracks. In this memorandum,however, only the wheeled agriculture tract-or sill be cnsidered (seeFig. 1) as this type is the most universally available, being considerablycheaper than a road-going unit.

5. There are three factors which give a separate prime mover apotential advantage over self-powered machines:

(1) A large variety of implements can be attached to the tractor which,apart from serving as a vehicle,has a separate power take-off(driven by the engine) which can provide auxiliary power to drive,for example, a hydraulic ram on a plough or a tipping trailer.

(2) When work with one implement has temporarily finished, anothercan be fitted, thus releasing the tractor for other work. Since thecost of operating the prime mover is usually far greater than theimplement operating cost, such an arrangement ensures maximumutilisation of expensive equipment. 0. the other hand, whenever nowork is available for a self-powered machine like an excavator orgrader, or whenever work is held-up as, say,during the loading ofa truck, the whole machine remains idle.

(5) Thetractor, being an unsprung and purely functional vehicle, isthe cheapest form of moving power unit which has yet been invented.

In short, a tractor is a cheap, flexible, mobile power unit, capable ofdoing a large variety of construction jobs and of being very effectivelyutilised.

4. The advantages of tractor power on farms has of course beenrecognised long since. Because of their widespread use in rural areas, andbecause they are therefore known and understood by many farmers, and sparesand repair facilities are widely available, tractors are particularly suitedto the sort of low-cost construction projects which may be carried out byfarm labor during agricultural slack periods (between sowing &nd harvestseasons, for example) when both labor and tractors may be otherwise idle.

5. Of course tractors may still be usefully employed on larger projectswhich, owing to the scale of investment in machinery, materials and organisa-tion, may have to continue under construction throughout agricultural workingperiods. In such cases the tractorswill need to be permanently allocated tothe project; they cannot be borrowed or hired from farmers to fit in with

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agricultural idle periods.

6. Although the variety of implements which may be attached to atractor has been mentioned as one of its advantages, this memorandum willconcentrate upon the operation of tractors with trailers for hauling bulkcivil construction materials such as earth, murum, road stone, aggregateand rock. Furthermore, because another of the advantages of a tractor isthat it can be kept in productive use by changing attachments, we shall onlyconsi4er "multiple-trailer operation", that is operation with more than onetrailer to each tractor, in such manner that the tractor is not delayed inthe loading area but merely exchanges an empty trailer for a full one.

7. Because the tractor is an unsprung vehicle its speed is limited,even on a good road. There is therefore a haul distance beyond which thetruck, because of its higher speed, and in spite of all its disadvantagesof inflexibility and delay during loading, can inevitably do the job morecheaply than tractor/trailers. The break-even distance varies enormouslyaccording to conditions and costing assumptions, but it is rarely thattractor/trailers can compete with trucks for hauls of over 10km (one way)while break-even distances are more commonly in the range of 1 to 5km.

8. Since the use of tractor/trailers is discussed here in thecontext of basically labor-intensive construction methods, it will beassumed that all loading is done by manual labor (in some cases assistedby loading platforms, hoppers, etc.). Loading by powered machines isignored.

9. This memorandum starts (in Chapter II) by considering the technicalrequirements of, and problems associated with, the operation of tractors withtrailers, in broad qualitative rather than in quantitative terms. The follow-ing three chapters (III, IV and V) bring together the factors which influencethe productivity and costs of such an operation. Mathematical relationshipsare introduced wherever they are considered helpful, but may be ignored byreaders who so wish. Chapter III explains how all the relevant factors areinter-related and how they are grouped into two sets, the first of which,revolving mainly around equipment selection, are considered in Chapter IV

(referring to detailed calculations and selection data in Appendix A), thesecond set of which, concerning organisation and gang balance, are consideredin Chapter V. Chapter VI concludes the subject of planning, productivityand cost, presenting a step-by-step method of calculating the relevant'figures, referring to example productivity and cost data contained inAppendix A. Chapter VII goes on to consider the factors affecting the break-even point between the use of tracks on the one hand and tractor/trailers onthe other, referring to example calculations in Appendix C, based on Indianexperience. The penultimate Chapter (VIII) gives some comments upon use oftractors with implements other than trailers; comments are mostly general,but more detail is given on the subject of ploughs and rippers, based upona short experiment with ploughs in India. The memorandum ends with ChapterIX containing the conclusions.

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II. TECENICAL FACTORS AFFECTING TRACTOR/TRAILER OPERATION

Operating Requirements

10. There are three distinct types of job which a tractor must performduring a trailer haul cycle:

(1) Eauling. The tractor must be able to haul the fully loaded trailerthroughout the length of the haul route, which may include softpatches and/or steep gradients, particularly near the loading andunloading points.

(2) Manoeuvring. The tractor must manoeuvre an empty trailer into itsplace for loading, and a loaded trailer into its place forunloading. Either or both may require reversing of the trailer.

(3) Coupling and uncoupling. The tractor must be able to unhook fromone trailer and hitch up to another, both in the loading area andpossibly also at the unloading point.

In addition to the above three tractor requirements the design of the trailersmust facilitate loading and unloading. Of course for a given trailer capacitythe more cycles which can be run during the day, the greater the productivity.The speed at which these three jobs can be carried out is therefore of thegreatest importance.

11. The relationships between technical factors and productivity andunit costs, where quantifiable, are discussed in the next chapter under"selection of equipment". This chapter is confined to a qualitativedescription of the factors affecting tractor/trailer operation.

Two-WAeel and Four-Wheel Trailers

12. A major decision, one which affects the hauling, manoeuvring andcoupling characteristics of the tractor/trailer unit.is the choice of trailertype. Tractor-drawn trailers can be classified as either two-wheel or four-wheel, and examples of both types are illustrated in Figs 1 and 2. Sometrailers have two axles close together near the back (tandem trailers,para. 32 and Fig. 2(a)), but as such trailers behave as two-wheel trailersthey will be considered as such in this memorandum. A four-wheel trailer willbe taken to mean a vehicle with one axle near each end (see Fig. 1 (c)).Principal pros and cons of the two types of trailer are described below.

13. Load transfer. The two-wheel trailer usually transfers aproportion of its weight to the tractor's rear wheels, since the trailerwheels are nomally near the back end. Traction is therefore increased, bya factor of anywhere between 2 and 3, depending upon the relative weightsof tractor and loaded trailer., Since-tractor pull on the rough roads andgradients frequently found ol-construction sites is comnonly limited byavailable traction between wheels and ground and not by engine

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power, this transfer of weight is of critical importance in obtaining maximuoutput from the tractor. Weight transfer from a factory designed two-wheeltrailer may vary from about one third of gross weight, for a small (3-tonne)trailer, to about a fifth for a 10-tonne trailer. The proportion is-generallylower forlarger trailers owing to the restrictions upon load transferdescribed below. It is most important that load transfer in fully loadedstate should not exceed these limits, otherwise the trailer will not be ableto carry its design load and productivity will suffer.

14. Limits to load transfer. A hydraulically operated tractor hookdesigned for the purpose will usually withstand safely a vertical force of some2 to 3 tonnes, and most trailers are designed within this limit. The safe loadon either the swinging or linkage drawbar of the tract6r is usually less. Ineither case the load transferred may be restricted not only by the strength ofthe hook or drawbar, but also by the tendency of the load to tip the tractorbackwards, though this can be countered by ballasting the front end (seepara. 23).

15. The anti-flip hitch (see Fig. 5) is an attachment specially designedto transfer the trailer load to a point slightly forward of the rear axle, sothat a small amount of load is transferred also to the front wheels. Owing tothe added stability imparted by such a hitch, greater loads may safely betransferred by the larger sizes of trailer. However this load is furtherrestricted by the strength of the rear tires, typically to between 2 tonnesweight for a small tractor and perhaps 5 tonnes for a large one (tractor plustransferred load) unless special tires are fitted. Finally the strength ofthe rear axle will normally limit this gross load to some 4 or 5 tonnes.

16. Pressure control. There is a device which enables the tractortemporarily to transfer some weight to its rear wheels from a four-wheeltrailer. It consists of a hydraulically operated arm attached to the lowerlinks, connected to a chain which passes round the trailer drawbar (see Fig.

3). By operating the pressure control lever the driver raises the arm, whichin turn tries to lift the front of the trailer, in effect loading the tractorrear wheels. Load transfer is again limited by the capacityof the tractordrawbar and by the vertical force which the trailer drawbar is designed towithstand. Coupling time will be increased by the need to attach and detachthe chain.

17. The caloacity of four-wheel trailers is,sometimes higher than that ofavailable two-wheelers. This does not apply so much to factory made equip-ment, but is often the case where only locally made trailers are available.

18. Reversing is a relatively simple operation with two-wheel trailers.To reverse a four-wheeler,however, requires great skill, skill possessed byvery few drivers. The job can be made somewhat easier by a hitch to the frontof the tractor so that the tractor is driven forward to reverse the trailer.Alternatively the pressure control device with arm and chain, referred to inpara. 16, may be used to lift the trailer front wheels clear off the groundso that it reverses like a two-wheeler. Application of this last method islimited by the load which can be safely transferred to the tractor and should

if possible be restricted to reversing empty trailers.

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19. Rolling resistance of a four-wheel trailer over soft ground will beless than that of a two-wheeler with the same gross weight, since the load perwheel is halved and the wheels have less tendency to sink in.

20. Coupling and uncoupling is a simpler operation with four-wheeltrailers; these are entirely self-supporting, and only the trailer drawbar needbe lifted, which can be done easily by hand. The drawbar load on a two-wheeler on the other hand is generally by design too great to be lifted by hand.Either the tractor must possess a hydraulically operated hook, or a sepazatemanual screw or hydraulic jack must be kept with the tractor. The formerrequires proper maintenance, the latter may easily be lost or otherwisemisplaced. If the tractor has no hydraulic hook, it is probably best to welda jack to the drawbar of each trailer (see Fig. 2 (b) and (c)). Xanual hitchingaided by trestles or chocks, is sometimes possible, but is both hazardous andtime consuming and is not recommended.

21. Drawbar facture. The combination of lifting and pulling forces inthe towbar of the two-wheel trailer induces greater stresses than in the tow-bar of a four-wheeler. The standard of fabrication and quality of steel usedneeds to be higher for a two-wheeler. (The high stresses led to fracture ofthe towbar of some local workshop fabricated two-wheel trailers at one sitein Africa, whereas no trouble was experienced during six months' work withthose manufactured in a modern factory in India).

Hauling

22. The three principal factors controlling haulage productivity aretractor power, trailer capacity and haul route condition. The relationshipbetween these three, and their balancing on the basis of the other factorsdescribed below, is discussed in Chapter III. Other incidental issuesconcerning hauling are mentioned below.

25. Ballast can be added to the tractor to increase weight on the rearwheels, to increase traction. Weight can also be added to the front to givegreater stability, for example to offset the backward tipping'effect causedby transferred trailer load. 3allast may be in the form of iron weights orwater in the rear wheels, or both. Nbst tractors are designed to acceptballasting. Rear-wheel loading, and hence traction, can be raised typicallyby a factor of between 2 and 3, but since the tractor is made heavier theincrease in useful pull is not so great; the drawbar pull may increase by afactor 1.5 to 2 on a tarmac or concrete road, less on a soft surface. Sincefuel consumption is also raised, ballasting should only be used as a lastresort. It is far better to obtain the necessary traction by transferringthe useful weight to the tractor wheels than by hauling useless dead loads.(The quantity of ballast which can safely be added will be reduced to theextent that weight is transferred from a trailer).

24. Tractor rear wheel load is also affected by the material of thewheels themselves which may be of heavy, cast iron, construction or light,pressed steel.

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25. Tractor tires can also have an important effect on the tractionobtained. Larger tires obtain better traction in soft conditions, but they aremore expensive.

26. Trailer tires can have a pronounced effect on the rolling resistanceof the trailer, if soft ground is encountered anywhere on the haul route.Where trailers are to be used to haul earth for embankments it may well beworth using high flotation tires (larger, lower pressure tires than normal).The alternative is to see that the haul route is well compacted throughout itslength and at all times (see also tandem trailers, para. 32).

27. Eillside haulage can be very dangerous on steep down gradients.The brakes on most tractors are not powerful enough to hold the weight of theloaded trailer, as well as the tractor itself, on down gradients of more thanabout 5 - 10Yo. The result is that the trailer pushes the tractor, and eitherthe pair accelerates out of control down the slope or the unit Ijack-knifes'and falls over sideways. Although most trailers are built with mechanicalhandbrakes these are very often broken off in service, and in any case thedriver is not in a position to exert very much force upon it. One solution isa hydraulic brake on the trailer wheels, opera'ed by an over-ride built intothe trailer drawbar, which starts to function as soon as the trailer begins topush the tractor. An over-ride brake can usually be adjusted to allow thetrailer-to be reversed on flat ground, but to reverse uphill or over roughground a chock will usually have to be inserted to render the over-rideinoperative (with the consequent danger that the driver will forget to removeit before setting off downhill).

Loading and Unloading

28. Tipping trailers are generally of the two-wheel type, though four-wheel tippers do exist. They are normally operated by a hydraulic ram,connected by hose to a coupling on the tractor's hydraulic power off-take, asillustrated in Figs 12(b) and 13. Each time the tractor couples up toanother trailer in the loading area this hose must also be detached andre-attached. It follows that the hydraulic arrangements on the tractor mustbe maintained in proper working order if tipping trailers are to functionusefully. Eose connections must also be compatible (couplings usually varybetween manufacturers).

29. Successful operation of tipping trailers also requires that thematerial mpty 0itself completely when the body is tipped. Some trailer bodiesonly tip to 50 , which is sufficient for granular materials but not alwaysenough for plastic soils. During the study in India soil of a silty clayvariety would sometimes stick to the trailer bottom, even though the soilwas fairly dry. A tipping angle of 550 to 600 should be sufficient in mostcases, but it will often be worthwhile carrying out a test to check. Theangle of tip may be increased by driving the tractor up a short ramp; or bywelding a spacer between the end of the ram and either the chassis or thetrailer bottom.

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30. Faulty operation of the tailgate can also increase unloading time.The tailgate refused to open so often at one site that the driver removed it,thereby reducing trailer capacity and hence productivity.

Pallet Trailers

31. Pallet-trailers without sides (see Fig. 2(o)) have not been used inthe study so fax. They have smaller capacity than box trailers of the samelength and breadth, but are easier to load and to unload. Loading height isreduced, and no time is spent raising and lowering sides, which can occupyseveral minutes per cycle if the sides are distorted. On short hauls, whereloading accounts for a substantial part of the cost, such trailers may proveworthwhile.

Tandem Trailers

32. Tandem trailers,having two axles near the back of the trailer (seeFig. 2(a)),have the advantage of four wheels to spread the load, and sodecrease rolling resistance on soft ground, while retaining the load transferand maznoeuvrability advantages of the two-wheeler. The wheels tend to scrubround corners, however, and tire wear is high. The effect is most pronouncedon very tight corners and, if ground conditions permit, the pressure controllift system can be used to take the weight off the frontmost axle while thevehicle rounds the corner. This requires considerable skill on the part of thedriver. In general tandem trailers are not recommended unless the ground insome parts is very soft and tight corners on hard ground can be avoided.

Rear-Dump Trailers

33. A specially constructed rear-dump trailer is available for use withtractors for earthmoving work (Fig. 4). It is designed to transfer not onlythe correct proportion of weight to the tractor rear wheels, but also someweight onto the front wheels to increase stability. The trailer has highflotation tires, and gives maximum manoeuvrability, the drawbar being designedto clear the tractor rear wheels when turning. Discharge is by tipping.However the trailer is relatively expensive, and may not be suitable for.manualloading owing to the high cost of loading delay when using labor.

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III. PLANNING FACTORS

34. It should be remembered that in this memorandum we areconsidering only manual loading and multiple trailer operation (i.e.more than one trailer per tractor). Much of the operational guidanceand productivity data presented below may nevertheless be of use toan engineer who may be forced by circumstance to work with one trailerto a tractor.

Planning Phases

35. The planning of a tractor/trailer operation involves twophases:

Phase A : The selection of equipment (tractors and trailers).Phase B : The 'establishment of a cycle in which material is

moved from loading point to unloading point withthe most efficient use of the three principalresources employed, viz. tcactor (with driver ,trailers and labor.

Since it is not practicable to maximise the efficiency ofuse of all three resources simultaneously, Phase B presents a problemof optimisation, of obtaining the best overall balance of utilisation.Usually, however, in low-wage economies the cost (in hourly terms) ofthe tractor will greatly exceed the cost of either labor .or trailers(1).The optimisation process of Phase B can therefore be simplified intotwo jobs:

(a) to maximise tractor efficiency; and then(b to check that the labor, and to a secondary extent

the trailers, are reasonably efficiently employed.

(')This is true even of short hauls, which give the highest laborto tractor cost ratio. In one experiment in India where haul lengthwas only 470m and wages U.S. 4.1/day, labor costs came to only 62%of tractor (and driver) costs,,.trailer costs representing 27% ofGractor costs. In another experiment where the haul length wasover 2km labor costs av;raged only 32% of tractor costs, but in thearea of this site the wage rate was exceptionally low at$0.33 perday.

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36. Phase A, selection of equipment, is in essence an exercisein choosing equipment which will itself maximise efficiency, which ineffect means to minimise haulage costs (as distinct: from loading andunloading, which are dealt with predominantly in Phase B). It isagain because the tractor is generally the most costly resource thatselection of equipment can be logically considered separately from thebalancing of resources, and that the former can be considered first.Of course there is some inter-dependence between the two phases, butthe distinction helps to clarify the complicated issues involved inplanning.

57. In countries where wages are very much higher.than U.S. $1/day,labor costs may approach or exceed tractor costs, and the argumentspresented in the previous paragraph may not apply. But in suchcases, it is likely that an engineer will begin to consider usingmechanical loading aids or mechanical loaders, which are beyond thescope of this memorandum.

38. To maximise tractor cost efficiency and to establish theoptimum tractor and trailer cycles the productivities-of both tractorand labor must be known. These productivities are largly determinedby a set of fifteen factors which are listed in Table 1. Some ofthese factors are variables, representing decisions which must bemade by the engineer planning the operation. The remaining factorsmay be variable in some circumstances, whereas in others they maybe fixed or limited by the available equipment or by organisationalpolicy. The factors are grou'ped in Table 1 according to whether theymost affect Phase A decisions or Phase B decisions.

39. Within each group of factors there is a lot of inter-dependence, which is why they have been so grouped. Furthermore itwould be impossible to carry out Phase B without a good estimate ofthe values of Phase A factors, which is why the latter should bedetermined first. However, there is limited feedback from Phase Bdecisions to Phase A: only the choice of loading and unloadingarrangements may have a significant effect upon equipment selection,and after Phase B has been completed the engineer should check thathis equipment is compatible with the loading and unloading arrangements.In reality the whole planning process is an iterative one, and at theend of it the values of all factors should be checked for compatibilityand adjusted where necessary. If the decisions are made roughly inthe order listed in Table 1, however, the need for major adjustmentsat the end will be minimised.

Table 1. Factors affecting productivity(1) and unit costs (2)

Phase A. Selection of equipment (factors primarily affecting haul costs)

1. Trailer capacity variable or fixed (3)

2. Tractor power variable or fixed (3)

5. Tractor maximum speed variable or fixed (3)

4. Number of trailer wheels and coupling arrangements variable or fixed (5)

5. Tractor ballasting variable

6. Condition of haul route variable or fixed (3)

7. Eaul route gradients variable or fixed (3)

8. Tipping or non-tipping trailers variable or fixed (3)

Phase B. Resource balancing(factors primarily affecting loading, unloadingand manoeuvring costs)

9. Labor organisation variable

10. Incentive payments variable or fixed (4)

11. Number of loading labor variable

12. Number of trailers per tractor (per'unit) variable

15. Number of unloading labor (if tippers not used) variable

14. Loading area arrangements variable

15. Unloading area arrangements variable

Notes:

(1) Productivity: the output (number of units of product) produced by one unitof input of the relevant resources (measured in hours, days, etc). In thiscase productivity might be defined:

- for haulage, in terms of tonnes (of material moved) per tractor/trailerhour;

- for loadine, in terms of tonnes per man-hour or per trailer-hour;

- for the whole task, in terms of tonnes per gang-hour, with a gang of1 tractor, so many trailers and so many laborers.

