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348 PULLEYS
PULLEYS.
The following empirical rules give arms of nice shape and
good proportions: When the diameter of the pulley is at least 4
times its face, use 6 arms for pulleys from 12 to 60 inches. For a
12-inch pulley make the arms li inches wide at the hub and add
-3*6 of an inch to the width of the arm for each inch the pulley
is increased in diameter.
Formula: h = D ~~ I2
+ i#. (See Fig. I.)
16
h=Width of arm in inches projected to center of hub.
D = Diameter of pulley in inches.
Example: Find width of arms at the hub for a 60-inch pulley.
Solution: h = 6o ~~ I2
+ ii = 4i inches.
16
The width of the arm at the rim should be three-fourths of
the width at the hub, and the thickness should be one-half of the
width for arms with segmental sections (see y Fig. 1) and four-
tenths of the width for elliptical form of section; (see * Fig. 1).
For double belts multiply these dimensions by 1.3.
3
Another formula is: I D X W
Fig. 1 h = 0.8 X
^- ^ n
h = Width of arm at the hub.
D = Diam. of pulley in inches.
W = W7
idth of pulley in inches.
n = Number of arms.
At the rim the width of the
arm is 0.85 times the width at
the hub. Thickness of arm
equals half the width.
Example :
Calculate the width of the
arms for a pulley 96" diameter,
30" face, having two sets of arms
with six arms in each set.
Solution: / 96X30 /
h = o.8xA’-fr = a8x \ 240 = 4.97
or practically 5 " wide at the hub,and 0.85 X 5 =4.25 =4^ at the rim.
Large, well rounded fillets must be used where the rim and
arms meet at m. (See Fig. 1.) For very wide pulleys, it is al-
ways better to use two sets of arms. For small pulleys, under
12 inches in diameter, 4 arms are better than 6, as they are less
liable to break while being cast. Using 4 arms, the width of the
arm at h, in a pulley 10 inches diameter, may be i-J in.; pul-
ley 8 inches in diameter, if in.; pulley 6 inches in diameter, ii
inch; and the thickness and taper as given above. When
pulley arms crack from shrinkage in casting, the trouble may
usually be prevented by either increasing the thickness of the
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