Study On The Formation Mechanism And Control Measures Of The Wheel Polygonization On High-Speed Emus Based On The Vibration Of Coupled Vehicle-Track System

In the past few years,the operating mileage of China's high-speed railway,as well as the running speed of high-speed trains increased rapidly.The high-speed railway technology changed from importation to independent innovation,while the high-order wheel polygonization appeared successively on high-speed EMUs.The high-order wheel polygonization not only deteriorated the ride comfort,but also threatened to the reliability of vehicle and track structure,which greatly raised the security risks and the operation and maintenance cost of high-speed railway.The wheel polygonization has become an obstacle limiting the development of China's high-speed railway.Wheel polygonization problem is a classic topic in railway engineering,however the formation mechanism of the high-order wheel polygonization is unclear,and there is still a lack of effective countermeasure to control wheel polygonization from the root.Therefore,it is of great engineering significance to study the formation mechanism and control measures of wheel high-order polygonization.Based on such background,this thesis focused on the EMUs' wheel high-order polygonization and carried out detailed experimental and simulation researches,the main works and conclusions are as follows:(1)The wheel roughness measurement as well as the long-term tracking test of EMUs are performed.The statistical analysis shows that the dominant order of the high-order wheel polygonization on China's EMUs is inversely proportional to the vehicle speed and directly proportional to the wheel diameter.The main characteristic frequencies of the polygonization of each EMU are within the frequency range of 550?650 Hz,then 300?400 Hz,which accords with the “frequency-fixing” mechanism.The “frequency-fixing” phenomenon is caused by natural resonance of vehicle track system.(2)A coupled vehicle-track rigid-flexible dynamic model of high-speed EMU is established considering the high frequency vibration of wheel and rail.A rotating flexible wheelset is modelled in Euler coordinate system.The sliding window method is improved using the least square criterion to simulate the vehicle travelling along an infinite long track efficiently without increasing the degree of freedom of the system,which can reflect the dynamic change of the support stiffness of the track longitudinally.And the dynamic model is validated by the measured vehicle-track steady-state responses.(3)Based on the coupled vehicle-track rigid-flexible dynamic model and Archard wear model,a long-term wheel polygonal wear iteration is developed.The growth progress of the high-order wheel polygonization on China's EMUs is reproduced.Then,the general law of the development of wheel polygonization is studied,which indicates that the evolution of wheel polygonization depends on the wheel initial irregularity,rather than the track irregularity.The wheel initial irregularity is the external cause of polygonal wear.The growth rate of polygonization is determined by the amplitude of wear fluctuation and the phase difference between wear fluctuation and wheel original irregularity.The closer the phase difference is to the opposite,the faster the polygonization grows.(4)A vertical coupled vehicle-track linear model is established to investigate the vibration characteristics of vehicle-track system under different coupling states in the frequency domain.The resonance modes associated with the generation of higher-order polygons are identified.And the effect of the vibration of coupled vehicle-track resonance modes on the polygonal wear is evaluated by introducing the Carter two-dimension contact theory and Archard wear model.The results suggest that the resonance frequencies of 3rd-order and 2nd-order rail vertical bending modes occurring between the wheels in a bogie are 650 Hz and 400 Hz respectively,which give rise to the periodic fluctuation of the wheel/rail normal force and wheel/rail wear depth.Furthermore,they also lead the phase difference between wear fluctuation and wheel original irregularity to be closest to the 180 degree at 565 Hz and 330 Hz.As a result,the growth rate of polygonization reaches highest at about 572 Hz and 342 Hz,which are consistent with the characteristic frequencies of high-order polygonization on China's EMUs Therefore,it is considered that 3rd-order and 2nd-order rail localized bending modes in the coupled vehicletrack system are the root causes of high-order polygonization on China's EMUs,and the characteristic frequencies of formed wheel polygonization will be slightly lower than the initial modal resonance frequencies.(5)The experimental verification of the proposed formation mechanism of wheel highorder polygonization is carried out by the on-site tests.On the one hand,through the coupled vehicle-track vibration test,the frequencies of rail localized bending resonances are captured on an actual line.On the other hand,a comprehensive comparison analysis is conducted between the wheel polygonization prediction and the long-term tracking test of a 250 km/h Emu,including the evolution process of polygonization,the root mean square development process and the spectrum change process of axle box acceleration.The results show that the simulation and measurement are in good agreement.In particular,it can be observed that the dominant frequency of the vibration gradually moves down from 650 Hz to 590 Hz in both measurement and simulation,which accords with the inference that "the characteristic frequencies of formed wheel polygonization will be slightly lower than the initial modal resonance frequencies" in the mechanism study.(6)Based on the formation mechanism of polygonization,the control measures are proposed from the perspective of external and root causes.On the one hand,the key parameters of coupled vehicle-track system are optimized,the simulation results indicate that in vehicle system,the wheelset flexible resonance makes little contribution to the wheel polygonal wear under normal conditions.Changing wheelbase can shift the order of wheel polygonization,while it cannot restrain the growth of polygonization.In the track system,increasing the railpad damping coefficient or reducing the fastener spacing can effectivity slow down the growth rate of the high-order polygonization.When the railpad damping coefficient is increased to more than 50 k N·s/m or the fastener spacing is reduced to less than 0.4 m,the wheel high-order polygonization can be basically eliminated.On the other hand,the influential weight of typical initial wheel surface irregularities on wheel high-order polygonization is analyzed.The result indicates that the wheel flat promotes the high-order polygonal wear developing along the flat rapidly,and the residual polygonization after re-profiling will significantly accelerate the development rate of polygonal wear on the adjacent wheel.Therefore,the above two should be strictly controlled.(7)A new idea of installing rail dynamic vibration absorber to attenuate the development of high-order polygonization is proposed theoretically.A single degree of freedom dynamic vibration absorber is designed to alter the phase-frequency characteristics of the coupled vehicletrack system.It is proved that the growth rate of high-order wheel polygonization can be reduced by more than 50% by installing the designed vibration absorbers,and the generation of highorder wheel polygonization can totally be avoided by shortening the installation spacing of the vibration absorbers.