The outer circlip constraint is a typical way for a wet multidisc clutch to limit the axial displacement of friction components. The pressure transmission mechanism in a clutch, excited by the concentrated reactive force of the outer circlip, is revealed by a simplified pressure calculation model. Moreover, a finite element model is constructed to investigate the contact pressure distributions on friction surfaces. Thermal analysis to determine the radial temperature distributions on friction surfaces is performed numerically. The computational results indicate that the concentrated reactive force contributes to the dissimilar distributions of the contact pressure along the radial and axial directions. The radial contact pressure considerably affects the temperature fields on friction surfaces, which is verified effectively by bench tests under the creeping condition. Both simulation and experimental results demonstrate that the outer circlip is identified to be one of the main reasons for the expansion of the radial temperature difference. In order to smooth the contact pressure in the radial direction, three new designs for a circlip are proposed, including an inner circlip, medial circlip, and both inner and outer circlips. Considering the utility and feasibility of the four cases, the design with both the inner and outer circlips is the most appropriate one.

Copper-based friction material (CFM) and paper-based friction material (PFM) are the two most commonly used clutch friction materials. The friction and wear characteristics of these two kinds of friction materials under dry conditions were investigated by the pin-on-disc test over a broad range of applied loads, rotating speeds and ambient temperatures. Before experiments, the running-in test was conducted to stabilize the coefficient of friction (COF) and wear amount of the test samples. After experiments, the metallographic micrographs of the tested samples were presented to investigate the wear mechanisms. Experimental results showed that both the COF and wear depth of the CFM are much greater than these of PFM. The COF of CFM decreases with the increase of applied load, and changes slightly with the variation of rotating speed, whereas it increases first and then decreases with the increase of ambient temperature. However, the COF of PFM decreases dramatically with the increase of the rotating speed and ambient temperature, while it remains stable at first and then decreases slowly as the applied load increases. Additionally, under such three working conditions, the wear depth of CFM changes linearly, while the wear depth of PFM varies greatly. This study can be used as a guide for selecting friction materials for clutches with different applications.

Considering the spline friction, the dynamic model of a multidisc clutch is established that permits the calculation of the contact pressure and friction torque acting at each friction surface; the thermal model is then developed to investigate the temperature difference between different friction pairs. The corresponding test bench is established, where the effect of inertia on the torque error is analyzed in detail. The simulated and measured results are compared to validate the numerical model and confirm the existence of spline friction. The results indicate that spline friction can lead to the attenuation of contact pressure on friction pairs along the axial direction. Therefore, there exists a significant temperature gradient in the axial direction, and the maximum temperature appears in the second separate plate near the piston. Moreover, the friction torque of a single pair decreases along the axial direction. Thus, as the number of friction pair increases, the total friction torque increases nonlinearly, and the average torque of single friction pair decreases gradually.