Study On Fretting Wear For Wind Turbine Pitch Bearing

With global warming and increasingly exhausted the fossil energy, wind energy, as a kind of renewable green energy, was paid highly attention by many countries. With the development of science and technology, Wind turbine which increasingly mature is a major wind energy utilization form, wind power industry has bright prospect in the future.The wind turbine works in the bad climate and in the field work, demanding free maintenance for20years. Wind turbine bearings as main components have great influence on the reliable operation of the wind turbine, therefore, high requirements proposed to bearings. Loads on pitch bearings are most complicated in all the wind power generator bearing. So analysizing of pitch bearing failure form, improving its reliability and service life is of important engineering significance.This paper focus on analyzing of the load and failure form of pitch bearings and main study contents of the paper are as follows:(1) In order to understand the mechanical characteristics of pitch bearings, firstly, external force of pitch bearing is analysed in wind turbine running process; and then, four-point contact ball bearing and crossed roller bearing load bearing mechanical characteristics are analyzed, which will is hepful to select type as pitch bearing; finally, load distribution on raceway of double row four-point contact ball bearing is calculated. And through calculation, it was found that the bearings have the largest capacity with certain negative bearing clearance;(2) Fretting mode and fretting damage form of bearing is analyzed on the basis of bearing load. The study found that pitch bearing fretting wear failure is the main failure mode; and fretting mode that take place on pitch bearing is complicated composite fretting,which can be divided into radial fretting, tangential fretting, torsional fretting, rolling fretting and Heathcote-silp fretting.(3) The composite fretting tests that composite of radial fretting and tangential fretting with ball-on-flat contact are carried out on the MTS890. It has been found that different force angle and hardness greatly influenced on the fretting wear damage. Tangential fretting wear is priority at small oblique angle, andhigh hardness material have good resistance to the fretting wear. On the contrary, radial fretting wear is priority at large oblique angle and low hardness material is excellent in reducing fretting.(4) In order to test pitch bearing the fretting wear, the bearings fretting wear test device is designed and constructed. Through this test device, fretting tests are performed at the different load, different oscillating angle and cycling times. The results show that load increases the fretting wear decreases, and oscillating angle increases, the fretting wear damage is serious; Wear increase obviously in early stage of the fretting, but with the increasing of number of cycles, increasment of wear demage tend to ease.(5) In order to study on effects of grease lubrication on bearing fretting wear, fretting tests are performed by test device under dry condition and grease lubrication condition. Experimental results show that grease lubrication can reduce the bearing fretting wear, and with the increasing of oscillating amplitude and decreasing of load the reduce effect of grease lubrication on wear increases; with the increasing of cycles, oxidation in the fretting wear gradually decreased; but opportunities of fatigue crack initiation under grease lubrication will increase at late stage of fretting.(6) According to test results of composite fretting composed radial and tangential fretting and bearing oscillating fretting, the reducing measures against the fretting wear in pitch bearing are put forward, including mechanical, lubrication and materials, and so on.In short, the mechanical characteristics and fretting failure form are analyzed in this paper, and the a series of Simulation fretting wear tests are carried out on bearing, and protective measures for pitch bearings against fretting wear are proposed based on the results of tests. The results of the study has important theoretical significance and practical value for pitch bearing types of structure design, manufacture, maintenance, reducing the fretting wear, increasing pitch bearing life and reliability.