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ELEMENTS OF METRIC GEAR TECHNOLOGY
There is a requirement of a minimum blank
width to make a helical gear span measurement. Let
bmin be the minimum value for blank width. Then
b min = sm sin bb + Db (10-5)
where bb is the helix angle at the base cylinder,
bb = tan –1 (tan b cos at)
= sin –1 (sin b cos an) (10-6)
From the above, we can determine that at least 3mm
of Db is required to make a stable measurement
of sm.
10.3 Over Pin (Ball) Measurement
As shown in Figures 10-6 and 10-7, measurement
is made over the outside of two pins that are
inserted in diametrically opposite tooth spaces, for
even tooth number gears; and as close as possible
for odd tooth number gears.
T-88
d
dp
Fig. 10-6 Even Number of Teeth
dm
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sm
b min bb
Fig. 10-5 Blank Width of Helical Gear
dp
Fig. 10-7 Odd Number of Teeth
The procedure for measuring a rack with a pin or a ball is as shown in Figure 10-9 by
putting pin or ball in the tooth space and using a micrometer between it and a reference
surface. Internal gears are similarly measured, except that the measurement is between the
pins. See Figure 10-10. Helical gears can only be measured with balls. In the case of a worm,
three pins are used, as shown in Figure 10-11. This is similar to the procedure of measuring
a screw thread. All these cases are discussed in detail in the following sections.
Note that gear literature uses “over pins” and “over wires” terminology interchangeably.
The “over wires” term is often associated with very fine pitch gears because the diameters
are accordingly small.
dm