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Materials
& Design
Materials and Design 28 (2007) 2148–2153
www.elsevier.com/locate/matdes
Experimental characterization of frictional behaviour of clutch facings using Pin-on-disk machine
M. Bezzazi a, A. Khamlichi a,*, A. Jabbouri a, P. Reis b, J.P. Davim b
a LMMS, De′partement de Physique, F.S.T.T, B.P. 416, Tangier, Morocco b Departamento de Engenharia Meca?nica, Universidade do Aveiro, Portugal
Received 24 January 2006
Available online 1 September 2006
Abstract
During the clutch engagement manoeuvre, sliding contact occurs between the pair of clutch facings mounted on the friction disk and the counter faces belonging to the flywheel and the pressure plate. The transmitted torque is proportional to the overall coe?cient of friction which depends essentially on temperature, normal pressure load and relative sliding velocity. In this work, performance of the friction coe?cient 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 actual clutch facings material has good fading resistance and a rather stable coe?cient of friction once running in phase is achieved.
2006 Elsevier Ltd. All rights reserved.
Keywords: Composites; Thermal; Mechanical; Friction behaviour; Clutch facings
1. Introduction
In a vehicle rotary motion is to be transmitted from the primary shaft (motor) to the secondary shaft (gear box) for intermittent periods only. To achieve that, a clutch is required between these two components. The function of the clutch is to produce a soft gradual increase in the angu-lar velocity of the driven shaft until full coupling between the motor and the gear box is achieved. Then, the clutch must act as a permanent coupling transmitting, from the driving shaft to the vehicle wheels, the entire mechanical power without subsequent slip [1].
During dry clutch engagement manoeuvre, a transient sliding contact situation occurs between the pair of clutch facings mounted on the friction disk and the counter faces belonging to the flywheel and the pressure plate. Through-
* Corresponding author. Address: 33, Re′sidence Al Isma?¨lia, Avenue des F.A.R., Appt. 15B, Tetouan, Morocco. Tel.: +212 67 79 50 68; fax: +212 39 35 07 02.
E-mail address: khamlichi7@hotmail.com (A. Khamlichi).
0261-3069/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.07.001
out this engagement process, heat is generated at the con-tact surface yielding temperature to rise [2]. Temperature rise may be very important in case of repetitive engage-ments. Good performances of the clutch facings material such as a stable and su?ciently high coe?cient of friction are required in order to operate engagements in an e?cient and regular way. Fading phenomenon which is due to a sudden decrease of the friction coe?cient as function of temperature must be avoided. Variations in time of the fric-tion coe?cient during the running in phase or due to mate-rial wear must also remain limited.
Other factors in addition to temperature might influence the coe?cient of friction level. They are related to hetero-geneities of the contact surfaces, chemical reactions or physical transformations. But, these mechanisms are both complex and non reproducible. Their e?ects could be, in practice, 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
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Europe [3], JIS D-4311 in case of Japan [4] and SAE J661a test in case of USA [5]. But, most of these tests are both lengthy and costly. This makes them di?cult to use in real research and development operations. So, they should be rather kept to the final stage in order to assess the material commercial specifications. Other basic tests which are more cost e?ective should be used in order to facilitate more quickly information about fiction and wear behaviours.
In case of friction products, the well known FAST test [5] is a quick and inexpensive test. However, it su?ers from the pathological apparition of glazing e?ect. This e?ect is often encountered for polymeric composite materials due to lack of cleaning the friction surface in the FAST test. Apparent glazing has in general nothing to do with real conditions of 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 most e?ective test configurations which allow a reliable and quick determination of coe?cient of friction and wear.
In this work, we have investigated the opportunity to use the Pin-on-disk test in order to study the dry tribolog-ical behaviour of commercial organic clutch facings which are manufactured according to the scatter wound proce-dure. For this purpose, special samples of clutch facings material having the form of disks and special pins made from steel were manufactured. The inverse operating con-figuration in comparison with the habitual technique of using the Pin-on-disk apparatus was chosen in order to per-form the friction and wear tests. These conditions are thought to enable the friction surface cleaning and to avoid artificial glazing.
Through this experiment, we have studied the friction coe?cient behaviour as function of temperature, sliding velocity and sliding distance. Load pressure was kept almost constant. Wear was also measured as function of the previous parameters.
The obtained results are presented in the following as charts giving the friction coe?cient, l, versus the sliding distance when temperature, T, and the sliding velocity, v, are set constant. Qualitative comparison with the classical SAE J661a test is then performed. They show in particular that glazing e?ect does not occur during testing. The fric-tion product is found to have a stable coe?cient of friction and does not su?er from fading phenomenon.
2. Clutch facing material description
2.1. Clutch facing material formulation and the manufacturing process
Organic clutch facings are composites witch are manu-factured in the most common case according to the scatter wound procedure. First a balanced formula is given. This consists of a fibre glass yarn fitted with a copper strip in order to enhance conductivity and to relieve heat evacua-tion from the facing surface during clutch operation. The
formula contains also the weight composition of the impregnating mixture which holds in general more than fif-teen raw materials. These materials are bonded by means of a phenolic resin which is put in the impregnant mixture with a proportion of about 30% by weight.
