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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.
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| RACEWAY
CURVATURE |
Raceway curvature ratio
(f) is the
ratio of the raceway radius (R) to the ball diameter
(Dw).
Inner raceway curvature
ratio fi = |
Ri |
 |
Dw |
|
where:
|
Dw = ball
diameter
Ri =
inner raceway radius
Ro =
outer raceway radius |
Outer raceway curvature
ratio fo = |
Ro |
|
Dw |
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fi, fo 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 = fi + fo -
1
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| 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
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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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