In some applications, allowable load seriously influences selection. Major variables affecting load capacity are ball size and number and internal geometry of the bearing.
Static: Primary consideration is given to static ratings. A bearing may be deformed permanently by high loads on nonrotating bearings or severe instantaneous loads on rotating bearings.
Radial static capacity for instrument bearings is defined as the load causing a permanent, plastic deformation of 0.001 in. of the ball diameter at the ball-raceway contact. This deformation degrades the smoothness and noise level of the bearing. However, in less critical applications, higher loads can be accommodated.
Static thrust-load rating is based on two considerations: 1. Load at which the ball-raceway stress approximates that calculated for the heaviest-loaded ball under the limiting static radial load. 2. Load causing the contact area between the ball and raceway to climb over the edge of the raceway shoulder. Thrust ratings vary with contact angle. Lower ratings will result from either low or high angles.
Dynamic: Dynamic load ratings are not directly applicable to life prediction of instrument bearings because little life-test data is available. However, metal-fatigue failure is seldom encountered in instrument applications.
Recommended use of dynamic load ratings for instrument bearings is limited to a comparison of relative capacity for different sizes or designs. Dynamic load capacity also may be used as a general guideline for the magnitude of operating bearing loads. A rule of thumb is: light loads, 0 to 5% of C factor; moderate loads, 5 to 15% of C factor; heavy loads, over 15% of C factor.
Not all manufacturers use the same load calculation methods. Thus, selection of a bearing because of a high advertised load rating is not recommended. The comparison for load-carrying ability should be based on the same formula. If a formula is not available, a fast, accurate comparison can be made by determining Nd2 values for each bearing where N = number of balls and d = ball diameter. The higher the Nd2 value, the higher the load capacity for bearings of similar design and material.