Engineering Data

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, Ultra-High 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.
Non-standard 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 ball-to-raceway 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 thrust-loaded 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


Ball bearings are preloaded for a variety of reasons:
  • To eliminate radial and axial looseness
  • To reduce operating noise
  • To improve positioning accuracy
  • To reduce repetitive runout
  • To reduce the possibility of damage from vibratory loading
  • To increase life and axial capacity
  • To increase stiffness

There are essentially two ways to preload a ball bearing, either by using a spring or through a solid stack of parts.

Spring preloading can consist of a coil spring or a wavy washer, which applies a force against the inner or outer ring of the non-interference fitted bearing in the assembly.

Since in a spring the load is fairly consistent over a wide range of compressed length, the use of a spring for preloading eliminates the need for holding tight location tolerances on machined parts. For example, retaining rings can be used in the spindle assembly, thus saving the cost of a locating shoulder, shims or threaded members. Normally a spring would not be used where the assembly must withstand reversing thrust loads.

A solid stack method may be used when precise location control is required. For example, in a precision motor, the use of built-in preload is suggested. Ball bearings with built-in preload are often referred to as duplex ball bearings. When the set of bearings is assembled, the thrust load needed to make the adjacent faces of the rings contact becomes the desired preload. Built-in preload helps satisfy the requirements of increased axial and radial stiffness and deflection control.

There are three methods of mounting preloaded duplex bearings: back-to-back, face-to-face and tandem.

Back-to-Back (DB)

When a back-to-back (DB) duplex pair is mounted, the outer rings abut and the inner rings are drawn together, providing maximum stiffness.

Face-to-Face (DF)

When face-to-face (DF) duplex pairs are mounted, the inner rings abut and the outer rings are drawn together, providing a higher radial and axial stiffness and accommodation of misalignment.

Tandem (DT)

With tandem (DT) pairs, both inner and outer rings abut and are capable of sharing a thrust load, providing increased thrust capacity.

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