Ball Bearings and Miniature Bearings: Dynaroll

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BALL BEARING - GEOMETRY

When a bearing is running under load, force is transmitted from one bearing raceway to the other through the balls. The contact area between a ball and the raceway (contact ellipse) may be very small, so that even moderate loads can produce very large stresses. These stresses can affect bearing performance and life dramatically, so that it is important to consider the internal geometry of the bearing before making a selection for a given application.

RACEWAY CURVATURE

Raceway curvature ratio (f) is the ratio of the raceway radius (R) to the ball diameter (D_w).

Inner raceway curvature ratio f_i =

R_i

D_w

where:

D_w = ball diameter
R_i = inner raceway radius
R_o = outer raceway radius

Outer raceway curvature ratio f_o =

R_o

D_w

f_i, f_o values are typically 0.56 ± 0.03 for small bearings where low torque is a primary requirement. It is not necessarily the same for both inner and outer raceways. Raceway curvature ratio is often referred to as a percentage f x 100%.

The total curvature of a bearing is defined as B = f_i + f_o - 1

RADIAL AND AXIAL PLAY

Bearings are assembled with a slight amount of looseness between the balls and the raceways. This allows the bearing to rotate smoothly but also affects the performance of the bearing in a given application. This looseness can be split into two components - radial and axial play.

RADIAL PLAY is the maximum displacement that one bearing ring can be displaced relative to the other ring in a direction perpendicular to the axis of rotation of the bearing.

Radial play = ∆_r

AXIAL PLAY, or end play, is the maximum relative displacement of the bearing rings, in a direction parallel to the axis of rotation.

Axial play = ∆_a

Radial and axial play are interdependent and are determined during the manufacture of the bearing. Typically, radial play is a purchasing specification.

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