(2) Unit Cost: the cost of one unit of output for an activity (say loading orhauling) or for a task (load/haul/unload). In this case unit costs might beexpressed in terms of US $ per tonne. Note that the cost of all resources usedmust be included.

(3) In some cases the values of these factors may be limited by the equipmentavailable to the engineer.

(4) In some cases this factor may be fixed by organisational policy.

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One Tractor per Unit

40. There is a sixteenth factor, which affects trailer utilisation;this is the number of tractors considered to constitute to one unit. Ifa unit is considered to comprise just one tractor and its associatedtrailers, then in some cases more trailers may be needed than in thecase where a unit contains two or more tractors. For instance theoptimum ratio of trailers to tractors might be calculated to be 2.5.Two units, each containing one tractor, would each then need 5 trailers(6 in all) whereas a unit of two tractors would need only 5 trailers.Such modifications complicate the planning procedure, however, andunless stated to the contrary, in this memorandum a unit is consideredto comprise one tractor only, with a whole number of trailers. Ofcourse there may be any number of units working simultaneously, dependingupon the scale of the job.

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IV. PLANNING (PHASE A) : SELECTION OF EQUIPIET

Introduction

41. The object of this chapter is first to explain the theoreticalbasis for selection of the most economic combination of tractor andtrailer for any given job, then to give practical guidance in makingthe selection, by considering the factors listed under Phase A inTable 1.

42. The selection method outlined is based entirely upon theperformance of the equipment while it is engaged on hauling (as opposedto loading or unloading) since:

(a) the majority of tradr/trailer costs are attributableto the haulage activity; and

(b) the haulage activity is likely to account for roughlyhalf the total cost of the work.

The bulk of the chapter is therefore devoted to the selection of themost economical combination of tractor, trailer and haul route. Asimplified representation of the argument, which is diV,ided into sectionslabelled A to G, appears in diagrammatic f6rm overleaf.

43. The general relationships between the various factors whichaffect the unit cost of haulage, such as speed, trailer payload andtractor power, are set out in the two sections following this (A and B).It is concluded in the next section (C) that the range of possibletractor/trailer combinations is limited by two parameters, viz. themaximum pull exerted by the tractor and the maximum speed obtainable.The influenece of the various factors which determine these two limits,in particular haul route condition-and gradient, are discussed inqualitative fashion in the following two sections (D and E).

44. Thenext section (F) leads on to the actual selection oftechnically feasible tractor/trailer combinations on the basis ofnumerically defined haul route condition and gradient. Th-,e process,which is based on a series of tables presented in Appendix A, canleave the engineer with a range of combinations of various tractorsizes and types, ballasting arrangements and route conditions. Thepenultimate section (G) gives guidelines as to how to narrow down thisrange by reference to the influence upon unit cost of the variablesjust mentioned.

45. The chapter ends with some comments upon the relative meritsof tipping and non-tipping trailers, an issue which has no effect uponhaulage costs and which can therefore 'be treated separately.

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A Unit cost & productivity and their relationship with load, speed and tractor power

Al Unit cost a function of hourly A2 Productivity of any given tractor a functionrate, payload, speed of payload and speed

3 'Intrinsio' speed/pull relationship a func-tion of equipment characteristics

'Externall speed/pull relationship a funct-of equipment and haul route characteris-

C Tractor/trailer/haul route selection criteria

C1 Pull criterion (poor part of haul route) C2 'Speed criterion' (good part of haul route)

2 Required pull function most often of tractive force, Required speed function of rimpull which issometimes of rimpull, which are determined by: determined by:

(a) equipment characteristics (a) equipment characterisiticsDower, payload, load transfer coeff, ballast, power, payload

Vtotal weight/tractive load ratio

(b) haul route characteristics (b) Haul route characteristicstractive coeff(C), g rolling resistance(r),gradient(g)

resistance coeff(R) = r + g, Resistance = r + ghaul route coLff(H) = C/R

Hence for traction, Hence,(1) haul route coeff>total weight tractive load, (1) rimpull t weight x resistance coeff,&and for _vimpull- -(2) rimoull)total weign- x resistance, coeff, where (2) speed power x transmission efficiency(3).M- ul! power E transmission efficiency speed quired rimpull

lNuerical solution of above three equations, to give F21 Numerical solution of above two equations, totractor/trailerhaul route combinations which satisfy give tractor/tr ler combinaticns which satisfyPull criterion' the 'speed criterion'

Ca

F3] Tecrollingyresistance(ac)orgradienr(g)

Resitanc coemb(Rna=ion+s

G Apply powerA ourly rate relationship and mther cost considerations, to obtaineconomically optimal tractor/trailer/haul. route combinations

Note: Block letters 'A', '3', etc refer to section in. Chapter IVtC11 means first part of section section C, etc.

Key to Relationships Explained in Chapter IV

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A. Relation of Speed, Load and Power to Unit Cost

46. The productiTity of the.tractor/trailer on the haulageactivity, for a given haul length, is proportional to the haul speedand the trailer payload (which should normally be about equal to thetrailer capacity):

Haulage produutivity O payload x speeddistance

The unit cost of haulage is equal to the hourly cost of operating theequipment (the "rate') divided by the productivity:

Unit haulage cost C) distance x (tractor+trailer) rate ..(2payldad x speed

47. Since speed may vary over different sections of the haulroute, this calculation should strictly be carried out separately foreach section with a different speed, and the sum of these costs shouldbe be added together (for full details of how to calculate unit costsand productivity see Chapter VI). For the purposes of this theoreticalpresentation the speed can be thought of as an 'average' speed over thewhole route, (both loaded and return).

48. To minimise unit haulage costs, for any given haul distance,it can be seen from the foregoing that it is necessary to maximise bothpayload and speed, and to minimise the rate. Since the t ractor ratewill usually be much greater than the trailer rate, the third require-ment can effectively be considered as minimising the tractor rate.Furthermore the tractor rate will be higher for more powerful tractors(though not necessarily in the same proportion, see para. 9.9). Therequirements for minimising unit haulage costs, over a given distance,can thus be restated:

(a) maximise payload;

(b) maximise haul speed; and

(c) minimise the rate

The last requirement is clearly in conflict with the first-two, and thefirst two are in conflict with each other because for any given tractorgreater loads mean lower speeds. We are therefore presented with anoptimisation problem, the solution of which forms the suject of theremainder of this chapter.

3. Speed, Load and Their Relationship

49. In this section we consider the first two requirements listedabove, namely maximisation of speed and load, and the relation betweenthe two, while ignoring for the time being the issue of varying tractorpower.

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50. The fintrinsic' speed/Tull characteristic of a tractor isdetermined by its engine, gearbox and transmission design, and relatestravel speed to rimpull in an inverse manner, ranging from a maximumspeed in top gear, witha low rimpull, to a maximum rimpull in bottomgear, with a corresponding low speed.

51. Rimpull is the driving force exerted at the point of con-tact between driving wheels and ground, and is the gross pull needed todrLve both tractor and trailer. Although it is conceptually useful itis difficult to measure and the pull is usually expressed in terms ofthe drawbar pull which is the net pull available to haul the trailer,after deducting the force required to move the tractor itself on levelground.

52. Circumstances external to the tractor itself may restricteither rimpull or travel speed (or both) to values below those givenin the intrinsic speed/pull characteristic:

(a) the maimm rimpull usually is limited bythe tractive force available at the driving wheels,which is in turn a function of the condition of thehaul route and the load on the driving wheels; or

(b) the top speed may be restricted by the bumpLnessthe haul route.

For a given set of conditions there is therefore an fexternalf speed/pull characteristic which for convenience of measurement can be expressedin terms of drawbar pull, not rimpull.

53. The 'external- travel speed/drawbar pull characteristic fora typical tractor (62 hp crankshaft rating) is plotted in Fig. 6(a).The relationship is valid only for the conditions specified, that isfor a surface of tarmacadam, and with the tractor ballasted by theamount shown in the figure. It can also be assumed that the gradientwas zero.

54. Five further points should be noted in relation to Fig. 6(a):

(1) Given the hard surface of the trial, the power loss between wheelsand drawbar (required to drive the tractor itself is fairlysmall, about 12% at low values of-wheelslip (see Fig. 6 (c)).Maximum rimpulls in the variou,--gars will not be much greaterthan the drawbar pulls shownw n

(~2) On the other hand on softer surfaces more power would be usedto move the tractor itself, resvlting in a reduction in drawbarpull. The maximum pull would be further reduced through theearlier onset of wheelslip.

(3) An unballasted tractor would give slightly greater speeds up toa maximum drawbar pull of (in this particular case) 2200kg,limited by the earlier onset of wheelslip (compared with 4100kgin the ballasted condition).

(2)In the lowest gears power loss is greater, owing to greater wheelslip,though rimpull could possibly be increased by obtaining more traction from atwo-wheel trailer.

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(4) Even when fully ballasted and on a hard surface, maximum pull was

limited by wheelslip (lack of traction), not by engine stall, in

the lowest gear.

(5) The graph only shows the first five gears, out of the eight

available on this particular model. Drawbar pull in higher gears

a.t the rated speed can be estimated with reasonable accuracy by

assuming that the maximum drawbar horsepower remains the same as

in fifth gear (about 55hybjallasted,.5hp unballasted); for

details see Appendix A.

C. Criteria for Selection of Tractor/Trailer Combination

55. Although the above example of a tractor speed/pull characteristic

has been given to help explain how the machine works it will seldom be

necessary in practice to calculate speeds and loads through the full

range of the characteristic. For most haul routes will consist- of a

long stretch of relatively good haul route, with short stretches of poor

route, often at either end.

56. On the long stretch of good route rolling resistances will be

low, enabling the tractor to haul fairly heavy loads at high speeds in a

high gear. More often than not speed will be limited by the bumpiness.

of the road rather than by the capacity of the tractor. On the short,

poor stretches, speed will be relatively unimportant, so the chief

criterion will be the maximum pull in bottom gear, whether limited by

engine stall or, more commonly,by traction (wheelslip).

57. In other words, the selection criteria can often be reduced

to two:

(a) the tractor must be able to pull the trailer over the

worst combination of gradient and rolling resistance

encountered anywhere along the haul route; and

(b) given that (a) is satisfied, the tractor'should be

able to pull the trailer over the main length of

haul route at a speed close to the maximum permitted

by the state of that route.

The next two sections discuss the determinants of these maximum loads

and speeds.

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D. Determinants of Maximum Pull

58. The full method of calculating pulls and speeds is set out inAppendix A. The basic principles are as follows:

(1 ) The force needed to move a given load can be calculated as aproportion of the total weight to be moved, the proportiondepending upon the character of the haul route:

Required driving force = total weight x resistance coeff

where Total weight = gross tractor weight, includingany ballast+ gross weight of loaded trailer .. (4)

and Resistance coefficient rolling resistance + gradient(- 100) .. (5)

(2) The maximum driving force available is limited by engine powerat the lowest possible travel speed (in bottom gear):

Maximum rimpull =enginepowerxtraismission efficiencytravel speed in lowest gear

Alternatively this figure may be read from manufacturers' testcurves, like the one shown in Fig. 6(a); these are based ondrawbar pull, which on a good surface, however, will not be muchlower than rimpull.

(3) The maximum driving force may be further restricted by traction:-

Maximum tractive force = tractive loadxtractive coefficient .. (7)where 'Tractive load* = total weight on tractor driving

wheels (assumed here to be rearwheels only) ..(8)

and 'Tractive coefficient' is a number which depends upon groundcondition

59. Thus the factors which determine the maximum load which can bepulled by a tractor may be listed as follows:

a) tractor horsepower;b) trailer capacity;c) trailer 'load transfer coefficient', which is itself a function of:

(i) number of wheels,(ii) tractor hitch, axle, *tire or drawbar limit (if any);

(d) ballasting of the tractor (this can also affect load transfer,which is limited by tractor front wheel lift in the ballastedstate);

e) tractive coefficient;f rolling resistance; andg) gradient.

Factors (e) and (f) are both functions of the haul route condition, while(e), (f) and (g) are all functions of the haul route characteristics. Theload limit is also affected by tractor and trailer tire sizes and pressures,

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but these effects will be ignored for simjlicity, average values beingassumed for all calculations. Some notes on the influence of the abovefactors are given below.

60. Tractor horsepower. In theory the force which can be exerted bya given tractor may be increased ad infinitum by use of lower gears, seeFig. 6(b). In practice however the greater the horsepower the greater isusually the maximum drawbar pull. Tractor horsepower will also determine,to a great extent, the tractor weight.

61. Trailer capacity, determines the potential payload, also, to agreat extent, the trailer unladen weight.

62. 'Trailer load transfer coefficient' is defined as the proportionof the trailer gross load which is transferred to the tractor rear wheels.It should be noted that some load can be transferred to the front wheels,using a special anti-flip device (see para. 15), but this proportion isnot included in the transfer coefficient. Values vary considerablyaccording to the design of the trailer. For a four-wheel trailer thecoefficient is zero unless a pressure control lift device is used. Thepermitted load on the tractor also varies according to the strength ofhitch or drawbar, rear axle and rear tyres. Typical figures are given inTable A3 of Appendix A.

63. 1tont wheel lift and ballasting. Since the usual point of attachmentof thetrailer load is behind the reaq axle, the trailer load tends to liftthe front wheels, thereby transferring some weight from front to back.This additional load reduces stability but helps traction. To counter theinstability a small amount of ballast should be added to the front wheels.Since both the additional gross tractor weight and the additio:-l 'ractiveload are small (no more than 10 to 15o) they can both be ignored, forsimplicity.

64. Load transfer limits.It is clearly advantageous for the trailer tobe designed to transfer as much load as possible to the,tractor, to increasetraction. If however, a trailer is selected which, whdn loaded, exceeds theload transfer limits set by the tractor hitch, axle or tire construction,then the trailer will have to be uqed with reduced loads, drasticallycurtailing the productivity obtained. It is most important, therefore, thatthe design load transfer from the trailer correspond to the tractor limits.It should be noted that there where use of the drawbar (rather than thehitch) is unavoidable, the transfer limit may be low enough to require aspecially designed trailer with wheels further forward than usual; possiblya four-wheel trailer would be preferable in such cases. Some typicalfigures of load transfer limits and transfer loads are given in Appendix A.They are only examples, however, and where possible precise figures shouldbe obtained for the equipment proposed to be used.

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65. Gradient (if uphill) acts to increase the pull required to movea given trailer load; the steeper the gradient, the more likely is themaximum load to be limited by engine stall than by wheelslip.

66. Haul route condition affects both rolling resistance and tractivecoefficient. Deteriorating condition of haul route reduces the load whichmay be pulled in two ways, (i) by increasing rolling resistance and (ii)by reducing the bractive coefficient. Table 2 (para. 79) lists the assumedranges of the values of rolling resistance and tractive coefficient forvarious route conditions.

67. Where traction, not rimpull, is the limiting parameter (which itusually is) equations (3.)and (7) in para. 58 can be amalgamated:

Maximum tractive force > required driving force .. (9). Tractive load x trac-

tive coefficient > total weight x resistance coeff ..(10)

i.e. Tractive coefficient total weightResistance coefficient tractive load

68. This last equation (11) is the condition which must be satisfiedfor the trailer to move, and is the mathematical expression of criterion(a) in para. 57 above, assuming that the rimpull requirement is satisfied.The ratio of resistance to tractive coefficient is entirely dependentupon the haul route characteristics since:

Resistance coeff.(R) = rolling resistance (r) + gradient(g) ..(12)Thus the effect of the two haul route characteristics (condition andgradient) on maximum pull can be expressed in terms of a single parameter:

'Haul route coefficient'= tractive coeff Cresistance coeff(R)

E. Determinants of Maximum Speed

69. Tractor speeds in top gear can range from around 15 to over 30 km/hdepending upon the model and the gearbox fitted (some have various optionsas to gearbox). It is only on an exceptionally smooth road, however, thata travel speed of more than 20 kmn/h can be obtained with reasonable comfortand safety. Such roads are rarely encountered on civil construction work,:so we can say that for practical purposes speeds will limited by poor sur-faces to a working average of no more than 16 kmA, often less.

70. Beyond the above statement the effect of 'bumpiness' upon speed istoo vague for any useful figures to be given here. The only sure way toestimate the maximum speed is to make a trial run.

71. The top speed may also be limited by available tractor power, if thetractor is hauling an unusually heavy trailer for its size, e.g. a 35-hp

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tractor hauling a-7-tonne trailer. In'such cases the maximum speed may becalculated in accordance with the method set out in Appendix A. Theprinciples of calculation are similar to those followed for calculatingmaximum load (para. 58) except that stage (3) can be ignored since tractionisunlikely to be a limiting factor at high speeds:

(1) Required rimpull = total weight x resistance coeff

(2) Calculated travel speed= engine power xtransmissionaefficiency..(14'required rimpull

(3) The calculated travel speed needs to be adjusted downward tocorrespond with the speed/oad characteristic.for the various gears(see Fig. A7 in Appendix A for details).

(4) Alternatively the travel speed may be read from a manufacturer'spull graph of the type shown in Fig. 6( a );such graphs are basedupon drawbat pull, not'rimpull,- bt of a reasonably good surfacethe difference between the two will not be very great.

72. The factors determining maximum speed are the same as those whichdetermine maximum pull except that, since traction will rarely be the limitingparameter, only factors (a), (b), (f) and (g) in para. 59 are of importance.

F. Selection of Technically Feasible Tractor/Trailer/Haul Route Combinations

73. .The object of this section is to give quantitative guideliies asto the range of technically feasible combinations of tractors and' trailersfor a given haul route. A combination is regarded as 'technically feasible?if it satisfies both the criteria listed in para. 57, which are restatedhere for convenience:

(a) the tractor must be able to pull the trailer over the worstcombination of gradient and rolling resistance encounteredanywhere along the haul route ('pull criterion'); and

(b) the tractor should be able to pull the trailer over the mainlength of haul route at a speed close to the maximum permittedby the state of that route ('speed criterion').

74. The calculations are presented in tables in Appendix A. Three stagesare involved; in the first two stages tractor/trailer/haul route combinationswhich satisfy the pull and speed criteria are calculated on the basis of theprinciples outlined in the preceding two sections and of the variables listedin para. 59 and referred to in para.72 respectively. In the third stagethose combinations which satisfy one criterion but not the other are eliminated,the remainder forming the list of 'technically feasible' solutions. Aselection of the latter, for a given range of haul route conditions, is givenin Tables 3 and 4 (para. 84).

75. In the first stages of the calculation , gross weights and tractiveloads are calculated for sixteen combinations of tractor and trailer. Because

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tractive load depends on trailer load transfer, four different degrees oftransfer are considered (outlined in para. 82). For each tractor/trailer/loadtransfer combination the calculation is checked to ensure that none of themechanical component f the tractor (drawbar, hitch, t±res and rear axle)are being overloaded . If calculation shows that the trailer load trans-ferred is too great for the tractor, that combination is dropped from theanalysis, except in the few cases where, by only slightly reducing trailerpayload, the trailer could be hauled safely and still carry more materialthan a smaller trailer.

76. Tractive load is plotted in Appendix A against gross weight for!each combination. Overlaid on this graph is then placed a set of plots, tothe same scalelof lines whose slopes correspond to the inverse of the haulroute coefficients for the various haul route conditions. Included oneach overlay is a series of 'cut-offt lines, one for each tractor, repre-senting the point at which rimpull takes over from traction as the factorlimiting drawbar pull. Each overlay refers to a different gradient. Fromthe correlation of gross weight/tractive load and haul route coefficient(see para. 68) the appropriate haul route condition and gradient for eachtractor/trailer combination can be read off; these are then tabulated inTable A7 of Appendix A.

