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BELTS. 335
Solution, by the last rule :
f = 8X25X8 = 1600 pounds.
Solution, by the first rule
:
At a speed of 3000 feet per minute such a belt will transmit
X8
30
formula
00 x 8
-g^j— = 48 horse-power, and calculating the pressure by the
p _ 3 X 33000 X H
p = 3 X 33000 X 48
= 15g4 ^
3000
Both rules give nearly the same result, and one is just as
correct as the other, as all such figuring is nothing more than
approximation at the best. The pressure on the bearings may
be a great deal more than calculated above. Sometimes the
pulleys are roughly made, belts are poor, and consequently the
coefficient of friction between belt and pulley is small, and as
the belt has to be a great deal tighter in order to do the work,
the pressure on the bearing will be greatly increased. Very
frequently, from pure ignorance or carelessness, belts .are made
very much tighter than necessary, and enormous sums of money
may be wasted in this way in large factories, as the steam
engines, at the expense of the coal pile, have to furnish power
not only to do the useful work, but also to overcome all the
friction produced by such over-strained belts, hot bearings, etc.
A belt will transmit more power over a good, smooth pulley
than over a rough one. When pulleys are covered with leather
a belt will transmit about 25% more power than it will when
running over bare iron pulleys, and in transmitting the same
power a much slacker belt may be used, thereby reducing the
friction in the bearings.
Special Arrangement of Belts.
By the use of suitable guide pulleys it is possible to connect
with belts shafts at almost any angle to each other. But
experience is required and care must be exercised to do it suc-
cessfully. When guide pulleys are used in order to change the
direction of a belt, always remember that when the belt is run-
ning the most pressure is thrown on the pulley guiding the
working part of the belt. This pulley is, therefore, very liable
to heat in its bearings,, if not designed to have bearing surface
enough and also to have proper means for oiling.
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