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BEVEL GEARS 397
The gears according to preceding table are supposed to have
6 arms. The arms are tapered in width ^ inch per inch toward
the rim. The thickness of the arms is half of their width, and
the section of the arms elliptical as shown in Fig. 10. The arms
are provided with rounded fillets both at the hub and at
the rim.
Width of Gear Wheels.
Gears with cast teeth are usually made narrower than gears
with cut teeth. In spur gears with cast teeth it is customary to
make the width of the gear four to five times the thickness of
the teeth, or twice the circular pitch.
Width of Gears With Cut Teeth.
The following rule is recommended by Brown & Sharpe
Mfg. Co. in their " Practical Treatise on Gearing":
Divide eight by the diametral pitch, and add one-fourth inch
to the quotient; the sum will be the width of face for the pitch
required.
Example.
What width of face is required for a gear of four pitch?
Solution
:
Face = f + \ = Z% inches.
For change gears on lathes where it is desirable not to have
faces very wide, the following rule may be used
:
Divide four by the diametral pitch and add one-half inch.
By the latter rule a four-pitch change gear would have but
a 1^-inch face.
BEVEL GEARS.
Fig. 1 1 is a diagram showing how to size bevel gear blanks.
First, lay off the pitch diameters of the two gears, which
may be calculated according to diametral pitch or to circular
pitch ; second, draw the pitch line of teeth ; third, lay off on the
back of the gear the line a b, square to the " pitch line of teeth
:"
fourth, on the line a b, lay off the dimensions of the teeth exactly
in the same manner as if it was for a spur gear.
If the gear is calculated according to circular pitch, find
dimensions of teeth by formulas on page 375, but if^ the gear is
calculated according to diametral pitch, find dimensions of teeth
in Table No. 49.
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