Machinery's Handbook, 31st Edition
2480 BEARING DESIGN AND APPLICATIONS popular type is the “cartridge” type of sealed grease ball bearing, many applications have requirements which dictate against them. Often, operating environments may subject bearings to temperatures too high for seals utilized in the more popular designs. If minute leakage or the accumulation of traces of dirt at seal lips cannot be tolerated by the applica tion (as in baking industry machinery), then the selections of bearings must be made with other sealing and lubrication systems in mind. High shaft speeds generally dictate bearing selection based on the need for cooling, the suppression of churning or aeration of conventional lubricants, and most important of all, the inherent speed limitations of certain bearing types. An example of the latter is the effect of cage design and of the roller-end thrust-flange contact on the lubrication require - ments in commercial taper roller bearings, which limit the speed they can endure and the thrust load they can carry. Reference to the manufacturers’ catalog and application- design manuals is recommended before making bearing selections. See Selecting a Suitable Lubricant on page 2512 for more information on this topic. Type of Mounting.— Many bearing installations are complicated because the best adapted type was not selected. Similarly, performance, reliability, and maintenance oper ations are restricted because the mounting was not thoroughly considered. There is no universally adaptable bearing for all needs. Careful reviews of the machine requirements should be made before designs are implemented. In many cases complicated machining, redundant shaft and housings, and use of an oversize bearing can be eliminated if the proper bearing in a well-thought-out mounting is chosen. Advantage should be taken of the many race variations available in “standard” series of bearings. Puller grooves, tapered sleeves, ranged outer races, split races, fully demount able rolling-element and cage assemblies, flexible mountings, hydraulic removal features, relubrication holes and grooves, and many other innovations are available beyond the obvious advantages which are inherent in the basic bearing types. Radial and Axial Clearance.— In designing the bearing mounting, a major consideration is to provide running clearances consistent with the requirements of the application. Race fits must be expected to absorb some of the original bearing clearance so that allowance should be made for approximately 80 percent of the actual interference showing up in the diameter of the race. This will increase for heavy, stiff housings or for extra light series races shrunk onto solid shafts, while light metal housings (aluminum, magnesium, or sheet metal) and tubular shafts with wall sections less than the race wall thickness will cause a lesser change in the race diameter. Where the application will impose heat losses through housing or shaft, or where a tem perature differential may be expected, allowances must be made in the proper direction to insure proper operating clearance. Some compromises are required in applications where the indicated modification cannot be fully accommodated without endangering the bear ing performance at lower speeds, during starting, or under lower temperature conditions than anticipated. Some leeway can be relied on with ball bearings since they can run with moderate preloads (0.0005 inch or 12.7 μm, max.) without affecting bearing life or tem perature rise. Roller bearings, however, have a lesser tolerance for preloading, and must be carefully controlled to avoid overheating and resulting self-destruction. In all critical applications axial and radial clearances should be checked with feeler gages or dial indicators to insure mounted clearances within tolerances established by the design engineer. Since chips, scores, race misalignment, shaft or housing denting, housing distortion, end cover (closure) off-squareness, and mismatch of rotor and housing axial dimensions can rob the bearing of clearance, careful checks of running clearance is recommended. For precision applications, taper-sleeve mountings, opposed ball or tapered-roller bear ings with adjustable or shimmed closures are employed to provide careful control of ra- dial and/or axial clearances. This practice requires skill and experience as well as the initial assistance of the bearing manufacturer’s field engineer.
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