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37© BEARINGS.
of the bearing, arrangements for oiling, etc. For common line
shafting from two to four inches in diameter, not making over
200 revolutions per minute, a pressure not exceeding forty-
pounds per square inch ought to work well. Greater pressure
or greater speed may make it difficult to keep the bearings cool.
Example.
What pressure may be allowed on a bearing twelve inches
long and four inches in diameter ?
Solution
:
Pressure = 4 X 12 X 40 = 1920 pounds.
In well constructed machinery there should not be any
trouble from heating, if the surface velocity and the pressure in
the bearings does not exceed the values given in the following
table :—
Metric Measure. English Measure.
Kilograms per Square
Centimeter.
Surface Velocity in
Meters per Minute.
Pounds per Square
Inch.
Surface Velocity in
Feet per Minute.
5
12
20
100
50
20
75
180
300
300
150
60
The bearings for machinery in general are constructed in
various ways and of different proportions, according to the de-
signer’s judgment, but it is a well-known fact that high-speed
machinery must have longer bearings than slow-speed ma-
chinery.
The length of the bearing will usually vary from one and
one-half to six times the diameter.
When the shafts are small (less than two inches in diameter),
and the speed is from 100 to 1000 revolutions per minute, the
following empirical formula may be used as a guide :
= ^( 1 + ^o)
L = length of bearing, d = diameter of bearing, n = num-
ber of revolutions per minute.
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