Study Of Brake Caliper Vibration For Emu Based On Stick-slip Friction

Frictional brake is one of the most important braking methods for EMU.However,frictional vibration appears frequently in the braking process of EMU.This kind of harmful vibration may reduce the stability and comfort of EMU.It may also cause damage to the bogie parts and the brake rigging.The brake caliper is both the source and the application of frictional vibration,superimposed by a variety of forces,making it the focus of this study.In this paper,the characteristics of frictional vibration of the brake caliper for EMU are analyzed based on stick-slip friction theory.The friction force elements are established according to the constant and negative slope friction coefficient in friction force-speed.The single degree of freedom model of stick-slip vibration is founded as well.The critical velocity of the system is analyzed and the influence factors of frictional vibration are also discussed.The research shows that frictional vibration appears when braking speed is lower than critical velocity.On the contrary,the system tends to be stable.Increasing the mass,stiffness and damping of the brake pad can reduce the critical velocity,which helps to suppress frictional vibration while braking.The frictional vibration would occur to the system in braking deceleration when braking speed is higher than critical velocity,leading to the continuous vibration phenomenon.In this paper,it is proved that the maximum static friction coefficient greater than dynamic friction coefficient causes stick-slip vibration.The primary cause of stick-slip vibration is not the negative slope friction coefficient in friction force-speed.On this basis,the brake caliper of CRH2 is taken as a prototype to establish a dynamic model of 1/4 bogie in SIMPACK.The law of disc brake friction when stick-slip vibration appears is considered and the force elements of static friction and dynamic friction are established.The condition of brake caliper vibration as well as the wheel rail force and pin force is obtained under different parameters,considering the track excitation.The simulation results show that the frequency of frictional vibration is about 50 Hz and the stick phenomenon of both sides and middle of the brake pad disappears successively with increasing braking speed.Frictional vibration results in the increase of vibration acceleration fluctuation of primary and secondary suspension,but not enough to influence the stability of the vehicle.Frictional vibration can also lead to the increase of the dynamic force of wheel rail contact and the deceleration inhomogeneity of wheel set.It is possible to relieve the impact to vehicle dynamics by reducing the positioning stiffness of brake rigging on the bogie properly.Normally,the track excitation has no significant effect to the frictional vibration.In this paper,the constrained modal analysis of the brake caliper of CRH2 is carried out.The first eight natural frequencies and modal shapes of the caliper are obtained.The influence between modal frequency and vibration frequency is also discussed.The analysis shows that the modal shape of the brake caliper which has a frequency of 160Hz-210 Hz shares the same vibration direction with stick-slip friction,without triggering any structural resonance of the brake caliper.