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BELTS. 337
Angle Belts.
The belt arrangement shown in Fig. 4 is usually called an
angle belt and is used to connect two shafts at an angle. Either
one, A or B, may be the driver, and
there are two guide pulleys (one for
each part of the belt at C), one of which,
of course, is on the driving part of the
belt.
Crossed belts, quarter-turn belts,
and angle belts must never be wide and
thin ; much better results are obtained
by narrow, double belts than by wide,
single ones.
Angle belts and quarter-turn belts
are frequently bothersome contrivances.
Their running is sometimes improved
by making a twist in the belt when
joining its ends : that is, lacing the flesh side of one end and the
hair side of the other end on the outside. This will prevent one
side of the belt from stretching more than the other.
Slipping of Belts.
Owing to the elasticity of belts, there must always be more
or less slip or " creep " of the belts on the pulleys. Under
favorable conditions it may be as low as 2%, but frequently the
slip is more. Therefore, if two shafts are connected by belts,
and both should have very nearly the same speed, the diameter
of the driver should be at least 2% larger than the diameter of
the driven pulley. When the driver is comparatively large in
diameter and the driven pulley is small, it is advisable to have
the driver from 2 to b% over size, in order to get the required
speed.
Tighteners on Belts.
If tighteners are used they should always be placed on the
slack part of the belt.
Velocity of Belts.
Belts are run at almost all velocities from less than 500 to
5000 feet per minute, but good practice indicates that whenever
possible main belts having to transmit quantities of power are
run most economically at a speed of 3000 to 4000 feet per min-
ute. At a higher speed both practice and theory seem to agree
that the loss due to the action of the centrifugal force in the belt
when passing around the pulley, and that the wear and tear is so
great when the speed is much over 4000 feet per minute that
there is not much practical gain in increasing the speed. But,
as a general rule, whenever possible the higher the belt speed
the more economical is the transmission as long as the belt
speed does not exceed the neighborhood of 4000 feet per minute.
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