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Analysis of differently sized prototypes of an MR clutch by performance indices

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2013 Smart Mater. Struct. 22 105009

(http://iopscience.iop.org/0964-1726/22/10/105009)

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Smart Mater. Struct. 22 (2013) 105009 (10pp) doi:10.1088/0964-1726/22/10/105009

Analysis of differently sized prototypes ofan MR clutch by performance indices

Francesco Bucchi1, Paola Forte1, Alessandro Franceschini2 andFrancesco Frendo1

1 Department of Civil and Industrial Engineering, University of Pisa, Largo Lucio Lazzarino,56122 Pisa, Italy2 Pierburg Pump Technology Italy S.p.A, Via S Orlando 12, 57123 Livorno, Italy

E-mail: frendo@ing.unipi.it

Received 3 May 2013, in final form 24 July 2013Published 4 September 2013Online at stacks.iop.org/SMS/22/105009

AbstractIn this paper we investigate the torque performances of three prototypes of amagnetorheological clutch, whose operation is based on the relative movement of a permanentmagnet with respect to a gap filled with magnetorheological fluid, are discussed. The clutchwas already been presented by the authors in other papers. The analysed prototypes are basedon the same layout and have different sizes (i.e. outer diameter and overall length). Two newperformance indices are introduced to evaluate the clutch design, and are proposed as analternative to the usually employed turn-up ratio for evaluating MR clutches and brakes.Measurements of the transmitted torque in different operating conditions were carried out on apurpose-built test bench. A brushless motor drove the primary shaft of the clutch, while thetransmitted torque was measured by a torque-meters up to 1500 rpm. The experimental resultsare discussed and compared to those expected on the basis of simulations, showing a fairlygood agreement. Some geometric parameters were found to have a great influence on themagnet shielding in the disengaged configuration.

(Some figures may appear in colour only in the online journal)

Nomenclature

h Fluid gap thicknessn Unit vector normal to the fluid surfacer Radial coordinater Normalized radial coordinates Axial coordinates Normalized axial coordinateA Mean gap surfaceD Average diameter of the cylindrical gapDA Average diameter of the cylindrical gap—

prototype ADi Inner diameter of the disc gapDe Outer diameter of the disc gapDj Average diameter of the cylindrical gap—

generic prototypeH⊥ Component of the magnetic field normal to the

fluidH Magnetic field vector

L Length of the cylindrical gapLA Length of the cylindrical gap—prototype ALj Length of the cylindrical gap—generic prototypeR Average radius of the cylindrical gapRj Average radius of the cylindrical gap—generic

prototypeT Total torqueTON Experimental engaged torqueTOFF Experimental disengaged torqueT Generic computed torqueTON Computed engaged torqueTON

A Computed engaged torque—prototype A

TONj Computed engaged torque—generic prototype

TOFF Computed disengaged torque

TOFFA Computed disengaged torque—prototype A

TOFFj Computed disengaged torque—generic proto-

type

10964-1726/13/105009+10$33.00 c© 2013 IOP Publishing Ltd Printed in the UK & the USA

Smart Mater. Struct. 22 (2013) 105009 F Bucchi et al

T0j Computed yield torque—generic prototype

T0A,c Computed yield torque—prototype A, cylindri-

cal gap

T0A,d Computed yield torque—prototype A, disc gap

Tvj Computed viscous torque—generic prototype

TvA,c Computed viscous torque—prototype A, cylin-

drical gap

TvA,d Computed viscous torque—prototype A, disc

gapTm

c Minimum ideal torque transmitted by a cylindri-cal gap

TMc Maximum transmissible torque by a cylindrical

gapTm

d Minimum ideal torque transmitted by a disc gap

TMd Maximum transmissible torque by a disc gap

Tm Minimum ideal torque

TM Maximum transmissible torque1ω Relative angular speedεON Exploitation indexεOFF Efficiency indexη Viscosityθ Angular coordinateξ Turn-up ratioτ Shear stressτA Shear stress—prototype Aτj Shear stress—generic prototypeτ0 Yield stress of the MR fluidτM Maximum MR fluid shear stressτ v Viscous stress

1. Introduction

Magnetorheological fluids (MRF) are smart materials. Theirbasic property is the capability of changing their rheologicalproperties in a very short time under the effect of a magneticfield. When not subjected to a magnetic field they behave asNewtonian fluids, showing a linear dependence of the shearstress on the shear rate (constant viscosity); in contrast, in thepresence of a magnetic field, they exhibit a rigid behaviourup to a threshold, beyond which they appear almost-perfectlyplastic. In the classical literature, the Bingham-plastic (BP) [1]and Herschel–Bulkley (HB) [2] mathematical models areusually adopted to describe the MRF constitutive propertyin activated conditions. A feature valid both for BP and HBmodels is the presence of a yield stress value, dependent onthe magnetic field, that has to be overcome before the fluidstarts to slip.

