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334 BELTS.
Solution
:
3 X33000X6° = 6600 pounds.
900
Example 2.
Suppose the diameters of the pulleys are increased until a
belt speed of 3000 feet per minute is obtained. What will then
be the pressure in the bearings caused by the belt when trans-
mitting 60 horse-power ?
Solution
:
t, 3 X 33000 X 60 nnoA ,
P = = 1980 pounds.
3000
^
By the above examples it is conclusively shown what a
great advantage there is in using pulleys so large in di-
ameter that proper belt speed is obtained. (See velocity of
belts, page 337).
The approximate pressure may also be very conveniently
obtained from the width of the belt, thus: For light single
belts, allow 1000 feet of belt speed per horse-power transmitted
per inch of width of belt. The effective pull in such a belt will
be 33 pounds per inch of width, and the pressure on the bearings
due to the belt will accordingly be 33 X 3 = 99 pounds per inch
of width of belt. For convenience, say 100 pounds pressure in
the bearings per inch of width of such belts. For belts where
800 running feet are allowed per horse-power per inch of width
of belt, this reasoning will give a pressure on the bearing equal
to 123^" pounds per inch of belt. For convenience, say 125
pounds pressure in the bearings per inch of width of such
belts. For belts where 600 running feet are allowed per horse-
power per inch of width, the pressure in the bearing is equal
to 165 pounds per inch of width of belt, and where the belt is
so heavy that only 500 feet of belt speed per horse-power per
inch of width is allowed, the pressure in the bearings will be
198 pounds per inch of width. A good, practical rule, which
can very easily be remembered, is, (when belts are in good order
and have the proper size and the proper tension)
:
Multiply weight of belt in ounces per square foot by eight
times the width of the belt in inches, and the product is
approximately the pressure in pounds upon the bearings caused
by the belt.
Example.
A belt is calculated with regard to the horse-power it has
to transmit under a given velocity, and found to be 8-inch
double belting, weighing 25 ounces per square foot. What pres-
sure will it cause on the bearings when working at proper
tension ?
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