Bezzazi_Experimental-characterization-of-frictional-behaviour-of-clutch-facings-using-Pin-on-disk-machine_2007.pdf

ofJabbouriF.S.T.T,nica,January1 SeptemberDuring the clutch engagement manoeuvre, sliding contact occurs between the pair of clutch facings mounted on the friction disk andthe counter faces belonging to the flywheel and the pressure plate. The transmitted torque is proportional to the overall coecient ofprimary shaft (motor) to the secondary shaft (gear box) forbelonging to the flywheel and the pressure plate. Through-ments. Good performances of the clutch facings materialcomplex and non reproducible. Their eects could be, inpractice, globally taken into account through the coe-cient of friction tolerances.To characterize the tribological behaviour of clutch fac-ings, a lot of standard laboratory tests are usually per-formed by the manufacturers: Raybestos tests in case of*Corresponding author. Address: 33, Residence Al Ismalia, Avenue desF.A.R., Appt. 15B, Tetouan, Morocco. Tel.: +212 67 79 50 68; fax: +21239 35 07 02.E-mail address: (A. Khamlichi).Materials and Design 28 (2007)Materialsintermittent periods only. To achieve that, a clutch isrequired between these two components. The function ofthe clutch is to produce a soft gradual increase in the angu-lar velocity of the driven shaft until full coupling betweenthe motor and the gear box is achieved. Then, the clutchmust act as a permanent coupling transmitting, from thedriving shaft to the vehicle wheels, the entire mechanicalpower without subsequent slip 1.During dry clutch engagement manoeuvre, a transientsliding contact situation occurs between the pair of clutchfacings mounted on the friction disk and the counter facessuch as a stable and suciently high coecient of frictionare required in order to operate engagements in an ecientand regular way. Fading phenomenon which is due to asudden decrease of the friction coecient as function oftemperature must be avoided. Variations in time of the fric-tion coecient during the running in phase or due to mate-rial wear must also remain limited.Other factors in addition to temperature might influencethe coecient of friction level. They are related to hetero-geneities of the contact surfaces, chemical reactions orphysical transformations. But, these mechanisms are bothfriction which depends essentially on temperature, normal pressure load and relative sliding velocity. In this work, performance ofthe friction coecient is investigated experimentally. Samples of a commercial clutch facings material have been tested using a Pin-on-disk apparatus. When the previous three parameters are preset constant, this machine provides automatic acquisition of friction coef-ficient and wear measurements. The obtained results are compared with the classical SAE J661a standard test. It is found that the actualclutch facings material has good fading resistance and a rather stable coecient of friction once running in phase is achieved.C211 2006 Elsevier Ltd. All rights reserved.Keywords: Composites; Thermal; Mechanical; Friction behaviour; Clutch facings1. IntroductionIn a vehicle rotary motion is to be transmitted from theout this engagement process, heat is generated at the con-tact surface yielding temperature to rise 2. Temperaturerise may be very important in case of repetitive engage-Experimental characterizationfacings using Pin-on-diskM. Bezzazia, A. Khamlichia,*, A.aLMMS, Departement de Physique,bDepartamento de Engenharia MecaReceived 24Available onlineAbstract0261-3069/$ - see front matter C211 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.matdes.2006.07.001frictional behaviour of clutchmachinea, P. Reisb, J.P. DavimbB.P. 416, Tangier, MoroccoUniversidade do Aveiro, P/locate/matdes21482153& DesignandEurope 3, JIS D-4311 in case of Japan 4 and SAE J661atest in case of USA 5. But, most of these tests are bothlengthy and costly. This makes them dicult to use in realresearch and development operations. So, they should berather kept to the final stage in order to assess the materialcommercial specifications. Other basic tests which are morecost eective should be used in order to facilitate morequickly information about fiction and wear behaviours.Incaseoffrictionproducts,thewellknownFASTtest5is a quick and inexpensive test. However, it suers from thepathological apparition of glazing eect. This eect is oftenencountered for polymeric composite materials due to lackof cleaning the friction surface