The first studies of MRFs date back to the 1940s.Nowadays their application is envisaged in clutches, brakesand dampers according to different operating modes [3]: theshear mode is exploited in clutches and brakes, the flowmode in shock-absorbers, and the squeeze mode in vibrationdampers [3–6]. In patents on MR clutches [7–12] severalsolutions are proposed, including single or multidisc, drum orcone shaped geometries. Usually, MR clutches have a disc or a

drum type geometry [8, 9] and the magnetic field is generatedby coils. The use of a permanent magnet [13] and hybridsystems [14] is uncommon for these applications.

In this paper three prototypes of an innovative MR clutchwith permanent magnet (PM) are presented. The activityis part of a wider research project, which was funded byRegione Toscana (Italy) and carried out in cooperation by twodepartments of the University of Pisa (Italy) (Department ofCivil and Industrial Engineering and Department of Energy,Systems, Territory and Constructions) and Pierburg PumpTechnology GmbH (Livorno, Italy). The aim of the projectwas a feasibility study of a clutch for the disengagementof auxiliary devices in an automotive powertrain in orderto reduce power consumption. A possible application wasidentified in the disengagement of the vacuum pump usedin the power-brake system when assistance is not necessary.When required, the device must re-engage quickly in orderfor the vacuum pump to work properly [15].

The design of the first prototype was described in aprevious work [16, 17] and is briefly summarized in thispaper; it was based on both electromagnetic FEM simulationsand operating functionality. MR fluids have several interestingproperties satisfying the requirements of the device. Theycan withstand different levels of shear stress, depending onmagnetic field, and they have a very short commutationtime (∼10 ms). Another relevant aspect for the envisagedapplication lies in the capability of the MR clutch oftransmitting torque without requiring any axial load. Theuse of a PM was selected to generate the magnetic field inorder to assure a fail-safe operation in the event of electricsupply failure. Moreover, a PM does not require energy togenerate the magnetic field, which is also convenient forthe considered application (reduction of fuel consumption).The MR fluid gap consists of a plane circular portion anda cylindrical part. The PM axial motion in its housingdetermines the clutch operation. Whether the magnet is in theengaged or the disengaged position, the magnetic field reachesthe fluid or passes around an appropriate shielding. Magnetmovement was not implemented on the analysed prototypes,but a fail-safe pneumatic actuation was designed for futureapplications (the pneumatic actuation principle was presentedin [18, 19]).

After the first prototype, two additional prototypes weremanufactured on the basis of geometrical similarity, in orderto analyse the effect of geometric parameters on the deviceproperties. The torque characteristics of the three prototypesare investigated in the present paper. For the envisagedapplication (overall efficiency of the powertrain), in additionto the minimum torque requirements with reference to theengaged condition, the device is required to have a lowdissipation when it is disengaged. For this aim, two newperformance indices were introduced in order to evaluate theMR fluid exploitation and the device efficiency in the engagedand disengaged condition, respectively. The proposed indicesare more specific and give additional information with respectto the turn-up ratio, which is usually used for MR clutches andbrakes [20].

The test bench which was set up to measure thetransmitted torque and the rotational speeds of the shafts

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Smart Mater. Struct. 22 (2013) 105009 F Bucchi et al

Table 1. Design specifications.

Outer diameter <70 mmLength <50 mmNominal operating torque 1 N mMaximum engaged torque 2.5 N mDisengaged torque at 1500 rpm <0.5 N mRotational speed <3000 rpm

during the tests [21] is briefly described in the paper; similartest benches for clutches and brakes can be found in [22–26].The experimental results for the three prototypes are thencompared and discussed with reference to the expectedtorques and on the basis of the introduced performanceindices.

