# Integrated Thermal and Dynamic Analysis of Dry Automotive Clutch Linings
## 4 Measurement of Coefficient of Friction and Thermal Transport Properties  
The coefficient of friction used in the current analysis is obtained through pin-on-disc tribometry. The frictional characterization of the clutch lining material is performed under realistic vehicle clutch engagement conditions. It was, therefore, ascertained that the pressures developed in the contact between the pin and the disc are similar to those in a real vehicle clutch contact. In addition, the rotational speed of the disc was adjusted so that the relative sliding speed between the pin and the disc was representative of that occurring between the actual clutch components in a real vehicle during take-off. Furthermore, the disc was equipped with a heating element that allowed for heating of the contact between the pin and disc to the bulk temperatures that exist in a real clutch contact in a vehicle [13]. The friction lining materials used for the tests were cut from a real vehicle clutch lining material. Table 11 in Appendix A provides information about the load and speed conditions in the real vehicle clutch contact and those simulated by the pin-on-disc tests. Each measurement included new samples and the tests for each sample were repeated six times [12; 13]. Following the completion of friction measurements, a carpet plot is produced, for frictional characteristics of the clutch lining (a sample of which was attached to the pin of the pin-on-disc tribometer) in contact with the surfaces of either the pressure plate or the flywheel, used as the disc under various operational conditions such as slip speed and clamp load (Figure 4). Figure 4 also shows a typical engagement path, depicted by the black line. The letters A and B represent the beginning and the end of the engagement process. This is a typical friction lining characteristic which varies according to the lining composition as well as its wear state (i.e., age). It is noteworthy that variation of coefficient of friction with slip speed mostly exhibits a negative slope. Therefore, depending on the negativity of this slope (according to lining type and wear state) and other operating conditions such as loss of clamp load and operating temperature, the propensity to judder alters. Figure 4: Characteristic 3D carpet plot of friction material with the typical engagement path. Here, two actual clutch samples are investigated. These include a new (unused) sample and a worn one, both of the same lining type. The purpose is to investigate the clutch lining performance at different stages of its life. The worn clutch sample is acquired from a passenger vehicle clutch after 100,000 km. This sample has undergone running-in wear and embedding processes, nearing its end of life. A detailed description of the acquired coefficient of friction (COF) through use of tribometry and the methodology followed are presented by [PERSON] et al. [12; 21]. Table 1 provides the results of tribometric measurements for the coefficient of friction variation with clutch interfacial slip speed at different surface temperatures of the disc, representing the surface of the pressure plate and made of the same steel with representative surface topography. The extrapolated linear regressed equations in each case are for all the results, representing lines of constant pressure (at full clamp load) in the overall frictional characteristic carpet map, similar to the one shown in Figure 4. The intercept of these lines corresponds to the static coefficient of friction. These show 30% higher static COF for the new lining material than that for its worn/used counterparts. All the results show a negative gradient for the COF variation with slip speed. However, the negative slope alters more rapidly with temperature for the case of the new lining.