Wind Turbine Gearbox Bearing Material Analysis to Study Crack Nucleation, Propagation, and Damage Mechanisms

摘要

Low reliability of wind turbine gearbox bearings result from wide variety of challenging operating conditions which may push the bearings beyond their designed limits. The occurrence of premature bearing failures (20% < L_(10) life) are attributed to specific failure mechanism that is known as white etching cracks (WECs). WECs are parallel or randomly oriented cracks under the subsurface bearing raceway which appear white upon etching, indicating an altered microstructure. The altered microstructure consists of Nano-crystalline grains about lOnm in diameter. White Etch Cracks (WECs) have been observed in through hardened martensitic, bainitic and case hardened AISI 52100 steel, nevertheless some steels are considered more durable and immune to WECs [1,2]. WECs can cause premature failures in rolling element bearings occurring as early as 1-20% of the calculated L_(10) life life [3]. WECs have been discovered in many rolling element bearing applications (generators, mill drives, dryers, cranes, industrial gearboxes) that are operating in conditions that bare no common denominator. Several hypotheses have been proposed to explain the mechanism of WEC's. Some of the influencing factors leading to premature failures due to WEC formation are listed below. 1. Hydrogen penetration into steel from lubricant decomposition, water ingress, stray current 2. Adiabatic shear banding due to transient loads and torque reversals 3. Tensile stresses in the bearing raceway 4. Stress cone, factors such as non-metallic inclusions, voids etc. 5. Shaft displacement and misalignments causing bending stress and impact loading In the present work, failed bearing from the 1.5 MW turbine gearbox are collected to analyze the microstructure alterations, understand the damage mechanism, and relate it to the existing hypothesis. Bearings from planet stage and high speed stage locations are examined using metallographic and microscopic techniques.