2. Design of the MR clutch

The design of the first MR prototype is briefly summarizedin the following. The MR clutch was designed according tothe specifications given in table 1. The nominal torque (ONcondition) necessary to drive auxiliary systems is about 1 N m;in addition, a relatively high torque, i.e. 2.5 N m, could berequested at vehicle start-up for short times, in the case oflow temperatures (due to the high viscosity of the lubricant inthe vacuum pump, which causes an increase of the absorbedtorque); the torque in the disengaged (OFF) condition mustbe as low as possible, in order to minimize losses. Additionalrequired specifications were the absence of any axial load onthe shafts, a fail-safe operation in the case of electric failure(e.g. due to exhausted batteries) and strict size constraints dueto the small volume available for the device. For the foreseenapplication the clutch should be directly coupled with thecamshaft, therefore the rotating speed limit is referred to the6000 rpm of the vehicle engine.

2.1. Preliminary design phase

Several geometric configurations of the clutch were firstlytaken into consideration: multidisc, coaxial cylindricalmultigap, powered by coils or by a PM. Each configurationand the related torque can be obtained as a combination of twobasic geometries, made up of a circular plane and a cylindricalfluid gap.

The shear stress τ in the MR fluid gap was estimated,according to the BP model, as the sum of an MR contributionτ0, depending on the component of the magnetic field H⊥,normally oriented with respect to the gap (figure 1), and of aviscous contribution τv, as described by equation (1):

τ = τ0(H⊥)+ τv = τ0(H⊥)+η1ωr

h(1)

where 1ω is the difference of angular speed between theshafts, r the radial coordinate, h the gap thickness and η

the viscosity of the fluid, which for the selected MR fluid isassumed constant and equal to 0.28 Pa s. Lord CorporationMRF140CG was selected as the MR fluid, as one with thehighest yield stress versus magnetic field characteristic.

Figure 1. Yield stress τ0 versus magnetic field H⊥—LordCorporation MRF140CG.

The magnetic field H for the analysed clutch geometrieswas estimated on the basis of electromagnetic finite element(FE) simulations, performed by the EFFE code [27] asdescribed in [19, 28]. The component of the magnetic fieldH⊥ to be used in equation (1) was obtained by the followingrelationship:

H⊥(r, s, θ) = H(r, s, θ) · n(r, s, θ) (2)

in which n(r, s, θ) is the unit vector normal to the fluid gapsurface, locally defined as a function of the radial, axial andangular coordinates r, s and θ .

The expected torque for each analysed geometry was thenobtained by numerical integration of the following equation:

T =∫

Aτ r dA (3)

where A represents the mean gap surface and τ is given byequation (1).

Solutions based on coils did not respect the fail-safespecification and were abandoned. Simulations on multidiscand coaxial cylindrical multigap geometries revealed that anadvantageous solution consisted in reducing the number ofactive surfaces, in order to minimize the viscous torque whenthe fluid is not magnetized (OFF condition), and in placingthem at the maximum radial distance from the longitudinalaxis, in order to achieve a high torque transmission in theengaged (ON) condition. In such a way, enough room couldalso be left to house a suitable size PM in order to obtain arelatively high magnetic flux in the MR gap.

2.2. Prototype definition

The final design of the MR clutch, which was reached afterseveral iterations, is shown in figures 2–5 and was made up ofa cylindrical and a plane gap.

Elements no. 1 and 8 represent the input and the outputshaft, respectively. The fluid gap, which is shown in red, canbe filled with MR fluid by removing the screws no. 9. The

3

Smart Mater. Struct. 22 (2013) 105009 F Bucchi et al

Figure 2. MR clutch drawing.

Figure 3. Assembled prototype.

seals no. 3 and 6 were used to avoid fluid leakage duringrotation.

The magnetic field is generated by the rare-earth NdFeBsintered cylindrical PM no. 4, visible in the left picture infigure 4, composed of four sectors and having four poles withdiametrical magnetization. It can move inside its cylindricalhousing along the rod no. 5. Particular attention was alsofocused on material selection, since the magnetic permeabilityof materials has a strong influence on the magnetic field andcan be used in order to direct it normally to the gap. In this

Figure 4. Primary and secondary group.

