pic: Over the rail delrin gearbox

This is an over the rail 3 cim/minicim gearbox that we have been working on this offseason. The plates will be cut on our lasercutter from delrin.

It’s got a first stage reduction of 12:72 and a second stage reduction of 42:48.

Here is the link to the model:

Looks like a cool way to save space! What is the stuff between your two red plates? Why use it over just spacers? What are the yellow things poking from your outer red plate?

Overall, Great gearbox!

My educated guesses:

  1. 1/2" square aluminum tubing, makes for an easy spacer
  2. 10-32 push to connect air fittings, for cooling motors between matches.
  1. Yes, and also adds stiffness since the plates are delrin.
  2. Or during the match :wink:

Based on the McMaster part numbers, it’s pneumatic fittings that attach to a hollow screw. Teams have used these in the past to provide airflow through the CIM casings to try and cool them from the inside.

EDIT: got sniped pretty bad

Looks pretty cool! Once concern though, have you taken into consideration the tolerances of the aluminum extrusion?

Looks neat! I get 6.83:1 as the total gear ratio, which makes sense for a 3 mini-cim gearbox. Further, there’s easy variation up or down by changing the two gears in the second stage.

A few more questions:

  • Why is there an extrusion spacer at the top but not the bottom? Is there any reason the bolts on those spacers aren’t more evenly spaced around the gearbox? (They’re much closer together at the corners)
  • Why is the bull gear (output gear) lightened when the cluster gear is solid? The forces/torques of the two gears are only 14% different, and it’s the bull gear which is likely to be taking the shock loads.

That’s VEXpro’s VersaKey geometry. Some gears only come with VersaKey.

  • The bottom of the gearbox will be flush against the drive rail (we run chain in tube) which will provide the flex support that the other extrusion spacers provide. The holes are much closer on the bottom because those bolts are what will be used to mount the gearbox directly to the tube.
  • What Ty said.

Hey Matt,

Looked over your design last night and it’s really cool! We’ve been trying to find a way to “fine tune” a single speed and this is a great solution (and some ideas we may adopt into our own potential design). With all the gears available from VEX these days, there are so many gear set options that add up to 90 (or some other number). Everything I saw made sense except for two things:

  1. what holes are you using for mounting to the tube? I see 4 different hole patterns: 1) the clearance holes for 10-32 on the outsides, 2) clearance holes for a 1/4" bolt, 3) tapped holes for 10-32 with a 1.875" c-c distance (looks like it could mate up with a WCP bearing block), and 4) the four-hole pattern around the output bearing.

  2. the 0.100" holes on the diagonal around all the bearing and motor holes.

Also, any plans on installing an encoder onto the output shaft backside (inboard side)?

  1. We plan on using the outside 10-32 clearance holes for mounting. The 1/4" holes are our current mounting pattern for the 2 cim ball shifter that I had modelled into it early. The other pattern is for mounting directly to a versa block if the need was there.

  2. The small holes around the motors and the first stage bearing are there for optional encoder mounts. The plan would be to pick a location and drill those holes out to the correct size to tap and mount.


Easily adjustable 2nd stage gear ratio.


Motors roughly even with outside of wheels.


Three mini CIMS grouped over 4" colson wheel.


Vastly expanded interior space for electronics and mechanisms.


Video of first powered testing.

That’s pretty nifty.

Regarding the air fittings to the motors: It looks like there is just a spot for air to enter, where does it exit? Also, has anyone used a cooling system like this and recorded motor temperature over time to see how well it works?

Are there belts or chains inside of the tube? If belts, do you have the ability to accurately machine bearing holes in the tube for the front/back wheels and get rid of the versablocks?

The two mounting fasteners for the motors have through holes down the center. One has an air fitting inline. That’s the inlet. The other is a drilled socket head cap screw. That’s the outlet.

Inside the tube are chains. We developed a fixed center to center chain drive a few years ago, and worked with a sponsor to have the rails CNC machined. Unfortunately, this year our sponsor has said they would be able to continue supporting the team. Our top priority this summer is to develop a new drive train design, or find another sponsor to machine the drive rails. We’re experimenting with the versa blocks after hearing good reports back from teams we’re friendly with using them. With them, we think we can make our own drive rails in house without relying on sponsors.

Now that you’ve had a month or two to play with this gearbox, how did you find the ~1/4" thick delrin plates for stiffness and stability? Do you think they’d hold up for a season, or is this just a prototype for eventual aluminum replacement?

So far the plates are holding up well, with the aluminum tubing between them the are extremely stiff(stiffer than the ball shifter casings). We are in the middle of switching them over to the competition robot to run in some offseason events, which will be the big test.

I think the key to using Delrin as a gearbox plate is to support the plate a lot more than you would with aluminum. So, rather than a couple of standoffs, the large support surface of the 1/2" tubing is a big help here.