Machinery's Handbook, 31st Edition
2478 BEARING DESIGN AND APPLICATIONS Starting or breakaway torques are also of interest in some situations. Breakaway torques tend to be between 1.5 and 1.8 times the running or kinetic torques. When evaluating the torque requirements of a system under design, it should be noted that other components of the bearing package, such as seals and closures, can increase the overall system torque significantly. Seal torques have been shown to vary from a fraction of the bearing torque to several times that torque. In addition, the torque values given can vary significantly when load, speed of rotation, temperature, or lubrication are outside normal ranges. For small instrument bearings friction torque has implications more critical than for larger types of bearings. These bearings have three operating friction torques to consider: starting torque, normal running torque, and peak running torque. These torque levels may vary between manufacturers and among lots from a given manufacturer. Instrument bearings are even more critically dependent on design features — radial play, retainer type, and race conformity — than larger bearings. Typical starting torque values for small bearings are given in the accompanying table, extracted from the New Departure General Catalog. Finally, if accurate control of friction torque is critical to a particular application, tests of the selected bearings should be conducted to evaluate performance. Starting Torque — ABEC7
Minimum Radial Play Range (inches) High Carbon Chrome Steel and All Miniatures Stainless Steel Except Miniatures
Max. Starting Torque (g cm)
Thrust Load (g)
Bearing Bore (in.)
0.10 0.14 0.18 0.22 0.40 0.45 0.50 0.63 0.50 0.63 0.75 0.95
75 75 75 75
0.0003–0.0005 0.0002–0.0004 0.0001–0.0003 0.0001–0.0003 0.0005–0.0008 0.0004–0.0006 0.0003–0.0005 0.0001–0.0003 0.0005–0.0008 0.0004–0.0006 0.0003–0.0005 0.0002–0.0004
—
0.125
0.0004–0.0006 0.0003–0.0005 0.0001–0.0003 0.0005–0.0008 0.0003–0.0005 0.0002–0.0004 0.0008–0.0011 — 0.0005–0.0008 0.0004–0.0006
400 400 400 400 400 400 400 400
0.1875–0.312
0.375
0.0003–0.0005 Selection of Ball and Roller Bearings.— As compared with sleeve bearings, ball and roller bearings offer the following advantages: 1) starting friction is low; 2) less axial space is required; 3) relatively accurate shaft alignment can be maintained; 4) both ra- dial and axial loads can be carried by certain types; 5) angle of load application is not restricted; 6) replacement is relatively easy; 7) comparatively heavy overloads can be carried momentarily; 8) lubrication is simple; and 9) design and application can be made with the assistance of bearing supplier engineers. In selecting a ball or roller bearing for a specific application five choices must be made: 1) the bearing series; 2) the type of bearing; 3) the size of bearing; 4) the method of lubri cation; and 5) the type of mounting. Naturally these considerations are modified or affected by the anticipated operating conditions, expected life, cost, and overhaul philosophy.
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