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3^8 SHAFTING.
The reaction at the pitch line between C and B, is also
21S8 pounds; therefore, the total pressure (besides the weight
of C, which is omitted in this calculation ) on both bearings will
be 2 X 2188 = 4376 pounds and the pressure of each bearing of
C wil« be 2188 pounds. Allowing a pressure on the bearings of
100 pounds per square inch, the necessary bearing surface
will be
91 go
— = 21.S8 square inches for each bearing.
Assuming the length of the bearing to be twice its diameter,
D X 2 D = 21.88
^21^8
2
D =Vio.y4
D — o.3 inches.
Calculating the size required with regard to transverse
strength by the formula on page 360,
3
D = A/
1 X 4376
_ = 3.12 inches.
144
Thus, a shaft 3.3 inches in diameter is Of ample size for
strength. The surface velocity of this shaft will be,
3.3X3.1416X40 ^,^
12
per minute, and at that velocity a pressure of 100 pounds per
square inch of bearing surface is very safe from liability of
heating if the bearing is well made and amply provided with oil.
Proportion of Keys.
The breadth of the key is usually made to be one-fourth of
the diameter of the shaft, and the thickness to be one-sixth of
the diameter of the shaft.
Keys and key-ways are usually made straight and should
always be a very good fit sidewise. Frequently set-screws are
used on top of keys in mill gearing. Sometimes in heavy ma-
chinery keys are made tapering in thickness, usually one-eighth
inch per foot of length. A corresponding taper is made in the
depth of the key-way in the hub. Key-ways in shafts are always
made straight.
For light and fine machinery taper keys are never used.
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