pic: Flat 3 CIM 2-Speed Gearbox



I’ve been playing around with the idea of a 3 CIM, 2-speed gearbox that is the height of the CIM for the past couple weeks and this is what I came up with.

Gearing:
Low gear: 8.23:1, 8 ft/s real-life speed with a 3.5” diameter wheel.
High gear: 3.43:1, 19.23 ft/s real-life speed with a 3.5” diameter wheel.
Shifter Spread: 2.40

I am well aware that 6 CIM drives are falling out style and that mini CIMs are all the rage these days. Therefore, this gearbox was designed so that replacing the CIMs with mini CIMs and swapping from 14 to 13 tooth pinions will result in nearly identical speeds.The CAD shows the CIM configuration because the packaging for the shifter cylinders looks better next to the longer CIM motors.

Overall I’m very happy with how the flat CIM gearbox’s packaging and ratios turned out. My main gripe with this gearbox is some of the cost inefficiencies, like having to use a VEX shifter shaft with the AndyMark dog gear.

The model can be downloaded here. Any feedback would be greatly appreciated and I’m more than happy to answer any questions anyone has.

That’s a cool setup! It solves some packaging problems. I also like the way the small red plate in the middle of the gearbox also serves to reinforce the middle pinion against the power of 2 motors going through it.

However, one motor disengages in low gear. A feature to stop brown-outs? :stuck_out_tongue:

Actually though, could be interesting to disconnect a motor in high gear to prevent brownouts while keeping a (slightly) higher acceleration for low gear.

What’s the difference in shifting force from a regular vex pancake cylinder?

I am well aware that 6 CIM drives are falling out style and that mini CIMs are all the rage these days. Therefore, this gearbox was designed so that replacing the CIMs with mini CIMs and swapping from 14 to 13 tooth pinions will result in nearly identical speeds.

Just so you know, it is actually more optimal to mix the motors together with identical pinions. They are designed for that specific purpose. There’s nothing magical about matching free speeds of motors exactly, versus more closely matching each other at other points on the power curve. So definitely stick to the same pinions on all motors, works just fine.

Currently that plate doesn’t support the middle CIM but that’s not a bad idea (assuming a bearing will fit).

Correct me if I’m wrong but I do not believe that the motor disengages when in low gear. When in low gear, the dog gear that is not engaged acts as an idler, transferring force from the outer motor to the pinion of the middle CIM. If this is not correct please let me know.

Those are 7/16 bore cylinder so at 60 PSI two of them extend with about 18 pounds of force while VEX’s pancake extends with roughly 26.5. To answer your question, 8.5 pounds or 32%.

Ah, my mistake. You’re correct.

I would agree with the fact that it “works just fine”. But I don’t think its “more optimal” to mix the motors without matching free speed.

The CIM has peak power output @ ~2670 RPM and peak efficiency @ ~4700 RPM

The miniCIM has peak power output @ ~2920 RPM and peak efficiency @ ~5000 RPM.

These are just two points you could be designing for. But in any case, you probably are aiming for the same point on the motor curve for all motors in a gearbox.

Does it matter? Not really. Just be careful with your wording :wink:

i interpreted the OP to mean build it with all CIMs or with all miniCIMS, and changing the pinions would give each one similar robot speeds.

This is a sweet layout! I am a big fan of this packaging and the fact that all the motors are even depth, in addition to being flat. The cylinder setup is what it is because of your super compact layout, but it still fits within CIM height so that is pretty awesome if you ask me.

The one recommendation I might make would be to extend the standoffs from the middle plate all the way to the outer plate to keep the main plates parallel more securely. I see it is pretty tight to the box tube, but I think that if it fits, that it would be helpful for rigidity and still light weight.

Also, if you were interested in minimizing the volume of the gearbox even more you could drop the bearing block and run #25 chain in tube. This is a pretty cool option, but I have heard a wide range of how well this has worked out for different people.

Overall an awesome design!

This is correct, sorry that was confusing.

I was definitely considering extending the standoffs across to the outer plate and probably would if this gearbox ever ends up getting made.

I have no experience running chain in tube so I decided to keep the sprockets external. I definitely agree that it could save a lot of space and the only thing it would really change in the design is the length of some standoffs and bolts.

Thank you!