Figure 5. Prototype components.

way it is possible to obtain high values of magnetic field inthe fluid when the PM is in the ON position (as represented infigure 2); on the other hand, when the magnet is in the OFFposition, element no. 7 acts as a shield for the MR fluid. Thealignment between shafts no. 1 and 8 is guaranteed by thebearings no. 2 and centring pins no. 11. The relative axialpositions of the parts are determined by the Seeger ring no.10, which also determines the thickness of the circular planepart of the fluid gap. The PM is in axial equilibrium both inthe ON and in the OFF conditions. During the experimentalcampaign the PM was located manually in either the ON orOFF position before each test; as mentioned in section 1, anautomatic pneumatic actuation will be implemented in futureprototypes [19]. An assessment of the magnetic axial force onthe magnet was performed, showing it to be lower than 50 Nfor each magnet position [19].

The yield stress distribution of the designed prototype,obtained on the basis of a 3D FE analysis and on the basisof the MR fluid properties, is shown in figure 6. As can beobserved, it results as being cyclically symmetrical due to thefour poles of which the magnet is composed; consequently,there are some regions in which the activation of the fluid issignificantly lower than in others.

For the selected geometry, equation (3) gives:

T = R2∫ L

0

∫ 2π

0τ ds dθ +

∫ R

0

∫ 2π

0τ r2 dr dθ (4)

where the first and second terms of the sum are related to thecylindrical and plane gap, respectively, R is the average radiusof the cylindrical gap and s and θ are the axial and angularcoordinates.

4

Smart Mater. Struct. 22 (2013) 105009 F Bucchi et al

Table 2. Geometrical properties and estimated characteristic torques. (Note: TON and TOFF calculated at 1500 rpm.)

Prototype D (mm) L (mm) TON (N m) TOFF (N m) TM (N m) Tm(1ω) (N m s rad−1)

A 57 14 3.14 0.11 6.66 0.000 68 1ωB 62 14 3.77 0.15 8.16 0.000 91 1ωC 57 19 4.08 0.13 8.03 0.000 80 1ω

Figure 6. Yield stress τ0 within the fluid gap—ON condition.

Figure 7. Built prototypes.

The torques referred to the ON and OFF conditions,obtained by numerical integration of equation (4), result as:

TON(1ω) = (3.03+ 0.000 681ω) N m (5)

TOFF(1ω) = 0.000 681ω N m (6)

where 1ω is expressed in rad s−1.

3. Prototype geometry

Once the first prototype was developed, two additionalprototypes were manufactured in order to evaluate anypossible influence of the geometry on the performance of theclutch. Those prototypes, which are represented in figure 7,differ from the first one, named as A in the figure, in theaverage diameter D (prototype B) and in the length L of thecylindrical gap (prototype C) as shown in table 2.

In order to get an evaluation of the performance of allprototypes, the expected torques TON

j and TOFFj of prototype j

(j = B, C), related to the engaged and disengaged conditions,were estimated on the basis of the results obtained for

prototype A, without performing any additional FE simulationof the magnetic field. To this aim, normalized radial and axialcoordinates r = r/Rj and s = s/Lj, related to the averageradius Rj and to the axial length Lj of the cylindrical gapof each prototype, were introduced, respectively. The samedistribution of the yield stress τj(r, s, θ), function of thenormalized radial and axial coordinates r, s and of the angularcoordinate θ was then assumed for all prototypes, with thatpreviously obtained for prototype A on the basis of FEanalysis and MR fluid properties being considered for thisaim.

With this assumption, considering equations (1) and (4),a linear dependence of the torques on the length L, togetherwith a quadratic and a cubic dependence of the MR and of theviscous torque, respectively, on the average gap diameter isobtained for the cylindrical gap. In a similar way, for the planeportion of the gap, a cubic and a fourth order dependence ofthe MR and of the viscous torque on the average diameter ofthe cylindrical gap are obtained, respectively. Therefore, withreference to figure 8, considering also that De ' D and Di �

De, the expected MR and viscous torques for the prototype jare given by:

TONj = T0

j + Tvj = T0

A,c

(Dj

DA

)2 ( Lj

LA

)+ T0

A,d

(Dj

DA

)3

+ TvA,c

(Dj

DA

)3 ( Lj

LA

)+ Tv

A,d

(Dj

DA

)4

(7)

and

TOFFj = Tv

j = TvA,c

(Dj

DA

)3 ( Lj

LA

)+ Tv

A,d

(Dj

DA

)4

. (8)

4. Prototype performance indices: fluid exploitationand clutch efficiency

In order to evaluate the design of an MR clutch or brakeand, in particular for the present case, to analyse thecapability of the clutch prototypes, two performance indicesare introduced to compare the actual magnetorheologicaltorque and the actual disengaged torque to two ideal referencetorques, i.e. the maximum magnetorheological torque and theminimum dissipative torque, which can be ideally achieved,respectively:

εON(1ω) =TON(1ω)− TOUT(1ω)

TM (9)

εOFF(1ω) =Tm(1ω)

TOFF(1ω). (10)

5

Smart Mater. Struct. 22 (2013) 105009 F Bucchi et al

Figure 8. Prototype geometry.

