By AST Bearings
This is Part 2 of a two-part article on radial play within ball bearings.
General Design Considerations:
The selection of the radial play value in a specific bearing application is an important design consideration. As described previously, radial play directly affects the contact angle and axial, or end, play of the bearing. In addition, in operation it is an important factor that has a significant influence on other factors such as noise, vibration, heat, stress, deflection, load distribution and fatigue life.
Fitting: A looser or greater value of radial play should be selected when the bearing is mounted using interference fits. The radial clearance in the bearing is reduced after mounting due to deformation of the inner or outer rings. With miniature bearings having very thin cross section rings, the radial play is reduced by approximately 80% of the actual amount of interference. The radial play after mounting is the primary design consideration. Therefore, a tolerance study of the mating components should be completed and compensation for the interference at maximum material condition must be made. For maximum life, a positive clearance after mounting is desired.
Loading: When a ball bearing is subjected to thrust loading, a higher contact angle will result in reduced ball to raceway stresses. Greater values of radial play result in higher values of contact angle. Under these conditions, this will provide longer bearing life, lower torque, and less axial deflection. In a pure thrust situation, a 15 increase in contact angle can result in over a 70% reduction in contact stress (ball-to-raceway).
When a ball bearing is subjected to pure radial loads (or radial load with low axial load), lower radial play is normally recommended. This distributes the load over a greater number of balls. However, particularly with miniature bearings, bearings with low radial play should not be subjected to interference fits. This can result in negative clearance and drastically reduce life.
Misalignment and Positioning: Higher values of radial play allow for greater misalignment and should be selected in cases where there is a high degree of shaft deflection. It should be noted, although a ball bearing has the ability to compensate (around 1 or less), misalignment greatly reduces bearing life. However, in cases with light loads, the impact of minor misalignment may be tolerable.
Tighter values of radial play will of course control and restrict radial movement. When axial positioning must be controlled or end play of “zero” is desired, it is recommended that the end play be removed by applying axial preload via shims, washers, or other assembly methods. Duplex bearings should be considered as well. It is not advisable to use low values of radial play to control end play.
Temperature: When a high temperature gradient exists between the inner and outer rings a looser radial play is recommended.
Speed: As mentioned previously, high values of radial play result in high values of contact angle. When a bearing is rotating, the set of balls (or ball complement) is rotating about the pitch circle of the bearing, each ball is rotating about its own axis, and gyratory moments are acting upon the balls. The magnitude of the gyratory moment is related to the contact angle. As rotational speed increases, the gyratory forces acting on the balls increases and there is sliding action between the balls and the raceways. This slippage results in lubricant film failure, increased heat, and potential premature failure. The balance between the benefits of contact angle reducing stress must be weighed against the potential for lubricant failure due to ball slippage.
Specifying Radial Play:
It is not practical, or perhaps even possible, to produce a group of bearings that all have the exact same amount of radial play. This is because all of the features of the bearing components (inner ring raceway, outer ring raceway, and balls) that effect radial play, have manufacturing tolerances associated with them. Manufacturers measure and sort the bearing rings and balls so that they can be “matched” during the assembly process to achieve a specific range of radial play for a group (or manufacturing lot) of bearings.
There are several ways to specify radial play. Radial play specifications are usually manufacturer dependent. Complete part number descriptions are detailed in a separate Technical Information Sheet (TIS). AST specifies radial play as follows: Metric design ball bearings (miniature and instrument) with a bore diameter of less than 10 mm.