| 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 (Dw).
Inner raceway curvature ratio fi = |
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where:
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Dw = ball diameter
Ri = inner raceway radius
Ro = outer raceway radius |
Outer raceway curvature ratio fo = |
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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 |
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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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| CONTACT ANGLE |
| Contact angle is the angle between a plane perpendicular to the bearing axis and a line joining the two contact points between the ball and the inner and outer raceways. The initial contact angle ao is defined as the contact angle when the bearing is subjected to a minimal axial load to remove the looseness resulting from radial play. The higher the radial play the higher ao. Further axial load will increase the contact angle see section on preload. |
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FORMULAE FOR INTIAL CONTACT ANGLE ao:
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INITIAL CONTACT ANGLE CHART ao DEGREES (fi = fo = 0.56)
BALL DIAMETER (Dw) |
SWC RADIAL PLAY CODE [ dimensions in inches (mm) ] |
MC2
(.0001 - .0003)
(.003 - .008) |
MC3
(.0002 - .0004)
(.005 - .010) |
MC4
(.0003 - .0005)
(.008 - .013) |
MC5
(.0005 - .0008)
(.013 - .020) |
.0236 & 0.6mm |
15.1o |
18.6o |
21.5o |
27.5o |
.0315 & 0.8mm |
13.3o |
16.3o |
18.9o |
24.1o |
.0394 & 1.0mm |
11.9o |
14.5o |
16.8o |
21.5o |
.0472 & 1.2mm |
10.8o |
13.2o |
15.3o |
19.5o |
.0625 (1/16th) |
9.3o |
11.4o |
13.2o |
16.9o |
.0787 & 2.0mm |
8.3o |
10.2o |
11.8o |
15.0o |
.0937 (3/32nd) |
7.6o |
9.4o |
10.8o |
13.8o |
.1250 (1/8th) |
6.6o |
8.1o |
9.4o |
11.9o |
.1406 (9/64th) |
6.2o |
7.6o |
8.8o |
11.2o |
.1562 (5/32nd) |
5.9o |
7.3o |
8.4o |
10.7o |
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| LAND AND PITCH DIAMETER |
| Bearing land diameters are given by the maximum outside diameter of the inner ring (li) and the minimum inside diameter of the outer ring (lo). Bearing land diameters are given in the individual bearing tables. |
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| Inner land diameter = li |
Outer land diameter = lo |
Bearing pitch diameter dp is the diameter of the circle that the center point of a ball travels during rotation of a bearing.
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Note: For pitch diameter calculations, use the land diameter values from the open (unshielded) bearing tables. |
Bearing pitch diameter dp~ |
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| FREE ANGLE OF MISALIGNMENT |
The free angle of misalignment (f) of a bearing is the maximum angle between the inner race and outer race axes freely allowed by the bearing. If a bearing is misaligned such that this angle is exceeded, then damage and premature failure can occur.
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| (dp >> Dw) |
