Machine Design

Bearing Deflection

Bearing deflection or yield is elastic deformation occurring between balls and raceways because of external loads. Deflection increases nonlinearly as external load increases.

To increase bearing stiffness (lower deflection rate) the factors to adjust are:

Contact angle -- increase.

Race curvature -- decrease or close up.

Ball size -- increase.

Ball complement -- increase.

For a low-contact-angle bearing, axial deflection per unit load is considerably higher than radial deflection per unit load. For most applications, radial yield rate is neglected and principal attention is given to axial loading and deflection.

Minimizing deflection: Contact-angle control has the largest influence on the yield rate and is also the easiest to adjust. Race curvature can also be adjusted, but with more difficulty. At higher contact angles, the influence of curvature on deflection becomes less significant.

Preloading: Bearings are preloaded to eliminate high initial deflections and achieve a constant (linear) deflection rate. Preloading is achieved by mounting two bearings in opposition to provide an internal load within the bearing.

The magnitude of this internal load is controlled by matching stickouts of each bearing or flush grinding of the rings under specified preload. Either the radial retainer or the angular-contact type of instrument bearing can be supplied as preloaded or duplex pairs.

Preloading bearings in either a back-to-back (DB) or face-to-face (DF) mounting arrangement allows for precise control of axial deflection rates, accurate positioning of the shaft relative to the housing, and increased radial load capacity. The DB mount is most common and provides high resistance to overturning moments. The DF mount has less resistance to moment loads, but is less sensitive to slight misalignments.

Design of a preloaded bearing pair for a specific application is based on maximum allowable axial-deflection rate and maximum applied external thrust.

Deflection rate of the preloaded pair is a function of the single-bearing characteristics, but can be altered by adjusting the preload level.

For proper operation, the unloading point of the preloaded pair must be greater than the maximum external thrust load. Value of the unloading point is determined principally by preload level and, to a lesser degree, by the single-bearing characteristics. As a general guideline, the unloading point is approximately three times preload level; however, exact determination must be made from the deflection curves.

TAGS: Bearings
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