Ball bearings are complex mechanisms, where ball race fit, radial clearance and ball complement affect the ball bearing's ability to support loads under various conditions. During assembly, the bearing manufacturer intentionally incorporates an internal clearance, or radial clearance, between the rolling elements (balls) and the races, resulting in radial clearance and axial play in the bearing.
Ball complement The number and diameter of balls in a bearing is commonly known as ball complement.
Radial play , or internal clearance, is the internal radial clearance in a bearing. It is the measured value of the total radial movement of the outer ring relative to the inner ring in a plane perpendicular to the bearing axis. Radial clearance is determined by the effective diameters of the raceways and the diameter of the balls.
Gauges test radial clearance by subjecting the bearing to a standard reference load, ensuring full contact between all bearing components.
Axial play or axial play is the maximum relative axial movement of the inner ring in relation to the outer ring.
Both radial clearance and axial clearance are specified independently of the ABEC tolerance classes for the rings. Radial clearance is usually specified as part of the bearing code.
In most applications using deep groove ball bearings, internal clearance, or play, is removed by applying an axial preload to the bearing pair, using shims, springs, locknuts, or other mounting techniques. This provides contact with the ball lane that ensures accurate rotation and reduces or eliminates ball slip and noise.
Contact angle in a ball bearing
A contact angle occurs when ball bearings are axially preloaded; is the angle formed by a plane perpendicular to the bearing axis and a line that joins the two points of contact between the ball and the inner and outer races. Additional axial loads increase the contact angle. As the radial clearance increases, the contact angle also increases.
General design considerations
There are numerous considerations regarding radial clearance values in a specific bearing application. Each choice affects noise, vibration and heat, among other factors.
Adaptation
When the bearing is mounted using interference fits, allow more radial clearance (radial clearance after mounting is of fundamental importance).
Loading
When a ball bearing is subjected to purely radial loads, a smaller radial clearance is recommended, which distributes the load over more balls.
Misalignment
When the degree of shaft deflection is large, increase the radial clearance, which allows greater misalignment
Temperature
When there is a high temperature gradient between the rings (inner, outer), a slower radial clearance is recommended.
Speed
High values of radial clearance lead to high values of contact angle. As the speed of rotation increases, the rotational forces on the balls increase, creating a sliding action between the balls and the raceways. This results in lubricating film failure, heat build-up and potential premature failure. The magnitude of the rotating moment is related to the contact angle. The balance between the benefits of reducing contact angle stress must be weighed against the risk of lubricant failure due to ball slip.
