How to attach timing belt pulley to CIM shaft

To run our intake rollers, we are planning to use a Mini CIM with timing belt pulley. The timing belt pulley is 18 tooth with 1/2" hex bore. We have a 8mm to hex shaft adapter. The problem we have is figuring out how to keep the adapter and pulley from sliding off the shaft. There is no set screw. Ideas we’ve had are:

  1. JBWeld it all together
  2. Drill through it all and insert a roll pin
  3. Drill and tap a set screw in pulley that extends also through adapter.

None of those sound great, hoping someone has a better idea.

Here’s what we are working with:

CIM to Hex Adapter -
Mini CIM -
18T x 9mm Wide Double Flange Aluminum Pulley (HTD 5mm, 1/2” Hex Bore) -

Have you seen how the motor pinion gears are retained on the kit of parts drivetrain gearbox? Might need to add a washer, but it should work…(also, be sure to add a spacer between the motor and the pulley, and use a key in the keyway)




use a lathe to cut a groove on the outside of the CIM to Hex Adapter, and use E-clips to secure the pulley.

That push on retaining ring looks like a simple solution to try. Sure wish I would have asked this question about 4 hours earlier, could have easily added it to my AndyMark order that just got packed.

you can probably find some type of 5/16" push nut at a local hardware store, it’s not a rare part.

You may need to gear the CIM to get the optimal speed on your intake. Check out the CIM to Versaplantary adapter


You’d need to cut the shaft or put a hole in the back plate of the nut, but yes.

An 18t HTD timing pulley pitch diameter is 1.128", so radius is 0.564". A minicim running 6krpm would be 100 rev/sec, so belt speed would be .564 in* 100 r/s * 2pi radians/rev * 1ft/12in = 29 ft/s. That sounds like a good intake speed to me; about twice the speed of a fast robot.

We used an 8mm shaft collar the last time we did something similar, it was easy and had no issues!

I’m pretty sure we still have some in the shop (even some spare hex adapters, probably) if you’ve got anyone coming up to the Cities anytime soon.

Edit to add picture:

1 Like

The gear ratio with the belt pulley’s is 2:1 and spinning 2" PVC pipe. The prototype we built in week 2 threw the power cells in a little two fast, but the motor was running 100%. We hope we can control the motor speed to get the power cells to land in our hopper.

During design I suggested to the team the robot should be able to drive 100% speed at a non-moving power cell and pick it up, not drive over it, and they did the math to come up with what we have. We doubt power cells will be sitting still much, but we defined our goal in way that was easy to understand and achieve.

As I type this, I just had the thought that there must be a max RPM at which different sizes and lengths of PVC pipe can spin before it shatters and becomes a projectile. Now I have something else to research…

1 Like

On a completely unrelated note… how thick is that polycarbonate? It looks like at least 3/8" to me. Seems like overkill?

Maybe it was a “hardened” design, for traveling over defenses in 2016?

But the drive is mecanum, the opposite of hardened

1 Like

2017 Steamworks, so no worry about defenses :slight_smile:

Yes, 3/8", and yes, probably overkill (especially since we ended up not using it at all during the season!). But we had the weight to spare, and it’s better to build something you know won’t break than it is to build on the edge and end up suffering :slight_smile: Here we are almost 3 years later, and it still works great and is a great demo robot (kids love trying to catch the hail of fuel it spits out).

1 Like