Gear pair with internal straight teeth


Module = mn = Image7 = Image8 or Image9
Normal pitch in mm = pn = mn.p or Image10
Normal base pitch in mm = pb = pn.cosa
Number of teeth = z = Image11 of Image12
Pitch diameter in mm = d = of Image13 or
Tip diameter in mm = da = mn.(z-2) of or d- Image14
Face width in mm = b =
Tooth depth in mm = h = 2, of 2,16.Image15 for a = 14°30’
= h = 2, or 2,25.Image15 for a = 20°
Addendum in mm = ha = mn or Image15
Dedendum in mm = hf = 1, or 1,16. Image15 (a = 14°30’)
= hf = 1, or 1,25. Image15 (a = 20°)
Thickness below the tooth in mm = H = approx. 1, minimum
Centre distance in mm = A = Image16
Root diameter = df = d+2,


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.