77. Owing to the indeterminacy of the haul route coefficient the haulroute condition.corresponding to a tractor/trailer combination can only beexpressed in terms of a range, or of probability. In the remainder of thischapter the work 'probable' refers to a greater-than-50/6 chance that apartipular tracto4V/railer combination will move on a given grade of haulroute(4-) or alternatively that it will move on a 'below average' example ofthat grade (i.e. if the haul route coefficient happens to be between theaverage and minimum for the grade of route). Conversely the wo=i 'possible'refers to a less-than-50% chance of moving, or alternatively that movementwill take place only if the haul route coefficient happens to be above theaverage for the grade of haul route.

78. The speed criterion is made operational by making the assumptionthat a speed of 16km/h should be maintained on a very good condition haulroute (5) on the level. Again the probabilities of each tractor/trailer__combination satisfying the criterion are listed CinTable A10 of Appendix A).

79. Assumed values of tractive coefficient and rolling resistance arelisted in Table 2.

To counter front-wheel instability it is assumed that a small amount ofballast would be added to the front of the tractor. This added load isignored in the calculation for the reason given in para. 63.

(4) Given the assumptions in Table 2.

(5) Equivalent to grade A (average or above average) in Table 2.

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Table 2: Tractive coefficient and rolling resistance

Haul Route Condition (HRC) Tractive Rollingave. Coefficient Resistance

Description Grade or (C) (r)range

Concrete, asphalt, compacted A ave. 0.9 0.055earth well maintained

min. 0.8 0.04

max. 1.0 0.07

Earth, poorly maintained B ave. 0.6 0.105(dry clay loam)

min. 0.5 0.07

max. 0.7 0.14

Earth, muddy, no maintenance C ave. 0.45 0.185(wet clay loam)

miu. 0.4 0.15

max. 0.5 0.22

Loose sand and gravel (wet) D ave. 0.35 0.255

min. 0.3 0.22

max. .0.4 0.29

Earth, soft, muddy, or dry E ave. 0.25 0.34loose sand

min. 0.2 0.28

max. 0.3 0.40

Notes:

(1) Figures apply to rubber-tired vehicles only.

(2) Figures for rolling resistance coefficients are probably conservative.

(3) Gravel surfaced roads engineered and well maintained, probably ERCgrade A; otherwise ERC grade B.

(4) The grades given in this table correspond very roughly to the routecondition codes given in Table 2 (p. 40) of Technical Memorandum No. 8in this series as follows:

grade A above average): Code 5grade A about average): Code 4grade A (below average: Code 3grade B : Code 2grade C : Code 1

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80. Equipment Assumptions. Four types of tractor are considered,35, 47, and 62 and 75 hp (crankshaft rating). Official test data formachines actually on the market were used as far as possible, though someestimates had to be made for the 35 hp model.

81 Four levels of trailer payload are considered, of 5, 5, 7 and10 tonnes capacity; the successive increases are by a factor of about1.5 each time. Unladen weights and load transfer coefficients are averagesfor the capacity concerned, but beyond the fact that they relate to factoryproduced trailers they do not refer to any particular model.

82. Degrees of load transfer are briefly as follows:

(a) zero, i.e. four-wheel trailer;

(b) 'minimun' i.e. two-wheel trailer on linkage drawbar, orfour-wheel trailer on linkage drawbar with pressure controllift;

(c) 'standard i.e. two-wheel trailer on automatic hitch,using standard tractor and trailer; and

(d) ?maxmum' i.e. two-wheel trailer giving up to 35o loadtransfer on automatic hitch, using specially strengthenedhitch and special tractor tires, so that load transfer islimited by tractor rear axle strength only.

For full details see Appendix A.

83. Ballasting is not considered in the exercise, since calculationshowed it to have only a small effect upon the size of trailer which canbe pulled in most circumstances.

84. Conclusions. Details of a large number of technically feasibletractor/trailer combinations for various haul routes and gradients are given

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in Appendix A. To give some simpler form of guidance to equipment selectionthe results for certain specified haul route conditions are summarised in amore general form in Tables 3 and 4. The figures in these tables are givenonly as a guide, and should be modified by the user in the light of experience.Some comments on Tables 3 and 4 and the additional conclusions from Appendix Afollow.

Table 5: Technically feasi l tractor/trailer.'coombinations, withtwo-wheel trailerU3 ) and average conditionsM ).

Tractor 3 tonne 5 tonne 7 tonne 10 tonne

35 hp OK OK on special FAILS - speed FAILS - rimpulltires only - speed

- tires- hitch

47hp OK OX (payload FAILS - tires FAILS - speedreduced to 750o - - tiresweak tires) - hitch

62hp OK OK OK OK on special hitchonly

75hP OK OK OK OK (payload reducedto 85% - weak hitch)

OK (tractor combination satisfiesconditions specified below)

OK with reservations

OK with special fitting

FAILS

Impossible (insufficient rimpull)

NOTES:

(1) Average Conditions:(a) Grade B haul route, traction probably sufficient at 5% gradient,

possibly sufficient at 10% gradient.(b) Speed 16 km/h on average grade A haul route (hard, well maintained

surface, rolling resistance coefficient 0,055).

(2) Load Transfer:'Minimum', 'standard' or 'maximum', i.e. using linkage drawbar or automatichitch,standard equipment, or special equipment where noted in table.

(3) Conditions also satisfied by 4-wheel trailer with pressure control lift.

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Table 4: Technically feasible tracto/ railer combinations, with four-wheeltrailer and good conditions

Trailer3 tonne 5 tonne 7 tonne 10 tonne

Tractor

35hp OK Probable

47hp OK Probable

62hp OK OK Probable

75hp Ox OK OK Probable

Notes: (1 ) 'Good Conditions':

a) Grade A haul route, traction definitely sufficient at50o gradient, possibly sufficient at 10% gradient.

b) Speed 16km/h on average grade A haul route (hard, wellmaintained surface, rolling resistance coefficient0.055).

(2) Load transfer: zero (for pressure control lift see Table 3).

(3) 'OKt': tractor/trailer combination satisfies above conditions.

(4) 'Probable': tractor/trailer combination has probably enoughtraction at 50o gradient, but definitely not a t 10% (grade Ahaul route).

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85. Given a typical haul route, consisting of a short stretch of poorlymaintained earth road (grade 3) with a maximum gradient between 5 and 101/,followed by a main haul route of well maintained hard earth (grade A, average),the appropriate trailers corresponding to the four tractor sizes follow thegeneral form:

- tractor 35 hp, trailer 3-tonne,-,- tractor 47 hp, trailer 5-tonne,- tractor 62 hp, trailer 7-tonne- tractor 75 bp, trailer 10-tonne-

86. However the 47 and 75 hp tractors are prevented from hauling fullyloaded trailers of 5 and 10 tonnes by tire and hitch limitations respectively.Special tires and hitch would solve these difficulties, but such specialequipment would also raise the capacity of the smaller, 35 and 62 hp tractors,to trailers of 5 and 10 tonnes respectively.

87. The figures suggest that the 35 and 47 hp tractors are fairlysimilar in performance, and that the same-is true-of the 62 and 75 hpmachines; but there appears to be a quantum jump between the two pairs. Itappears that the two tractors in a pair will do roughly similar jobs, thebigger of the two merely doing it somewhat better. The explanation may be,in part at least, that the smaller two tractors have,as standard, similar,and much weaker, tires than the other pair; and that they all have similaraxle strengths.

88. According to the definitions of haul route given in Table 2 noteven the biggest of the tractors analysed can pull the smallest trailer on agrade D or grade E haul route, on the level; though all four tractors couldprobably move the 3-tonne trailer on a level grade C route.

89. The 'zero load transfer' figures clearly show up the poor performanceof four-wheel trailers (assuming no pressure control lift); for guch a trailerto equal the performance of a two-wheeler under the same.conditions the haulroute must be improved by one 'grade' (Table 2). For this reason Table 4is based on a grade A haul route (compared with grade B for Table 3). Nofour-wheel trailer can be worked, fully loaded, on a grade C route and at a5% gradient it requires a 75 hp tractor to have more than a 50% chance ofshifting a 3-tonne trailer.

90. The linkage drawbar ('minimum load transfer') gives similar resultsto the standard automatic hitch for the 62 and 75 hp tracors. But a 47 hptractor is limited to 3-tonne trailers, and a 35 hp machine is not onlysimilarly restricted, but can only draw it with a 70% payload.

91. Deterioration in haul route condition dramatically reduces the loadwhich can be pulled by a given tractor: a change of one grade (from A to B,or from B to C) make the difference between a 10--tonne and a 3-tonne trailer.

92. Improvements to the route cause acorrespondi:ng increase in the loadwhich can in theory be handled; but in the case of the 35 and 47 hp tractorsthis load will normally be limited by available rimpull (35 hp model) and byspeed requirements (35 and 47 hp models) to a 7-tonne trailer. In any casespecial tires and hitches would be needed to carry the additional loadtransferred from a two-wheel trailer,

93. This last remarks points to one of the few advantages of a four-wheel trailer,viz. with a good (grade A) level haul route, a 35 or a 47 hptractor, with no special fittings, can pull very heavy loads, up to 7-tonnetrailrs on an asphalt (6) road at 16 km/h even 10-tonners at lower speeds.However the route needs to be-good, and level, throughout its length.

94. A change in gradient of between 10 and 15Yo is needed to offseta change in haul route condition of one grade, to compensate for the changein haul :ute coefficient and traction availability. However the steeperthe gradient the more likely is the rimpull to become the limiting factorrather than the traction.

95. Generally, nevertheless, the speed requirement limits the loadbefore the rimpull does; the latter is very seldom a limiting factor.

96. Ballast generally makes Only a small improvement, to tractorperformance, since overall weight is raised as well as tractive load.Significant improvement is confined to four-wheel trbailers and to smalltwo-wheelers (3r-tonAie), also to low gradi!ents. With heavier loads and.steepergradients ballasting often reduces the load which can be pulled becauserimpull is not sufficient and the engine stalls. On the smaller ( 35 and 47hp) tractors the rear-wheel load in the fully ballasted condition is aboutequal to the maximum load which may be placed on the standard. tires, soballasting can only be done if four-wheel trailers (zero load transfer)are to be used, or if special tires are fitted.

(6)Assumed rolling resistance 0.04

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G. Selection of Economically Optimal Tractor/Trailer/Haul Route Combination

97. The last section explained how to select the right size of trailerfor a particular tractor (or vice versa), for a given type of trailer andhaul route. The object of this section is to help reduce this range byreference to the unit cost of haulage. The subject is tackled under threeheadings which group together the variables discussed above:

(a) tractor size (or power) and trailer size;(b) the choice between two - and four - wheel trailers; andc) ti Cost ofimpriving Jhe_ condiio anda=adiqnt -

.of the haul route.

98. Tractor power and trailer size. Earlier in this chapter (paras.46 - 48) it was pointed out that for a given haul distance:

Unit haulage costOC tractor.rate .....................(15)payload x speed

and that it is necessary to optimise between rate, payload and speed. Theproduct of load and speed is a rough measure of the tractor power needed,so the above proportionality may be reduced to the following:

Unit costcc tractor ratetractor power (approximately).........(16)

99. Now the hourly rate of operating a tractor, excluding the cost ofthe driver and fuel, is roughly proportional to its initial cost and avail-able information suggests that the ratio of initial cost to rated powerdecreases somewhat with increasing power, up to about 75 hp, as indicatedby the following values:

Rated hp Initial Cost, (f.o.b. US $)(8) Initial Cost ($ per hp)

47 6600 140

62 8000 129

75 9000 121

100. In the above example a 60/o increase in horsepower is accompaniedby a 37% increase in initial cost, or a 14/ reduction in initial co't perunit power. The proportionate increase in initial cost in an importing countrywill be lower, since freight charges do not rise in proportion to horsepower.

Fuel costs tend to remain proportional to power output, which means that theyincrease with horsepower faster than other operating costs. Driver costs,while not being constant, rise less than proportionally to horsepower, but

(7) This ignores the rate (hourly cost) of the trailer, which is usuallysmall and also likely to be roughly proportional to the tractor rate.

(8) February,1976, prices. P.o.b. - 'Free on b6a'dd 'hip in British port, butexcluding freight charges.

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in low wage economies driver costs represent only a small proportion of thetotal.

101. Overall it is probably fair to state that in most developingcountries, by doubling the tractor rated power from 35hp to 70hp, operatingcosts, including fuel and driver, will be increased by around 75%; and theratio of t9tal operating costs to rated power will be correspondingly reducedby 10-15% 9),implying a reduction in unit cost by a similar 10-15% margin.

102. For a given tractor it may be noted that an increase in trailer sizeby one step, (e.g. from 5 to 7 tonnes) will lead to a unit cost reduction ofsome 200/o after allowing for some reduction in speed of the larger trailer.Conversely, for a given trailer a reduction in tractor size by one step (e.g.from 62 to 47 hp) will reduce unit cost by around 15%,again after allowingfor some speed reduction. (The above remarks apply only to technically feasiblecombinations, as defined in the previous section.)

103. The conclusion is therefore that unit costs will be somewhatreduced by selecting a higher powered tractor, other things being equal.However the margin of reduction is unlikely to be very great (of the orderof 10-15% for a doubling of tractor horsepower) andbef iTially-selecting alarger tractor the following checks must be made:

(a) that sufficient traction is available to utilise thetractor's potential;

(b) that the available trailers can make equally effectiveuse of the more powerful tractor's power as can smaller atrailers with a smaller tractor;

(c) that increased wheel loadings (compared with smallertractors and trailers) do not require inordinately moreexpensive improvements or repairs to the haul route andloading and unloading areas; and

(d) that where the tractor is to be employed on jobs otherthan haulage, its greater power output can 1e effectivelyused on these other jobs.

104. In general, however, where there is a choice of technically feasibletractor/trailer combinations then, subject to the above checks.the largestavailable trailer should be selected, along with the appropriate tractor.

105. Number of wheels. The relative advantages and disadvantages oftwo-and four-wheel trailers were discussed fully in Chapter II. Brieflythe two-wheel trailer has the two big advantages of affording greatertraction to the tractor and of being more easily manoeuvred. By use ofthe pressure control device it may be possible to transfer some weightfrom a four-wheeler, alternatively the tractor can be ballasted. Howeverit will always be possible to haul greater loads in a two-wheeler than

(9) In countries with particularly high wages (driver wage more thanUS $2 per day, say ) or with particularly low fuel costs, the rise in opera-ting costs will be less.

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in a four-wheeler under similar conditions (see the tables in Appendix A),and for this reason two-wheel trailers are generally to be preferred.

106. If, however, there is no two-wheel trailer on site large enoughfully to utilise the power of the available tractor, but such trailercapacity is available in a four-wheel version, then it may be worth usingthe larger four-wheel trailer even if it means incurring some additionalcosts. Such costs could be by way of increased manoeuvring time (using a fronthitch), by clearing a larger loading area to avoid the need to reverse thetrailer (see Chapter V) or by improving ground conditions to offset thereduced traction.

4 107. It may also be found that locally fabricated two-wheel trailersare not reliable enough, in which case four-wheelers could prove more economic.

108. Haul route imDrovement. Table A7 in Appendix A may be used notonly to estimate which tractor/trailer combinations can work on a givenhaul route, but also to see where an improvement in either the condition.or the gradient of the haulroute may permit use of either a larger or asmaller tractor. It should be noted that generally the most severe res-triction in trailer payload, is set by the maximum available traction on poor surfacesand/or steep gradients in short lengths of route; such short lengths maybe improved at a relatively little cost and to great advantage compared withimprovement of the complete route.

109. Of course any improvement will cost some money, and this must be off-set by savings in unit cost of transporting material.' As a very rough ruleof thumb, the savings in unit haulage costs, allowing for some reduction inhaul speeds, may be taken as follows for a haul length k km:

(a) for an increase in trailer size by one step (e.g. 5 to 7tonnes), $0.02 k/tonne;

(b) for a reduction in tractor size by one step (e.g. 62 to47 hp), $0.012 k/tonne.

In fact the cost reductions may be as much as twice or as little as halfthese figures, but they give a rough guide. Given the length of haul routeand the quantity to be moved, the total savings made by increasing trailersize or reducing tractor size can be calculated. These can then be com-pared with the estimated cost of making the improvements to the route, plusany additional cost of maintaining the route to the improved standardduring the time required to move the estimated quantity of material (sayone season's work).

Tipping or Non-tipping Trailers

110. Although trailers constitute an item of equipment, and as suchthe decision as to which type lies within Phase A of the planning.process,to some extent the decision may also depend upon the outcome of Phase B,which is outlined in the next Chapter (Y).

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111. Tipping trailers should generally be selected if they are available.Not only does the fast unloading time reduce delay to the tractor, andhence costs,but the unloading labor gang can also be dispensed with(unloading labor are almost inevitably inefficiently deployed) and withoutunloading labor it is simpler to organise the loading labor gang and toinstitute a workable piecework system as explained in Chapter V. Theadditional cost of the hydraulic tipping arrangement will normally berepaid by the savings in tractor time alone, even on long hauls withrelatively few work cycles per day.

112. There are two provisos however. First it is vital that the tippingarrangements should function properly from the outset of the job and thatit should continue to do so. For technical details see Chapter II, butbriefly the requirements are that:

(a) the hydraulic arrangements on tractor and trailermust be compatible;

(b) the trailer tailgate should open and clooe properly;

(c) the angle of tip should be sufficient to empty thetrailer completely; and

(d) maintenance facilities should be avs.ilable.

The more complicated the mechanica9 arrangements (as in the rather sophisticatedtrailers shown in Fig. 13 for example) the greater the effort needed to maintainthe equipment in good working- order.

113. A small amount of malfunctioning of the tipping arrangements(including material sticking to the trailer) will very quickly nullifythe benefits, by requiring the presence of unloading labor who may have tobe brought at short notice from another work site, upsetting the plannedlabor balance.

114. The second proviso for successful tipper operation is that thedriver be prepared to take advantage of the time savings. On a long haulroute, with a cycle of time of around an hour, the driver is likely totake ten minutes' break every cycle; if so, he may as well take it whilethe trailer is being unloaded, as anywhere. On the other hand with shortercycles where manual unloading would occupy a greater proportion of thecycle time, even frequent driver rests would not prevent a tipping trailerfrom effecting substantial savings in tractor costs. Finally, driverson piece rates can normally be expected to try to get the maximum numberof cycles in a day's work, taking occasional rests only during the oddhold-up.

115. To sum up, tipping trailers are particularly valuable for workon short cycles (haul empty and return less than 30 minutes, say), orwhere piece rates are in operation, or both. They are less valuable withdaily paid workers and drivers, especially for cycles nearing an hour orso; and if the mechanical arrangements are unreliable they should beavoided.

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V. PLANNING (PEASE B): OR=ANISATION AND BALANCING OF RESOURCES

116. Phase A of the planning process should have been completed withdecisions on equipment more or less finalisedalthough the equipmentselected should be checked against the outcome of Phase B it is unlikelythat any major re-adjustment will be necessary. On the basis of Phase Adecisions the tractor cycle time can be estimated fairly accurately, onlythe manoeuvring times and (in the case of non-tippers) unloading timesbeing dependent upon Phase B decisions.-

117. The tractor cycle time is one of the key determinants of theoptimum balance of tractor, trailer and labor resources, but beforebeginning the optimisation exercise it is necessary to consider thequestion of labor organisation and incentive payments, which have animportant bearing upon labor productivity and equipment delay times.

Labor Organisation

118. The principal question concerning labor organisation is whetherthe loading and unloading labor work is to be done by separate gangs or byone gang which rides to and fro on the trailer between loading and unload-ing points. The second alternative, though wasteful of labor time, isoften used in conjunction with track transport, having the advantage ofsimplicity of organisation:one truck, on6 gang. The same advantage ofsimplicity would apply in the case of single-trailer working. But withmultiple-trailer working there is nothing to be gained from having loadersride on the trailers, so we shall consider in depth only the first methodof labor organisation,that is separate gangs for loading and unloading. Iftipping trailers are used, of course,there is no problem since no unloadinglabor are required.