in the FAST test. Apparentglazing has in general nothing to do with real conditionsof product working because the friction surface is perma-nently cleaned in real servicing life of friction products.In the special case of metal pairs 6, and PEEK-CF30/steel 7,8 the Pin-on-disc has proven to be one of the mosteective test configurations which allow a reliable andquick determination of coecient of friction and wear.In this work, we have investigated the opportunity touse the Pin-on-disk test in order to study the dry tribolog-ical behaviour of commercial organic clutch facings whichare manufactured according to the scatter wound proce-dure. For this purpose, special samples of clutch facingsmaterial having the form of disks and special pins madefrom steel were manufactured. The inverse operating con-figuration in comparison with the habitual technique ofusing the Pin-on-disk apparatus was chosen in order to per-form the friction and wear tests. These conditions arethought to enable the friction surface cleaning and to avoidartificial glazing.Through this experiment, we have studied the frictioncoecient behaviour as function of temperature, slidingvelocity and sliding distance. Load pressure was keptalmost constant. Wear was also measured as function ofthe previous parameters.The obtained results are presented in the following ascharts giving the friction coecient, l, versus the slidingdistance when temperature, T, and the sliding velocity, v,are set constant. Qualitative comparison with the classicalSAE J661a test is then performed. They show in particularthat glazing eect does not occur during testing. The fric-tion product is found to have a stable coecient of frictionand does not suer from fading phenomenon.2. Clutch facing material description2.1. Clutch facing material formulation and themanufacturing processOrganic clutch facings are composites witch are manu-factured in the most common case according to the scatterwound procedure. First a balanced formula is given. Thisconsists of a fibre glass yarn fitted with a copper strip inM. Bezzazi et al. / Materialsorder to enhance conductivity and to relieve heat evacua-tion from the facing surface during clutch operation. Theformula contains also the weight composition of theimpregnating mixture which holds in general more than fif-teen raw materials. These materials are bonded by meansof a phenolic resin which is put in the impregnant mixturewith a proportion of about 30% by weight.The manufacturing steps as described in 5 are asfollows:(1) Impregnation. A yarn is placed into an impregnatingtank which is filled with the impregnant solution. Vis-cosity is modified by adding organic solvent in orderto improve the mixture absorption by yarn.(2) Drain and preliminary bake. Rack of impregnatedyarn is lifted from impregnant and allowed to drainin an air circulating oven to remove the extra solventand to produce a relatively sti yarn.(3) Preforming. This needs an automatic preformingmachine and special platens. The dry yarn is forcedto form a tracing which enables even distribution ofmatter over the preform surface.(4) Flat press and final bake. The preform is put into aheated mould and highly pressed during a short cur-ing cycle. Then, rings are stacked in piles on a flat sur-face and under weights to keep flat and placed into anoven during a long and smooth curing cycle.(5) Grinding to thickness and turn and bore. Grinding isdone on a double disc grinder. Then the rings areplaced on the boring bar in order to have the desireddiameters.After all these operations, anti corrosion surface treat-ment of clutch facings is achieved and drilling in accordwith a specified drawing can be performed. The clutch fac-ings are then ready to fit a friction disk.2.2. Clutch facing morphologyThe clutch facing morphology is in general very hetero-geneous. But it could roughly be defined by the threedimensional tracing achieved by the impregnated yarn dur-ing the preforming operation. Some irregularities mayaect the overall yarn tracing due to certain deformationsoccurring during moulding or because of material removalneeded in grinding operations. Yarn tracing results in theactual preforming machine from superposing two basicmovements of the pipe: a radial sinusoidal translation withangular frequency x1and a uniform circular rotation withspeed x2, a = x1/x2defines the preforming ratio. Thisparameter controls to a large extent the clutch facingmorphology.For some