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404 WORMS AND WORM GEARS
Increasing the diameter of the worm decreases the angle of
the teeth in the worm gear.
Decreasing the diameter of the worm increases the angle of
the teeth in the worm gear.
This angle is most conveniently obtained by drawing a dia-
gram as shown in Fig. 13.
Draw a line / m, equal to 3^ times the length of line b; this
line will be equal to the length of the circumference of the pitch
diameter of the screw. Erect the perpendicular, m o, equal to
the lead of thescrew. Connect the points / and by theline / o y
and the angle s is the angle of the teeth on the worm gear.
Caution.—When cutting a worm gear, be careful not to
lay the angle of the teeth in the wrong direction.
The diameter of worm gears is usually calculated according
to circular pitch, for convenience in cutting the worm with the
same gears as used for ordinary screw cutting in a lathe.
When a worm gear has comparatively few teeth, the flank
of the tooth will be undercut by the hob; to prevent this in a
measure, it is customary tojiave the blank somewhat over size,
so that from five-eighths to three-fourths of the depth of the tooth
may be outside the pitch line.
The form of teeth is usually involute, and the thread
on a worm screw is constructed of the same shape as the teeth
in a rack. Fig. 14 shows the shape of tooth and the table
gives the dimensions of finishing tool for the most common
pitches.
The surface speed of a worm screw ought not to exceed 300
feet per minute.
Table No. 5 2 is calculated by the following formulas. (See
Fig. 14.)
fig. 14
P– Circular pitch.
N- 1
’’
P
3.1416
M —
P
/ =4=-
a —: P X 0.3183
d =: P X 0.3683
D =-a + d
S = P X 0.5
3 = P X 0.31
C = P X 0.335
h = ^ + 4
k = P X 0.1
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