Incentive Payments

119. With track or tractor and single-trailer operation, with loadersriding on vehicles to unload, operation of a piecework system is very simple.Payment is made by truck load and shared between the members of the gang.Piecework payment may be more difficult to organise-with multiple-traileroperation, since there are two gangs of labor working at different places.They cannot keep an eye on each other, nor on the tractor driver, who maynot himself be on piecework. There is therefore more chance of hostilitiesbreaking out. (Arguments developed on a site in India between a loadinggang and an unloading gang working a rail system, who were in sight of eachother, separated by only 100 metres.)

120. Nevertheless there are advantages to piecework payment, and sucha system should if practicable be instituted. For not only may unit laborcosts be lower than with daily-paid labor (usually output increases some-what more than payment) but, more importantly, trailer loading times maybe reduced, allowing the use of a smaller number of trailers. Moreover,

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the higher general level of efficiency observed on sites where pieceworkis in operation may lead to better utilisation of the tractor. Thisutilisation may be further increased since pieceworkers are often willingto work more than 8 hours a day.

121. If piecework is to be successful, then gang size (loaders plusunloaders) should be kept to the minimum compatible with full utilisationof the tractor. This is a further argument in favor of considering onlyone tractor and its trailers to constitute one 'unit'. Use of tippingtrailers further simplifies the problem by doing away with unloading labor.

122. Taskwork should be avoided since its effect is only to increasethe hourly productivity of daily-paid labor by reducing daily hours worked.The result is to reduce equipment utilisation.

Balancing of Resources: Separate Loading and Unloading Gangs

123. Loaders. In the case of separate loading and unloading gangs,there is no conflict between maximising the degree of utilisation of thetwo most costly resource groups, the tractor and the loading labor. Itis merely a question of balancing the gang of labor with the tractor cycletime which may be calculated according to the method described in thenext chapter; only manoeuvring, coupling and uncoupling times should beallowed for at the loading end, and unloading time and manoeuvring timeat the other. Within the tractor cycle time the labor should be able toload one trailer, with an allowance for normal rest.

If (a) trailer capacity (10) = q tonne' (11)b) labor input coefficient = i man-hr (WT)/tonnec tractor cycle time t mins (WT),

Then number of loading labor N. = _60cg.men

c

Periodic obstruction of loading during tractor manoeuvring time is accountedfor in the working time productivity figure.

124. The calculated number of men may be fractional. The figure shouldnormally be adjusted up to the nearest whole number since the cost of delayto the tractor will generally be greater than the cost of delay to the gangof labor.

125. Trailers. If the calculated number of loading laborers can allbe effectively employed at one time loading one trailer, then only twotrailers per tractor are needed. One is hauled and unloaded while theother is loaded, and vice versa. The upper labor limit may lie anywherebetween 4 and about 10, depending upon the method.

(io) Input coefficient: reciprocal of productivity, see note (1) toTable 1 above.

WT = Working Time, see footnote (14) paragraph 147.

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126. In the case of shorter tractor cycles the required number ofloaders may not all be able to work effectively in one place, in whichcase additional trailers are needed. At any one time one trailer onlyis being hauled or unloaded, the others are being loaded. Suppose forexample that 16 loaders are required but that there is working space foronly 6 men to one trailer.

16 - 6 = 2.7

2.7 rounded upwards gives 3. So 3 trailers are needed in addition to theone being hauled, making a fleet of 4 in all. See Fig. 7 for a diagram-matic representation of this operation.

127. Having calculated the number of trailers required, it is nownecessary to check that there is enough space for them all in the loadingarea (factor 14 in fable 1). If not, a compromise will have to be struckbetween efficient use of resources and loading area congestion. Alter-natively it may be better to select another source of material whereloading can be more efficiently 'organised.

128. Unloaders. To optimise the number of unloading labor is a littlemore complicated since the cost of delay to the tractor will usually haveto be considered. In theory the loaded trailer could be uncoupled forunloading, wbile the tractor returns to the loading point with an alreadyunloaded trailer. In practice this arrangement is rarely economic because:

(a) unloading requires much less effort than loading and canusually be accomplished in 5 to 10 minutes, little more thanthe time spent in uncoupling, manoeuvring and coupling;

(b) the unloading point, particularly in the case of road works,is often congested and does not allow space for the necessarymanoeuvring; and

(c) the saving in tractor delay costs is unlikely to offset theextra cost of the additional trailer needed."

129. The sort of situation where uncoupling of trailers for unloadingis most likely to be worthwhile is in the movement of earth for embankments,where leads are short and the cost of a few minutes' delay to the tractoris high in relation to the total (though of course tipping trailers aregreatly to be preferred in such situations). If trailers do not have dropsides, then it may also be worthwhile to uncouple.

150. If in the end it be judged, after calculating the unloading timeas explained in the paragraph overleaf, that it would be economic to swaptrailers at the unloading point, then the gang balance calculation shouldbe carried out in the same way as the loading gang balance calculation.

131. Where it is planned that the tractor should remain coupled to thetrailer during inloading,the minimum overall cost occurs when the number ofunloaders is so adjusted that the cost of delay to the tractor plus trailer

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during unloading is equal to the cost of delay to the labor waiting inbetween trailer arrivals.(12)

132. In practice a calculation on this basis will generally lead toan unloading time of 5 to 10 minutes, with a gang of 3 to 5 men, perhapsmore on a 7 or a 10-tonne trailer. These figures pre-suppose a fast rateof working during this short period, considerably faster than that observedover long time spans. It has been observed that even daily paid workersdo in fact work much harder for this short spell, so long as adequaterest is allowed afterwards. The working time input coefficients inAppendix B can be used to plan overall time allowances per cycle, includingrest: 'Fast unload' time input coefficients may be as low as a third ofthe normal working time figures, or even less.

133. Given that sufficient rest allowance is planned for, there aretwo ways in which excess unloader idle time may be reduced. If the jobis big enough to employ two or more tractors then the same gang can unloadtrailers hauled by all the tractors (though the optimum gang size mayalso be increased). This arrangement may interfere, however, with theoperating of a piecework system, which is easier to operate with onelabor gang per tractor. Alternatively (or perhaps in addition) theunloading labor may be given other jobs to do in between unloading trailers(such as spreading or stacking the unloaded material) in which caseoptimum gang size may again be increased.

134. Generally, it will be difficult to employ more than five peoplein unloading a trailer, however, and in most cases it will be sufficientto plan for about 4 unloaders and to dispefse with optimising calculations.Having specified the gang size it is nonetheless nedessary to calculatethe unloading time (using 'fast unload' input coefficients, since the timeis short) so as to provide an estimate of tractor cycle time for thepurpose of balancing the loading gang.

Balancing of Resources: Loaders Ride on Vehicles to Unload

135. Such an arrangement will only be viable on a small job involvingprobably only one tractor. Since the unloading labor (or replacementsfor them) have to return with the tractor (having exchange'd an empty fora full trailer) immediately upon arriving back at the loading end, it isimmaterial whether they remain at the unloading point between trailerarrivals or whether they travel back and forth on the trailer (see Fig. 7).Possible advantages of having the labor ride back and forth are:

(a) a piecework system may be easier to operate; and

(b) groups of labor can take turns to 'rest' (they usuallyenjoy the apparently uncomfortable ride), alternatingtrips to unload with bouts of loading.

On the other hand if the labor ride on the vehicle they cannot do otherjobs, such as spreading, in between unloading one trailer and the next.

(12) See paragraph 151 Step 4 (Chapter VI) for full calculation.

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136. 'he optimum number of unloaders to ride with the vehicle iscalculated as for separate groups (see above); 3 or 4 will be normal.Any higher number would usually be wasteful in labor. The total gangwill be equal to the number required to.load, plus the number of unloaders(who will be missing from the loading area 1000o of the time).

Loading Arrangments, Coupling and Manoeuvring Times

137, Rough calculations of the numbers of tractors, trailers andlaborers, based upon assumptions as to the values of the 15 factors, shouldfirst be made (as explained above), to assess how many loading bays areneeded, and how large they should be. However, given that there is enoughspace (or enough space can be made) to permit the required number of-trailersit will generally be preferable to organise the site so that loading takesplace from one side of the trailer only.

138. Tonminiumise trailer manoeuvring time and to obtain greatest laborproductivity, the following rules should be observed where possible:

(1) Loading should be from one side of the trailer only (plus, possibly,the end). Loading from both sides usually involves more walking forloaders (Fig. 10 (c)) and more delay in raising the trailer side.With a loading bench it is rarely feasible anyway.

(2) Hauling of the material before loading should be minimised. Fig. 10 (c)shows an example of excessive walking. Wherever possible trailersshould be directly loaded by shovel or hoe as shown in Fig. 10 (a)& (b). If this is impossible, loading by gravity chute (if terrainpermits) is to be preferred, and if the haul length must exceed about

15 m either chutes or wheelbarrows may be used with advantage,particularly if there is gravity aid.

(3) If the total quantity of work is considerable (say three months' work)then loading should be from an excavated earth bench or from a plat-from at least as high as the trailer bottom say 1 m), preferably ashigh as the top of the sides (Fig. 11 (a) & (c)). For stone productsa hopper may be worth building. There is one proviso:use of a load-ing bench should not unduly extend the manual haul distance.

(4) Reversing of trailers should be avoided where foru-wheel trailersare in use.

(5) Steep gradients for the trailer should be avoided, both uphill anddownhill. Uphill gradient8 (loaded) mean reduced trailer load (andhence lower productivity); downhill gradients require more sophisti-cated trailer brakes than are normally provided (see Chapter II).As far as possible gradients should be limited to between 5 and 100/,both uphill and downhill.

(6) Manoeuvring areas, particularly if reversing cannot be avoided, shouldbe as flat as possible.

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159, Figs 8 and 9 show some suitable arrangements for sites whereall the previous rules can conveniently be followed. Mhrum and stonequarries are often sited on gentle hillsides typified by Fig. 8. Thesimplest and most economical arrangement has a series of loading benches,between 1 and 2 m high, running in a line along a contour of the hill.The spacing between trailers should be just sufficient to allow a tractorand trailer to pull in or out without reversing the trailer. Eow hargethis space should be depends upon the skill of the driver, the type oftrailer and coupling arrangement, and the terrain. The slope is utilisedto give gravity aid to the loaders.

140. Fig. 9 shows possible arrangements in flat terrain, which istypical of the borrow area for high embankment earthworks. There is noslope to aid loading, so a trench is first cut through the borrow area,to the final depth of the pit, with entry and exit ramps, and wide enoughfor two trailers to pass; thereafter the trailer always stands in thebottom of the pit during loading, so the loading height is minimised.

141* Of course there are many situations where it may be impossibleor inconvenient to follow the six rules, and to some extent there isconflict between them. For example, a loading bench in flat terrainnecessitates use of sloped ramps; and the construction of a loading benchrepresents a certain investment which may only be justified if it is usedto load a given quantity of material, which in turn may imply a requiredminimum radius (manual haul length) from the trailer.

142 , In situations where any of the rules cannot be observed, thefollowing points should be borne in mind:

(1) In-confined sites where the number of trailers which can be accommodatedis limited, more labor may have to be employed in loading each trailerso as to keep the tractor occupied. The assumed labor productivity maythen have to be reduced to allow for the effect of congestion. Inextreme cases another loading site should be chosen.

(2) In a confined site two-wheel trailers should be chosen if at allpossible, for easier manoeuvring, since trailers will almost certainlyhave to be reversed.

(3) If steep gradients cannot be avoided, then use two-wheel trailers forgreater traction.

(4) If the quantity of work is large enough it could be worth spendingmoney to reduce local gradients in the loading area.

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Unloading Arrangements

143. In most civil construction works the unloading areas tend to becharacterised by man-made obstructions, as distinct from the naturalobstacles of the loading areas. The road construction site is probablythe most congested, but high embankments for canals or dams can also pre-sent problems as access will be determined, and then limited, by thepositions selected for the ramps.

144. Important factors to take into account are:

(1) If non-tippers are used then, unless they are uncoupled, unloadingshould be from both sides and backend (sides and end down as shown inFig. 12 (a)) to minimise delay to the tractor.

(2) The post and pin arrangements securing the (cellapsible) trailer sidesand ends should be regularly maintained to ensure trouble freeoperation. (This is particularly important when hauling rock, whicheasily leads to distortion of the trailer sides). Pins should beattached to posts with chains and a hammer and lever should be keptwith the tractor. Otherwise several minutes per cycle can easily bewasted and if the job of lowering and raising sides becomes toointractable the laborers will try to unload with sides up, thus wastingmore time.

(3) As with loading areas, reversing of four-wheel trailers should be%avoided, steep gradients (over 10% on hard ground, 5% on softer ground)should be avoided and manoeuvring areas should be as flat as possible )(eg. Fig. 12 (b)).

(4) Material should be spread into layers (or stacked in heaps) as soon aspossible after inloading so as to give the following tractor a reason-able running surface.

(5) It should be remembered that the trailer may itself constitute anobstacle to other construction work, while it is being unloaded.

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VI. PRODUCTIVITY AND UNIT COST CALCULATIONS

145. This chapter describes how to calculate the productivity of tractor/trailer haulage and the associated unit costs. Some work elements and hourlyoperating rates derived from observations in India are given in Appendix B,for use where no reliable local data is available. It is emphasised, though,that any data given in this memorandum should be supplemented or replacedby data observed in the locality concerned. In some cases it may be possibleto carry out site trials to obtain certain data, such as tractor speeds,before planning the operation.

Time Definitions and Conversion Factors

146. The basic definitions of working time, non-working time, availabletime, lost time and total time correspond to those given in TechnicalMemorandum No. 8 (Field Manual), as do the lost time and site factors('s' and 't') which relate to them. (13)

147. When using these quantities in the estimate of the unit costs ofa tractor/trailer operation, however, three definitional complicationsarise:

The practical tractor Wcle time will generally be greater thanthe 'ideal' cycle time, hich is the sum of the calculated elementsof haul, unload, manoeuvre working times, etc., to account foroccasional operati9nal problems and obstacles, short periods ofdriver rest, etc. 14)

(2) The unloading labor may work at an abnormally fast pace for the fewminutes required to unload a trailer, and for purposes of calculatingthe gang size in Chapter V this is 'working time'. Even afterallowing for a period of exceptional rest, however,,the loaders mayhave to wait for a considerable time between successive trailers;but the complete tractor cycle time may also be correctly definedas 'working time' for them, if there is no way that alternativework can be conveniently organised during the waiting periods.

(13) Briefly, working time (WT) is defined as Available Time (AT) lessNon-Working Time (NT), where NT is the time when the resource is notfurthering the progress of the work (e.g. late starts or unnecessary restbreaks). AT is defined as the Total Time (TT) less the Lost Time (LT),where TT is the period during which work on site would normally be doneand LT is the time when production may not be possible or practical. Fulldefinitions and explanations of these time categories are given inTechnical Memorandum No. 8 of this series.

(14)(4 In Technical Memorandum No. 8 all such delays, up to a total of15 minutes per hour, may be subsumed by an observer under "working time".

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(3) For productivity assessment the tractor working time should be takenas the time during which work cycles are performed; but for costingpurposes working time:

(i) will include running time to and from depot, or betweentasks (generally accounted for under overheads);

(ii) may exclude short breaks during the cycle when the engine isturned off (e.g. during unloading), the inclusion or exclusionof such breaks depending upon the method of observation.

148. For simplicity the working time of all resources will be defined asthe 'TASK WORKING TIME', which will be taken as synonymous with the practicaltractor cycle time. Then the site factor (s) also applies equally to allresources:

Task site factor (s) = task working timeavailable time

except that for calculating tractor operatingcosts ONLY a separate site factor (S is usedC

where s = tractor running timecavailable time

149. The following further definitions are added:

Pract ca cycletime (15) (t C) = ideal cycle time (ti)

+ cycle allowance (a) mins.

Cycle factor () = t c/ti = It + a/t

Fast unloadallowance (b) unload time at'normal WT rate mins.

less actual unload time

Fast unloadfactor (g) = actual unload time

unload time at normal 'WIT rate

(Note: See para. 151 Step 4 for application of 'g q).

(15) See footnote (16) para. 150.

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Calculation Steps

150. The calculations are set out as a series of steps, the major quantities,in order of calculation, being:

(16)(a) tractor cycle time ,

(b) tractor cycles per day,

(c) input coefficients (labor, trailers, tractor(s)),

(d) rates (i.e. hourly resource costs), and

(e unit costs.

151. Table 5, which appears at the end of this chapter (p. 48) lists allthe variables which appear in the calculations, along with the correspondingsymbols and units. In this table may also be found some of the more frequentlyrequired equivalences between variables. The calculation steps should there-fore be read in conjunction with Table 5.

Step 1. Calculate haul time (th). Estimate average speed or, having firstselected the tractor, trailer and haul route, as outlined in Chapter IV,calculate the travel speed over various sections of haul route in accordancewith para. 71.

th 1 +t2 +t3 etc.

1 +2

Setc. x 60 mins,3 )

summing over the whole haul route, loaded and return.

or th= 2 Iv x 60 mins

SteD 2. Estimate combined manoeuvring, uncoupling and counlingtime in loading area (tml). mins

Step 3. Estimate manoeuvring time in unloading area (includinguncoupling times if applicable) (tM) mins

(16) Note that trailer cycle time is longer than the tractor cycle time,so the cycle must be specified in terms of the relevant resource. In thismemorandum 'cycle time' means tractor cycle time unless otherwise specified.

Step 4. Estimate unload time (tu) if tippers are to be used. If loading isto be done manually, without uncoupling trailers, calculate optimumunload time, taking the input coefficient (j) from Appendix B, andapplying, the 'fast unload' factor (g) suggested therein, using theunits and symbols set out in Table 5, that is:

No. of unloaders................Nu

labor rate......................r1 $/man-hr (WT)

labor input coefficient.........gj man-hr (WT)/tonne

trailer payload... ............ tonne

tractor/trailer rate, idle......r. $/hrIr

unload time... .............. t U mins/cycle

tractor cycle less unload time..t mins/cycle(i.e. labor delay time) r

then: tractor delay time = t = 60,qgj/N mins/cycleu u

so: combined delay costs = Nu r1 tr/60

(labor)

+ qgjrir/NU $/cycle

This quantity is minimum when both labor and equipment delay costsare equal,

i.e. when Nu r1 t to = rir qgj/Nu

i.e. when Nu2 = r. qgj 60r t1r

Hence optimum unload time,

t =60 qgjt xntu r 1 minsr.

of if N is fixed,u

tu = 60 qgj/NU mins

If trailers are to be uncoupled for unloading time, unload time shouldbe calculated in accordance with Step 7.

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Step _. Calculate ideal tractor cycle time (t ) from the above elements:1

fih ml mu u mn

This represents the minimum cycle time in ideal circumstances.

Step 6. Practical cycle time(17) (tc). Convert the ideal cycle time to theaverage likely to be obtained during working time, by estimatingthe cycle allowance (a) or factor (f). Note that if the tractor ismanually unloaded the driver can take rest during unloading, so theallowance may be reduced.

tc i + a = ft1 mins

Step 7. Calculate the optimum balance of labor and trailers per tractor, asdescribed in Chapter V. Use labor (WT) input coefficients fromAppendix B for loaders, also for unloaders where trailers are un-coupled for unloading, taking the relevant loading and unloadingheights. (For manual unloading without uncoupling, see Step 4.).

N1 = 60 qi/t c

N = 60 qj/t

(trailers uncoupled to unload).

Step 8. At this point input coefficients for all resources can now be cal-culated in terms of working time, which is the most convenient formfor calculating unit costs. The engineer will probably wish however,to estimate the number of cycles (and hence output) obtained in anormal day's working. A 'NORMAL DAY' will be here taken to mean aday during which there is no lost time (i.e. it consists entirely ofavailable time) and so the total output over a period of, say, amonth, must be adjusted downward by applying the lost time factor(t) to the 'normal day's' output.

Step 9. Establish the number of site hours (AT) in a 'normal day' (h).

This will normally be 8, but piecework or double shift working mayincrease it.

(17) See footnote (13) para. 146.