The manufacturing steps as described in [5] are as follows:
(1) Impregnation. A yarn is placed into an impregnating tank which is filled with the impregnant solution. Vis-cosity is modified by adding organic solvent in order to improve the mixture absorption by yarn.
(2) Drain and preliminary bake. Rack of impregnated yarn is lifted from impregnant and allowed to drain in an air circulating oven to remove the extra solvent and to produce a relatively sti? yarn.
(3) Preforming. This needs an automatic preforming machine and special platens. The dry yarn is forced to form a tracing which enables even distribution of matter over the preform surface.
(4) Flat press and final bake. The preform is put into a heated 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 an oven during a long and smooth curing cycle.
(5) Grinding to thickness and turn and bore. Grinding is done on a double disc grinder. Then the rings are placed on the boring bar in order to have the desired diameters.
After all these operations, anti corrosion surface treat-ment of clutch facings is achieved and drilling in accord with a specified drawing can be performed. The clutch fac-ings are then ready to fit a friction disk.
2.2. Clutch facing morphology
The clutch facing morphology is in general very hetero-geneous. But it could roughly be defined by the three dimensional tracing achieved by the impregnated yarn dur-ing the preforming operation. Some irregularities may a?ect the overall yarn tracing due to certain deformations occurring during moulding or because of material removal needed in grinding operations. Yarn tracing results in the actual preforming machine from superposing two basic movements of the pipe: a radial sinusoidal translation with angular frequency x1 and a uniform circular rotation with speed x2, a = x1/x2 defines the preforming ratio. This parameter controls to a large extent the clutch facing morphology.
For some given material formulation and for some com-mercial reference application of the clutch facing which corresponds to outer radius Ro, inner radius Ri and thick-ness e, the facing texture depends on how the preform is outfitted.
Using polar coordinates (r, h), the location of an arbi-trary point of the yarn tracing is given, as studied in [2],
2150 M. Bezzazi et al. / Materials and Design 28 (2007) 2148–2153
Fig. 1. Clutch facing morphology for a = 4.27.
by r(t) = Rm + h sin(x1t) and h(t) = x2t, where Rm = (Ro + Ri)/2 is the mean radius of the clutch facing
and h = (Ro Ri)/2.
Fig. 1 gives the clutch
facing morphology where
Ro = 100 mm, Ri = 67 mm,
e = 3.35 mm, x2 = 180 rpm
and a = 4.27. The chosen value of a enables optimal mate-rial distribution over the clutch facing surface, [2].
3. Tribological tests
3.1. Tribometer Pin-on-disk
Friction and wear tests, on polymeric composite clutch facings material under unlubricated conditions, were per-formed on a Pin-on-disk tribometer PLINT TE67HT. The objective was to evaluate the behaviour of the compos-ite material/steel pair under the e?ect of sliding distance, sliding velocity, normal load and temperature.
The tribometer, as shown in Fig. 2, consists of a loading stationary pin sliding against a rotating disk with its axis
Fig. 2. Tribometer PLINT TE67HT.
perpendicular to the disk. The test disk is mounted on the disk carrier spindle and secured by a central bolt. The pins specimens were clamped in the collet chuck and fixed in a loading arm. The pins stayed over the disks with two freedom degrees: a vertical one, which allows normal load 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 a tachogenerator feedback ensured the stable running speeds. An indication of wear process in the pin/disk contact was given by a linear potentiometer mounted in the pneumatic pin-loading piston.
3.2. Tribological conditions
DIN Ck45K steel pins (flat-ended) with a diameter of 8 mm and a length of 67.8 mm were machined. All pins have the following chemical composition (wt.%): 0.45%C, 0.25%Si, 0.65%Mn and present a hardness value of 230 HB. Before testing the pins were ultrasonically cleaned in an acetone bath.
Organic friction material discs with 8 mm thickness and 76 mm of diameter were manufactured and used as counter face.
Table 1 shows the tribological conditions used on tests to evaluate the behaviour of composite material/steel pair. These conditions were chosen so that the load pressure P remains almost constant during the tests. Two test temper-atures were chosen: ambient temperature and 100 LC. Tem-perature is stabilized by a controlled heating system. All the experiments were performed under dry sliding condi-tions with a relative humidity of about 50%.
3.3. Chase machine and standard SAE J661a
The Chase machine is well established as a means of determining friction levels for clutch facings material. A sample for Chase testing has the dimensions 25.4 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 by a drag test where the test drum temperature is allowed to rise to 550 LF (288 LC). During the recovery part of the test the drum is allowed to cool, and the brake is applied, and friction readings taken, at 100 LF (37.8 LC) intervals. The wear portion of the test consists of 100 applications at 400 LF (204.4 LC), 20 s on, 10 s o?. This is followed by a second fade and recovery test. Similar to the first, but with
Table 1
Tribological conditions used on tests
Sliding velocity (m/s)
Sliding distance (m)
3
1000
1
1000
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temperatures going up to 650 LF (343.3 LC). Finally a base-line like the one at the beginning is performed. Test sam-ples are weighed and thickness is measured before and after testing to get some idea of wear.
