I am trying to install an optical encoder that interfaces with a conveyor belt's drive shaft. The current setup is as follows: The central DC motor has a left-handed helical gear. This central helical gear interfaced with a right-handed plastic helical gear with the following dimensions:

Teeth #: 38 teeth

Outer Diameter: 34 mm

Bore: 6mm

I calculated the Module as OD/# of teeth + 2, ignoring the cosine of the helical angle (most vendors ignore it anyways). This gives me a module of 0.85, which is very uncommon, and I can't find anything online like it!

Here are my questions:

  1. Am I calculating the Module correctly?
  2. Is 0.85 Module very uncommon where it's most likely custom-made? or am I just looking in the wrong places?
  3. Could an imperial-to-metric conversion be the problem with why I am not getting a whole number for the module?


  • 1
    $\begingroup$ Honestly. look harder. Really hard. It may not be easy or cheap as gears are infinite and varied, and many are not common stock while others are custom. You are already into rare gear types (helical), and a plastic one at that. An unusual combination. I'm surprised you even managed to find plastic helical gears in for sale, to be honest so I am not surprised you can't find a matching pitch. You might need it custom made. $\endgroup$
    – DKNguyen
    Commented Nov 29, 2022 at 15:43
  • $\begingroup$ @jsotola, I am so sorry, not sure why my questions did not get published earlier on. I edited my post and added my questions. Thanks for your willingness to help! $\endgroup$ Commented Nov 30, 2022 at 15:44
  • $\begingroup$ @DKNguyen, thank you! This gives me a good perspective on how uncommon this might be. I can now stop banging my head against the wall and go about 3D printing my own or something. I am not experienced with gears and was hoping that there is some sort of standard that I don't know about. $\endgroup$ Commented Nov 30, 2022 at 15:50
  • 1
    $\begingroup$ You might have calculated the module wrong. It is not OD/# teeth, it is the pitch diameter / number of teeth. The pitch diameter is not the same as OD, it is somewhere in between the OD and diameter at the root of the teeth. $\endgroup$ Commented Dec 12, 2022 at 9:06

1 Answer 1


Considering how rare this helical gear might be, I decided that I needed to make my own. Luckily, I can get away with a plastic version so I 3D printed a design that I made from scratch. It worked!

  • $\begingroup$ If you want to go convert a 3D printed object to metal: youtube.com/watch?v=GJOOLH9ZP2I $\endgroup$
    – DKNguyen
    Commented Dec 15, 2022 at 14:39
  • $\begingroup$ @DKNguyen, this is super useful, thank you for sharing! $\endgroup$ Commented Dec 16, 2022 at 16:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.