6. All circular measurements of a stub tooth gear,
STUB TOOTH GEARS
including the number of teeth and the pitch
Stub tooth gears are widely used throughout the
diameter, are the same as those of a spur gear.
automotive industry in transmissions because their
To find the pitch diameter, divide the number of
great strength enables them to transmit maximum
teeth (NT) by the diametral pitch (DP).
power. Cranes and rock crushers are examples of
type of gear has a 20-degree pressure angle and is short
and thick. A stub tooth gear compared to other gears has
7. To find the circular pitch (CP), divide ! (3.1416)
a shorter addendum (ADD). This results in a stronger
by the diametral pitch (DP).
tooth, but causes the gears to operate with more noise.
Stub tooth gears come in two forms. One form has
straight teeth, like spur gears. The other form has teeth
similar to those on helical gears. Gears with helically
8. To find the circular thickness (CT), divide
shaped teeth are used when smooth operation is
1.5708 by the diametral pitch (DP).
The basic rule for spur, helical, and bevel gears, "for
every tooth on the gear, there is a circular pitch (CP) on
the pitch circle" also applies to stub tooth gearing
FELLOWS STUB TOOTH
We will discuss two stub tooth gearing systems: the
American Standard System and the Fellows Stub Tooth
This system was introduced by the Fellows Stub
AMERICAN STANDARD SYSTEM
Tooth Gear Company. It uses a 20-degree pressure
This system bases tooth dimensions on specific
angle and is based on the use of two diametral pitches
(DP). In the formulas we will use, the numerator (DPL)
is the circular measurement which consists of the pitch
1. The tooth depth or whole depth (WD) equals 1.8
diameter (PD) and the number of teeth (NT). The
divided by the diametral pitch (DP).
denominator (DPS) is the radial measurement.
There are eight standard pitches in this system.
They are 4/5, 5/7, 6/8, 7/9, 8/10, 9/11, 10/12, and 12/14.
2. To find the outside diameter (OD), add 1.6 to the
number of teeth and then divide by the diametral
The formulas for the basic dimensions are as
3. To find the addendum (ADD), divide 0.8 by the
diametral pitch (DP).
4. To find the clearance (CL), divide 0.2 by the
diametral pitch (DP).
5. When the addendum (ADD) is added to the
clearance (CL) the result is the dedendum
DED = ADD + CL