CIM + Belts Best practices

Let me start off with by priors. I love gears. I love chain & sprockets. Pulleys and belts? Not a fan.

I know I am wrong to be so biased against them but gears and sprockets have been good to me.

I am going to try to remedy that situation. I want to design and build a chassis using belts in the offseason. Detroit was a week ago… …time to turn the page and start driving CAD.

Where to start? Can anyone point me to some best practices for using a belt on a CIM shaft (or similar – Mini-CIM, NEO, Versa Planetary, CIM-sim, …).

What belt size? what pulleys? tensioning or tight by design? Point to good examples in CAD or photos is always helpful.

Help teach an old dog new tricks.

Thanks in advance.

Old Joe


HTD 5mm is often the standard in FRC. HTD 3mm is not uncommon, but is typically only used for the first stage on a 775pro gearbox.

I recommend tensioning every single belt and chain run you have - belt stretch/wear can happen, and additionally this helps to account for inevitable design mistakes.

Are you planning on using belts just to drive the wheels, or also in the gearbox? If it’s for the gearbox, 192’s 2014 drive gearbox is a favorite of mine:

They use the differing sets of mounting holes for the CIMS to tension the gearbox - one pair is +.001, another is +.003, and so on.

As far as source, VEX Pro has a decent stock, but if you’re after something in particular B&B Manufacturing is the way to go.

When designing belt paths, you can use WCP’s calculator for simple paths ( However, for more complex paths, you need to use the pitch diameters and a driving path length dimension (if you want a SW tutorial on this, I’m happy to make a follow-up post).


Great idea using the hole arrays to modify the belt tension.

How much tensioning to you typically have to add in? If you use discrete tension stages (like the hole pairs in 192’s drive), what is the spacing between steps?

If you’re example numbers weren’t pulled out of the air, then 192 would have +0.001", +0.003", +0.005", +0.007", 0.009". Oh, new question, would that be changes to CENTER TO CENTER distance or should the adjustments be in BELT LENGTH?

ANOTHER QUESTION. when I work with chain, I work hard to make sure I get at least 120 Degrees of wrap on the sprocket (it isn’t chiseled into stone or anything but it is a good rule of thumb - if you have less that that you are going to need to have the chain super tight and even then you are probably going to regret not having more at some point in the season). Is there a similar minimal wrap angle for belts?


Typically the additions are measured in the C-C distance. I definitely pulled the numbers out of thin air. Most likely, I lowballed the adds there - it’s possible that they’re adding as much as .01" on each step, and this would likely be empirical data. There might be something more helpful in the thread - I believe they also posted one of their master sketches in the thread I linked.

120 degress is a pretty good mininum wrap angle for belts. Due to the tooth profile, you can be slightly more aggressive on low wrap, but more wrap is always better, especially in high torque or cases with shock loading, to prevent slippage.

5 of the 12 pinion teeth in engagement is a good rule of thumb, I think. That limits minimum belt length as illustrated below. [Use the calculator that ClayTownR linked above.]

Shorter belts may be ok if you use a tensioner. We have used exact C-C (adding 0.001 inch or 0.03 mm) without tensioners, but only for short belts as shown below.

Why are my spidey senses tingling?

The only CIM shaft belt pulley I can find is this one from AndyMark?
# 12 Tooth Pulley for 5 mm HTD Belt am-3263

If belts on CIMs are such a good idea why does Vex, REV, WCP, etc sell versions as well???


Manufacturers’ design guides are generally a good place to start.

Look here.

WCP-0076 is similar to the Andymark pinion you linked.

Or WCP-0077 for a 15mm wide version (which is probably overkill, but still)

Perfect. Thanks. Not sure why I didn’t see that WCP pulley. Probably just the photo only showed hex bore.

I feel a lot better knowing that WCP has a CIM shaft pulley. I am all for trying new things but when you are trying to play catch up, starting from a design philosophy that is too far outside the main stream feels like a recipe for failure.

Keep the tips coming. I have a lot to learn…

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When we are building with many of the same pulley; e.g., for a drive train on multiple robots, we buy timing pulley stock and use a lathe to part-off the pulleys needed. Stock with tooth counts that are multiples of three will center on a typical lathe chuck nicely. If flanges are required we make them from thin plastic and attach using small screws into the stock face. [Note: flanges on the ready-made pulleys are not integral; they are press-fit sheet metal that will pop off easily when shock loaded.]


cough cough The kitbot chassis uses belts cough cough
You could use that? Maybe?

You can also just use this adapter to use any 1/2 Hex pulley. We did it in 2018 with our swerve and had zero issues

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