Bearing Lubricants
Bearing Loads
In some applications, allowable load seriously influences bearing
selection. Major variables affecting load capacity are ball size
and number and internal geometry of the bearing.
Static: Primary consideration is given to static ratings. A bearing may be deformed permanently by high loads on nonrotating bearings or severe instantaneous loads on rotating bearings.
Radial static capacity bearing for instrument bearings
is defined as the load causing a permanent, plastic deformation
of 0.001 in. of the ball diameter at the ball-raceway contact. This
deformation degrades the smoothness and noise level of the bearing.
However, in less critical applications, higher loads can be accommodated.
Static thrust-load rating is based on two considerations:
1. Load at which the ball-raceway stress approximates that calculated
for the heaviest-loaded ball under the limiting static radial load.
2. Bearing load causing the contact area between the ball and raceway
to climb over the edge of the raceway shoulder. Thrust ratings vary
with contact angle. Lower ratings will result from either low or
high angles.
Dynamic Bearing: Dynamic load ratings are not directly applicable
to life prediction of instrument bearings because little life-test
data is available. However, metal-fatigue failure is seldom encountered
in instrument applications.
Recommended use of dynamic load ratings for an instrument bearing
is limited to a comparison of relative capacity for different sizes
or bearing designs. Dynamic load capacity also may be used as a
general guideline for the magnitude of operating bearing loads.
A rule of thumb is: light bearing loads, 0 to 5% of C factor; moderate
bearing loads, 5 to 15% of C factor; heavy bearing loads, over 15%
of C factor.
Not all bearing manufacturers use the same load calculation methods.
Thus, selection of a bearing because of a high advertised load rating
is not recommended. The comparison for any bearing's load-carrying
ability should be based on the same formula. If a formula is not
available, a fast, accurate comparison can be made by determining
Nd2 values for each bearing where N = number
of balls and d = ball diameter. The higher the Nd2
value, the higher the load capacity for bearings of similar design
and material.
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