# Gear pair with internal straight teeth

 Module = mn = = or Normal pitch in mm = pn = mn.p or Normal base pitch in mm = pb = pn.cosa Number of teeth = z = of Pitch diameter in mm = d = z.mn of or da+2.mn Tip diameter in mm = da = mn.(z-2) of d-2.mn or d- Face width in mm = b = 10.mn Tooth depth in mm = h = 2,16.mn of 2,16. for a = 14°30’ = h = 2,25.mn or 2,25. for a = 20° Addendum in mm = ha = mn or Dedendum in mm = hf = 1,16.mn or 1,16. (a = 14°30’) = hf = 1,25.mn or 1,25.  (a = 20°) Thickness below the tooth in mm = H = approx. 1,6.mn minimum Centre distance in mm = A = Root diameter = df = d+2,5.mn

## NOTES

At toothing of internal gear on a Fellows- or similar machine through gear cutters the obtained dedendum is 1.25 m instead of 1.16. m.

In a transmission with internal gears there are no special precautions to take when it comes to an internal gear with a large number of teeth and a pinion with a small number of teeth. On the other hand, where the teeth numbers approach each other at a reduction, there will be a moment that gripping is no longer possible.

One assumes that for a normal gear with pressure angle 14°30′, this difference must amount to at least 12 teeth; for stub gear (reduced tooth depth) and with pressure angle 20° this difference can be reduced to 7 teeth.

The chart below allows a quick way to define the maximum number of teeth of the cutter for cutting of gears with internal teeth.

Z0: maximum number of teeth of the cutter.
Z1: number of teeth of the gear with internal teeth.