Tribology Online, 8, 1 (2013) 1-6. ISSN 1881-2198

DOI 10.2474/trol.8.1

Copyright © 2013 Japanese Society of Tribologists 1

Short Communication

An Experimental Investigation of Oil Film Temperatures in Elliptical Profile Journal Bearing

Rakesh Sehgal1)*, Amit Chauhan2) and Rajesh Kumar Sharma1)

1)Department of Mechanical Engineering, NIT

Hamirpur-177005 (H. P.), India 2)Department of Mechanical Engineering, UIET, Panjab University

Chandigarh, India *Corresponding author: rakeshsehgal.nitham@gmail.com

( Manuscript received 12 February 2012; accepted 24 October 2012; published 31 January 2013 )

( Presented at Technical Session in the International Tribology Conference Hiroshima 2011 )

Hydrodynamic journal bearings experience significant variation in oil film temperature. Reliable data of operating temperatures in these journal bearings are very useful and important for practical bearing designers and mathematical modellers. Here, an elliptical journal bearing has been tested to access the temperature rise at oil-bush interface with three grade oils at loads varying from 100 N to 600 N and speeds = 3000, 3500, 4000 rpm at constant oil supply pressure. The results show that with increase in load at constant speed and pressure, and with increase in speed at constant load and pressure, the oil film temperature increases in the central plane of both the lobes of the bearing for all grade oils under study. Further, it is observed that under the given operating conditions, oil 2 gives the coolest operation of the elliptical bearing under analysis. Keywords: elliptical bearing, oil-film temperature, journal bearing test rig

1. Introduction

Thermal changes significantly affect the bearing performance as lubricant viscosity is a strong function of temperature. Moreover, excessive rise in temperature can cause oxidation of the lubricant and, consequently, lead to failure of the bearing. Researchers have tried to study the behaviour of the lubricant film by adopting various approaches of simulation and reported that the temperature rise is quite high in circular bearings as they operate with single active oil film. This encouraged to the development of bearings with non-circular profile which operate with more than one active oil film. This feature accounts for the superior stiffness, damping, low power losses, and reduced temperature in oil film as compared to the circular bearings. Out of non-circular bearings, elliptical, offset-halves, and three-lobe configurations are the most common. Among these non-circular bearings the elliptical bearings are commonly used in turbo-sets of small and medium ratings. The so-called elliptical bearing is actually made up of two circular arcs whose centers are displaced along a common vertical straight line from the centre of the bearing, as shown in Fig. 1.

Only a limited experimental work has been reported for the thermal effects in lobed bearing configurations such as offset-halves and elliptical/lemon bore journal

bearings thus leading to the non availability of the standard procedures/charts for their design. To maintain brevity only few recent experimental and theoretical works are reviewed here. Pinkus and Lynn [1] have presented an analysis of elliptical journal bearings based on the numerical solution of Reynolds equation for finite bearings with the assumption that there is no heat loss to the surroundings. The solution of the differential equation was supplemented by additional work on the nature of the oil flow, power loss, and eccentricity in elliptical bearings. Fitzgerald and Neal [2] presented

Fig. 1 Schematic diagram of elliptical journal bearing

Rakesh Sehgal, Amit Chauhan and Rajesh Kumar Sharma

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some experimental data for 76 mm diameter two-axial groove circular bearings. They observed that the axial temperature variation was negligible but the circumferential temperature variation could be very significant. Hussain et al. [3] have predicted the temperature distribution in non-circular journal bearings: two-lobe, elliptical, and orthogonally displaced. The prediction of temperature by the authors is based on a two-dimensional treatment following Mc Callion’s approach (an approach in which the Reynolds and energy equations in oil film are decoupled by neglecting all pressure terms in the energy equation). Mishra et al. [4] considered the non-circularity in bearing bore to be elliptical and made a comparison with the circular case to analyse the effect of irregularity. It was observed that with increasing non-circularity the pressure gets reduced and temperature rise is less in case of journal bearing with higher non-circularity value. Ostayen and Beek [5] have carried out a finite element analysis of a lemon-bore journal bearing. The thermohydrodynamic model presented by the authors is an inverse model, that is, a model in which the shaft eccentricity and attitude angle are calculated given a certain known and prescribed load and load angle. In analysis carried by authors, care has been taken to accurately model the heat to and from the oil supplied and the model is used to check the design of the lemon bearings in a specific naval application. Sehgal et al. [6] presented a comparative theoretical analysis of three types of hydrodynamic journal bearing configurations namely, circular, axial groove, and offset-halves. It was observed that the offset bearing runs cooler than an equivalent circular bearing with axial grooves.

