Addendum Modification and Addendum Modification Coefficient.

With the pinion gear normally having fewer teeth than its mating gear, it has a disadvantage of being the weaker of the two components.  To compensate for this inherent weakness, the addendum of the pinion can be made stronger by increasing its tooth thickness.  The pinion’s mating gear is normally adjusted to compensate, thereby making the gear's tooth thinner.  The process that controls this adjustment also varies the length of the addendum and dedendum.  If the pinion's mating gear does not compensate for this adjustment then the center distance of the pair will be enlarged or reduced.  Under certain circumstances, this can be used to the designer's advantage.

It is recommended that the designer think in the terms of the coefficient rather than the simpler length adjustment.  Once the designer is accustomed to thinking in terms of the coefficient, then the size of the diametral pitch (or Module) does not need to be considered.  For example, a 0.25 addendum modification coefficient will affect a 12 diametral pitch gear the same way it will affect a 24 diametral pitch gear, proportionally for the size of diametral pitch.  The simpler addendum modification is a derivative of the diametral pitch (or module) and the coefficient.

Also see the Balance Addendum Modification section.