4. Experimental friction charts obtained by Pin-on-disk machine
The obtained experimental results with the Pin-on-disk machine are depicted in the following figures as curves giv-ing the friction coe?cient l versus the sliding distance.
Fig. 4(a) and (b) are associated with almost the same experimental conditions but the sample disk was returned in case of Fig. 4(a), so the friction surface material in con-tact with the pin is the backward surface in the first case and the forward surface in the second case. This surface was used also for the subsequent tests. One can notice that the friction coe?cient becomes stable in Fig. 4(a) and (b) for high sliding distances. But the reached levels are slightly di?erent: 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 coe?cient behaviour is quite di?erent from one face to 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 are considered 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 coe?cient can be formulated in case of Fig. 4(c)–(e). The running in phase is found in these last cases to be shorter. Explanation of this could be related to the amount of 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 the disk lateral side. This temperature is not equal to that of the friction surface between the pin and the disk. More-over, this last temperature is always higher than the lateral disk temperature due to heating resulting from the fric-tional work. Since the generated heat increases with increasing sliding velocity, the spot temperature at the con-tact surface between the pin and the friction material will increase if sliding velocity is increased.
Organic polymeric friction materials sliding against steel or cast iron are known to have a friction coe?cient which decreases with temperature after reaching a maximum [9]. This is in agreement with Table 2 results since the friction coe?cient is in all cases decreasing with increasing sliding velocity or temperature. E?ect of sliding velocity variations is found also to be higher than that associated with temper-ature variations.
Finally, one can notice that in all the previous tests the friction coe?cient behaviour is rather stable and does not show huge fluctuations from the mean level. The friction coe?cient level remains also high even for the hard condi-tions corresponding to v = 3 m/s. For the special clutch
Table 2
Computed average coe?cient of friction
Sliding velocity
Temperature
Pressure
Friction
(m/s)
(LC)
(N)
coe?cient
1
100
51
0.27
1
100
53
0.33
3
100
56
0.19
1
200
58
0.32
3
200
51
0.17
facing material having been tested, fading is found to be outside the range of parameters investigated in the present study.
5. Comparison with other testing results
Fig. 5 gives the friction coe?cient versus temperature during the seven phases of the SAE J661a test. The pres-sure load acting on the clutch facing sample is almost the same than that used in the Pin-on-disk testing. Tempera-ture is measured on the drum, Fig. 3. So, one could expect to find a significant di?erence between temperature used in the Pin-on-disk test and temperature given in Fig. 5. Another di?erence between these two tests is due to the counter friction surface used in the SAE J661a which is made from cast iron, whereas for the Pin-on-disk it is made from steel. Also, samples used in the SAE J661a are only 4 mm thick and are taken from daily life production whereas the tested disks on the Pin-on-disk machine are 8 mm thick. Di?erences between these two tests regarding the coe?cient of friction level are expected to exceed the above 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 coe?cient of friction behaviour as function of temperature is well characterized by the Pin-on-disk tests, Fig. 4(a)–(e). In fact, to reach the same average level of the friction coe?cient as in these last tests, temperature could be shifted right in Fig. 5. Taking into account coe?cient of friction tolerances, the tempera-ture variation is found actually to exceed 100 LC. This fact
ω
P
μP
Fig. 3. Chase machine sketch.
2152 M. Bezzazi et al. / Materials and Design 28 (2007) 2148–2153
1.0
0.8
0.6
μ
0.4
0.2
0.0
0
101
201
302
402
503
603
704
804
905
Sliding distance (m)
1.0
0.8
0.6
μ
0.4
0.2
0.0
0
101
201
302
402
503
603
704
804
905
Sliding distance (m)
1.0
0.8
0.6
μ
0.4
0.2
0.0
0
101
201
302
402
503
704
804
905
Sliding distance (m)
1.0
0.8
0.6
μ
0.4
0.2
0.0
101
201
302
402
503
603
704
804
905
0
Sliding distance (m)
1.00
0.80
0.60
μ
0.40
0.20
0.00
0
101
201
302
402
503
603
704
804
903
Sliding distance (m)
Fig. 4. Friction coe?cient versus sliding distance (a) T = 100 LC; v = 1 m/s; P = 51 N. (b) T = 100 LC; v = 1 m/s; P = 53 N. (c) T = 100 LC; v = 3 m/s; P = 56 N. (d) T = 200 LC; v = 1 m/s; P = 58 N. (e) T = 200 LC; v = 3 m/s; P = 51 N.
1
0.9
0.8
coefficient
0.7
0.6
0.5
7
4
2
Friction
0.4
3
5
0.3
1
6