Lathes Page: 86
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53-55 AIR CORPS
c. The size of gear teeth is commonly measured by what is known
as diametral pitch. This is the number of teeth contained in a pitch
diameter of 1 inch. For example, if there are 12 teeth in this measure-
ment, the diametral pitch is 12; if there are 8 teeth the diametral pitch
is 8, etc. In figure 76, there are 16 teeth of 8 diametral pitch in the
small gear, making a pitch diameter of 2 inches, and 24 teeth of the
same diametral pitch in the large gear making its pitch diameter 3
inches. From this calculation it may be readily seen that the ratio
of the gears illustrated is 2 to 3 or 1 to 1.5. The following formula
may be applied to any spur gear:
. Number of teeth
Pitch diameter= m
d. By referring to figure 76, it may be noted that a certain portion
of the gear tooth extends beyond the pitch circle. This portion is
known as the addendum. The height of the addendum is always
equal to one divided by the diametral pitch. In order to obtain the
outside diameter of a spur gear blank, two addenda must be added
to the pitch diameter; therefore:
Outside diameter= pitch diameter+2 addenda
_number of teeth +2
54. Helical gears.-a. The teeth of helical gears are cut across
the outer rim of a gear blank similarly to screw threads, and the
angle of cut is referred to as the helix angle. Helical gears may be
used to connect either parallel shafts or shafts set at any noninter-
secting angle. They have greater strength than spur gears for a
given size as there are more teeth in mesh at any one time. For this
same reason, they are also more quiet and smooth in operation.
b. The same general rules that have been given for plain spur
gears are also applicable to helical gears, although as the angle of
spiral increases the pitch diameter also increases. The pitch diameter
of helical gears may be calculated by means of the following formula:
number of teeth
Pitch diameter= secant of helix angleber of teeth
normal diametral pitch
c. As in spur gearing, the outside diameter of a helical gear is ob-
tained by adding two addenda to the pitch diameter.
55. Bevel gears.-a. Gears of this type are used to transmit
power when shafts are not parallel. They can be made to set at any
angle, although the 90 position is the one most commonly used.
Right angle bevel gears that have the same number of teeth are
called miter gears.
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/88/: accessed April 28, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.