Fatigue and Wear in Rolling and Sliding Contacts

Shravan Janakiraman

Research output: Book/ReportPh.D. thesis

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The REWIND project was conceptualized to "perform strategic research at thehighest level in the field of material-manufacturing-properties-performance ofmetallic components in the rotor and drive train in large wind turbines, withthe ultimate aim of enhancing the reliability and arriving at an improved lifeexpectancy prediction of such components."One of the focus areas of the REWIND project is to study the failure of themain bearings in a wind turbine and suggest improvements to improve theirlifetime.This PhD project is focused on two areas : Lubrication and rolling contactfatigue.The main bearing supports the main shaft, which connects the rotor to thegearbox. The main bearing is a rolling element bearing containing sphericalrolling elements. The loads on a main bearings are very high, which leads toa lubrication regime called elastohydrodynamic lubrication (EHL). Under theEHL regime, the pressures in the lubricant are large enough to elastically deformcontacting surfaces. EHL usually occurs between the inner ring and therolling element because the non-conformal contact leads to a high interfacialpressure. The EHL film thickness is very small, usually less than 1 micrometer.It is intended to increase the film thickness, so as to ensure there is no contactbetween the roller and the raceway. Under lower loads (loads less than EHLloads) it has been observed that axial grooves help to increase the film thicknessat certain optimum operating conditions. It is believed that these groovesact as reservoirs of lubricant and can emit excess lubricant to increase the filmthickness. However the performance of these grooved surfaces have not beenstudied under EHL loads. So in this Ph.D. project, rolling-sliding, lubricated tests are performed to study the tribologial behaviour of axially grooved ringsunder EHL loads. Multigrid models simulating the rolling of a single groovedsurface against an infinite half-plane are coded. The results from this modelare used to explain certain experimental results.The results show that underEHL loads, the grooves do not appear to prove beneficial in improving the filmthickness. They might improve the film thickness at certain optimum runningconditions, but it is tough to ascertain what those conditions are.The main bearings also undergo rolling contact fatigue failure. The main bearingsexperience premature fatigue failure in both onshore and offshore windturbines. Their failure is characterized by the formation of nanosized ferritegrains called white etching areas (WEAs). These grains surround the fatiguecracks and turn white when etched with Nital. Hence they are named WhiteEtching Areas. It has also been proven in past studies that under high temperaturesand pressures, lubricants can react with a fresh steel surface to decomposeand generate hydrogen gas. So it is assumed that hydrogen is generated whenthe lubricant in a main bearing reacts with the steel surface. This hydrogenthen enters the steel surface and causes embrittlement. To study the fatiguefailure of these bearings, it was decided to replicate these failures in the lab.So a test rig was built to conduct lubricated RCF tests on 100Cr6 bearing steelrings. The loads applied were similar to those experienced by the main bearingin a wind turbine. Hydrogen is infused into the steel ring by immersing the ringin a solution of aqueous ammonium thiocyanate. The tests are then run untilthe ring cracks. Once the rings have cracked, they are etched and observedunder a microscope. The results show that White Etching Cracks (WECs) canbe simulated using the test rig.The main bearings are usually press-fit onto the main shaft to ensure a tight fitbetween the inner ring and the shaft. This press-fit introduces a tensile Hoopstress and a compressive radial stress on the inner ring. To simulate this on thetest rig, the steel ring specimens are press fit onto the shaft introducing tensileHoop stresses. The effect of two values of tensile Hoop stress on the fatigue lifeof the bearing is studied. It is observed that the fatigue life decreases as thetensile Hoop stress is increased.
Original languageEnglish
PublisherDTU Mechanical Engineering
Number of pages168
ISBN (Electronic)978-87-7475-409-1
Publication statusPublished - 2014
SeriesDCAMM Special Report

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