Gear Terminology: Pitch circle and module

module = d/T

where, d= pitch circle dia & t = no of teeth.

So, if module or d changes the centre to centre distance between gears will change. If this deviation is within limit, then gears being having involute profile it won't matter much. but if it is beyiond tolerance zone it will cause excessive digging in at tooth base.

If a gear train is existing, there is no way to change the 'pitch circle dia', from its original value.

Module, yes, you can go for different value. For a particular pitch circle dia, you can go for less or more number of teeth depending up on the size/standard of gear tooth geometry.

First I will make the assumptiont that we are talking about involute gears.

Pitch circle is the circles defined by the point of tangency of two meshing gears. Module is millimeters of pitch diameter per tooth and it defines the tooth thickness in millimeters at the point of tangency. Two meshing gears must have the same module and the ratio is determined by the number of teeth in the respective gears. Module can not change. The respective pitch circles can change by increasing or decreasing the center distance. The ratio of the pitch circles can not change.

The bottom line is once the gears are manufactured the module can not change. The pitch circles can change by varing the mounting or center distance. The net result is that the amount of backlash or free movement of the gear set is the only thing that changes if the pitch circles deviate from the original value.

Yes. For example, if you half the module and double the number of teeth, we change the module without effecting pitch circles or center distance. In redesigning an existing gearbox, this could be useful. d=m*t, so d also equals (m/2) * 2t. And of course, by doubling all teeth, gear ratios the same. We could also subtract 1 driver tooth as long as we add 1 driven tooth. In this case you ratio changes to (n+1)/(n-1) - module and center distance remains unchanged. Helical gearing can change your pitch circles witout effecting your module or gear ratio by introducing a helix angle, but center distance changes as a function of cosine of the helix angle.

The module is fixed by the tool you use to make the gear. So, why there would be a variation?

You are right. The module is what it is. It don't change. The ratio don't change. The only thing that can vary is the backlash or interference at the gear mesh. If it changes to much you lose conjugate action.

Yes. My last reply obviously entails new sets of gears. Of course. Once a gear is cut, ya can't change the pitch!