【文章內(nèi)容簡介】 nt minimal degree of damage. The rate of failure propagation is therefore important. If the degree of damage at test termination is consistent among test elements, the only variation between the experimental and theoretical lives is the lag in failure detection. In standard throughhardened bearing steels the failure propagation rate is quite rapid under endurance test conditions, and this is not a major factor, considering the typical dispersion of endurance test data and the degree of confidence obtained from statistical analysis. This may not, however, be the case with other experimental materials or with surfacehardened steels or steels produced by experimental techniques. Care must be used when evaluating these latter results and particularly when paring the experimental lives with those obtained from standard steel lots. The ultimate means of ensuring that an endurance test series was adequately controlled is the conduct of a posttest analysis. This detailed examination of all the tested bearings uses highmagnification optical inspection, highermagnification scanning electron microscopy, metallurgical and dimensional examinations, and chemical evaluations as required. The characteristics of the failures are examined to establish their origins and the residual surface conditions are evaluated for indications of extraneous effects that may have influenced the bearing life. This technique allows the experimenter to ensure that the data are indeed valid. The posttest analysis is, by definition, after the fact. To provide control throughout the test series and to eliminate all questionable areas, the experimenter should conduct a preliminary study whenever a bearing is removed from the test machine. In this portion of the investigation each bearing is examined optically at magnifications up to 30 ? for indications of improper or outofcontrol test parameters. Examples of the types of indications that can be observed are given 3 in Figs. . 2 .AVOIDING FAILURES The best way to handle bearing failures is to avoid them． This can be done in the selection process by recognizing critical performance characteristics． These include noise， starting and running torque， stiffness， nonrepetitive run out， and radial and axial play． In some applications, these items are so critical that specifying an ABEC level alone is not sufficient． Torque requirements are determined by the lubricant， retainer， raceway quality(roundness cross curvature and surface finish)， and whether seals or shields are used． Lubricant viscosity must be selected carefully because inappropriate lubricant， especially in miniature bearings， causes excessive torque． Also， different lubricants have varying noise characteristics that should be matched to the application. For example， greases produce more noise than oil． Nonrepetitive run out(NRR)occurs during rotation as a random eccentricity between the inner and outer races， much like a cam action． NRR can be caused by retainer tolerance or eccentricities of the race ways and balls． Unlike repetitive run out, no pensation can be made for NRR. NRR is reflected in the cost of the bearing． It is mon in the industry to provide different bearing types and grades for specific applications． For example， a bearing with an NRR of less than is used when minimal run out is needed， such as in disk— drive spindle motors． Similarly，machine— tool spindles tolerate only minimal deflections to maintain precision cuts． Consequently, bearings are manufactured with low NRR just for machinetool applications． Contamination is unavoidable in many indus