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Step 10. Calculatelthe number of practical cycles per normal day (n), usingthe task site factor (s). Note that if the tractor has to return tothe depot each evening, s (and so WTT) will be lower than otherwise (18)

n = sh/t

This number will be fractional. For simplicity it may be roundeddown (or up) to a whole number; but in fact the number of cycleswill vary from day to day, so the fractional value of n calculatedmay be taken as an average over a period of days.

Step 11. Calculate normal daily output

Normal daily output = nq tonne/day

Step 12. Summarise resources and out-ut for one unit (normally one tractor,but it may be more):

tractors N no. (generally 1)

trailers - N no.r

lao(19)labor0q N no.luno

tractor cycle time t mins

load per'cycle q tonne

(18) Even though time running to and from depot is classified as workingtime for purposes of calculating tractor operating costs.

(19) If unloaders' work is split between two or more units, or if itincludes work extraneous to the tractor haulage task, then the number ofunloaders included in the labor gang should be apportioned accordingly. Ifunloaders spend time, say, spreading or stacking, then such activities maybe included within the definition of the task.

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Step 13. Calculate input coefficients for the LEU(20) task:

(1) tractor(s): kT = to hr(WT)/tonne6 0q

(2) trailers: kr =Nr to hr(WT)/tonne

NT 6 0q

(3) labor: k(1 )N n t man-hr (WT)/tonne

1T 60q

If input coefficients are required in terms of available time, simplydivide the above quantities by (s),

e.g. labor input coeff. = kl/s man-hr(AT)/tonne

Step 14. Tractor fate. To convert input coefficients to unit costs theoperating cost ('rate') of the equipment must be known. If figuresare laid down by the organisation, take these hourly figures, in-cluding fuel and oil, to which the driver's wages may or may nothave to be added; or they may include a fixed hourly cost fordepreciation, maintenance, taxes, etc. plus a fuel cost based onengine running tige. Alternatively the method'described in TechnicalMemorandum No. 10 22 ) recommends a two-part charge for depreciation,etc., consisting of a fixed hourly rate for up to the planned numberof hours per month or season, plus a lower rate for hours in excessof those planned; fuel and oil and driver's wages are separat:9lycharged. However the figures m3y be specified, they must be conve'rtedinto rates corresponding either to available time or to working time,preferably the latter.

Step 15. Establish tractor costing site factor (s). This,allows for timerunning to or from depot, and travellingobetween sites, to be classi-fied as working time (WTo), though the actual cost of such runningtime may be allocated to overheads, not to the specific task.

(20) See footnote (19). L - Load, H -Haul, U - Unload.(22) Technical Memorandum No. 10 of this series entitled "A System ofDeriving Rental Charges for Construction Equipment".

(21) Note that this is not necessarily equal to (i + j), owing to unloaderidle time.

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Step 16. Calculate operator rate (ro) in terms of working time:

r = monthly wages $/hr(WT )s tm h

C

The wage includes driver plus any assistant attached to the tractor.

Either Step 17a, if fuel is costed separately:

let fuel rate =r $/hr(cT

and let average depreciation,etc. rate = rd $/hr(WT)

then overall operating rate,r= r + r + r $/hr(WTrT o dC

(unless rd includes operator cost, in which case ro is ignored).

Or Step 17b, if separate working (r ) and idle rates (r.) are specified. Insuch a case the former wilL include fuel costs. The overall costincurred per working hour is:

r WT + r. NTw c1 cy

WT0

which may be re-written as: w + r. (1/sc 1)

Thus overall operating rate

rT =r +r + r. (1/s - 1 $/hr(WT)'

(unless r and r. both include operator cost, in which case ris ignoreA). 1

Step 18. Trailer rate (rr), if not specified by the organisation, can becalculated in accordance with Technical Memorandum No. 10. Alter-natively, see Appendix B for 'rule of thumb'. The rate is normallygiven in terms of available time (Rr). Then

R6 -r/s $/(WT)

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Step 19. Labor rate (rl). Labor is usually employed and paid on a total timebasis, that is,-they are paid even though work may be stopped owingto breakdowns, rain, etc. ('lost time'). In some cases labor may notbe paid for complete days' stoppages, or they may be moved to differentworksites in case of breakdowns; but it is better to err on the con-servative side and assume that they are paid on the basis of totaltime. Then:

(1) R = monthly wage (RM a1 $/man-day(AT)

tm (i.e. per 'normal day')

(2) r=R =R(2) m $/man-hr(WT )s h s tmhc c

Step 20. Unit Costs. For each resource, tractors, trailers, labor, multiplythe input coefficient (from Step 13) by the relevant hourly rate.Both quantities must be expressed in the same kind of time: if inputcoefficient is expressed in terms of working time, so must the rate:

(1) tractor(s) u = rTkT $/tonne

(2) trailers: u = r k $/tonner rr

(3) labor: ul = rlk $/tonne

(4) total unit cost: U = + ur + ul $/tonne

If preferred, calculation of the labor or trailer, components ofunit cost can be done using available time rates and input coefficients;the end result is identical. Tractor unit costs must, however,be calculated using working time rates, owing to the discrepancybetween tractor costing site factor and task site factor.

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Timc, eo t . )e: dtal th,o, V1l. recourcea hr I. -itc per dAy or month e.AVible tiMe, all reasourac AT(='d+UT) do.

Workinc ti=e/nonworkizg time:

task level ""T, tT hr or minn per day or cycletractor co3tin? 'TO, NTI do.

Tractor cycla allowance a mine per tractor cycleFast unload allowance b mins per trailer loadLost time factor (all resources) t(=AT/'T ) (dimensionless)

Task cite factor (aIl resources) s(='/AT) do.

Tractor costing site factor so(=llAT do.

Tractor cycle factor (=1+at 1 do.Fast unload factor g(1-b/t0) do.

No. of site hours per noal day h hr(AT)/day

No. of working d3va per month m days/month(TT)

Tractor Cycle Time Slements Manoeuvre, hitch etc. in loading area t mine (ideal )/cycle

Manoeuvre, hitch etc. in unloading area " do.

Unload do.(Notes tu= 0 if trailer is detached to unload)

Eaul:

elements over short distances t +t2 +t, etc do.over whole route, loaded or empty t( ti*t2 +t, etc) do.

Tractor cycle time (ideal) t(=t +t +tu+ t) do.Tractor cycle time (practical) t (=t+at=ft) do.

Cycle time less unload time tr (tm a+tmu ) mins (ideal )/Cycle)No. of tractor cycles per day n(=sh/t 0 ) cycles (practical)/day

Tractor ?roductivitv Factors Tractor/trailer speed:

elements over short distances vj,7 2 '7 3 , etc. km/h

average loaded km/h

average empty Ve km/h

average loaded and empty 2 ) km/h

Haul leugth: a

elements over short distances 11,12113, etc. km

whole route (one way) 1 (= j(1 +12+1, etc))ca

Trailer pay load q tonneGang Sire Number of loading la:cor per unit 1 (no.

Number of unloading labor per unit Iu (no.)

Total labor per unit . 1 = 1 +u) (no.

Number of trailers per unit N (no.rNumber of tractors per unit T (no.)

InTut Coefficients Labor, loading -man-br(T)/tonne

Labor, unloading * do.Labor, complete task k (2) do.

Trailer kr trailer-hr(W )/tonneTractor k tractor1r(W Ytonne

?ates Labor (on monthly basis) a 3/month(T')Labor (on daily basi3) Rl(=a1 t,) :/man-Jay(AT)Labor (on hourly basis) rl S/hr(%TTrailer Rr 3/hr(AT)Trailer rr S/hr(%T )Tractor, while working only r.? S/hr(WTC)Tractor, -.hile idle only ri 3/hr(NT)Tractor, total cost per working hour rT(=,$T +r,,,IT

'MT

Tractor operator (incl.assistant) ro 3/hr('-TcTractor fuel rrh( )Tractor depreciation, etc (ave. ) d/hr(,TcTractor plus 1 trailer (idle) r1r(. T +R) S/hr(N'T)

Unit Cont of Cn"lIete Task Labor only U,(= rlkl) 3/tonneTrailers only u.(=rc) /tonneTractor only U(= rk )'-/tonne

Total unit coot U (= u 1 1u"11-) S/tonne

Full definiins of -zme components and time factors are given in Technical MemorandumNo. S (-ield Taual).

(2) kr-i+j, owing to unloader idle zine (see text, Chapter 7).

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VII. COMPARISON OF TRACTOR/TRAILERS WITH TRUCKS

Break-Even Haul Distance

152. A convenient way to compare the efficiency of tractor/trailers with tracks is to calculate the unit costs of the haulage taskover various distances by the two methods. The haulage task can besplit into two components:

(a) the haul activity, the costs of which-vary withhaul distance, and which will henceforth be referredto as 'variable costs';

(b) the load and unload activities, the costs of whichare independent of haul length and which will bereferred to as the 'fixed costs!.

153. The advantage of a tractor/trailer is that the fixed costsare usually lower than for truck haulage, while the trucks generallypossess the advantage of lower variable costs. Thus tractor/trailers aremore likely to prove cheaper for short hauls, trucks for longer hauls, andthere is normally, therefore, a break-even haul distance where costs arethe same by either mode.

154. The actual break-even distance depends upon the values ofthe fixed and variable costs for.both haulage modes; tractor haulage iscommonly found to be che-per up to 1 or 2 kn, sometimes upto5andoccasionally up to10 u m2 m)

Factors Affecting 3reak-Even Point

155. As has been seen in Chapters IV, V and VI, the cost ofvehicle haulage is influenced by a large number of parameters : a fewgeneral comments only will be offered on the subject of the most significantof these.

156. Haul costs, payload and haul route condition. Initial costsand hourly operating costs per unit payload tend to be fairly similar forboth truck and tractor/trailex. For example a 10-tonne truck would costaround twice as much to operate as a tractor hauling a 5-tonne trailer,possibly slightly less. On a particularly good haul route, with easygradients (assuming that the route is good throughout its length) a tractor

(23) There are unusual cases where fixed costs are lower by truck, in whichcase truck haulage is cheaper over all haul distances and,vice versa, othercases where variable costs are lower by tractor, making tractor haulagecheaper regardless of haul distance.

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may be able to haul an above-average size of trailer, thus reducing itscost-to-load ratio. The converse, however)does not apply, since on aparticularly poor haul route or on a steep gradient a reduced trailerpayload will probably be matched by reduced truck loading.

157. Haul costs, speed and hau route condition. Given averagehaul routes, however, and roughly equal cost-to-load ratios for the twomodes, the unit costs of the haul activity are usually lower for a truckbecause, being a sprung vehicle, it can generally travel faster than atractor.' The better the haul route the bigger the relative differencein speed between truck and tractor; on a good,surfaced road the truck maytravel at up to three times the speed of the tractor (say 60 and 20 km/hour respectively), while on a rutted earth track both types of vehiclemight be constrained to similar, much lower, speeds. Thus the haul routecondition can play an important role in the cost comparison.

158. Fixed costs - labor costs. Labor costs (for loading andunloading) do not differ greatly between the two haulage modes, thoughthey will.usually be somewhat lower for tractor/trailers since:

(a) trailer loading height is somewhat less; and

(b) it is easier to organise a tractor/trailer operationto avoid labor sitting idle travelling on vehiclesbetween loading and unloading points.

159. Fixed costs - vehicle delay costs. It is in the avoidingof vehicle delay that tractor/trailers possess. their biggest potentialadvantage. A truck may often spend half its time on site idle whilebeing loaded or unloaded. Of course no fuel is consumed during this idletime, and it must be remembered that some time is spent by the tractor incoupling and uncoupling; but on short hauls (upto 2km, say) delay costsfor a truck can often represent a quarter to a half of its total operatingcost.

160. Delay costs for a truck will be reduced if loading areaefficiency is increased by paying piece rates or by establishing loadingplatforms or loading hoppers. Similarly tipping to unload reduces delaycosts for both truck and tractor. In a tractor/trailer operation delaycosts are increased by the addition of greater numbers of trailers toeach unit.

Influence of Costing Method

161. Tractors are almost invariably costed on an hourly basis(see Chapter VI). A truck may be charged on the basis of either time, orof distance travelled, or on a combined time/distance basis. In the caseof hourly costing (for tractor or for truck) there may be a single rateor there may be a separate rate for working time and idle time.

162. Distance basis. Where truck costs are based upon distancealone, delay costs are calculated to be zero : incorrectly, of course,since the costs of interest, fixed maintenance facilities,taxes and operator

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are still being incurred. The conclusion is almost invariably that thetruck is more economical than the tractor/trailer, which suffers an'unfair' disadvantage.

163. A single hourly charge, however, gives an 'unfair' advantageto the tractor, since it leads to an over-estimate of the truck delay costs.Fuel costs may or may not be included in the hourly charge; even if fuel isaccounted for separately, the hourly charge will include the full rates formaintenance and depreciation, which should in fact be lower when the vehicleis not running. Nevertheless, as long as fuel is accounted for separately,the error incurred when using a single hourly charge for the truck iscertainly less than that involved in using a distance based cost.

164. Composite charging methods are more appropriate for givinga true comparison of the relative costs of the two haulage modes. Thereshould be two rates, ideally:

(a) an hourly 'site rate', chargeable for all hours duringwhich the machine is on site; and

(b) 'a 'working' rate, which may be either time or distancebased, in either case reflecting the marginal cost ofrunning the vehicle, over and above ownership costs(preferably rates for tractor/and truck should bebased both on time or both on distance).

Example Costing Exercise

165. A comparative costing exercise, carried out in India, andusing assumptions related to public works organisations in that country,is presented in Appendix C. Only equipment costs were considered, laborcosts we7e ignored.

166. Sixteen different sets of assumptions were considered inthe exercise; for each set unit cost/haul distance graphs are plotted inAppendix C. The values of the assumed parameters are set out in detailin the appendix, but the variables can be summarised as follows:

Assumption (Al) : low vehicle initial costs, low interest (120o), lowrepair factors associated with short life.

(A2) high vehicle initial costs, high interest (24/) highrepair cost factors associated with long life.

(31) : high utilisation, high truck payload (double trailerpayload), low load/unload times.

(32) : low utilisation, low truck payload (equal to trailer),high load/unload times.

(Cl) : low speeds, corresponding to fair haul route.

(C2) : high speeds, corresponding to good haul route.

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(l) : new vehicles.

(D2) : old vehicles (at the end of their economic life).

Unit costs were assessed for both truck and tractor/trailer operation foreach possible combination of A, B, C, and D (16 in all). There are certaintheoretical shortcomings in the method adopted, in the exercise, but itserves as a useful illustration.

167. Equipment rates are made up of two parts:

1) Time rate, consisting of:-

(i) interest on outstanding capital value(24)(ii) fixed costs (tax, insurance),(iii) driver's wages.

(2) Useage (distance based) rate, consisting of:-

(i) repair and maintenance charge(25)(ii) fuel and oil,

(iii) time cost (sum of (1) above) during running time.

The first rate is applied during loading and unloading periods, the secondduring the haul activity.

168. The calculations were all carried out in terms of Indianrupees. The results, plotted in Appendix C, have been converted to US $at the rate of Rs 8.00 to each US $ 1.00.

169. Main conclusions from the costing exercise are that, giventhe assumptions chosen, and in particular taking the assumption that thetruck carried twice the payload of the tractor/trailer (B.)

( Break even haul distances are all in the range 1 to 3 km.

(2) Break-even distances are greater with new vehicles (2-3km)than with old ones (1 - 1.5 km), because the interest chargesfor the old vehicles are lower (consider a contractor withan old truck : he is not worried about it standing idle).

(24) This item distinguishes between new and old vehicles, the latterincurring lower interest costs and therefore lower delay costs (althoughrepair costs, which are a function of distance travelled, will be higher).

(25) This item is taken as a fixed quantity per km run throughout the lifeof the vehicle. This is strictly incorrect (it gets higher, the older thevehicle) but the error makes little difference in the comparative cost.

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Break-even distances are about 30 greater with the higherinitial cost and interest assumptions, for new vehicles,but for old vehicles initial cost and interest rates makeno difference. Again the reason is higher delay costs forthe truck.

4) Variation in running speed, within the range considered(7.5 - 12.5 km/h for tractors, 10 - 20 km/h for trucks)has only a small effect upon absolute unit costs (costsare around 15 - 20o higher at the lower end of the speedrange than at the higher).

170. If truck and tractor/trailer payloads are equal, as assumedin (B2) above, tractor/trailer haulage is cheaper than truck haulage forall possible combinations of other assumptions, up to thelimiting hauldistance considered (10 km).

171. Although graphs in Appendix C do not show specifically theeffect of low utilisation and high delay time combined with high truck payload(double the trailer payload) it can be deduced from the graphs that break-even haul distances uider such conditions range from 3 - 4 km for oldvehicles to 5 - 7 km for new; the effect of variation in speed or in set('A) variables is relatively slight.

172. It can thus be seen that, assuming that variations in initialcost, interest rates, repair costs, and vehicle life apply in like fashionto both trucks and tractors, the major determinants of break-even hauldistance are firstly payload, secondly the age of the vehicles (greater agefavouring trucks). The effect of a third factor which probably has animnortant influence, speed, was not properly evaluated in the study, whichlimited itself to a maximum truck speed of 20 km/h. Likewise the influenceof the method of charging was not evaluated, since only one method wasconsidered.

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VIII. OTEER TRACTOR IMPLEMENTS

173. In addition to the trailer, a wide range of implements may be

attached to a tractor. Most of these were originally developed for

agricultural purposes, but sane are also applicable to civil construction

work on a medium scale; others are designed specifically for construction

jobs. Implements of interest to the engineer include: low-loading trailers

or platforms; water tankers; ploughs and rippers; earth-moving attachments

such as soil scoops and scrapers, dozer and grader blades, and loaders; androllers, both towed and tractor mounted. A short note on each follows but

for full details consult tractor manufacturer6l catalogues, or consult

manufacturers directly.

174. When planning work with various implements, it should be remembered

that the fitting time must be taken into account when assessing productivity;

there will be a minimum economic time during which each implement must work

before fitting a different tool. This minimum will be longer for implements

which take longer to fit, such as a front-end loader.

Haulage Attachments

175. Low loading trailers and platforms are available in many varieties,

ranging from a platform which clips onto the back of the tractor (Fig. 14(b)),

to an independent pallet trailer (Fig. 2 (c)). They are useful for carting

small pieces of construction hardware around the site (tools, wheelbarrows,

etc.) and for transport of packaged materials such as cement, bitumen and

bricks.

176. Jib crane attachments are also available as a loading and unloading

aid, see'Fig. 14 (a).

177. Water tankers (Fig. 14 (c)) are frequently needed to fetch water

for compaction. Sizes from 400 up to 1000 litres are common, in either

two- or four-wheel versions. The considerations that apply to trailers

apply equally to such tankers.

Ploughs and Rippers

178. Ploughs, rippers and subsoilers (Figs 15 and 16) are basically

agricultural implements, but they can sometimes be used successfully for

loosening soil in borrow areas for earthworks. There are three important

differences between agricultural ploughing and earthwork excavation in

civil construction:

(1) Earth to be loosened in borrow areas is often much harder than

the soil found on farmland which is constantly being worked,

particularly since construction work tends to be concentrated

in the dry, hot seasons.

(2) In addition to loosening, a plough is designed to turn over the

soil and to aerate it, activities which are unnecessary inexcavation and wasteful of energy.

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(3) A farmer wishes to plough an extensive area but to no greatdepth, whereas excavation in borrow areas is concentrated insmall areas but proceeds typically to a depth of a meter.

A full discussion of the use of ploughs and rippers and of the effectof water upon soil hardness, based on studies carried out in India,appears in Technical Memoranda No*1-9 -of thiseries entitled "Excavation".Some important points are noted below.~

179. Penetration. It is important to obtain the greatest possibledepth of penetration. If the depth of the loosened layer is too small

(less than about 200 mm), the soil becomes difficult for laborers to

load. Even at 200 mm depth there may still be problems: work may have

to be spread over an inconveniently large area, particularly on small

embankments where quantities per metre run are small, and measurementof workers' output becomes awkward.

