gear

You can usually tell the pitch P by laying the gear on a tooth-size chart (there is one in Machinery's Handbook, p. 1769 in the 23rd ed., and some gear catalogs have them). If there's any chance that it's a metric module gear, be careful that you use the right chart.

To find the diameter is a little trickier. Does the machine part, or the mating parts of the machine, have a center that your gear teeth pivot about? You can measure the radius from this. Otherwise you can calculate it geometrically. The addendum (distance from the pitch circle to the outer diameter of the gear) is basically the reciprocal of the pitch. So, for example, a 16-pitch gear would have an addendum of about 1/16 inch. If you trace a pitch line along the existing teeth and then measure the chord length c between the ends and the and the height h from the center of the chord to the center of the arc, then you can calculate the diameter D from

D = (c^2 + 4h^2) / 4h

Recall that the theoretical whole gear must have an integral number of teeth N and that D = N/P, so see what integer value of N gives the closest value of D to your calculation and use that as your pitch diameter.

Determining the pressure angle is really tricky. There are charts for checking gears by putting rods of a known diameter between teeth on opposite sides of the gear and measuring the outside dimension, but the difference between the measurements of a 14-1/2 deg. gear and a 20 deg. gear is so small that it is really difficult to determine the angle without a doubt, especially on used gears. Short of meshing the gear with another gear of known pressure angle, I've never really found a foolproof method of determining this. Perhaps someone else in the group has a clever method?

Don Kansas City

thanks don carrey