Micro-pitting Analysis of Rolling Bearings in Operating Conditions

Abstract

Micro-pitting is surface distress caused at times by poor lubrication and slippage on the rolling contact surfaces of bearings. Using low-viscosity oil to improve efficiency of rotary machines decreases the lubrication performance of the bearing, which significantly causes the frequent occurrence of micro-pitting. Many studies have been conducted to address this issue in several ways. However, previous works did not consider the effect of the mechanical elements surrounding the bearing that operate in conjunction with the bearing in rotary machines. Therefore, the conditions covered in previous works differ from actual operating conditions of a rolling bearing. This study was conducted to perform a sensitivity analysis by selecting factors that influence lubrication and slip motion under operating conditions, to prevent early failure of rotary machines caused by micro-pitting of the rolling bearing. This study selected all factors that could cause micro-pitting by using a fault tree analysis. As a result, it was revealed that operating clearance and out-of-roundness of outer ring were the factors that influence the micro-pitting through numerical analysis. These were the factors that were effected by housing and shaft which were surroundings of rolling bearing. The load and speed, which were identified as two main factors in previous work, and the operating clearance and out-of-roundness were altogether considered in this study to verify which factor is more sensitive to the micro-pitting. The analysis showed that operating clearance and out-of-roundness of outer ring, among other factors, had the greatest impact on micro-pitting. Uneven load distribution stemming from increased clearance and roundness caused local increases in contact forces, reducing film thickness of the rolling surface. The uneven load distribution also affect to radical changes in slip velocity. Therefore, clearance and roundness were found to also be factors affecting micro-pitting, besides speed and load identified in previous works. Consequently, it is recommended to maintain a operating clearance and out-of-roundness small enough not to affect the life of a rolling bearing to keep the bearing load distribution even.