180, Cross bunds.. The maintenance of cross bunds in the pits

(sometimes needed to prevent flood erosion) can present a problem

where tractors are used to excavate. If the complete depth can be

out in one pass the problem is lessened, since the tractor remains

at ground surface level throughout the operation. If the work is done

in two layers, b=nds may be rebuilt for the top half only. With more

than two layers, bunds will have to be rebuilt starting nearly from the

full depth of the pit.

181. Equipment and productivity. To overcome soil hardness and.to

avoid wasteful tuning over of the soil, a ripper is preferable to a

plough in excavation work, since it simply cuts the soil. A ripper,

particularly a 'subsoiler', will also penetrate to a greater depth, one

manufacturer claiming between 600 and 800 mm for a subsoiler. If cuts

to a depth of 500 mm can be made, then a complete borrow area may be

loosened in two passes of the ripper; this compares f a plough which,

as observed in action in India on a stiff (grade 4) 2 clay l

penetrated to a maximum of only 200 mm with a 48 hp tractor.

182. To obtain maximum effectiveness from the tractor, as with

trailer operation, it is necessary that the plough or ripper linkage

be designed so as to force as much weight as possible onto the tractor

rear wheels (the tractor can also be ballasted). 4The (limited) experience

from the India study also si,ggests that if the soil is hard enough to be

worth loosening by machine L28),then a 35 hp tractor may be too small

(26) See Technical Memorandum No. 8 (Field Manual) for soil hardness

classification.

(27) Penetration in very stiff (Grade 5) soil with a 35 hp tractorvaried between 30 and 80 mm.

(28) Probably grade 4 or a hard grade 3 as defined in TechnicalMemorandum No. 8 (Field Manual).

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(much better results were obtained with a 48 hp model although the soil

was a little softer and the design of plough was different in the latter

case).

(29)183. Tn an attempt to improve productivity in very stiff clay soil

in the Indian study, water was spread the evening before ploughing. Even

with large quantities of water spread (72 litre/m2 ) depth of water

penetration was only100 mm and the increase in ploughing productivity was

slight, not sufficient to offset the cost of bringing the water a distance

of 1 km. It is unlikely that water spreading can prove economic, except

where the water is available close to the site so that it may be -broughtmanually.

184. The economics of soil ripping depend not only upon the rate of

output while the ripper is actually cutting, but equally critically upon

the ratio of cutting time to the overall working time of the tractor

(including turning, running between cuts and coupling). To keep coupling

time to a minimum the linkage ball joints must be properly maintained; even

so, the job may take a few minutes. The ratio of cutting to total working

time becomes greater with longer runs, but if runs are too long the work

may become inconveniently dispersed. Experience in India suggested that

the ratio of cutting time to working time is unlikely to exceed 50o in

practice, and it will often be less.

Earth Moving Attachments

185. Soil scoops and scrapers (Figs 17, 18 and 19) may-be fitted to

the rear of a tractor, using its hydraulic linkage arrangements. Scoops

of up to 1.2 m3 capacity are available, and can be used as scrapers. For

maximum effectiveness the scoop connections should be designed so as to

force the maximum amount of weight onto the tractor rear wheels; this means

that the scoop should generally be drawn forward behind the tractor,

although some models are so designed as to be able to travel backwards,

for such jobs as backfilling. Control may be either by position (to

maintain constant depth of sub-grade) or by draught (to maintain uniform

loading rate). Dumping can sometimes be done by a trip system operatedfrom the driver's seat.

186. Dozer and grader (Figs 20 and 21) blades are available, alsotmulti-purposer blades which can either doze or, with a grader wheel

accessory attached, grade to a fixed level. It is usually possible

to adjust such things as blade pitch, sideways offset, angle to the

horizontal (for ditching) and length. Some models may be reversed for

back filling. While rear mounted blades are most common, and are often

designed to force more weight onto tractor rear wheels, front mounted

blades are also manufactured. Scarifying attachments can be used to help

levelling.

(29) Cohesive grade 5.

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187. Bucket loader (Fig. 22) attachments may be fitted to the frontor rear end of a tractor; they are operated by rams connected to thetractor0s hydraulic power system. One such model has a capacity of 0.3 m(about 500 kg of loose material), a cutting depth varying between 165 and400 mm, depending upon the tractor used, it can exert a cutting force ofabout 1500 kg f., and may be fitted to almost any size of tractor.

Rollers

188. Towed rollers (Fig. 23 (b)) are available in both smooth-wheel andsheepsfoot styles. They are most suited to compaction of soils: rollingof stone aggregates carries the danger of cutting the tractor 'tirey ,particularly during the early stages of compaction; while in surfacing workthe tires get covered with bitumen. The weight of roller which may be towedobviously depends upon the nature of the soil; some rollers can be filledwith water, thus allowing variation in weight. As with all implements,traction is improved if the design helps force more weight onto the tractor'srear wheels (so long as the rear axle and tires do not become overloaded).

189. Tractor mounted rollers (Fig. 23 (a)) are manufactured in India,possibly in other countries too. The tractor climbs up a short ramp ontothe roller base, which is driven by chain from a mechanical power take-offspigot on the tractor. The Indian models seihup t6 8 tonnes, and can bedriven, on most types of work, by a 35 hp tractor, giving a total weight ofup-tb-t tonnes. Because the tractor wheels are clear of the ground thisroller can be used for surfacing and stone base course compaction.

190. Mounting, however, can take an hour or more, so it is not worthusing such a roller for less than half a day at a time, preferably a fullday. Herein lies its main disadvantage, since on many smaller constructionjobs a roller is only needed intermittently for short spells of an hour ortwo at a time.

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Ix CONCLUSIONS

191. The organisation of an efficient tractor/trailer operation is

complicated by the wide range of options available, in both tractor and

trailer, which gives rise to a large number of possible combinations

of the two. In addition to a range of possible sizes of both tractor

and trailer, there may also be choice in the selection of thethird major variable affecting performance, namely the degree of load

transfer between trailer and tractor.

192. The degree of load transfer is of the greatest importance in

ensuring that the tractor has sufficient traction to avoid slipping

if any part of the haul rate is in poor condition or suffers from

steep gradients. Thus two-wheel trailers, with wheels placed near

the rear, are generally to be preferred. Four-wheel trailers in some

cases, however, may be more robustly built or have greater capacity than

available two-wheel models, and can perform as well as the latter on

good, level haul routes. If four-wheel trailers must be used in soft

conditions or steep gradients, traction can be increased by a pressure

control lift device or, less effectively, by ballasting the tractor.

Load transfer from a two-wheel trailer is limited by the strength of

the hitch and the rear axle and ti-es on the tractor.

193. For very rough guidance, on a typical haul route consisting of

a sho3Tt length of rough but compacted soil and a 5 to lo gradient,

followed by a longer section of well maintained, fairly level road,

the following two-wheel trailer sizes (in terms of payload) are appropriate:

- 35 hp tractor : 3-tonne (possibly 5-tonne)

- 47 hp tractor : probably 5-tonne.

- 62 hp tractor : 7-tonne

- 75 hp tractor probably 10-tonne

194. The quality of the haul route in the (usually) short sections

in the loading and unloading areas has an important effect upon the

optimal payload for a given tractor, so it may often be worthwhile

spending money to improve both condition and, gradient in these short

sections.

195. Greatest economic benefits are obtained from a tractor when

it works in combination with two or more trailers, so that it is never

delayed by having to wait for a trailer to be loaded. In organising a

tractor/trailer operation it is therefore important to ensure that

coupling and uncoupling can be done quickly and reliably, also that

manoeuvring times'are not excessive. Use of tipping trailers can be

advantageous, particularly on short hauls or where payment is by piece

rate: but the advantage is lost totally if the tipping and tailgate

mechanisms are the slightest bit unreliable, since construction schediles

axe upset.

-58-

196. A tractor/trailer operation is most easily planned around thecycle time of the tractor. First the most appropriate combination oftractor, trailer and haul route is selected, so as to maximise theefficiency of the tractor, the most costly resource employed. Additionof estimated coupling, manoeuvring and unloading times to the haul timegives the tractor cycle time. This is the time during which the loadinglabor-must fill another trailer, hence the number required can becalculated. From the size of the loading gang the number of.trailersneeded (if more than two) can be calculated.

197. Unit costs can also be calculated on the basis of the tractorcycle time, due allowance being made for hold-ups, rest periods and othernon-working time.

198. Haulage of material by tractor/trailer is generally cheaperthan by truck over short distances, in the region of 1 to 5 km, sincethe operation can be organised so that the tractor need not stand idlewhile a trailer is loaded. The actual break-even distance is -affectedgreatly by travel speed and the load carried; a good haul route givesan advantage to trucks, which can travel very much faster than tractorson a good road. The method used to compute the equipment hire rate alsohas a great influence over the calculated break -even distance: if vehiclesare charged according to distance travelled only, the true cost of loadingdelay is not charged, and trucks are likely to appear to be cheaper thantractor/trailers over very short distances; a true comparison is given onlyby a two-part charging system which reflects interest costs as well asrunning costs.a

199. In addition to trailers there are many other implements whichcan be attached to a tractor. But, because the wheeled tractor and itsattachments were developed originally for agricultural purposes, existingdesigns are not always ideally suited to civil construction work, andthere is considerable scope for improvement.

- 59-

C(a) 3 hp TockorTowing wo-whee\ Vrailer

C13;

- Tra etr factorg knadec' .ë

1 . -

3)shp. Tracor

ow\m 9 TWo -wheei rai\er(neha

e ncý

F our whe,e,\ nn\er

toilI m'taler

-ig. \Tratorraoe mode.

Fig. i Tractors ard Trailers

* - -

- I' -4 - s.*4 I ~5

-. · s

r 44- r

* .v4

c oo C rednneb

4- 44. .

rr --. r

- . - -. ,

Photgraps bycoutesyof Msse -Feguso Ud

Lowerlinks TCoupler

D Cn

R 4D iC ITtegalve (169Porre, Drawbr

Forces on hrctor induced 69 pressure conh-<3) IiPV device.-- --- Forces on Iraller drawbar induced 6 9 pressure conVro\ lii-cdevje.

- - -- orrces on lroýler induced b9 pressure con\-rol llP dev-c e.

All forces shown ns po,sikve evcepb Por Croan- wheel \i - 1orc F.

By considering forces first on trailer drawbar,

then on tracto-x, it can be seen that the effect

of the device is to induce a lift force C onthe drawbar which is greater than the drawbar

reaction D; the result is a net upward force

T exerted on the trailer and a corresponding

equal downward force on the tractor; thisappears as two components, an even greater

additional downward force on the rear wheels Phoora<ph 69 courbey oFMasse9 -Ferc9 son~ L MR, balanced by a (smaller) lift on the front

wheels, F. The nearer the chain to the trailer

the greater the resultant downward force on the tractor.

Figure 3. Pressure Control Lift Device

I7

The hitch point between tractor and trailer is aheadof and below the tractor's rear axie.

WEIGHT TRANSFER POINT

PRESSURE PRESSURE

CONVENTIONAL DRAWBAR COUPLING GOOSENECK COUPLING

0

STEERING HUMPING' lVTPIO ROCKINGPIVOT

PIVOTDumpers are available with thefollowing variations

6,12 or 20 Ion capacity,

2,1 4 or 6 wheel drive with hydraulic drivefor rear dumper wheels.Earth or rock body

Tailgate or open ended

Fixed Hitch Quick Hitch Quick hilch or fixed hitch coupling

Diagrams by courtesy of the Harold Poole Group Lid., UK.

Fig. 4. Rear Iump Trailer (Catalogue)

No load is imposed on thehydraulics while transporting.The tractor hydraulics areused to couple up the tractorand trailer but once in thetransport position thehydraulic lin kage is leftcompletely free to move.

RECOMMENDED NEWDRAWB.AR POSITION RONT OF TRAILER

- TYPICAL TRAILERToDRAWBAR<TO BE REMOVED ..-

23' 315, 2<4

18.4/rs3O 10s

The Hitch transfers the weight on the traiier drawbaron to the front pivot points of the 3point linkage,underneoah Ihe. rear eýxle oE ýhe hroctor.Trxnsler;ng) lrhe w.e.9hil this fer For 1.orclirnproves 1*rac.Hon For beJer perFormncce cnd

HMPING ROCINGPIO IV T5a 6Zes fhe ractor so Rmt' ikcon accepi- 6argeroAr OAQ LOADr r

1. Part of the load is distributed to the front and rear tractor g h n rIrm .wheels as the attachment point is in front of the rear axle centre.

3. Steenng pivot

4. Hucking pivot rr v

3.e A.te ee pivotorIexlr't rol ers . on Ib l-k e le

w;nk spc!, ffleov3 :as linlccgobo cda> cei

-lrld Pos;Gc p tbdn 6 4 . I.: .F AIt~ ~'elink cw~sv J.he dm Jw'lý- zuppa

4. Rocking pivot ren-d . ýo r wel-di"É fl d:rowvjr,cýndi r% qvatsli Ørntc

ex drøcý=r i4~~

D"grarnz cond Phehecgnapi 6t) courý)~ 'The R-cro Fole Creup. Ud. W.<.

F~ig- 5 An.ti-Flip Hlitch (Catalogue)

1/

Sth gear

12

10

8

ti gear

E

4 rd gear

.2n gear

1st gear

(a) Travel speed 01000 2000 3000 4000 5000

Drawbar pull , kg •

60

41h gear

Sih gear rd Igear

40

30-

30 1s1 gear

20-

CL10

(b) Drawbar horsepower

40

20

(c) Slip jj 01000 2000 3000 4000 5000

note for test data seé DrQwbar pult kg.

Table 6 Drawbar te51 On larmacadam, with ballasi.

ipig. 6 Performani-ce Characteristics of a 62 hp Tractor

PRIELIMMLIJARY WORK TRACTOR CYCLE INMIBER(START OF DAY)

091 2 3 4

c c c cTRACTOR 3 3 MS [ 1:] ! M,* I U :--:11FE. /. L:U::N"\U'' :HE,/ HL U\:HE.:. H./

:4c c c cTRI1LERS A I//.... H/ / . L-c~: \

c cB 111 M E ED ///////////////HL.:N\\ '.HE: // L /Lw

H 7

-A77OURl :

n10AIIG GATG a [/A 1/ ///// R (///LOAO TR B//A-/7/ [A j 'T

C+

0 LOADMG~ GANGI b EDj '1/ EDOAD T -4ZXZZ 10A D TRZZ AbNLOADLIG GU~G El DE D []E DE DE ~' DE i D u E , DE

IATERIAL MOVEfNTTRåCTOR CYLE 1 ///// /////w- /// ¯:~ \. \ 0 0

XTRACOR CYCLE 2 ////// / // / /:. HL.u 0 00 0(D

TRACTOR CYGLE 3 ///////////// L ////////A Hii:- O 0 00TRAC201R CYCLE 4

/L /:HL•.:

- - NOTES

HL Haul loaded~ including manoeuvring (1) Before tractor can begin hauling, the first trailer (A) must be loaded. DuringF7-7. 11E Haul empty in unloading area this firat loading period the tractor, trailers B & C, loading gang b andunloading gang must inevitably stand idle ("wait")C Couple/uncouple/manoeuvre (in loading area) (2) Trailer & loading labour wait time : with a perfectly balanced group of resourcesL Load this will be zero. In practice either trailer & labour will be slightly under-utilised, or the tractor - preferably the former, as shown here.U Unload

(3) Unloader wait time : this time may be used for spreading or stacking work.w Wait (idle) (4) All activities are expressed in terms of working time.0 0 s Spread or stack (possibly)

HILLS[DE QUARRY

Graviy-feed .Gravity feed,-

MAIN ROAD

(a) Without Reversing

-Gravily feed

Gravity fZed

b) With Reversing

Traclor/trailer man~oeuvre (exaimple)

~ Roadway

.Edge of loaiding bench

Lacter position of edge of loaiding bench (possible)

Contour line

Possible trailer loaiding positions

x Minimum disance between trailers, determined by tria0 5 tol0m)

Stage1

Stage 2

Stage 4

Stage 4

Vertical Section Through Borrow Pit. etc

key: • Edges of borrow and filt areas

Ground between borrow ama.s to be left uncut

Stage(I) cul & houl route

.....--... . Edges of st age (2 G) cuts etc. up remrps ecasy-- Zngle (5%/)(stage (i)»

lraalor/'lail r troShort hcut,T t r tro rossibly hauled

etc.- ( i mtc. an ally.

0

\

~down ra rnmsi g

ddw

Plan of Borrow Area dowc

Fig. 9 Loading Arrangement in -Borrow Pit (Týheory)

(q) Earthwork(ndlik)

( M A~urr Q uQrrg 4

Lýd> c' sIzýv

TrQ;Iýer rnxce

~ &)Mururn CDuarrg

OA,

- -

I~ (I Nt

Fig. 10 Loadinag Arrangementbs(Traditional Prac-Lice)

--- )

- Lx

-(Q) MUrurn Quorrg

) Mururn QuQrrg

CQ)o) (6),e Quc;rry

& a linc)

I V

(c) o5ne Quorrg(Indiac)fU

L ad inc3b e)t. P(r V ený cü

k44,

bewabooI. c.4Lesy

onuc ed oren.

-- ?

Fig. 11 Loading Arrangements (Experimental)

*p

(ý t)cQdinC Mururn b L -oc> \nd r cý\ eie e and L&cck-- nre o ide c e

Fig. 12 Unloading Arrangements

(a) ch lveJ Paor pper

i.;-Z .. rV5

-- 44 'Leve] P nr-,or -h iper

(4 5 konne cQ CA'k

*Vl

Wigk 141h Level Ti ppe-,r

Automoe1c. Toc%ýe Opem'In9 (cSide pn nc To age

Fig. 13 Special Tipping Trailers (Catalogue)

a) ib Crane-(Caioii

By courtesy bOf

Ferguson lid. -

Fig. s Tr14 nrpr ter

P \cQVo m (cicii 9 u

c)~Q Va e\r Tonker

Fig 14 Other Transport Attachments

(cO) 7..0 rn cu E ná SHGF CILg)

4 -

-å

()50 ýo loory\YYN Cub in vert) ýHFF clag s nC4c)

36 hF ¢*C.dorU

Fig. 15 Ploughing (Experimental)

'4r

gi

ipe

~i RI

N

rr- g - 7bie -t g

Pheroegres he>a couÆess - ::Zisseg Wryu->, Ltd

Fig. 16 Subsoilers (Catalogue)

tCz

~~. r1 - -tMi4

Reor durnpi n9 Foruord urnp n19

Scoriier ~eel-h ond reorcoztcr whe&

Scroping

,4t-I 5.~j

b> coopes9

Fargson oL.

