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STRENGTH OF GEAR TEETH 415
Note : First that the gear is cast iron and that the pinion is
machinery steel ; therefore, on account of the material, the teeth
in the gear are the weaker. If both the gear and the pinion had
been of the same kind of material, the pinion would, on account
of the shape of its teeth, have been the weaker.
Also note that the sizes are given in diametral pitch and the
strength must, therefore, be calculated by table No. 57.
The value found in the table for ioo teeth, at a speed of
1800 feet per minute, is 741 pounds, and at 2400 feet per minute
630 pounds, a difference of in pounds. Therefore, by inter-
polation of these values we have :
164 X in ,
741 2 = 711 pounds.
Thus, the value corresponding to a speed of 1964 feet per
minute is 711 pounds.
711 X 2^-
The working strength = = 444 pounds.
In the above calculation we multiply by i\ because the face
of the gear is 2\ inches, and we divide by 4 because the gear is 4
diametral pitch.
Example 2.
Find the working strength and also the horse-power
of a cast iron spur gear having 20 teeth, 3 inches circular pitch
and 7 inches face, running at a speed of 600 feet per minute.
Solution :
The value given in table No. 56 for a cast iron gear of 20
teeth and one inch circular pitch, running at 600 feet per minute,
is 360 pounds, or 6.54 horse-power.
Thus
:
Working strength =360 X 7 X 3 = 7560 pounds.
Power transmitted =6. 54 X 3 X 7 = 137 horse-power.
Example 3.
If a pair of cast iron mitre gears, 3 diametral pitch, 36 teeth,
2 inches face, 14! degrees involute form of teeth, run at a speed
of 1200 feet per minute, what is the safe working strength and
how many horse-power will they transmit with safety ?
The large pitch diameter is 12 inches and the small pitch
diameter is 8£ inches (very nearly). The formative number will
in this case be 1.4 X 36 = 50 teeth. In Table No. 57 we find
that the constant for 50 teeth at a speed of 1200 feet per min. is
844 pounds and the horse-power is 30.7. Thus:
Working strength = 844 X 2 X 8| = ^ pounds>
3 X 12
Power transmitted = 3°-7 X 2 X 8^ _ horse-power.
3X12
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