# Re: Internal Helical Gear

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Posted by DanO on November 26, 2002 at 15:03:14:

In Reply to: Re: Internal Helical Gear posted by James on November 25, 2002 at 21:06:10:

I understand the terminology issue in describing diameters. I had described for you what is the 'root diameter' in some circles, and you are looking for the inside diameter (the counterpart to the outside diameter). The classic way to consider addendum, dedendum, working depth and what we call the d+f or whole depth plus clearance of the External gear uses the proportion 1/Pn for addendum(Pn=Normal Diametral Pitch or 25.4/Normal module), 1.157/Pn for dedendum, 2/Pn for working depth, and 2.157 for d+f. This implies the equation for inside diameter of the Internal mate is P.D. minus twice the 1/Pn given in the working depth, leaving clearance of .157/Pn with the root of the external. This is described in Buckingham's Manual of Gear Design: Helical and Sprial Gears published 1937 by The Industrial Press. This was given for both 14 1/2 degree and 20 degree gears. Your I.D. then becomes P.D. of the internal gear minus 2 times the normal diametral pitch.

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