\ r

SCt: scoopmn9 Trcmepor%

Fig. 17 Soil Sooops (Catalogue)

*S

24ý k

-*r ri

S--ej

't -wA -

'bl2

~~ -

11

F--- rs y L n U cA

Fig. -c.ourYee -(Catalogue\

Fig. 18 Scrers (Catalogue)

긔

dip

1w1w1 NS1i4

' 14

Pr

4-k:

I. 4-ff

liv

C» kj v- L)

Fig. 20 Dozer Attachments (Cataloglue)

IVI

Q w -

Xl --

Fig- 2 -ae -(

Phe yophb 9 cour s OG E.C. MHIarn~ and ComlpQrn U-d. (.,

Fig. 21 Grader Attachmen.ts (Cataloguie)

* . - ' 7 ~ - -

i fi

!-C

"r Pv-c-

Figm. 2.--- Font-en - -"a>ScDn (a1re

Fig. 22 Front-end Loaders (Catalogue )

(a)Three Views oE c Tpc-or-Mounbed Rol er

Ph&lo9raphs of Trac&er-noun-e.d roll.c by9cocurt-es9 of kmrenon, Encieerenc3corpo~roon L -d. (i ndicn)

ià

(b) lowedSheeps3ao Co lle

FI 2Rl

Fig. 23 Rollers

APPENDIX A

Selection of Ap ropriate Tractor/Trailer Combinations

Tables

Al. Tractive, Resistance and Haul Route Coefficients forVarious Gradients

A2. Tractor Loading Capacities

A3. Trailer Loading Capacities

A4. Definition of Degree of Load Transfer

A5. Load Transfer for Tractor/Trailer Combinations

A6. Gross Weights and Tractive Loads for Tractor/TrailerCombinations

A7. Limiting Haul Route Condition for Various Tractor/TrailerCombinations

A8. Maximum Drawbar Pull at 16 km/h

A9. Maximum Trailer Gross Weights at 16 km/h

A10. Tractor/Trailer Combinations Which Fulfill Speed Criterion

Figures

Al. Gross Weights & Tractive Loads for Tractor/TrailerCombinations

A2. Resistance/Tractive Coefficient Ratios at Zero Gradient

A3. Resistance/Tractive Coefficient Ratios at 5Yo GradientA4. Resistance/Tractive Coefficient Ratios at 10o Gradient

A5. Resistance/Tractive Coefficient Ratios at 15o GradientA6. Resistance/Tractive Coefficient Ratios at 20% Gradient

A7. Estimation of Rimor Drawbar Pull from Travel Speed

A-Dendix APage 1

Selection of Appropriate Tractor/Trailer Combinations

Al. Chapter IV of the main text refers to an exercise inselection of technically feasible and economically optimal tractor/trailer/haul route conditions from a set of five groups of variables:-

4 tractor sizes;4 trailer sizes;4 degrees of trailer load transfer;5 grades of haul route condition; and5 values of (uphill) gradient.

The data and calculations for this exercise, and the detailed resultsof it, are contained in this appendix. The principles underlyingthe calculations and the broad conclusions are presented inChapter IV of the main text.

A2. The data and associated calculations are set out in aseries of tables. Tables Al to A7 are concerned with the applica-tion of the 'pull criterion' (para. 75, main text), Tables A8 toA10 with the 'speed criterion'.

A3. Table Al calculates the haul route coefficients (ie.ratioof tractive to resistance coefficient) for the 25 selected haulroute condition/gradient combinations.

A4. Table A2 lists the tractor data, that is:

(a) front and rear wheel load and gross tractor weight;

(b) load limits on rear axle, rear tires, standardautomatic hitch and linkage drawbar;

(c) leverage ratio of hitch, or drawbar, to frontwheel about centre of rear wheel; and

Wd maximrN rimpull.

From this data it calculates axle, tire and front wheel stabilitylimits on trailer load transfer. The stability limits are cal-cilated just for interest: since instability can be countered byapplying a small weight to the front of the tractor the issue ishenceforward ignored. The question of ballast on the rear wheelsis also ignored, for reasons outlined in the main text.

A5. Table A3 presents the trailer data, i.e. weights andtransfer loads.

A6. Table A4 lists and explains the four assumed degrees ofload transfer from trailer to tractor.

A7 Table A5 brings together the various limits to loadtransfer, listed for the tractors in Table A2 and for the trailersin Table A3, and for each of the 64 tractor/trailer/degree ofload transfer combinations lists both tractor limit and trailer

Appendix APage 2

limit, comparing the two, to specify the lower value as the actualtransfer load. The limiting factor is stated. Where the tractorlimit is substantially below the trailer transfer load (whichlatter is fixed by the wheel position), that combination is thence-forth ignored, on the grounds that the trailer would have to runonly partly loaded. Where the tractor limit is marginally belowthe trailer transfer load the lower figure is entered as the actualtransfer load in brackets, to show that the trailer is not runningfully loaded but that it still carries significantly more loadthan a smaller trailer.

A8. Table A6 lists the tractor and trailer contributionsto both gross weight and tractor rear wheel load (tractive load)and calculates the total weights and tractive loads.

A9. The next stage is to compare the haul route coefficientfor each particular route with the ratio of.total tractive load tototal weight for each equipment combination. This job is carriedout with the aid of Figs. Al to A6 inclusive. Onto Fig. Al areplotted the total tractive loads and total weights. Overlaid onthis plot, successively, are Figs. A2 to A6 which show the ratioof resistance to tractive coefficient (inverse of haul routecoefficient) drawn to the same scale, one for each value of gradient.If the point representing the equipment combination lies abovethe line representing a particular haul route condition, then theequipment will be able to move in the lowest gear on that haulroute (see main text, paras 76 and 77, for full explanation).

A10. For a given gradient, the condition of haul route(graded A to E) on which each equipment combination can move canbe read from Fig. Al in conjunction with the appropriate overlay,in terms of probability of being able to move. These limitinghaul route condition figures (which are those which satisfy the'pull criterion') are set out in Table A7 (see notes to that table).Selected values from Table A7 are given in Tables 3 and 4 of themain text.

All. The first step in the application of the 'speed criterion'is to calculate the maximum available drawbar pull for each tractor sizeat a speed of 16 km/h. This is done in Table A8; the calculationincludes a correction to allow for the stepped form of the speed/pull graph, which is explained in Fig'. A7.

A12. Table A9 converts these drawbar pulls to trailer grossweights by dividing the former by the rolling resistance of thehaul route. It is assumed that the condition will be grade A('good'), since bumpiness is likely to restrict speeds to wellbelow 16 km/h on poorer haul rou-tes. Within the possible rangefor a grade A route (Table Al) three values of rolling resistanceare considered, the maximum ('worst'), the average and the minimum('best'). Table A9 gives suggested types of surface correspondingto each.

A1. In theory the above calculations should be carried out interms of rimpull, not drawbar pull, and total (tractor plus trailer)

Appendix APage 3

weight, to allow for variation in the power required to shift thetractor on different haul routes. But since this power loss issmall compared with the total available power in this case, it canbe ignored without significant error.' When selecting appropriatetractor/trailer combinations, in marginal cases the next largesttractor (or next smallest trailer) is selected.

A14. Table A10 lists the sizes of trailer which can be pulledat 16 km/h by each of the four tractor sizes, over each of thethree quality levels of a grade A haul route referred to above(para A12), assuming that the ground is level.

Table Al. Tractive, Resistance & Haul Route Coefficients for Various Gradients

Resistance Coefficient Haul Route CoefficientHaul Route Condition (ERC) (R) (x 100) Trac- (H = C/.R)Combination Gradient tive Gradientof R & C Coeff.

Description Code Values 0 50o 10% 150/ 20% (C) 0 5o 10% 15% 20%

Concrete, asphalt, compacted A Min R/Max C 4 9 14 19 24 1.0 25.0 11.1 7.1 5.3 4.2earth well maintained Ave R/Ave C 5.5 10.5 15.5 20.5 25.5 0.9 16.5 8.6 5.8 4.5 5.5Max/l/Mm C 7 12 17 22 27 0.8 11.4 6.7 4.7 3.6 5.0

Earth, poorly maintained B Min R/Max C 7 12 17 22 27 0.7 10.0 5.8 4.1 3.2 2.6(dry clay loam) Ave R/Ave C 10.5 15.5 20.5 25.5 30.5 0.6 5.7 3.9 2.9 2.3 2.0Max R/Min C 14 19 24 29 34 0.5 3.6 2.6 2.1 1.7 1.5

Earth, muddy, no maintenance C Min R/Max C 15 20 25 30 0.5 3.3 2.5 2.0 1.7(wet clay loam) Ave,R/Ave C 18.5 23.5 28.5 33.5 0.45 2.4 1.9 1.6 1.3Max R/Min C 22 27 32 57 0.4 1.8 1.5 1.5 1.1

Loose sand and gravel (wet) D Min R/Max C 22 0.4 1.8Ave R/Ave C 25.5 0.35 1.4Max R/Min C 29 0.3 1.0

Earth, soft, muddy, or dry E Min R/Max C 28 0.5 1.1loose sand Ave R/Ave ,C 54 0.25 0.7Max R/Min C 40 0.2 0.5

Notes: 1) Resistance coefficient = rolling resistance plus gradient (o/ 100).(2) Some figures are omitted where haul route coefficient is so low that no tractor/trailer combination

can.move in such conditions.3) Figures apply to rubber tired vehicles only.4) Figures for resistance coefficients are probably conservative.5) Gravel surfaced roads:

engineered and well maintained, probably HRC code A; (Dotherwise HRC code B.

Table A2. Tractor Loading Canacities An-endix A

Load (tonne ) or ratio withunballasted tractor

Item35h0 47h0 62hp 75hp

Weiahts (unballasted)Front wheel load (tonne) (a) 0,6* 0.6 0.8 0.9NET TRACTIVE LOAD(tonne) (b) 02* 0 1 1.8

(= rear wheel load)GROSS TRACTOR WEIGET (tonne X(c=a+b) . 1 2.1 2.7

Load Transfer LimitsAxle & Tires

Max.permissible rearaxle load (tonne) (d) 4.0* 4.5 4.5 4.5

Max. permissibl reartire load 12 ) (tonne Xe) 2.2* 2.4 5.1 5.6

A=LE LIMIT ON TRANSFERLOAD (tonnle) (f=d-b) .101j K1

STANIARD TIRE LIMIT ONTRANSFER LOAD (tonne) (g"e-b) .o* l

Stability (front wheel lift):Leverage raiq at hitch

(w/x)L3) (g) 9.0* - 9.1 10.4 10.4*Leverage ratiq at drawbar

(w/y)(.4 ) (h) 2.4* 2.4 - -STA3ILITY LTIMIT (N TRANSFER

LOAD ON EITCH 5) (tonne) (j=ag/2) (2.7) (2.8) (..2) (4.6 )STABILITY LIMIT ON TRA)SER(

LOAD ON DLAWBAR (5,6 (fm%(k=ah/2) (0. )* (0.8) (1.1) (09)STANDARD HITCH ;IT ON

TRANSFER LOAD )_(tonne) (1) n 2 2 2e

IR BAR IMIT ON TRANTSrLOADj6) (tonne) (m) 1.0* 1.2

R i m -mu l l ( o) ni _ - .MAX. RIMPULL (approx.

Notes: 1) denotes estimated figures. Other figures taken frm officialtest reports, with some modification for drawbar anomaliesbetween 62 & 75 hp models.

2) Permissible tire load up to 16km/h; at higher speeds, reducethese values by 17%.

3) w = tractor wheelbase, x = distance of hitching hook behindrear axle.

4) y = distance between point on drawbar giving 7aAimum mechanicaladvantage (for trailer lift) and rear axle. See also notes(3) and (6 ).

5) Stability limits in unballasted state can be ignored for practicalpurposes, because front wheel lift can be countered by a smallamount of ballast on front wheels.

6) Linkage drawbar, taken at point giving maximum mechanical advan-tage and therefore maximum lift. Swinging drawbar would givegreater stability.

7 These stability limits taken direct from test reports (half forceneeded to lift front of un'allasted tractor).

8) Hitch limit may be slightly exceeded on occasion.9) Maximum rim7ull taken as eaual to irnwbar pull on level taraac

road, ballasted (conservative estimate).10) Underlined figures, opposite items in capitals, are carried forward

to Tables 5 and 6.

Appendix APage 6

Table A3. Trailer LoadingCapacities

Load (tonne) or Coefficient

Item Trailer Capacity (tonne)

5 5 7 10

Weights

Payload (tonne) (a) 5.0 5.0 7.0 10.0

Unladen weight (1 )(tonne) (b) 0.9 1.5 2.0 3.0GROSS TRAILER WEIGHT (c=a+b) 3.9 6.5 9.0 13.0

(tonne)

Transfer Loads

'Standard' trans;egcoefficient 2 ) (d) 0.55 0.30 0.23 0.20

'STANARD' SFERLOAD2 (tonne) (e=cd) 1.3 1.9 - 2.1 2.6

'Maximum' transf rcoefficient 4) (f) 0.33 0.33 0.35 0.33

'MAXIUME' TRANSFER LOAD (g-cf) 1 2.2 . O(tonne)

Notes: (1) Unladen weights are typical values, they do not refer to aparticular trailer.

(2) 'Standard' transfer loads refer to actual models manufacturedin UK, and the coefficients are derived from these loads.

(3) Variable wheel position: 2.6 tonnes is the load transferredwith wheels in rearmost position.

(4) Theoretical maximum, by designing rear wheels farther back,and assuming tractor is able to accept these increased loads.

(5) Underlined figures, opposite items in capitals, are carriedforward to Table 5 and 6.

Table A4. Definition of Degree! of Load, Transfer

Number Trailer (1) Loadin 2 ) Pressure Tractor Tractor PossibleDegree of of wheel position point contr9 1 rear rear limits toload trailer lift 1) tires axle loadtransfer wheels transfer

(a) Zero 4 - Standard hitch no standard standard noneor linkage drawbaror swinging drawbar

(b ) ' Minimum'-'either 4 - linkage drawbar yes stadard standard drawbar, tiresor 2 standard linkage drawbar - ) )

(c) 'Standard' 2 standard standard hitch - standard standard tires, hitch

(d) 'Maximum' 2 to give up to Yo special hitch.- - special standard axle only (byload transfer definition)

Notes: (1) A dash (-) means the item is irrelevant.

(2) Standard hitch standard automatic (hydraulically operated) hitch supplied with tractor.Special hitch = specially strengthened automatic hitch (see Table A5 for load sustained).

(3) Actual position of wheels depends upon the capacity of the tractor rear axle to acceptthe transferred load: if tractor cannot accept 33%, wheels are then assumed to be positionedso as just to load tractor re6r axle to its limit, with loaded trailer.

(4) Special tires only needed for 35 and 47 hp tractors (see Table A2).

(5) 'Maximum' means for the purpose of the definition given above, only. It would be possibleto obtain even greater load transfer by strengthening the tractor rear axle.

A-pendix APage 8

Table AS. Load Transfer for Tractor/Trailer CombinationE

Degree of Load TransferTrco!(a) (b ) (c ) k(d.)

Tractor/ 7mMinimum' Stanard' MaximumTrailer - F F 60 F-o F acCD Da() r- C0 WD~ C D~Combination H 4 4- H H r CH H H - H 41

d+ -1Cd M 4"0 MoP C - 0 M 4n 0d(tractor ro 02A -P r2 4--unballasted) a 0 H PH 0 0 P - H -H 0 0 -H 0 0 H -Hd

tonne tonne tonne - tonne tonne - tonne tonne -

35hp tractor -1.0 d '1.3 t 3.1 atrailers 3-t 0 1.3 (1.0) d 1.3 1.3 1.3 1.35-t 0 1.9 x dt 1.9 x t 2.2 2.27-t 0 2.1 x dt 2.1 x t 3.0 3.0

10-t 0 2.6 x dt 2.6 x th 4.3 3.1 a

47hp tractor 1.2 d 1.5 t 3.6 atrailers 3-t 0 1.3 (1.2) d 1.3 1.3 1.3 1.3

5-t 0 1.9 x dt 1.9 (1.5) t 2.2 2.27-t 0 2.1 & dt 2.1 x t 3.0 3.0

10-t 0 2.6 x dt 2.6 x th 4.3 3.6 a

6 2hp tractor 2.__ d 2. h 3.2 atrailers 3-t 0 1.3 1.3 1.3 1.3 1.3 1.3

5-t 0 1.9 1.9 1.9 1.9 2.2 2.27-t 0 2.1 2.1 2.1 2.1 3.0 3.0

10-t 0 2.6 (2.3) d. 2.6 (2.3) h 4.3 3.2 a

75hp tractor 2.3 d 2.3 h 2.7 atrailers 3-t 0 1.3 1.3 1.3 1.3 1.3 1.35-t 0 1.9 1.9 1.9 1.9 2.2 2.2

7-t 0 2.1 2.1 2.1 2.1 3.0 2.7 a10-t 0 2.6 (2.3) d 2.6 (2.3) h 4.3 2.7 a

Notes: (1) See Table A4 for definition of degree of load transfer.

(2) See Table A2 for tractor transfer limits.

(3) See Table A3 for trailer transfer limits.

(4) x - Tragtor limit to load transfer lower than trailer limit, sotractor cannot accept fully loaded trailer - but see note (5).

(5) ( ) - Transfer load given in brackets where it is limited bytractor, so that trailer runs only part loaded, but where thispart load is still greater than the full load of a smaller trailer.

(6) In the case of maximum load transfer it is assumed that wheretractor axle is the limiting factor the trailer wheels are sopositioned that actual load transfer from loaded trailer equalsthe tractor axle limit.

(7) Limiting factor:d-drawbar h - hitch t - tires a - axle

If not stated limiting factor is the trailer load transfer coefficient.

(8) Actual load transfer - lower value of tractor and trailer limits.

(9) Tractor limit on load transfer - it is assumed that any front wheelinstability is countered by a small amount of ballasting, not takeninto account in the load figures (Table A6).

Table A6. Gross Weights & Tractive Loads for Tractor/Trailer Combinations

35hp tractor 47hp tractor 62hp tractor 75hp tractor

trailer size trailer size trailer size trailer size3-t 5-t 7-z 10-t 3-t 5-t 7-t 10-t 3-t 5-t 7-t 10-t 3-t 5-t 7-t 10-t

TRACTIVE LOADS(a) Zero Load Transfer

Total tractive load tonne 90- 0. Os9i -2 0-9 0--9 .22.±2 13 L3 113.3 1- 3 1.8 1.8 1.8 1.8

(b) Minimum Load TransferTractor self load tonne 0.9 0.9 1.3 1.3 1.3 1.3 1.8 1..8 1.8 1.8Trailer transfer " (1.0) 1.2 1.3 1.9 2.1 (2.3) 1.3 1.9 2.1 (2.3)Total tractive load (1.9) 2.1 2.6 .j 2 .4 (3.f) 3.1 3.7 .2 (A1)

(c) Standard Load TransferTractor self load tonne 0.9 0.9 0.9 1.3 1.3 1.3 1.3 1.8 1.8 1.8 1.8Trailer transfer " 1.3 1.3(1.5 1.3 1.9 2.1 2.3 1.3 1.9 2.1 2.3Total tractive load " 2.2 2.2 (2. 2. 6 3-2 . A 3 ) L. 1i -9 .

(d) Maximum Load TransferTractor self load tonne 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 1.3 1.3 1.3 1.3 1.8 1.8 1.8 1.8Trailer transfer " 1.3 2.2 3.0 3.1 1.3 2.2 3.0 3.6 1.3 2.2 3.0 3.2 1.3 2.2 2.7 2.7Total tractive load " 2.2 3- 3*9 4.0 2.2 3- 39 4- 2.6 .4. -5 -.1 4-0 4-5 4-.

GROSS -WEIGHTSTractor tonne 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 2.1 2.1. 2.1 2.1 2.7 2.7 2.7 2.7Trailer (loaded) 3.9 6.5 9.0 13.0 3.9. 6.5 9.0 13.0 3.9 6.5 9.0 13.0 3.9 6.5 9.0 13.0Total weight 5.4 8.0 1Q.5 14.5 5.4. 8.0 10.5 14.5 6.0 8.6 11.1 15.1 6.6 9.2 11.7 15.7

d-d

g (D

Apendix APage 10

Table A7. Limiting Haul Route Condition for Various Tractor/Trailer Combinations

Limiting Haul route condition for specified combination oftractor, trailer, degree of load transfer and gradient

Trailer size 5-tonne 5-tonne 7-tonne 10-tonne

Load transfer -+ zero st'd max. zero st'd max. zero st'd max. zero st'd max.Tractor Gra'dient

size % U-D

0 (B)1A(c) (c)B

5 A A *nX

35hp 10 QIx x x

15 X x X X(B JAt xt

20 x x xxB) ( A Xt Xt

0 (B) .Anc c) (c)()

5 A ( X (c B -B * Bi

47hp 10 A X X X

15 XX A*X X * x

20 X X A A x Xt0 (B) (C) (c) A

5 ((C)AA

62hp 10 A(A x x

20 , X x ( A x A_ (A

(B ( (B) AA5 .(B c) c ( C C B

75hp 10 ( ) A ) B

15 A X X

20 A () xX (3) - (X

For notes see following page.

