Experimental and analytical study of gear micropitting initiation and propagation under varying loading conditions

Abstract

Micropitting damage is one of the failure modes commonly observed in gears leading to destructive failures, which in turn results in unplanned shutdown and expensive replacement, such as those observed in wind turbine gearboxes. This study investigates gear micropitting initiation and propagation when subjected to varying torque loads under a constant rotational speed. The study employs both experimental gear testing and analytical evaluation based on the ISO Technical Report of Gear Micropitting, ISO/TR 15144-1:2010 and the recently revised ISO/TR 15144-1:2014. Initiation and propagation of micropitting are assessed in testing by quantifying the development of micropits and their progressive rate after specific numbers of running cycles at step-up torque levels. The analytical study is conducted to validate the prediction of micropitting using the ISO/TR recommended procedures by comparing the results with the occurrence of micropits in the tested gears.The gear test results show that micropitting initiates at the pinion dedendum but escalates at the addendum, because of the greater severity of progressive micropitting at the dedendum of the mating wheel where the tip relief area first comes into mesh. The analytical results, based on varying surface roughness measurements obtained from the tested gears, confirm that the maximum contact stresses and minimum specific lubricant film thicknesses occur in these regions. The specific lubricant film thickness varies considerably because of changes of surface roughness after gears are subjected to various running cycles under varying torque levels. It has found that the excessive loading, gear tooth micro-geometry, surface roughness and lubricant film thickness are the main factors affecting micropitting.