Failure Mechanism Of Grease Lubricated ZEV Contact With Impact Loading

The zero-entraining velocity(ZEV)working condition,i.e.,an opposite sliding working condition in which the two contact surfaces move in opposite direction but the same speed,widely exists in ball screws and cageless rolling element bearings.Due to the opposite sliding motion,thermal rise is usually prominent and thus the formation of reliable lubri cant film and the life of the lubricant film are of great significance on the long and reliable working of the ball screws and cageless rolling element bearings.In industrial applications,varying load problems are common.Aiming at such kind of problems,the author employed a ball-disk test rig with varying load by optical interferometric technique,using PLC programming to control the variations of the velocities and applied load.During the experiment,an industrial camera was used to capture the image of the lubricant film between the ball and the disk.After the experiment,the thickness of the oil film at the cross-section in the contact area was measured by a DIIM software.This thesis is composed of the following three parts: the influence of variable load on the distribution of oil film formation in the contact area,the research on the lubrication failure mechanism of oil/grease lubricated contact under the condition of fixed load and surface velocity,and failure analysis of grease lubrication wit h impact load.Firstly,the distribution of the dimpled oil film on the zero-entrainment elastohydrodynamic lubrication surface under loading conditions was investigated.The distribution of the "temperature-viscosity wedge" dimpled oil film was observed by varying the load,velocity and loading cycle.In the case of loading,when the surface velocity is relatively low and the load is relatively small,the regularity of the oil film is not strong.Only when the load increases to a certain extent,the "temperature-viscosity wedge" effect will take effect,forming a centrally dimpled oil film.The occurrence time of a central dimple advances with the increase of surface velocity.Under higher speed conditions,a small central dimple will rapidly evolve into a larger one.The comparison with steady-state results shows that time-varying effect delays the "temperature-viscosity wedge" effect.Secondly,the problem of rapid lubrication failure of lubricant/grease lubrication film under steady-state conditions was studied,and some surface textures were made on the steel ball to improve the lubrication performance.By observing the oil film changes and lubrication failure of grease and oil at low speed(0.004m/s),moderate speed(0.04m/s)and high speed(0.4m/s),it was found that under low speed conditions,even if the surface and speed are all constant values,the transition between the cavitation zone and the starveling area also leads to a time-dependent problem.Therefore,effective lubrication is maintained for a very short time before surface abrasion.When the surface velocity increases,the enhanced "temperature-viscosity wedge" effect will significantly prolong the effective lubrication time.Compared with pure rolling condition,the life of the grease lubric ation film is still longer than that of oil lubrication at low speeds under opposite sliding motion,but the effect of grease is reflected in high speed conditions.Under grease and oil lubrication conditions,the oil grooves on both sides of the contact a rea can effectively extend the life of the lubricating film.Finally,the effect of load change on grease lubrication film formation and lubrication life is studied.The effect of load variation,speed and load cycle on the surface dishing and the breakdown of grease thickener fiber clusters was investigated.The experimental results show that no obvious zero-entrainment surface depression will occur under low-speed and medium-speed variable load conditions,and the thickener will quickly decompose until direct contact occurs.Under high-speed conditions,there will be a horseshoe-shaped oil film at the beginning of the movement,and a stable zero-entrainment surface depression will occur at high speeds.