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PULLEYS 349
rim of the pattern or by reducing the size of the hub, or
both; it will also help the matter to remove the core and the
sand from the hub as soon as possible after the pulley is cast,
and leave the casting in the sand undisturbed until cool.
When the diameter of the shaft is less than 4 inches, the di-
ameter of the hub is usually made twice the diameter of the
shaft. When shafts are over 4 inches in diameter the hub of
the pulley is usually made a little less than twice the diameter
of the shaft. The length of the hub may be made three-fourths
the width of the rim, for a tight pulley, and five-fourths the width
of the rim for a loose pulley.
The thickness of rim, measured at the edge, is usually
:
For pulleys under 12 inches in diameter, T\ inch.
For pulleys from 12 to 24 inches in diameter, % inch.
For pulleys from 24 to 36 inches in diameter, T
5
g inch.
For pulleys from 36 to 48 inches in diameter, T
7
g inch.
For pulleys from 48 to 60 inches in diameter, x
/
2 inch.
For double belts increase the thickness of the rim one-
eighth of an inch.
The thickness in the middle may be about lj£ times the
thickness at the edge.
Pulleys which are to run at high velocity ought to be turned
both inside and outside, in order to be in good balance. Pulleys
to go on line shafts ought to be made in halves, so that they can
be put on and taken off the shaft with convenience. Pulleys on
which the belts are to be shifted must be a little over twice as
wide as the belt, and they should be turned straight across the
face on the outside. Pulleys on which the belts are not to be
shifted ought to be only 1.2 times as wide as the belt, and they
should be turned crowned across the face; that is, the outside
diameter of the pulley must be largest in the middle. A
straight taper should be turned each way from near the middle to
the edges, and the following proportions will give good results:
Pulleys under six inches wide, 2^-inch taper per foot.
Pulleys from 6 to 12 inches wide, %-mc\\ taper per foot.
Pulleys from 12 to 18 inches wide, ^-inch taper per foot.
When pulleys are turned in a lathe where the tail-stock can be
set over, a taper of ^-inch per foot is practically obtained when
the tail-stock is set over -jVinch per 1 inch length of arbor. For
instance, if a crown pulley is to be turned %-inch per foot, and
the arbor is 12 inches long, the back center must be set over |f
= ^rinch. If the arbor had been 14 inches long the back cen-
ter would have had to be set over || = TVmcn to obtain the
same result.
All pulleys must be well rounded on the edges. They must
also be carefully balanced,especially if they are to run at high
speed. Loose pulleys ought to have longer hubs than tight pulleys
They ought never to have hubs shorter than the width of the rim,
and must always be provided with means for oiling.
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