Machinery's Handbook, 31st Edition
MOUNTING BEARINGS
2469
Bearing Mounting Practice Because of their inherent design and material rigidity, rolling contact bearings must be mounted with careful control of their alignment and runout. Medium-speed or slower (400,000 DN values or less where D is the bearing bore in millimeters and N is the bear- ing speed in revolutions per minute), and medium to light load ( C/P values of 7 or greater where C is the bearing specific dynamic capacity in pounds and P is the average bearing load in pounds) applications can endure misalignments equivalent to those acceptable for high-capacity, precision journal bearings utilizing hard bearing materials such as silver, copper-lead, or aluminum. In no case, however, should the maximum shaft deflection exceed .001 inch per inch (or mm per mm) for well-crowned roller bearings, and .003 inch per inch (or mm per mm) for deep-groove ball-bearings. Except for self-aligning ball- bearings and spherical or barrel roller bearings, all other types require shaft alignments with deflections no greater than .0002 inch per inch (or mm per mm). With preloaded ball bearings, this same limit is recommended as a maximum. Close-clearance tapered bear- ings or thrust bearings of most types require the same shaft alignment also. Of major importance for all bearings requiring good reliability is the location of the races on the shaft and in the housing. Assembly methods must insure: 1) that the faces are square before the cavity is closed; 2) that the cover face is square to the shoulder and pulled in evenly; and 3) that it will be located by a face parallel to it when finally seated against the housing. These requirements are shown in the accompanying Table 22. In applications not con trolled by automatic tooling with closely controlled fixtures and bolt torquing mecha nisms, races should be checked for squareness by sweeping with a dial indicator mounted as shown below. For commercial applications with moderate life and reliability require ments, outer race runouts should be held to .0005 inch per inch (or mm per mm) of radius and inner race runout to .0004 inch per inch (or mm per mm) of radius. In preloaded and precision applications, these tolerances must be cut in half. In regard to the question of alignment, it must be recognized that rolling-contact bearings, being made of fully-hard ened steel, do not wear in as may certain journal bearings when carefully applied and ini tially operated. Likewise, rolling contact bearings absorb relatively little deflection when loaded to C/P values of 6 or less. At such stress levels the rolling element-race deformation is generally not over .0002 inch (5.08 m m). Consequently, proper mounting and control of shaft deflections are imperative for reliable bearing performance. Aside from inadequate lubrication, these factors are the most frequent causes of premature bearing failures. Mountings for Precision and Quiet-Running Applications.— In applications of rolling- element bearings where vibration or smoothness of operation is critical, special precau tions must be taken to eliminate those conditions which can serve to initiate radial and axial motions. These exciting forces can result in shaft excursions which are in resonance with shaft or housing components over a range of frequencies from well below shaft speed to as much as 100 times above it. The more sensitive the configuration, the greater is the need for precision bearings and mountings to be used. Precision bearings are normally made to much closer tolerances than standard and therefore benefit from better finishing techniques. Special inspection operations are required, however, to provide races and rolling elements with smoothness and runouts compatible with the needs of the application. Similarly, shafts and housings must be care- fully controlled. Among the important elements to be controlled are shaft, race, and housing roundness; squareness of faces, diameters, shoulders, and rolling paths. Though not readily appreci ated, grinding chatter, lobular and compensating out-of-roundness, waviness, and flats of less than 0.0005 inch (0.013 mm) deviation from the average or mean diameter can cause significant roughness. To detect these and insure the selection of good pieces, three-point electronic indicator inspection must be made. For ultra-precise or quiet applications,
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