# Gear coupling and clearance between pitch surfaces

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?

• do you expect much improvement over simply computing the PCD from tooth count and module? Aug 21 '15 at 21:55
• Yes, I do. Because I know that the coupling may simply jam if I set too tight. Aug 22 '15 at 6:16
• 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. Aug 22 '15 at 9:56

You may have to convert your module ($m$) to diametral pitch (inches) ($P$):
$P = \dfrac{25.4}{m}$
Your centre distance is of course: $\dfrac{mN_1}{2} + \dfrac{mN_2}{2} = \dfrac{(0.4)(24)}{2} + \dfrac{(0.4)(120)}{2} = 28.8 \text{ mm}$