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GEAR TEETH. 387
Involute Teeth.
Suppose a strap is fastened at a and b on the two round discs
in Fig. 2. If the disc b is turned in the
direction of the arrow, the strap will move
in a straight line from c, toward d. This
motion will cause the disc a to rotate with
exactly the same surface speed as the disc
b, but in the opposite direction.
Suppose, further, that to the under side
of the disc a (see Fig. 3) is fastened a piece
of sheet brass ft, or other suitable material
of somewhat larger diameter than disc a,
and that a scratch awl is fastened in the
strap at the point m ; then by turning the
disc b in the direction of h to b, and
the strap moving with it, being kept tight
by the resistance of disc a, the scratch awl
will trace on the brass plate the curve from
m to k, but if the discs are moving in the
opposite direction, the scratch awl will
trace the curve from m to K. Take an-
other brass plate and do the same thing with the other disc, and
a similar curve will be produced. In these two brass plates the
stock may be filed away carefully, following the curves as shown
in Fig. 4. The discs are laid to match each other and free to
—-R
fig.
fig. 3.
swing on their centers ; turning the disc a in the direction of the
arrow, it will give motion to b, and both discs will move with the
same speed in exactly the same manner as if they were connected
by the strap as shown in Fig. 2.
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