Research On Forming Theory And Process Method In Precision Helical Rolling Forming Of High Speed Rail Bearing' Rolling Elements

High speed rail bearing not only should meet the basic requirements such as support,load transmitting,positioning accuracy,and ensure the sports,but also meet the long service life,low vibration,low noise and high stability.As the key part of high speed rail bearing,the tapered roller has a directly effect on the performance of bearings in harsh service conditions.The helical rolling process is an advanced plastic rotary parts forming technology for producing tapered roller.However,due to the lack of research on the helical rolling process for tapered rollers and the complexity of the helical rolling process,the current study about metal flow law,deformation mechanism and damage mechanisms of tapered roller in helical rolling process is not clear,which severely constrains the application of helical rolling process in producing tapered rollers.Based on the process principle and deformation characteristic of helical rolling process of high speed rail bearing's rolling elements,a 3D coupled macroscopic and microscopic behaviors finite element model was implemented with the help of SIMUFACT software in this study.And the key modeling technologies of hot helical rolling process are detailedly described.According to the results of the finite element model,the distribution and evolution laws of plastic strain,stress,rolling force,temperature,dynamic recrystallization,grain size were revealed.Furthermore,the effects of the rolling parameters such as rolling temperature,cross angle of rolls and rotation speed of rolls were discussed.Through the research of stress state and the characteristics in the process of helical rolling,a modified Lemaitre model was adopted by considering of considering the damage growth under negative hydrostatic stress state and coupling the effects of deformation temperature and strain rate based on the Zener-Hollomon coefficient.Then,the modified model was implemented in Simufact via a user subroutine.Using the modified Lemaitre model,finite element simulations were conducted to predict the distribution of the central damage in helical rolling.In order to verify the reliability of the damage model,a finite element model for steel-balls was establishedto compared with the experimental results.Moreover,the influence of different rolling parameters on central damage were analyzed.Due to the groove structure of single-groove helical rolling process,a new parametric model design method was used to establish the 3D model of helical roll based on PROE.And then,with the help of SIMUFACT,a 3D coupled macroscopic and microscopic behaviors finite element model of single-groove helical rolling process was implemented.Based on the simulation results,the single-groove helical rolling process was compared with the traditional helical rolling process by the distribution and evolution laws of microstructure and macroscopic thermodynamic.According to the results of 3D coupled macroscopic and microscopic behaviors finite element model,some conclusions can be obtained.The plastic deformation are mainly occurred on the surface and the area connect the neck,and the effective plasic strain in small end of tapered roller is larger than that in big end.After rolling,the temperature of surface decreased rapidly,and the central temperature has little change,the temperature connect the neck has a slight increase.The function of refining grains helical rolling process were obviously.The strainedcondition of central area was three-dimensional pulling stress,and the shearing stress changes from positive to negative alternately,which leads to that the crackle occurred easily on the central area.