Appendix APage 11

Notes to Table A7

(1) The number in each cell represents the worst haul route in whichthe corresponding tractor/trailer/gradient/ballast combination willwork, in the lowest tractor gear. A number without brackets or circleshows that the tractor/trailer combination will operate through thecomplete range of soil conditions (tractive coefficient and rollingresistance) which may be found in a haul route of that category, asdefined in Table Al.

(2) A circled figure shows that the combination will probably work ina haul rate of that condition number, i.e. that it will work overmore than half the possible range of conditions. A bracketed figureindicates that the combination will possibly work in a haul-routeof that condition number, i.e. that it will work over less than halfthe possible range of conditions. For example, the table shows thata 62hp tractor, working with a 5 tonne payload trailer with standardload transfer, on a zero gradient, can possibly pull the loadedtrailer on a condition C haul route (and therefore definitely canpull it on a condition B route). With maximum load transfer thesame combination can probably work on a condition C route (andtherefore definitely also on a condition B route).

(3) X means that the tractor/trailer combination cannot work even on acondition A haul route, as defined in table Al.

(4) t means that the limiting factor is not wheelslip (which is thecase with the remaining figures) but rimpull (engine stall).

(5) * denotes cases where the tractor limit to load transfer is lower thanthe trailer Imit, so the trailer would run only partly loaded. Inmarginal cases, where the difference between tract6r and trailer limitsto load transfer is small enough for it still to be economic to use thetrailer partly loaded, a haul route condition code is entered in thetable; otherwise no code is entered.

A--endix APage 12

Table A8. Maximum Drawbar Pull at 16km/h

Tractor Size. (Crankshaft hp)

Item Unit Symbol35 47 62 75

Test Conditions:

Gear 5th 5th 6th

Drawbar power db-hp (a) 28.0* 40.8 57.1 64.6

Engine speed rpm (b) 2290 2091 2023Rated engine speed rpm (c) 2250 2000 2000

Lowest gear for 16km/h 7th 7th 7th

Max.speed in gear (atrated engine speed) km/h (d) 16* 19.8 17.2 18.3Drawbar pull at 16km/h kg-f (e = 27.5a) 478* 554 869 957

db

Estimated on basis of 800 of crankshaft hp @ assumed top speed

of 1.6km/h.

0l

Table A9. Maximum Trailer Gross Weights at 16 km/h

Rough equivalent Maximum trailer gross weight (tonne) which

Haul route Rolling code in Technical Suggested can be pulled at 16 km/h at zero gradientCondition Resistance Memorandum No. 8, surface 35 hP 47hP 62hp 75hp

(r) Table 2 (P-40) condition tractor tractor tractor tractor

Grade A - Compacted earth,'Worst'within 0.07 3 average main- 6.8* 7.9 12.4 13.7range tenance

Grade A - Compacted earth,'Average' 0.055 4 well maintained 8.7* 10.1 15.8 17.4within range and levelled

Grade A - Asphalt or'Best' within 0.04 5 concrete, well 11.9* 13.9 21.7 23.9range maintained

Notes: (1) Maximum trailer gross weight = maximum drawbar pull ? r

(2) * calculated from estimated tractor data

vL

Table A10. Tractor/Trailer Combinations Which Fulfill Speed Criterion

Rough equivalent Largest trailer which can be pulled atHaul route Rolling code in Technical Suggested 16 km/h at zero gradientCondition resistance Memorandum No. 8, surface 35 hP 47 hp 62 hp 75 hp

(r) Table 2 (p.40) condition tractor tractor tractor tractor

Grade A - Compacted earth, 3-t 5-t 7-t 7-t'Worst' within 0.07 3 averag& main- (5-t) (10-t)range tenance

Grade A - Compacted earth,'Average' 0.055 4 well maintained 5-t 7-t 10-t 10-twithin range and levelled

Grade A - Asphalt or 7-t 7-t 10-t 10-t'Best' 0.04 5 concrete, well (10-t)within range maintained

Note: Trailer sizes in brackets are marginal cases.

t' dI.

-T 10

9

7

3- tonne 5-tonne ,,7-tonne 10-tonne6 trailers -railers I trailers trailers

o s.c:

So 7 x47 x62 75i- 4 - × o7 ×5 X 35 .75

12,×62 .62 .o 0 o623~~~6 -o632.s -

-3 " ý 36 2 35,47

> 35,47--ý35,47 o(47)

(35)' ý75 A75 å75 A75<å 6 2 A62 A62 e A62

1 -35, 47 35,47 A35,47 A3547

o o

CL 0 1 2 3 4 . 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Total weight ( tonnes)

jå 35 3 5 hp tractor zero load transfer35' 3 5 hp Iractor minimurn load transfer

no 35 35 hp tractor siandard load transfer

O x 35 35 hp tractor maximum' load transfer

If ligure is in bracketc trailer cannot run fully locided.

10 Gradieøt 0 \

LD0

NRJ 8ç

0

6

kil

2- -Ji

C3

n~ rn a xH

0-0 1 2 3 59 10 i 13 14 i5 16 17 18 19 20

TRACTIVE COEFF C c-)

0 ~ KE 23 3i67 8 9 1 1 1 3 / 5 1 7 1 9 2

4R..B. Slope of line represents inverse of klaul Roule Coelficient (e R/C) for Hqul Roult Condition B

Range ol H for a particular Hau Route Condition

Limiting enginq pull for .35hp tractor (approximate)

NOTE:- For zero gradient, resistance coeff, ( R) w folling resiilance (r)

스

스

스

Gradient 20*19 up.

10cr)

9

7-

nCL m

6

...............................n4 35

3

2-

o.0 1 2 3 4 5 6 7 8 9 10 12 14 15 16 17 18 19 20

TRACUVE CO EFF CKEY

HA-C.B. Stope ot line represents inverse of haul Route Coefficient tie RIC) for Havl Route Condilion B -u

Range of H for a parlicular Haul Route Condition

1-imiting engine pult for 35 bp tractor (approximalt)

긔

APPMMIX B

Productivity and Cost Data

Figure B1 Loading & Unloading Labour InputCoefficients

AmDendix BPage 1

Productivity and Cost Data

Some approximate data for use in calculating productivities andunit costs are given in this appendix. Data is based on obser-vations where applicable; figures not based on observation aremarked (*).

1. Tractor speeds (km/h)

loaded empty(2Haul route condition A 12-20* 15-20*

B 5-12* 6-15*

C 2- 5* 2- 6*

2. Manoeuvring, etc., times (minutes)

4-wheel trailer and/orhydraulic hitch on 2-wheel trailer using

2-wheeler drawbar or screw jack

Unhitch: 0.05 to 2.0*

Hitch: 0.2 to 3.0*

Manoeuvre tractor for coupling: 0.2 (open site) to

0.5 (restricted site)

Manoeuvre trailer into load/unload position and/or reverse:

0 (direct approach) to 3.0 (reversing in restricted site).

3. Unload time (minutes)

Tipping trailers: 0.8 mins (more if mechanics not perfect)Manual unloading (assuming Idequate rest between successive

trailer arrivals): f3)

(1) Use graph in Fig. B1:

(a) for drop side trailers set "loading ht." to-0.3 m;

(b) for fixed side trailers set "loading ht'. to +1.Om(to allow for difficulty in unloading 'the lastlittle bit');

For both (a) and (b) apply fast unload f)ctor g = 0.3 to0.6 (the higher figure for boulders). (4)

For notes see page 2.

Avoendix B

Page 2(2) For preliminary estimating'rule of thumb,

assume 5-10 mins, depending on conditions, withabout 1 man per tonne payload (assuming dropside trailer).

4. Tractor cycle factor (f)

Assume 1.1* (manual unloading, good equipment,uncongested sites)

1.2* (tipping trailers, good equipment,uncongested sites, ormanual unloading, poor equipment and/orcongested sites)

1.3* (tipping trailers, poor equipment and/orcongested sites )

Alternatively, estimate the tractor cycle allowance inminutes per cycle (at).

5. Loading inout coefficient - see Fig. B1.

6. Site factor (s)

Should be established from site observations or records.If no information is availablf, take:

4

0.6 for daily paid work, average supervision,

0.9 for piecework, good supervision.

7. Tractor Initial Costs

Typical initial costs of tractors (February 1976) are:

35hp US $4,500 in India ($ equivalent) (pstimate)

47hp US $6,000 in UK (8 equivalent)62hp US $8,000 in UK " "

75hp US $9,000 in K92hp US $10,600 in USA.

Prices are "free on board" ship in a port of the countryquoted, but exclude freight charges from that country toa foreign port.

8. Trailer rate (R r)

Since tire cost forms a high proportion of the trailercost, a high maintenance factor of about 1000 is recom-mended. Allow a further 50% for interest charges overthe relatively long life of such a piece of equipment. Thus:

rate -^- initial cost x 2.5* $/hr (AT)e=ected life (.hr, AT)

Notes: (1) * denotes suggested figures, not based on observation.(2 See Table 2 (main text).

(3) See para 132 (main text).4 -In 'one India study, 4-tonne trailers containing murum

were unloaded typically in 8 minutes by a maximum of 4daily paid men, i.e. L.13 man-hr (WT)/tonne; supervisionwas poor in tP-is case.

Appendix B

Page 3

1.5Take loading height - 0 -3 mfor unloading dropzidetrailer.

Take loading height=l-Omfor un(oading fixed

- side trailer

-0

-0-4 0 04 0.8 1-2 16 2Loading Height ( mi)

Notas:- (1) This graph is ta-an from Technical Mmorand=: No. 20 on "Loading and Unloading". Forlimitations In usLnE the graph see paragrph 13 or that memoranz-um.

(2) Actual unload ti- may be calCulated usin Invut zoefi:cienta betwee. 0.3 and 0.6 times thevalue showi In the graph (see Chapter 71 of text).

(3) Vorking time (W) is defined as Available V-e (AT) less gon-Vorking Tima (NT), where tiT is thetime when the resource is not £.rthering the rogvess or tho work (e.g. late s:a·ta vvur-ecessaryrest breaks). ÄT is defined as the Total Ti-e (TT) less the Lost Time (LT) where n s the per,dduring which work on site would notally te dona and LT :s the tie when production :.y not beposeible or practical. Full definizions and explanations of these time categories are given inTacnical Memorandu No. 8 of this series.

(4) Line A refers to good organisation and superision with a piecewor .payment =ethod.

(5) Line a refers to average (fair) organisatIon and sumervision with a daily wage payment method.

(6) Analytioal results for relationship under dirferent combinations of payment method and supervisionlevels are:-

Payment Method Suervision Level

Daily paid or taskork + Poor or fairLeily paid or taskntork + Good or very good

Piecework + Poor or fairPiecevurk + Good or very good

Fig. Bl. Loading and Unloading In-mut Coefficients

APPENIIX C

Costing Exercise: Tractor/Trailers vs Trucks,

Table C1. Assumed Fixed Parameters

Table C2. Assumed Variable Parameters

Figure C1. Unit Costs of Haulage by Truck &-by Tractor/Trailer

(a & b Conditions)

Figure C2. Unit Costs of Haulage by Truck & by Tractor/Trailer

(c & d Conditions)

Appendix C

Costing Exercise: Tractor/Trailers vs Trucks

The theoretical exercise presented below was carried out in India in 1974. The values ofassumed. parameters (with the exception of interest) were selected to reflect currentIndian conditions. Conclusions are written into the Chapter VII of the main text.

Table C1. Assumed Fixed Parameters

Item Unit Symbol Values AssumedTruck Tractor

Driver wage Rs/month d 350 350Fixed charges (tax, insurance, etc): /

Vehicle Rs/month f 353 150Trailer Rs/month f' 10

Fuel and oil ("POL") charge Rs/km p 0.20 0.40Residual value of ,vhicle or trailer as

% of c or ' 1 ) % v 20 20

Note: C, C' = cost of new vehicle at site (Table C2)

Table C2. Assumed Variable Parameters

Item Unit Symbol alues AssumedTrack. Tractor

Group (A) assumptions: (Al) (A2) (Al) (A2)

Cost of new vehicle on site Rs/1000 c 76 100 30 45Cbst of new trier on site Rs/1000 c' - - 8 11Repair factor 1 vehicle Y r 60 150 60 150Repair factor, trailer / r' - - 80 150Economic life, vehicle km/1000 k 120 240 90 150Economic life, trailer km/1000 kt - - 90 150Interest rate, per month Y p.m. i 1 2 1 2

Group (3) assumptions: (B1) (32) (31) (B2)

Monthly working hours hr/month m 200 150 200 150Vehicle (load + unload) time hr/m3 t 0.125 0.25 0.1 0.2Payload m3/trip 6 3 3 3

Group (C) assumptions: (Cl) (C2) (Cl) (C2)

Average running speed(2) km/h s 10 20 7.5 12.5

Group (D) assumptions: (D1) (D2) (D1) (D2)Age of vehicle new old new oldDistance already travelled) ile ) x (0) (k) (0) (k)during its life) ) trailer ) x - 0) k)

General:

Haul distance (one way) km y continuously variable

Notes:(1)Repair factor: the repair cost during the economic life of the vehicle as % of cost

to be depreciated (c-v).(2) Average running speed includes time spent manoeuvring, coupling trailers, etc. The

lower (Cl) speeds correspond to a fair haul route (GradeB), the higher (C2) valuesto a good haul route (Grade A).

(3) "New" means unused. "Old" means at the end of its economic life, i.e.it has run'k' km.

A-Pendix CPage 2

Calculations

Trucks

Time Interest oi outstanding capital value = 10000 (1.8x) s/monthCost: 100 k K/ot

driver cost = TV

fixed costs - = f

Total Time Cost -10ic (1-0.8x )+d+f 1 Rs/hrk m

Distance repair and maintenance = 0.8c.r. .1 Rs/kmCost: 100 k

fuel and oil p

time cost while running =Oic(1-0. 8x)+d+f] Ik ms

Total Distance Cost = p+0.8cr+ 10ic(1-0.8x)+d+f 1(incl. time element) 100k k j ms

Rs/km

Cost per i 3 (truck and driver only):

while loading and unloading: 10ic (1-0.8x) +d+I. t ./m-k m

while hauling: p+0.8cr. Z +1 0Ok] L

1Oic(1-o.8x)+d+f .2 Ks/mLk JIas

Total Truck Cost (incl. driver):

p+.Bcr . 2 + Oci(1-0.8x)+d+f.ft+2. 1 Rs/ 3

P 100k 1 1-]L k [JU2tsJ m

Appendix CPage 3

Tractors and Trailers

No. of trailers per tractor = 1+ trailer (load + unload) timehaul time

-1+ I 2

Add 20% to ensure maximum use of tractor,

No. trailers required per tractor = 1.2 (1 + :t5/ = . sa

Time interest on outstanding capital valuecost:

i_ 1000 1-0.8x + 1000no' 1-0.8x' Rs/month100 k kI

driver cost = d

fixed costs = f + nf'

total time cost = 1i i(1-0.8x+ ne' 1-0.82 +d+f+nf'}.1 Rs/hr

' I I

Distance repair & = 0.8 Cr + c'r' (only 1 trailerCost maintenance 100k look'i running) Rs/km

fuel and oil = P Rs/kmtime cost whilerunning = flOi j c(1-0.8x + (1 -0.8x' +d+f+f' 1 Rs/km

SL \ k\ k .ms

I(1 trailer running with tractor)

Total Distance Cost = p + 0.8 cr ++ 'r' +f10i [(1-0.8x)(tractor + 1 trailer) 100k 100k' t L k/

+0 1-0.8x') +d+f+f} .1 Rs/kmk'/ ms

Costs per m3 (tractor, driver, trailers only):

p + 0.8 ( r + c'r' ) + 10i [ 1-0.8x) + ' 1-0,8x' +d+f+f 1

+ [( -1)o'. (1-0.8x' + (n-1) f' . n-1 Rs/

1.2

0-70 - 0-70

SET (a) CONTITIONS SET (b) CONDITIONS

0.60 Assumptions (Al), (Bl) 0-60- Assuptions (A2), (l)

0

0

0 0-50 0-30-

-0x

(D

Pi '0 20- 0-20-

CTYoCtor / troilers - raCtor / troilers

Gi Trucks

0 -4,1_ _ _ _ _ _ 00 -_ _ _ _ _ _ _

(D 2 3 4 5 1 2 3 4 59 Haut Distance, km Haul Distance, km

New vehicles, slow (fair haul routej Note: US 1 1.00 = Indian Rs 8.00 RD

- -New vehicles, fast (good haul route)

-.... - Old vehicles, slow (fair haul route)Hl-

o-- Old vehicles, fast (good haul route)

0 Breåak-even point

1-75 1-75,

ci1-.50-. 1·50 .SET (c) CONDITIONS SET (d) CONDITIONSAssumptions (Al), (B2) Assumptions (A2),(B2)

0

1.25. 1.25.0

0 c%

1.00 1-00

0700

0-75- -75

Trucks 00k0-5

0o50 Trucks~0-50- 5-0 0 0-50- /-

X 5- , .-.25,

JPiH

1-3-

.. .. - .. - Ol e i s lr

SvrctorI t eiilers fast (oaaueut

(D 0 2 46 8 10 0 2 4 6 8 10Haut Distance. km H aju l D ist an ce, kmO________ New vehicles, slow (fair haul route) Nota: US 1.00 = Indian Ra 8.00 1d x

- ____New vehicles, faat (good haul route)(D0

0___- Old vehielas, slow (fair haul route)Old vehioles, fast '(good haul route)

Copies of other memoranda in this series (with the latest revisions, whereapplicable) or individual revisions may be obtained from:

Transportation DepartmentInternational Bank for Reconstructionand Development

1818 H Street N. W.,Washington, D. C. 20433'USA

Technical Memoranda published in this series to date are as follows:

N=rber Title Dated Revisions

1. Comparison of AlternativeDesign Wheelbarrows forHaulage in Civil ConstructionTasks Jan. '75

2 ~ Increasing Output of Manual .Excavation by Work Re-organization: An example ofPassing Place Construction on

- a Mountain Road Jani. ' 7 5

3 Comparison of Different Modes Completely revisedof Haulage in Earthworks Jan. 175 and re-issued

June 1975

4 Effect of Health andNutrition Status of Road Con-struction Workers in NorthernIndia on Productivity Jan. '75

5 Comparison of Hand Laid and Supplement issuedMachine Laid Road Surface Feb. '75 Feb-ruary 1976

6 Haulage with lift of MaterialsLifting Sand by Rcpeway Feb. '75

7 Productivity Rates of Earth- Supplement issuedmoving Machines May '75 August 1975

8 Collection of ProductivityData from Civil ConstructionProjects July '75

9 'Report of First Road0Demon:t rction Proj oct August '75

56

Yumber Title Dated Revisions

10 A System of Deriving Rental Chargesfor Construction Equipmenc Aug. '75

11 A Literature Review of theErgonomnics of Labor-IntensiveCivil Construction Aug. '75

12 Haulage by Headbaskets, ShoulderYokes and other 'fanual LoadCarrying Methods Oct. '75

13 The Use of Wheelbarrows in CivilConstruction Oct. '75

14." Hardware Research Sumzary Oct. 175

15 The Planning and Control ofProduction, Productivity andCosts in Civil, Construction Pro-jects . Oc. '75

6 Level Cranes Oct. '75

17 Compaction Dec. '75

18 Spreading Activities in CivilConstruction Dec. '75

19 Excavation Feb. '76

20 Loading and Unloading Åctivities Feb. '76

21 A Literature Revie- of che WorkOutput of Animals wich ParticularReference to their Usa in CivilConstruct,io£1.. Feb. '76

2? '' Haulage using Aerial Ropcsays June '76 Supersedes . M.Uo. 6 which is no

longer circulain-,

23 The Use of Rail Systers in CivilConstruccion June '76

24 The Use oE Åg.rcultural Tracfor/Trailer Combin-tion- June '76

25 A Aregne Produelion Juna '76

26 The Rela..tionzhip of flutri tionand ,oalth z: Wrker Producivity±n 1:-y7M 77

27 Haulage Using Animals in Civil Construction Feb. '78

28 Hand Tools for Earthworks and Stone Breaking Feb. '78