Ball Bearings

Ball bearings are used to provide smooth, low friction motion in rotary applications. Ball bearings types include Radial Ball Bearings (Deep Groove and Angular Contact) and Thrust Ball Bearings. Radial ball bearings are designed to carry both radial and axial (thrust) loads, while thrust ball bearings are designed for pure thrust loads only.  Radial, or deep groove bearings, can take radial and axial loads to varying degrees but are used when the primary load is in the radial direction.  They consist of an inner ring, an outer ring, balls and sometimes a cage to contain and separate the balls.  Angular contact ball bearings are designed such that a contact angle between the races and the balls is formed when the bearing is in use.  The major design characteristic of this type of ball bearing is that one or both or the ring races have one shoulder higher than the other.  In order for these bearings to function properly, they must be assembled with a thrust load.  This loading (or preload) creates a line of contact (or contact angle) between the inner race, the ball and the outer race. The preload can be built in to the bearing or created when the bearing is inserted into an assembly.  The contact angle varies from 15° to 40° and is measured relative to a line running perpendicular to the bearing axis.  Angular contact bearings are one-directional thrust bearings that can withstand heavy thrust loads and moderate radial loads.  Thrust ball bearings are designed for pure thrust loads.  These bearings can handle little or no radial loads.  The rolling element can be a ball, needle, or roller.  Slewing ring or turntable bearings can accommodate axial, radial and moment loads.  They are not mounted in a housing or on a shaft, but instead are mounted directly to a seating surface.  The inner and outer rings are supplied with mounting holes.  The inner ring, outer ring, or both may have integral gears.  These bearings are referred to as tabletop bearings, turntable bearings, and slewing rings.

Important dimensions to consider when specifying bearings include bore, outside diameter, and overall width.  The bearing industry uses a standard number system for bearings with metric diameter bores.  For bore sizes 04 and up multiply by 5 to obtain the bore in millimeters.  If the bore is a hex this refers to the dimension across the flats.  If the bore is tapered this refers to the smaller diameter.  The outside diameter of the bearing includes the housing if a housed unit, but excludes the flange if a flanged bearing.  The outer ring width is the overall width of the outside of the bearing.  The overall width of the bearing or bearing assembly includes the locking collar, if present.  Important operating specifications to consider when searching for bearings include rated speed, dynamic axial or thrust load, and dynamic radial load.  The rated speed for a bearing running with grease lubrication is lower than a bearing with oil lubrication.  The dynamic axial or thrust load is the calculated constant axial load, which a group of identical bearings with stationary outer rings can theoretically endure for a rating life of 1 million revolutions of the inner ring.  The dynamic radial load is the calculated constant radial load, which a group of identical bearings with stationary outer rings can theoretically endure for a rating life of 1 million revolutions of the inner ring.

Bearing quality is rated by ABEC (Annular Bearing Engineers' Committee).  These ratings classify different accuracy and tolerance ranges for ball bearings. The higher the ABEC number the tighter the bearing tolerances.  ANSI -American National Standards Institute and AFBMA - Anti-friction Bearing Manufacturers Association specifies other standards.  Special materials and plating options to consider for ball bearings include stainless steel, plastic, ceramic hybrid, ceramic races, and platings.  Special applications for bearings include airframe or aircraft control, aerospace, agriculture, automotive, ball screw support, instrument or miniature, medical or dental, military, pump, spindle or super precision, and sporting and consumer goods.