I have a couple of spur gears (0.4 mm module, ISO8/AGMA9 precision, 24 and 120 teeth). I want to make their backlash as small as possible. But also I don't want them to jam. Where can I get the normal clearance between their pitch surfaces (and the tolerance of the clearance)?

Maybe there is some handbook to use?

  • $\begingroup$ do you expect much improvement over simply computing the PCD from tooth count and module? $\endgroup$ Aug 21, 2015 at 21:55
  • $\begingroup$ Yes, I do. Because I know that the coupling may simply jam if I set too tight. $\endgroup$
    – Felix
    Aug 22, 2015 at 6:16
  • $\begingroup$ If you're using the computed PCD as in @pauloz1980's answer, correctly made gears won't be too tight. If you want to reduce PCD below this to reduce backlash further, look at how clock and watchmakers adjust PCD, using a "depthing tool" to test the running of the actual gears before drilling the pivot holes in the watch. $\endgroup$ Aug 22, 2015 at 9:56

1 Answer 1


The Machinery's Handbook is pretty amazing when it comes to this sort of thing. Go to pg2067 and that's the section on backlash. Depending on whether your spur gears are coarse or fine pitch the tolerance and "recommended or proper" amounts are shown. These are Table 1 (pg2068) and Table 3 (pg2072).

You may have to convert your module ($m$) to diametral pitch (inches) ($P$):

$P = \dfrac{25.4}{m}$

Your centre distance is of course: $\dfrac{mN_1}{2} + \dfrac{mN_2}{2} = \dfrac{(0.4)(24)}{2} + \dfrac{(0.4)(120)}{2} = 28.8 \text{ mm}$

Also your module is quite small! You must be dealing with tiny gears - if not then I'd double check your calculations.


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