Review of the existing literature reveals that a very little experimental work was related to the elliptical journal bearings. In present work, the thermal effects in an elliptical journal bearing (machined from Methyl Methacrylite) have been studied experimentally using three grade oils namely: Hydrol 68, Mak 2T, and Mak Multigrade at loads/stress varying from 100 N / 0.0235 N/mm2 to 600 N / 0.141 N/mm2 and speeds = 3000, 3500, 4000 rpm. Hydrol 68 oil is used as a general purpose lubricant oil, antiwear hydraulic oil and extreme pressure gear oil whereas Mak 2T oil is used for lubrication of engines and mainly for lubrication of 2-stroke engines. Mak Multigrade oil is recommended for use in heavy duty commercial vehicles, light commercial vehicles and multi-utility vehicles [7-10]. These commercial grade oils have been considered in the present analysis based on the availability in the local market and are named as oil 1, oil 2, and oil 3. The experiments were carried out on a specially designed journal bearing setup.

2. Experimental Setup

The test rig (Fig. 2) is a versatile apparatus, easy to operate with provision to measure temperature on

middle and ends at every 45 degrees angular position on the circumference of bearing. The temperature is measured at 22 different points on the circumference of the bearing using temperature sensors (PT-100). In addition, one sensor is fixed to measure oil inlet temperature. Temperature sensors were calibrated using standard resistance box. These were removed from the bearing one by one, connected to standard resistances of 100, 109.75, 119.5, 129.25 and 139 ohms and checked for expected temperatures of 0, 25, 50, 75 and 100°C on the display of the resistance box. The corresponding gain factors were then calculated and added to data acquisition software LabVIEW. The accuracy of the temperature sensors is 0.01. The journal made of C45 steel material is mounted horizontally on two pedestal bearings and is rotated by a motor through 1:2 ratio pulleys, to attain maximum speed up to 5700 rpm. A proximity sensor fixed on the journal, senses its speed. A chrome plated journal sleeve is tightened at middle portion of journal with lock nuts and bearing slides over it. A radial load is applied on bearing by 1:5 loading lever. The test rig is designed to apply a maximum radial load up to 700 N. An rpm sensor disc with square shape is fixed to journal end and rotates along with it. The measured data of oil temperature and speed obtained during the experimentation in analog form is converted into digital form and preprocessed on the instrumentation card. The data received is displayed and stored on the PC using LabVIEW based software WINDUCOM 2005. An oil sump is provided beneath the bearing for collecting the used oil which flows into metallic tank for its re-circulation. Viscosity of lubricating oil is determined by using FUNGILAB Digital Readout Viscometer. The resistance generated by the test material on this spindle is directly proportional to its viscosity. Measuring range of the viscosity of this model is 6 cP to 2 × 106 cP.

Fig. 2 Journal bearing test rig

An Experimental Investigation of Oil Film Temperatures in Elliptical Profile Journal Bearing

Japanese Society of Tribologists (http://www.tribology.jp/) Tribology Online, Vol. 8, No. 1 (2013) / 3

3. Experimental programme

In present experimentation, an elliptical journal bearing (Fig. 3) was tested using three grade oils: oil 1, oil 2, and oil 3 at loads varying from 100 N to 600 N and speeds = 3000, 3500, and 4000 rpm at constant oil inlet pressure of 0.3 MPa and oil inlet temperature of 30°C. The oil was supplied through two oil grooves at 90° to the vertical loading line.

