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SHAFTING. 367
Distance Between the Bearings.
Jack shafts should always have bearings as near the pulleys
as possible.
Ordinary line shafts, as given in Table No. 41, and shafts
for simply transmitting power, may have the distance between
the hangers as given in the following table :
TABLE No. 43.
Diameter of Shaft in Inches. iy
2 to ik 2 to 2 l
/
2 2^ to 4
Distance between bearings in feet 6/
2 8 10
Shafts for Idlers.
Shafts for idlers (see C, Fig. 1) have very little torsional
stress and the distance between the bearings may also be very
short, so that even with a great transverse load such a shaft
may be of comparatively small diameter as far as requirements
for strength is concerned. In such a shaft there is great danger
of trouble from hot bearings ; therefore, in designing, it is very
important to make its diameter and the length of the bearing of
such proportions that excessive pressure per square inch of
bearing surface is avoided.
Example.
Twenty-five horse-power is
through idler C. ( See Fig. 1 ).
are 36 inches in diameter and
make 40 revolutions per minute.
What is the necessary diameter
of shaft C, which is supported
by two bearings one foot apart
and carrying a gear 48 inches
in diameter placed at the mid-
dle between the bearings.
Solution
:
The velocity on pitch line of gear A will be
40 X 36 X 3.1416
to be transmitted from A to B
The gears on shafts A and B
Fig.
12
= 377 feet per minute.
25 horse-power = 33,000 X 25 = 825,000 foot-pounds.
The pressure at the pitch line of A transferred to Cwill be
825000
377
2188 pounds.
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