| With the increasing of urban population,traffic jam has occurred in big cities in China.In order to solve this problem,urban rail transit develops rapidly,environmentally friendly and economically.At present,the braking system of our metro still uses tread braking.The tread braking can give the wheels no round modification.Meanwhile,tread braking also brings some hazards.The tread braking will transform the kinetic energy of the vehicle into heat energy and transmit it to the wheels,causing the wheels to have thermal damage.As an important component of wheel of metro,its reliability is directly related to the safe operation of the vehicle.Therefore,it is very necessary to carry out the simulation analysis of the brake heat load of the wheel tread braking.Taking a metro vehicle as the research object,based on the theory of heat transfer and fracture mechanics,the thermal and coupled finite element method is used to analyze the distribution of temperature and stress field and the influence of heat of braking on the surface thermal crack of tread.First,using the commercial finite element software ANSYS,a three dimensional finite element model of the metro wheel is built to consider the change of material properties with the temperature.The heat capacity of the new wheel and wear to limited wheel in three consecutive emergency braking,normal operation braking and round-trip operation braking is calculated and analyzed,and whether the wheel tread meets the thermal damage requirement under this working condition is evaluated.The analysis results show that: in the above three kinds of operating conditions,the train speed of 80 km/h under the condition of only considering the air brake(pure tread brake),three consecutive emergency braking,normal operation(one-way)braking conditions and round-trip operation under braking,and new wheel and wear to limit wheel meet the highest surface temperature requirements and the requirements of wheel tread damage.Secondly,three dimensional finite element model of coupled thermo-mechanical of metro wheel is established by the ANSYS,considering the tread brake caused by thermal stress and mechanical load wave superposition and calculating the stress response of tread braking of the new wheel under vehicle respectively with straight line,curve,and turnout in an emergency,to analyze the causes of car spokes plate stress larger position,finally the Haigh-Goodman fatigue limit and wheel dynamic stress amplitude evaluation method to evaluate the fatigue strength.The results show that the fatigue strength of the wheel spokes can meet the requirements under the two conditions of mechanical load and engine coupling,but the addition of brake thermal load makes the wheel spokes more easily.In the three working conditions of straight line,curve and turnout,the fatigue of wheel and spokes is more obvious when crossing the switch,followed by straight line and finally curve.At last,the finite element model of thermal load of the tread braking with hot crack is established,and the influence of the thermal physical properties of the material on the temperature change is taken into account in the model.Research the influence on braking heat to tread thermal crack in an emergency under the new wheel and effect of contact area of tread and brake shoe,axle load,tread wear on thermal stress intensity factor at the crack tip.The analysis results show that: 1)the contact area between the wheel tread and the brake shoe is very significant to the thermal crack propagation of the tread.2)the influence of axle weight on thermal crack propagation is very significant;3)the grinding of wheel tread has certain influence on the thermal crack propagation,and the maximum stress intensity factor of the wheels is 21% higher than that of the new wheel. |
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