Recently it has been reported that bearing damages of gear boxes for wind power generators are often caused by wear and failure due to electric discharges. Electric discharges also occur in gear boxes for automobiles but do not end up in failure. Therefore, electric discharge simulation tests were carried on the surfaces of bearing balls that had been operated well for a long time. The test results showed that the brown film on the surface of used bearing balls was non-crystalline carbon film and that the film protected the surface metal structure from melting and damaging when electric discharge was caused. The voltage from sliding was estimated as roughly 0.05 kV, however, this suppression was observed at 1.5 kV, which far exceeded this estimation. Depositing this non-crystalline carbon film or similar diamond-like carbon (DLC) film on the sliding surface would protect the surface from damage by electric discharge.
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