Research On The Fatigue Life Of Large Pitch Bearing With A Hardend Raceway Of Laser Surface Hardening

With the rapid development of global wind power,the operational reliability of wind turbines has been widely valued.As a key component,pitch bearing plays an important role in the safe operation and high reliability of the entire machine.There are many factors affecting the fatigue life of pitch bearing.Contact fatigue is one of the most important forms of failure.The calculation method of ordinary small bearings is not suitable for large bearings.This paper studies large 3.4MW pitch bearing for megawatt wind turbine,focusing on the contact stress and fatigue life calculation considering the external loading modeling and hardened layer depth.The results provide some reference and theoretical guidance for design and manufacture of pitch bearing,which has important engineering significance.The main research contents are as follows:1.The aerodynamic model of the wind turbine is studied based on the blade element momentum theory and the main design conditions of load analysis are defined.The GH Bladed software is used to calculate the timeseries loads at the blade roots under various working conditions,and the limit load list and LDD spectrum on the 3.4MW pitch bearing is obtained,which provides load data for the mechanical analysis and fatigue life study of the pitch bearing.2.Considering the effect of laser surface hardening,the raceway is divided into three parts: a case layer,transition layer and core layer.The local-contact finite element model is established.The linear elastic stress is analyzed based on the Hertz theory,the results of which has a good agreement with theoretical value,verifying the model accuracy.On this basis,the effect of hardened layer depth on equivalent stress and contact stress is further studied.The results show that for the linear elastic material model,the maximum equivalent stress and the maximum shear stress reduce and move down with a deeper hardened layer,meanwhile the maximum contact stress decreases,and the overall stress condition of the pitch bearing raceway improves.3.Since parts of the raceway contact area may enter the plastic deformation zone under service condition,the elastoplastic stress-strain relationship based on the above local model is introduced to calculate the elastoplastic contact stress.On this basis,the effects of hardened layer depth,contact loading and friction coefficient on the contact stress and equivalent stress are deeply studied.The results show that considering the plasticity of hardened layer,the maximum contact stress and the maximum equivalent stress on the raceway are less.For elastoplastic materials,increasing case layer depth and decreasing contact load and friction factor of contact surface would lead to a smaller amplitude of contact stress as well as a smaller maximum and a lower position of equivalent stress and shear stress,which is beneficial to the improvement of the force condition on the raceway surface.In addition,this study indicates that the distribution path of the equivalent stress is almost not affected by the case layer depth once passing the peak.Therefore,the case layer has to reach the depth that is the allowable value of core stress so that the stress on the core of the raceway does not exceed the equivalent allowable value.4.The L-P formula is used to calculate the fatigue life of pitch bearing.Taking the effect of laser hardened layer of pitch bearing into consideration,the strain-fatigue life of the raceway is calculated using n Code software based on the above local elastoplastic material model and stress calculation results,meanwhile the effect of hardened layer,contact loading and surface roughness on the bearing fatigue life is analyzed.The results show that the fatigue life of pitch bearing raceway can be significantly improved by increasing the case layer depth,reducing contact loading,and decreasing the roughness of contact surface.