In these equations, TON(1ω) and TOFF(1ω) are the exper-imentally measured torque in the engaged and disengagedconditions respectively; TOUT(1ω) is the transmitted torque,which is measured with the magnet removed from the clutch;TM represents the ideal magnetorheological torque that canbe obtained if the maximum shear stress τM is exploitedthroughout the whole gap; and Tm(1ω) is the minimumtorque due to the viscous stresses in the fluid, which isideally dissipated in the disengaged condition in the case ofperfect shielding of the magnet and negligible friction losses.TOUT differs from TOFF, since the latter includes any possiblecontribution due to the spurious magnetic field, in the case ofnon-perfect shielding.

Therefore, with the exploitation index, εON, the actualmagnetorheological torque of a given prototype is comparedto the maximum torque which could be ideally achieved forthe considered geometry. On the other hand, the efficiencyindex εOFF evaluates the actual disengaged torque with respectto the minimum torque that would be obtained in the ideal caseof no friction or losses due to spurious magnetic fields.

The indices εON and εOFF are bound in the range [0 1] inthe absence of shear thinning or thickening phenomena and,separately, evaluate the performance of a given MR clutchor brake design in the engaged and disengaged condition,respectively. For this reason they provide a more in-depthinsight with respect to the usually employed turn-up ratioξ , which is simply defined as the ratio TON/TOFF(1ω)

(see [20]). The turn-up ratio, indeed, cannot be comparedto any absolute reference value and thus cannot be used toevaluate a given design and to show how improvements canbe achieved, i.e. with reference to the ON or to the OFFcondition. Moreover, it can be seen that, if TOUT can beneglected in comparison with TON, the turn-up ratio can beobtained on the basis of the previously defined indices, by thefollowing approximate expression:

ξ(1ω) =TON(1ω)

TOFF(1ω)≈ εON(1ω)εOFF(1ω)

TM

Tm(1ω). (11)

Figure 9. Test bench.

The maximum magnetorheological torque TM of thedeveloped prototypes A–C was obtained by equation (3),considering the maximum shear stress τM (τM

≈ 55 kPa forthe selected fluid, see figure 1) and the given geometries. Thefollowing relationship was obtained:

TM= TM

d + TMc = πτ

M

(D3

e − D3i

12+

D2

2L

)(12)

where TMd and TM

c are the maximum magnetorheologicaltorques that can be obtained by the plane and the cylindricalportion of the gap.

At the same time, the minimum torque Tm(1ω), whichwas ideally dissipated in the disengaged condition due to theonly viscous contribution, can be evaluated by replacing τin equation (3) with the viscous stress τv. In this case thefollowing relationship was obtained:

Tm(1ω) = Tmd (1ω)+ Tm

c (1ω)

= πη1ω

h

(D4

e − D4i

32+

D3

4L

). (13)

By considering the actual geometry of the threeprototypes, the numerical values given in table 2 wereobtained.

5. Experimental testing of the three prototypes

The test bench shown in figure 9 was developed in orderto obtain the actual torque characteristics of the clutchprototypes. A detailed description of the bench and testingprocedures is reported in [19, 21].

The input and output shafts no. 2 and no. 6 wereconnected to the asynchronous motor no. 7 and to thebrushless motor no. 1 by the orientable joints no. 8,respectively. The MR clutch no. 4 was positioned in thecentre; a torque-meter, no. 3, is enclosed in the primary groupto measure the torque. The shaft speeds were measured byencoders. The encoder on the primary shaft was embedded inthe brushless motor, no. 1, while for the secondary shaft it wasthe element no. 5. Three orientable bearings were employedto support the shafts, one of which (the one on the left inthe figure) was mounted on an elastic rubber pad to preventhigh internal loads as a consequence of overconstraining.During the test campaign the MR clutch prototypes weretested according to different operating modes, i.e. as clutches(with both the input and the output shafts in rotation) and

