the Diametral Pitch of herringbone gear?

Question

what is the difference between 8D.P and 8D.P linear?
what is the difference between 20(degree) P.A and 20(degree) P.A linear?
what is normal HELIX ANGLE for the herringbone gear?

Answer 1

Diametral pitch DP is an inverse length measurement; the number of teeth divided by the diameter; a 40-tooth gear with a 5-inch diameter has a DP = 40/5 = 8 per inch of diameter. Circular pitch CP is a length measurement; the circumference divided by the number of teeth; the same gear would have a CP = 5*pi/40 = .3943 inches of circumference. See ref. 1. CP is sometimes referred to as a linear measure since it is of length rather than inverse length.
Refs. 2 and 3 contain much information on gear theory. You may find a normal helix angle value in those. Ref. 4 is a large collection of documents and links including some on gear design and industry standards.