given material formulation and for some com-mercial reference application of the clutch facing whichcorresponds to outer radius Ro, inner radius Riand thick-ness e, the facing texture depends on how the preform isoutfitted.Design 28 (2007) 21482153 2149Using polar coordinates (r,h), the location of an arbi-trary point of the yarn tracing is given, as studied in 2,by r(t)=Rm+ hsin(x1t) and h(t)=x2t, whereperpendicular to the disk. The test disk is mounted onthe disk carrier spindle and secured by a central bolt. Thepins specimens were clamped in the collet chuck and fixedin a loading arm. The pins stayed over the disks with twofreedom degrees: a vertical one, which allows normal loadFig. 1. Clutch facing morphology for a = 4.27.2150 M. Bezzazi et al. / Materials and Design 28 (2007) 21482153Rm=(Ro+ Ri)/2 is the mean radius of the clutch facingand h =(RoC0Ri)/2.Fig. 1 gives the clutch facing morphology whereRo= 100 mm, Ri= 67 mm, e = 3.35 mm, x2= 180 rpmand a = 4.27. The chosen value of a enables optimal mate-rial distribution over the clutch facing surface, 2.3. Tribological tests3.1. Tribometer Pin-on-diskFriction and wear tests, on polymeric composite clutchfacings material under unlubricated conditions, were per-formed on a Pin-on-disk tribometer PLINT TE67HTC210.The objective was to evaluate the behaviour of the compos-ite material/steel pair under the eect of sliding distance,sliding velocity, normal load and temperature.The tribometer, as shown in Fig. 2, consists of a loadingstationary pin sliding against a rotating disk with its axisFig. 2. Tribometer PLINT TE67HTC210.application by direct contact with the surface of the disk,and a horizontal one, for friction measurement. The nor-mal load applied on the pin is provided by a pneumatic sys-tem with a compression load cell. A motor with atachogenerator feedback ensured the stable running speeds.An indication of wear process in the pin/disk contact wasgiven by a linear potentiometer mounted in the pneumaticpin-loading piston.3.2. Tribological conditionsDIN Ck45K steel pins (flat-ended) with a diameter of8 mm and a length of 67.8 mm were machined. All pinshave the following chemical composition (wt.%): 0.45%C,0.25%Si, 0.65%Mn and present a hardness value of230 HB. Before testing the pins were ultrasonically cleanedin an acetone bath.Organic friction material discs with 8 mm thickness and76 mm of diameter were manufactured and used as counterface.Table 1 shows the tribological conditions used on teststo evaluate the behaviour of composite material/steel pair.These conditions were chosen so that the load pressure Premains almost constant during the tests. Two test temper-atures were chosen: ambient temperature and 100 C176C. Tem-perature is stabilized by a controlled heating system. Allthe experiments were performed under dry sliding condi-tions with a relative humidity of about 50%.3.3. Chase machine and standard SAE J661aThe Chase machine is well established as a means ofdetermining friction levels for clutch facings material. Asample for Chase testing has the dimensions25.4 mm 25.4 mm 6 mm. The test procedure is accord-ing to SAE standard J661a. It begins with a bedding in(running in) of 20 applications, 10 s on, 20 s o, with fric-tion readings at every fifth application. This is followed bya drag test where the test drum temperature is allowed torise to 550 C176F (288 C176C). During the recovery part of the testthe drum is allowed to cool, and the brake is applied, andfriction readings taken, at 100 C176F (37.8 C176C) intervals. Thewear portion of the test consists of 100 applications at400 C176F (204.4 C176C), 20 s on, 10 s o. This is followed by asecond fade and recovery test. Similar to the first, but withTable 1Tribological conditions used on testsSliding velocity (m/s) Sliding distance (m)3 10001 1000facing material having been tested, fading is found to beoutside the range of parameters investigated in the presentstudy.PPTable 2Computed average coecient of frictionSliding velocity(m/s)Temperature(C176C)Pressure(N)Frictioncoecient1 100 51 0.271 100 53 0.333 100 56 0.191 200 58 0.323 200 51 0.17and Design 28 (2007) 21482153 2151temperatures going up to 650 C176F (343.3 C176C). Finally a base-line like the one at the beginning is performed. Test sam-ples are weighed and thickness is measured before andafter testing to get some idea of wear.4. Experimental friction charts obtained by Pin-on-diskmachineThe obtained experimental results with