6

Smart Mater. Struct. 22 (2013) 105009 F Bucchi et al

Figure 10. Characterization of prototype A.

as brakes (with one of the shafts locked), but no particulardifference was found in the measured characteristics (torqueversus relative angular speed) in the explored rotating speedrange; for this reason the results obtained in the brakingoperating mode will be shown. In this case a fixed systemreplaced the asynchronous motor no. 7 and prevented therotation of the shaft no. 6. With this configuration, a constantslope speed profile was imposed to the brushless motor andthe torque transmitted by the shaft no. 2 was measured bythe torque-meter no. 3. The rotational speed of the shaft wasmeasured by an encoder integrated in the brushless motor andit was verified that the actual rotating speed differed from therequired speed by less than 1%. Results are reported up to amaximum relative speed of 1500 rpm.

The transmitted torque was measured for each of thedeveloped prototypes in four different clutch configurations:

(1) without the MR fluid and with the PM removed from theclutch, in order to get only the friction contribution to themeasured torque;

(2) with the gap filled and the PM removed from theclutch (OUT condition), to get the friction and viscouscontribution to the transmitted torque;

(3) with the gap filled and the PM in the OFF position, toget the torque transmitted in the disengaged condition andevaluate any contribution due to spurious magnetizationof the fluid (unproper MR gap shielding);

(4) with the gap filled and the PM in the ON position, to getthe torque transmitted in the engaged condition.

Each test was carried out at room temperature and lastedabout 20 s; due to the shortness of the tests, any possibletemperature effect on the fluid was neglected. The effect oftemperature on the torque characteristic is discussed in [29].

6. Results and discussion

The results obtained with the three prototypes are shownin figures 10–12, respectively, together with the expected

Figure 11. Characterization of prototype B.

Figure 12. Characterization of prototype C.

torques, calculated considering the BP model (equations(5)–(8)). Each experimental curve was obtained as the averageof five acquisitions. The relative standard deviation obtainedfrom the measured data was less than a few percent andgives an evidence of the good repeatability of the test. Thecomputed torques did not consider any friction contribution.

The torque measured without the MR fluid and PM is dueto the sliding friction of the seals and to the rolling resistanceof the bearings. By testing the clutches filled with MR fluidand without the PM (OUT) it is possible to isolate the viscouscontribution of the unmagnetized fluid. When the PM is in theOFF position, the clutch is disengaged. The torque transmittedin this configuration is usually higher than in the OUTcondition because the MR fluid gap is not completely shieldedfrom the magnetic field and the spurious magnetic field causesan undesired, though small, transmission of torque.

In all cases, as expected, when the PM is in the ONposition and the clutch is engaged, the transmitted torquewas the highest possible. Figures 10–12 show a fairly goodagreement between the expected and the experimental torques

7

Smart Mater. Struct. 22 (2013) 105009 F Bucchi et al

Figure 13. Comparison between computed (dashed lines) andexperimental net torques for the engaged clutch.

in the engaged condition and between the expected andexperimental viscous torques. In addition, for the prototypesA and B, a very small difference between the measured OUTand OFF (disengaged) torques was observed, indicating a veryeffective shielding of the magnet in the disengaged condition.In contrast, for prototype C, which was somewhat longerthan prototype A, a too high spurious magnetization wasobserved. This shows how, in this case, the shielding effect ofelement no. 7 (figure 2) could not be properly obtained simplyby similarity and a detailed FE magnetic analysis becomesmandatory. Moreover, for this prototype the magnetic field inthe disengaged condition caused MR fluid leakage through theseals, and for this reason tests were interrupted at 500 rpm.

In order to deepen the analysis and verify the accuracyof the predicted magnetorheological torques (estimated on thebasis of the FE simulation carried out on prototype A) themagnetorheological torque for each prototype was obtainedby subtracting the torque relative to the OUT condition fromthat relative to the ON condition. The comparison is shownin figure 13. As can be seen, for all the prototypes the yieldtorque was significantly overestimated, which was attributedto an inappropriate definition of the yield strength given inthe MR fluid data sheet. However, a fairly good assessment ofthe engaged torque for 1ω > 500 rpm was obtained by thepresented methodology.