4. Results and discussion

Input parameters and properties of the oils under study are given in Table 1 and Table 2 respectively. The results obtained experimentally have been plotted in Figs. 4-7. It is observed that:

1. For all grade oils under study, the temperature of bearing increases with increase in the load at same speed and pressure. Also, the temperature of bearing has been observed to be increasing with increase in speed at same load and pressure for each grade oil. Temperature rise is less at high loads and is nearly equal at high speeds, when compared with temperature rise at low loads and

speeds. This can be attributed to the more oil being pumped into the system which has been measured directly with the help of a flow meter fitted to the test rig. For lower lobe, the maximum oil temperature rise is observed as 24.5°C for oil 3 at 4000 rpm and 600 N, and minimum rise is observed as 16.6°C for oil 2 at 3000 rpm and 100 N. For

Fig. 3 Elliptical journal bearing with RTD sensors

Table 2 Properties of the oils under study

Oil 1 (Hydrol 68)

Oil 2 (Mak 2T)

Oil 3 (Mak Multigrade)

Viscosity, µ (at T0 = 33°C) 0.075 Pa·s 0.065 Pa·s 0.200 Pa·s Viscosity, µ (at T0 = 100°C) 0.00771 Pa·s 0.004861 Pa·s 0.01239 Pa·s Density, ρ 880 kg/m3 868 kg/m3 885 kg/m3 Thermal conductivity**, Koil 0.126 W/m·°C 0.126 W/m·°C 0.126 W/m·°C Viscosity index* 98 135 110 Flash point*, °C 230 94 200 Pour point*, °C -9 -24 -21 Barus viscosity-pressure index**, α 2.3 e-8 Pa-1 Temperature viscosity- coefficient**, γ 0.034 K-1 Thermal conductivity of bush**, Kbush 0.22 W/m·°C Coefficient of thermal expansion of bush, hbush

75 e-6 K-1

*[7-10]; ** [6]

Table 1 Input parameters

Parameter Dimension Tolerance Roughness

Outer diameter of the bearing, OD 85 mm ±0.2 mm 10 µm

Maximum inner diameter of the bearing, DImax 65.4 mm ±0.2 mm 10 µm

Minimum inner diameter of the bearing, DImin 65.2 mm ±0.2 mm 10 µm

Length, L 65 mm ±0.2 mm 10 µm Radial Clearance, C 300 µm ±50 µm Minimum Clearance, Cm 200 µm ±50 µm Oil hole 6.35 mm ±0.15 mm Relative sensor position 45° ±1°

Rakesh Sehgal, Amit Chauhan and Rajesh Kumar Sharma

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upper lobe, the maximum oil temperature rise is found to be 21.4°C for oil 3 at 4000 rpm and 600 N, and minimum rise is found to be 13.4°C for oil 2 at 3000 rpm and 100 N. Oil temperature rise is highest for oil 3, which is probably due to the fact that this oil is highly viscous in nature which results in high frictional heat generation because of rubbing between oil layers. The oil temperature variations shown in Figs. 4 (a & b)-6 (a & b) starts with a higher value of oil temperature than the oil inlet temperature and this may be because of the recirculation and hot oil mixing during the experimentation.

2. As expected across the width of the bearing, oil film temperature is higher at central plane of the bearing as compared to ends of the bearing for all the grade oils under study. Maximum film temperature rise at the centre of the bearing is observed as 18.4°C for oil 3, 17.5°C for oil 2 and 17.8°C for oil 1 respectively at 600 N and 4000 rpm. This can be attributed to fact that oil 3 is highly viscous in nature which results in high frictional heat generation because of the rubbing between oil layers. It has also been found that increase in load at the same speed results in the increase in oil film temperature of bearing.

3. Flow rate is 6.8 l/min for oil 1, 7.3 l/min for Oil 2, and 9.0 l/min for oil 3 at 3000 rpm and 100 N, and

Fig. 4 Circumferential variation of oil film temperature in the central plane of elliptical journal bearing at 3000 rpm and 0.3 MPa pressure at load = 100 and 600 N

Fig. 5 Circumferential variation of oil film temperature in the central plane of elliptical journal bearing at 3500 rpm and 0.3 MPa pressure at load = 100 and 600 N

Fig. 6 Circumferential variation of oil film temperature in the central plane of elliptical journal bearing at 4000 rpm and 0.3 MPa pressure at load = 100 and 600 N

An Experimental Investigation of Oil Film Temperatures in Elliptical Profile Journal Bearing

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is 8.8 l/min for oil 1, 8.9 l/min for oil 2, and 11.3 l/min for oil 3 at 4000 rpm and 600 N. Flow rate for oil 3 is large at high speeds and loads as its viscosity decreases sharply with temperature in comparison to the other grade oils considered.

