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how do i match gears that will work together by gear tooth/ spur sizes, i have no clue? Answered

hello, i recently bought some shaft gears for the motors for my r/c tank project, in case you don't know, a shaft gear is the spur gear that goes onto the motor! anyway, i am back to start working on my tank again, and am searching for some large gears , but i have no idea aside from physically comparing the two, of what gear is the correct size for my gear. how do i figure out if two spur gears are compatible or not? someone please help!


What have you bought ? You need to match the Modulus, if they are metric gears, or the DP if they are imperial.


Hi Steve,

I have a quick question. How do you calculate the modulus of a rack (shown below)? As I understand, modulus = (reference diameter) / (number of teeth). However, the reference diameter is infinite for a rack which means the modulus is also infinite. Is this correct? I have a feeling I'm missing something


Joe K


Hi Jak,

Modulus is then calculated by considering the spur that drives the rack.

ah, i get ti now, i just searched the ebay listing and it has a modulus of 0.6. does this mean it will only be compatible with other gears, with a modulus of 0.6?

for some reason i cant find any secondary gears ( big gear that turns a wheel, being turned by small motor gear), using the key words, modulus or mod 0.6, at least not for under $20 each, any ideas?
after ill give you best answer

How many teeth are you looking for ? Now you know the Mod, you can work out all you need gear-ratio wise.

I have some sources, but they are all in the UK.

Of course, you CAN make gears, if you have the right tools - a dividing head and a milling machine, its tedious though.

I'd love to have a small gear making machine at work. We use a lot of gears in the 0.6 ..1 mod.