The previously defined fluid exploitation index (εON) andthe clutch efficiency index (εOFF) are shown in figures 14 and15. For all prototypes the fluid exploitation resulted in therange [0.25 0.35] at yielding and in the range [0.40 0.55] inslip condition. Such relatively low values were due both tothe magnetic field, which had a cyclical periodicity, and to thevery low magnetization of the MR fluid in the circular planegap (see figure 6).

On the other hand, the εOFF index (figure 15) showshow seals and bearings give a friction contribution which issignificantly higher than the viscous contribution of the MRfluid, especially at low speed. For prototype C, in particular,

Figure 14. Fluid exploitation index (εON).

Figure 15. Efficiency index in disengaged condition (εOFF).

the spurious magnetization gave an additional contribution tolosses which resulted in even lower index values.

Finally, the turn-up ratio ξ , defined as the TON–TOFF

ratio, is given in figure 16. As already discussed, itmainly represents a relative indication of the torque gainfrom the disengaged to the engaged condition and itsinterpretation is somewhat ambiguous, since it is neitherreferred to any precise value nor bounded. In contrast theexploitation and efficiency indices give precise informationabout improvements that can be obtained with reference tothe engaged and disengaged conditions, separately.

7. Conclusions

The theoretical and experimental characterization of thetorque transmission of three prototypes of an MR fluid clutchwith permanent magnet were presented and discussed inthe paper. For the envisaged application the clutch has tobe capable of transmitting a given requested torque whenengaged, and to have a very small dissipation if disengaged.For this reason two new indices were introduced and used

8

Smart Mater. Struct. 22 (2013) 105009 F Bucchi et al

Figure 16. Turn-up ratio (ξ ).

to independently evaluate the prototype performances withreference to the engaged and disengaged conditions. Fromthis point of view the newly proposed indices are much moreuseful than the turn-up ratio, since they give a precise andquantitative indication about the possibility of improving agiven design, with reference to the ON and to the OFF state.The analysis allows one to point out the importance of themagnetic field distribution and intensity in determining theMR fluid exploitation. The exploitation index was between40% and 55%, indicating the possibility of further designimprovement. The relatively low exploitation index wasmainly due to the low yield stress achieved in the plane portionof the gap and marginally due to the circular periodicity of theyield stress in the cylindrical portion of the gap.

With reference to the disengaged condition, it was shownhow the efficiency was affected by friction losses and bythe spurious magnetization of the fluid. At 1500 rpm theefficiency index was about 20% for two of the prototypes,while one of them showed a poor shielding of the MR gap,giving a very low efficiency (few %).

The exploitation and efficiency indices appeared moreappropriate than the sometimes used turn-up ratio in givingan in-depth evaluation of the performance of an MR clutch orbrake, and are proposed for future design evaluations.

The obtained experimental results validated the FE-based analytical model but pointed out the limits of thegeometric similarity assumption and therefore the needof specific magnetic FE analyses to accurately predicttorque transmission in all conditions. At the same time,the experimental activity confirmed the feasibility of thedeveloped MR device for the selected application (i.e. forthe vacuum pump in diesel engine), in compliance with theconstraints of small available volume, axial load absenceand fail-safe operation. The obtained torque guarantees thatnormally, in the engaged condition, there is no slip, andthen there is no dissipation, with the exception of a shortengagement transient. The study of an automatic pneumaticactuation for the foreseen specific application and of the

temperature effects on the torque transmission is currentlyunder way.

Acknowledgments

The authors wish to thank Regione Toscana for funding theresearch activity and Pierburg Pump Technology (Livorno,Italy) for the technical support given during the projectdevelopment.

References

[1] Bingham E 1916 An Investigation of the Laws of Plastic Flowvol 13 (Gaithersburg, MD: US Bureau of StandardsBulletin) pp 309–53

[2] Herschel W and Bulkley R 1926 Konsistenzmessungen vongummi-benzollsungen Kolloid. Zh. 39 291–300

[3] Olabi A and Grunwald A 2007 Design and application ofmagneto-rheological fluid Mater. Des. 28 2658–64

[4] Forte P, Paterno M and Rustighi E 2004 A magnetorheologicalfluid damper for rotor applications Int. J. Rotating Mach.10 175–82

[5] Carmignani C, Forte P and Rustighi E 2006 Design of a novelmagneto-rheological squeeze-film damper Smart Mater.Struct. 15 164–70

