| 摘要 |
In view of the fact that, when backup roll bearings which have been found satisfactory in the purity thereof in the sample evaluation through observations using a microscope are checked many times for the top surface layer portions of the raceway surfaces thereof according to an ultrasonic detecting method, on quite rare occasions, there can be found a bearing which contains one or more inclusions each having a size of as large as several hundreds mum, and such inclusions give rise to occurrence of a short-life bearing as a finished product, a range expressed by 2% Da depthxraceway surface is used as a test piece volume and the size of a non-metallic inclusion existing in the test piece volume is restricted to a length less than 500 mum, preferably, less than 100 mum. Here, the term "2% Da depth" means a depth up to 2% of a rolling element mean diameter from the surface of an inner or outer race of a bearing.
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