Spherical roller bearings (SRBs) utilized in the gearboxes of wind turbine generators are known to be especially susceptible to premature failure due to low cycle micropitting of the raceways. Micropitting in rolling element bearings is believed to arise from significant roller/raceway sliding in thin film lubrication conditions. Roller/raceway sliding occurs in SRBs as a consequence of their geometry, and almost all the bearings in wind turbine gearboxes operate in thin film (or low lambda) lubrication conditions. There is currently no accepted solution to mitigate micropitting in wind turbine gearboxes that are equipped with SRBs. Since WC/a-C:H coatings on rolling elements have been effectively used to solve wear issues encountered by SRBs in other industrial applications, these coatings have been offered as a solution to low cycle micropitting in wind turbine gearbox SRBs.
This research plan has been developed to test the hypothesis that a WC/a-C:H coating will mitigate or eliminate micropitting such as that experienced by SRBs in wind turbine gearboxes. The laboratory tool that is used to create micropitting on test specimens is the PCS Instruments Micropitting Rig (PCS MPR). The MPR is a three-contact disc machine in which there are three rings of equal diameter positioned at 120 degrees apart with a smaller diameter roller located in the middle and in contact with all the rings. This arrangement allows the test roller to be subjected to a large number of rolling contact cycles in a short period of time and hence significantly reduces testing time. At a typical entrainment speed of 3.5m/s, the central test roller will experience
approximately one million contact cycles per hour. Since the controls of the PCS MPR allow the speed, slide-roll ratio, temperature, and load to be automatically and independently controlled, the thin film lubrication and slide/roll ratio conditions that generate micropitting on SRBs can be reproduced in the laboratory. Most wind turbine gearboxes operate with a synthetic ISO-320 lubricating oil with anti-wear and extreme pressure additives. However, to ensure thin film lubrication conditions necessary for micropitting experiments were performed on the MPR using an ISO-10 base oil.
Baseline tribological testing were performed using untreated SAE 52100 rings and the roller. The targeted surface finish on the rings and the rollers varied from about 0.2 to about 0.6 micrometer Ra, and the entire surface topography was quantified using a Zygo 7300 3D optical profilometer. The sets of roller and rings were tested on the MPR using a range of slide/roll ratios from 0.0 to +/- 10% at contact stresses up to about 3 GPa. The number of cycles needed to generate the onset of micropitting was recorded and some tests were repeated up to three times. Results of micropitting tests on steel/steel, steel/WC/a-C:H and WC/a-C:H/steel contacts were compared with a tribological conversion coating; black oxide. Black oxide is a surface treatment that converts the surface of ferrous alloys to magnetite (Fe3O4). It has been utilized to reduce wear and corrosion of rolling element bearings and gears, and its use has become especially widespread on roller bearings used in the gearboxes of modular wind turbines. It has been reported that black oxide might have a lower friction coefficient than steel, which may reduce shear stresses due to friction, dampen vibrations, or possibly prevent the diffusion of hydrogen.
Committee: Gary Doll, Professor (Advisor); Evans Ryan, Doctor (Committee Member); Binienda Wieslaw, Doctor (Committee Member); Dong Yalin, Doctor (Committee Member); Menzemer Craig, Doctor (Committee Member); Sancaktar Erol , Doctor (Committee Member)
Keywords: DLC coating, Tribology, Bearing, Micropitting, Black Oxide, Surface Fatigue, Frictional Heat, Flash Temprature, Hertzian Contact