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388 GEAR TEETH.
The curve on these two discs represents the form of a gear
tooth in the involute system.*
The line h g, Fig. 4, is called the line of pressure or the line
of action. The circles, P and P, are the pitch circles. The line
B R shows the direction of motion of the teeth at the moment
they are passing the center line, c c.
Approximate Construction of Involute Teeth.
It will be noticed that the line of pressure, h g, forms an
angle with the line B R. This angle is usually taken as 14>£
degrees. This makes the diameter of the base circle, £, (see
Fig. 5) equal to 0.968 times the diameter of the pitch circle.
The base circle gg, in Fig. 5, corresponds to the disc in Fig. 3,
and the line of pressure in Figs. 5 and 6 corresponds to the strap
in Fig. 2. The line of pressure, h g, Fig. 5, is 75^ degrees to
the center
line,/c.
A perpen-
dicular i s
ere c t e d
from the
line h g,
through the
center, c.
Using the
point of in-
tersection
at i as cen-
ter, the
tooth is
drawn sim-
ply bya cir-
cular arc. This will, in practical work for small gears having
more than twenty teeth, correspond nearly enough to the
true involute, which was illustrated by means of the strap, disc
and scratch awl, as explained in Figs. 2, 3 and 4.
When the gear has less than twenty teeth, and is constructed
by circular arcs, as shown in Fig. 5, the top of the tooth will be
too thin ; but the top of the tooth will be too thick to clear in
the rack, if the true involute curve is used.
When the teeth are of true involute curve, a smaller gear
than twenty-five teeth will not run freely in a rack having straight
teeth slanting 14^ degrees. (See Figs. 6 and 7). Therefore,
*The way to actually draw this curve on paper by means of drawing instru-
ments is explained on page 192. This way explained here, using the disc on the
strap, is merely for illustrating and explaining principles, and serves well for that
purpose, but would be inconvenient to use in actual construction of gear teeth. In
actual v/ork one tooth is carefully constructed, and templets and cutters are made
and used, as was explained for Cycloid Gears, pa~e 385.
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