[6] Zapomel J, Ferfecki P and Forte P 2012 A computationalinvestigation of the transient response of an unbalancedrigid rotor flexibly supported and damped by shortmagnetorheological squeeze film dampers Smart Mater.Struct. 21 105011

[7] Rabinow J 1951 Magnetic fluid torque and force transmittingdevice US Patent Office 2575360

[8] Gopalswany S and Jones G 1998 Magnetorheologicaltransmission clutch US Patent Office 5823309

[9] Wheals J, Turner A, Ramsay K and Hey J 2005 Clutchesincluding a magnetorheological fluid WO PatentSpecification 2005/036009 A1

[10] Steinwender H 2011 Magnetorheological clutch US PatentOffice 7870939 B2

[11] Gopalswamy S, Linzell S M and Jones G L 1998Magnetorheological fluid clutch with minimized reluctanceUS Patent Office 5845752

[12] Usoro P B, Smith A L, Kao C, Moser G and Sommer G 2011Magnetorheological fluid clutch US Patent Office6318531 B1

[13] Johnston G L, Kruckemeyer W C and Longhouse R E 1998Magnetorheological passive clutch US Patent Office5848678

[14] Saito T and Ikeda H 2007 Development of a normally closedtype of magnetorheological clutch and its application tosafe torque control system of human-collaborative robotJ. Intell. Mater. Syst. Struct. 18 1181–5

[15] Bucchi F, Forte P, Frendo F and Squarcini R 2012A magnetorheological clutch for efficient automotiveauxiliary device actuation Fract. Struct. Integ. 23 62–74

[16] Armenio G, Bartalesi E, Bucchi F, Ferri A, Forte P, Frendo F,Rizzo R and Squarcini R 2011 Mechanical combustionengine driven fluid pump European Patent114251176.2–2423

[17] Bucchi F, Forte P, Frendo F, Bartalesi E and Squarcini R 2011Progetto e sperimentazione di una frizione a fluidimagnetoreologici XL Convegno Nazionale AIAS Palermo(075) (in Italian)

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Smart Mater. Struct. 22 (2013) 105009 F Bucchi et al

[18] Bartalesi E, Bucchi F and Squarcini R 2012 Vacuum actuationfor axial movement of a magnet in a magnetorheologicalclutch European Patent Office patent pending

[19] Bucchi F, Forte P, Frendo F, Musolino A and Rizzo R 2012A fail-safe magnetorheological clutch excited by permanentmagnets design and prototype testing J. Intell. Mater. Syst.Struct. in review

[20] Genc S 2002 Synthesis and properties of magnetorheological(MR) fluids PhD Thesis University of Pittsburgh

[21] Bucchi F, Forte P and Frendo F 2012 Experimentalcharacterization of a permanent magnet magnetorheologicalclutch for automotive applications ESDA Proc. Nantes(ESDA2012-82284)

[22] Nguyen Q H and Choi S B 2010 Optimal design of anautomotive magnetorheological brake consideringgeometric dimensions and zero-field friction heat SmartMater. Struct. 19 115024

[23] Li W H and Du H 2003 Design and experimental evaluation ofa magnetorheological brake Int. J. Adv. Manuf. Technol.21 508–15

[24] Kavlicoglu B M, Gordaninejad F, Evrensel C A,Cobanoglu N, Liu Y and Fuchs A 2002 A high torquemagnetorheological fluid clutch Proc. SPIE4697 393–400

[25] Kavlicoglu B M, Gordaninejad F, Evrensel C A, Fuchs A andKorol G 2006 A semi-active, high torque,magnetorheological fluid limited slip differential clutchTrans. ASME. J. Vib. Acoust. 128 604–10

[26] Kavlicoglu N C, Kavlicoglu B M, Liu Y, Evrensel C A,Fuchs A, Korol G and Gordaninejad F 2007 Response timeand performance of a high-torque magneto-rheological fluidlimited slip differential clutch Smart Mater. Struct.16 149–59

[27] EFFE 2009 Effe v2.00, User Manual Bathwick ElectricalDesign Ltd

[28] Rizzo R, Musolino A, Bucchi F, Forte P and Frendo F 2013Magnetic fem design of a fail-safe magnetorheologicalclutch excited by permanent magnets and its experimentalvalidation IEEE Trans. Energy Convers. submitted

[29] Bucchi F, Forte P and Frendo F 2013 Thermal effects on thetorque characteristics of a magnetorheological clutch Mech.Adv. Mater. Struct. in press

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