the Pin-on-diskmachine are depicted in the following figures as curves giv-ing the friction coecient l versus the sliding distance.Fig. 4(a) and (b) are associated with almost the sameexperimental conditions but the sample disk was returnedin case of Fig. 4(a), so the friction surface material in con-tact with the pin is the backward surface in the first caseand the forward surface in the second case. This surfacewas used also for the subsequent tests. One can notice thatthe friction coecient becomes stable in Fig. 4(a) and (b)for high sliding distances. But the reached levels are slightlydierent: 0.27 for Fig. 4(a) and 0.33 for Fig. 4(b). However,during the running in phase (estimated at 200 m) the fric-tion coecient behaviour is quite dierent from one faceto the other. This is mainly due to the manufacturing pro-cess and to the heterogeneous texture of the friction mate-rial surface as depicted in Fig. 1. These variations areconsidered here as defining tolerances of the friction coe-cient. After the running in phase they are found to be lim-ited to 0.06.The same remarks about the stable behaviour of the fric-tion coecient can be formulated in case of Fig. 4(c)(e).The running in phase is found in these last cases to beshorter. Explanation of this could be related to the amountof the frictional work which is here higher, so only a smal-ler sliding distance is required.It should be noticed also that temperature measure-ments were performed during Pin-on-disk testing on thedisk lateral side. This temperature is not equal to that ofthe friction surface between the pin and the disk. More-over, this last temperature is always higher than the lateraldisk temperature due to heating resulting from the fric-tional work. Since the generated heat increases withincreasing sliding velocity, the spot temperature at the con-tact surface between the pin and the friction material willincrease if sliding velocity is increased.Organic polymeric friction materials sliding against steelor cast iron are known to have a friction coecient whichdecreases with temperature after reaching a maximum 9.This is in agreement with Table 2 results since the frictioncoecient is in all cases decreasing with increasing slidingvelocity or temperature. Eect of sliding velocity variationsis found also to be higher than that associated with temper-ature variations.Finally, one can notice that in all the previous tests thefriction coecient behaviour is rather stable and does notshow huge fluctuations from the mean level. The frictionM. Bezzazi et al. / Materialscoecient level remains also high even for the hard condi-tions corresponding to v = 3 m/s. For the special clutch5. Comparison with other testing resultsFig. 5 gives the friction coecient versus temperatureduring the seven phases of the SAE J661a test. The pres-sure load acting on the clutch facing sample is almost thesame than that used in the Pin-on-disk testing. Tempera-ture is measured on the drum, Fig. 3. So, one could expectto find a significant dierence between temperature used inthe Pin-on-disk test and temperature given in Fig. 5.Another dierence between these two tests is due to thecounter friction surface used in the SAE J661a which ismade from cast iron, whereas for the Pin-on-disk it is madefrom steel. Also, samples used in the SAE J661a are only4 mm thick and are taken from daily life productionwhereas the tested disks on the Pin-on-disk machine are8 mm thick. Dierences between these two tests regardingthe coecient of friction level are expected to exceed theabove mentioned tolerance of 0.06.From the previous discussion it results that only a qual-itative comparison could be done between Figs. 4(a)(e)and 5. This however shows that the coecient of frictionbehaviour as function of temperature is well characterizedby the Pin-on-disk tests, Fig. 4(a)(e). In fact, to reach thesame average level of the friction coecient as in these lasttests, temperature could be shifted right in Fig. 5. Takinginto account coecient of friction tolerances, the tempera-ture variation is found actually to exceed 100 C176C. This factFig. 3. Chase machine sketch.0.00.81.00 101 201 302 402 503 603 704 804 905Sliding distance (m)0.00.81.00 101 201 302 402 503 603 704 804 905Sliding distance (m)0.00.81.00 101 201 302 402 503 704 804 905Sliding distance (m)Sliding distance (m)0.00.81.00 101 201 302 402 503 603 704 804 9050.000.200.400.600.801.000 101 201 302 402 503 603 704 804 903Sliding dista