3. Operation at normal (reference) center distance a

External spur gear

Consider figure 7 . We have shown a normal gear on the left, and a corrected gear with no variation in center distance on the right.

The common center distance is the reference center distance a; the reference pitch circles are merged with the operating pitch circles; the operating pressure angle corresponds to the pressure angle α ₀ of the generating rack. On the primitive circles, the pinion and wheel teeth have the same pitch: π m ₀ .