Radial & Axial Play, Raceway Curvature & Contact Angle
Radial and Axial Play  Raceway Curvature  Contact Angle  Key Formulas
When a ball bearing is running under a load, force is transmitted from one bearing ring to the other through the balls. Since the contact area between each ball and the rings is relatively small, moderate loads can produce stresses of tens, even hundreds of thousands of pounds per square inch. These internal stresses have a significant impact on bearing life and performance. Thus the internal geometry of a bearing—its radial play, raceway curvature and contact angle—must be carefully chosen so loads can be distributed for optimal performance.
Radial and Axial Play
Most ball bearings are assembled in such a way that a slight amount of looseness exists between the balls and the raceways. This looseness is referred to as radial play and axial play. Radial play is the maximum distance that one bearing ring can be displaced with respect to the other, in a direction perpendicular to the bearing axis when the bearing is in an unmounted state. Axial play, or end play, is the maximum relative displacement, in a direction parallel to the bearing axis, between the two rings of an unmounted ball bearing.
Since radial play and axial play are both consequences of the same degree of looseness between the components, they bear a mutual dependence. Yet their values are usually quite different in magnitude. Radial play can often vary between .0002 and .0020, while axial play may range from .001 to .010. The suggested radial play ranges for typical applications should always be consulted when a device is in the initial design phase.
Suggested Radial Play
Typical Application 
Suggested
Radial Play
(inches) 
Small Precision High Speed Electric Motors 
.0005 to .0008 
Tape Guides, Belt Guides, Low Speed 
.0002 to .0005 
Tape Guides, Belt Guides, High Speed 
.0005 to .0008 
Gyro Gimbals, Horizontal Axis 
.0002 to .0005 
Gyro Gimbals, Vertical Axis 
.0005 to .0008 
Precision Gear Trains, Low Speed Electric Motors, Synchros and Servos 
.0002 to .0005 
Gyro Spin Bearings, UltraHigh Speed Turbines and Spindles 
Consult factory 
In most ball bearing applications, radial play is functionally more critical than axial play. While radial play has become the standard purchasing specification, you may also specify axial play requirements. Keep in mind, however, the values of radial play and axial play for any given bearing design are mathematically interdependent, and that radial play is affected by any interference fit between the shaft and bearing I.D. or between the housing and bearing O.D., as shown in the Table of Recommended Fits. Since the important condition is the actual radial play remaining after assembly of the complete device, the radial play specification for the bearing must be modified in accordance with the discussion in the mounting and coding section.
Standard Radial Play Ranges
Description 
Radial Play Range* 
NHBB Code 
Tight
Normal
Loose
Extra Loose 
.0001 to .0003
.0002 to .0005
.0005 to .0008
.0008 to .0011 
P13
P25
P58
P811 
*Measurement in inches.
Nonstandard ranges may be specified.
Raceway Curvature
Raceway curvature is the ratio of the raceway radius to ball diameter. Raceway curvature values typically are either 52 to 54 percent or 57 percent. The lower 52 to 54 percent curvature implies close balltoraceway conformity and is useful in applications where heavy loads are encountered. The higher 57 percent curvature is more suitable for torque sensitive applications.
Contact Angle
Contact angle is the angle between a plane perpendicular to the ball bearing axis and a line joining the two points where the ball makes contact with the inner and outer raceways. The initial contact angle of the bearing is directly related to radial play—the higher the radial play, the higher the contact angle. The Table of Contact Angles as shown gives nominal values under no load.
For support of pure radial loads, a low contact angle is desirable; where thrust loading is predominant, a higher contact angle is recommended.
The contact angle of thrustloaded bearings provides an indication of ball position inside the raceways. When a thrust load is applied to a ball bearing, the balls will move away from the median planes of the raceways and assume positions somewhere between the deepest portions of the raceways and their edges.
Table of Contact Angles
Ball Size
Db 
Radial Play Code 
P25 
P58 
P811 
.025
1/32 & 0.8mm
1mm
3/64
1/16 
18°
16 1/2°
14 1/2°
14°
12° 
24 1/2°
22°
20°
18°
16° 
30°
27°
24°
21°
19° 
3/32
1/8
9/64
5/32
3/16 
9 1/2°
12 1/2°
12°
11°
10° 
13°
17°
16°
15°
14° 
15 1/2°
20°
19 1/2°
18 1/2°
16 1/2° 
The contact angle is given for the mean radial play of the range shown i.e., for P25 (.0002 to .0005)—contact angle is given for .00035. Contact angle is affected by raceway curvature. For your specific application consult with factory.
Key Formulas
