Tutorial 8 - Selection of Ball Bearings

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Tutorial 8 - Selection of Ball Bearings
Goals Calculate rated loads of bearings
Select a bearing pack from the SKF catalog
Incorporate manufacturing concepts into bearing
design.
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Problem Statement
Given A gear is mounted on a 1.5 inch diameter
shaft and will be supported by two deep groove
roller bearings to ensure it meshes properly with
a pinion. The bearings must support loads of 400
pounds each and a combined thrust force of 50
lbs. The shaft rotates at a maximum speed of 300
rev/min and must have an L10 life of 50 x 103
hours with a static safety factor of 1.5.
Find Select a bearing for this application.
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General Comments
A large number of bearing manufacturers are in
existence which can provide a bearing for this
application. While one could obtain additional
bearing catalogs, this analysis is restricted to
selecting a bearing from Table 13.6. The design
approach is identical to selecting a bearing from
any other manufacturers catalog, and is
therefore representative of a designers task.
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Calculate the equivalent load, P.
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Calculation of Equivalent Load
The equivalent load is P 265 pounds
See the next slide for details of the analysis!
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Analysis Details
The equivalent load is given by Equation (13.86)
as PXPrYPa. X and Y are found from Table 13.8
to be 0.6 and 0.5, respectively, for a radial
deep-groove ball bearing. The radial loads on
each bearing are given as 400 pounds, and the
total thrust load is 50 pounds. There is no
indication that the thrust load is evenly shared
or is entirely borne by one bearing. A worst
case analysis would suggest that the situation
where one bearing pack supports the entire thrust
load should be analyzed. The equivalent load is
then P(0.6)(400lbs)(0.5)(50lbs)265 lbs
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Select a bearing based on this load rating.
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Bearing Selection
A 6208 series bearing is best for this
application.
See the next slide for details of the analysis!
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Analysis Details
The safety factor forces us to consider only
the bearing packs with basic load ratings of 1.5
x 265 lbs 400 lbs. A number of bearing packs
from the SKF catalog can support the basic load
rating required in this application. Indeed,
the loads are not very large for the size of
shaft which is used. Often times shafts are
designed so that the angular deflection is small
under the torques experienced in application,
and long shafts are quite large in diameter.
Note that a 6404 series bearing, for example,
has a sufficient load rating. But to fit this
bearing onto the shaft would require machining
the shaft to a 0.78 in diameter and then
compromise the stiffness or strength of the
shaft. Even if the bearing were located at the
edge of the shaft, this is not a good design
practice, as a scale drawing of the shaft will
readily indicate. To fit onto the existing
shaft, a bearing bore (db) of 1.5748 is
selected. Of the bearings with this bore, the
6208 series bearing is the smallest bearing with
a high enough basic load rating. In theory,
there is nothing wrong with selecting a 1.7717
inch bore (or larger) bearing as long as a proper
sleeve is provided for mounting the bearing onto
the shaft. However, there is an added expense
to this approach. Good design practice would
now suggest that the shaft design be reviewed and
modifications made, if practical, to allow
rapid mounting and exposure of the bearing packs.

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Concluding Remarks
A common problem was encountered here, where
small bore bearing packs would have sufficient
capacity to support the design loads, but would
have compromised the shaft on which they are
mounted. This problem illustrated the notion
that the mounting of bearings onto shafts must
play an integral role in their selection.