Damage mechanisms of spline teeth of friction plates under non-uniform impact

The fatigue fracture failure of friction plates seriously affects the safety of high-power density transmission systems due to the non-uniform impact with the inner hub. However, the crack initiation inducement and underlying damage mechanisms are still confused so far, which has critical significance for improving the service life. To address this problem, the matrix metallographic structure of friction plates was observed by Scanning Electron Microscope (SEM), which excludes internal defects within the organization. Then, an impact dynamic model was established to investigate the collision laws between spline teeth and the inner hub. The effect of the inner hub's speed fluctuations on the impact force and acceleration was analyzed, and an analytical relationship between tooth root stress, impact force, and gear parameters was established. A large module simulation prototype test rig was designed based on the proposed equivalent model to verify the impact dynamic model. Results show that the majority of collisions between spline teeth and the inner hub are "catch-up collisions", which well explains the nonuniform impact phenomenon. The "frontal collisions" will only occur when the speed of the inner hub is lower than that of the friction plate. The higher cumulative frequency of tensile stress on the pull side of spline teeth roots by "catch-up collisions" reveals the underlying mechanism for the differential extent of cracks' damage. The minimum bending stress of the teeth root is 125.5 MPa at a pressure angle of 32 degrees, which shows a linear increase with tooth clearance. The fatigue fracture of friction plates has been effectively suppressed after structural optimization.