Spur gear corrected tooth

Spur gear teeth are manufactured by either involute profile or cycloidal profile. Most of the gears are manufactured by involute profile with 20° pressure angle. When two gears are in mesh at one instant there is a chance to mate involute portion with non-involute portion of mating gear. This phenomenon is known as INTERFERENCE and occurs when the number of teeth on the smaller of the two meshing gears is less than a required minimum. To avoid interference we can have undercutting, but this is not a suitable solution as undercutting leads to weakening of tooth at its base. In this situation Corrected gears are used. In corrected gears Cutter rack is shifted upwards or downwards.

PCD and MOD

Main article: Gear § Standard_pitches_and_the_module_system

In the case of Module (MOD) 4.0 spur gears:

Normal spur gears (over 17 teeth) have a Pitch circle diameter (PCD) equal to MOD x Number of Teeth.^[1]
Corrected spur gears (under 17 teeth) have a PCD equal to MOD x Number of Teeth + MOD.

There are two types of corrected gears:

S0 gearing (x1+x2=zero)
S gearing (x1+x2 not equal to zero)

References

↑ http://www.metrication.com/engineering/gears.html

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