4. It has been observed that the nature/trend of oil film temperature distribution along the circumference of bearing is very close to the oil film temperature distribution obtained by the theoretical model developed by Chauhan et al. [6] as shown in Fig. 8 for oil 2. In Fig. 8, the oil inlet temperature has been taken as 44.5°C which is the oil inlet temperature obtained during

experimentation at 3000 rpm and 600 N. The oil film temperatures obtained experimentally has been observed to be on higher side than theoretical one, which may be due to the oil recirculation, and mixing of hot oil at the oil inlet.

5. Conclusion

It is concluded that the thermal behaviour of the elliptical profile journal bearing is affected significantly by rotational speed, loads and type of oil used. Flow rate increases markedly with speed and load. For lower lobe, the maximum oil temperature rise is 24.5°C for oil 3 at 4000 rpm and 600 N, and minimum oil temperature rise is 16.6°C for oil 2 at 3000 rpm and 100 N. For upper lobe, the maximum oil temperature rise is 21.4°C for oil 3 at 4000 rpm and 600 N and minimum oil temperature rise is 13.4°C for oil 2 at 3000 rpm and 100 N. Across the width of bearing, oil temperature is highest for oil 3. Therefore, under the operating conditions (load up to 600 N and speed up to 4000 rpm), it is more appropriate to use oil 2, as the elliptical journal bearing will run coolest, thus providing the desired operational accuracies. A reasonably good agreement in the nature of oil film temperature has been observed between the experimental and the theoretical results for oil 2.

Nomenclature

e Eccentricity, m h Film thickness for elliptical journal bearing, m hbush Coefficient of thermal expansion of bush, K-1

n Journal Speed, rpm ofr Oil flow rate, l/min P Film Pressure, Pa R Radius of journal, mm RL, RU Radius of lower lobe and upper lobe of the

bearing, mm

Fig. 7 Temperature distribution (°C) in elliptical

journal bearing at 4000 rpm at 600 N for Oil 1, Oil 2 and Oil 3

Fig. 8 Comparison of oil film temperature in the central plane of elliptical journal bearing obtained experimentally with the available results of Chauhan et al. (2010) for Oil 2 [N = 3000 rpm, µ = 0.038 Pa·s, C = 300 µm, L = 0.065 m, DImin = 0.0652 m, Cm = 200 µm, Oil inlet temperature = 44.5°C, Load = 600 N]

Rakesh Sehgal, Amit Chauhan and Rajesh Kumar Sharma

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T0 Oil inlet temperature, °C W Load, N θ Angle measured from the horizontal split axis

in the direction of rotation, degree ω Angular velocity of shaft, rad/s

References

[1] Pinkus, O. and Lynn, M., “Analysis of Elliptical Bearings,” Trans. ASME, 55-LUB-22, 1956, 965-973.

[2] Fitzgerald, M. K. and Neal, P. B., “Temperature Distributions and Heat Transfer in Journal Bearings,” Trans. ASME, J. Tribol., 114, 1992, 122-130.

[3] Hussain, A., Mistry, K., Biswas, S. and Athre, K., “Thermal Analysis of Non-Circular Bearing,” Trans. ASME, J. Tribol., 118, 1996, 246-254.

[4] Mishra, P. C., Pandey, R. K. and Athre, K., “Temperature Profile of an Elliptic Bore Journal Bearing,” Tribology International, 40, 3, 2007, 453-458.

[5] Ostayen, R. A. J. V. and Beek, A. V., “Thermal Modelling of the Lemon-Bore Hydrodynamic Bearing,” Tribology International, 42, 2009, 23-32.

[6] Chauhan, A., Sehgal, R. and Sharma, R. K., “Thermohydrodynamic Analysis of Elliptical Journal Bearing with Different Grade Oils,” Tribology International, 43, 2010, 1970-1977.

[7] www.bharatpetroleum.com [8] www.bharatpetroleum.com/wheels/pop_ups/mak2t

.htm [9] www.bharatpetroleum.com/wheels/pop_ups/makm

ultigrade.htm [10] www.maklubes.com