Compact 3CIM Single Speed Gearbox Review

FIRST General Forum
1

My team is experimenting with custom gearboxes and WCD this summer. This is a potential design for it and I’m looking to get some feedback and/or concerns with my design.

Specs:
Gear Ratio: 5.2381:1
Weight: 10.7lbs w/ 3 cims
Theoretical top speed: 15.92 ft/s

Link to cad model (GrabCad): https://grabcad.com/library/compact-3cim-wcd-gearbox-1
Link to renders (Imgur): http://imgur.com/a/1mkqi

2

Looks pretty solid all around.
Note:

  1. Your shafts can be a bit longer to take full advantage of the bearings. Right now they only stick about halfway into them.
  2. You could make the plates slightly larger if you want to add a plastic cover around it.
  3. For the “powercoated” look, click on the beach ball up top, set the appearance to “polished aluminum” on the right, and click on the color right above black on the left. I’ve found that gives good results (credits to Chak).
  4. What made you go with bolts and nuts over making the back plate tapped?
  5. It wouldn’t be too much trouble to make this shifting I think, given the space you have between plates. You might want to look into that.

That’s all I can see right now. Overall it looks like a great job! Keep up the good work.

3

I would be interested to know if Griffin has considered the pros/cons of shifting and decided against adding that capability. If so, would you mind sharing your thought process?

4
  1. We were trying to maximize use of COTS, so those are AndyMark AMS Cluster Hex Shaft (am-0302).
  2. Is there any reason to add a plastic cover? Seems like unnecessary extra weight.
  3. I came back part way through the render queue and regretted using the powdercoat texture, looks awful. Oh well.
  4. There is only one bolt that uses a nut, the rest of the bolts go directly into the cims. I thought I’d be better off with a locknut vs. a tapped hole.
  5. We currently have the parts for a single speed configuration, although I think a dogshifter would be possible. As long as the edge of the CIM doesn’t interfere with the standoffs holding the pancake piston.
5

Shifting gearboxes are very game specific, if you don’t need it, why go through the trouble of using one? My team has consistently used Kitbot the past few years, so we decided a single speed WCD was a good place to start and easier for newer students to design.

We do however have a WCD 2CIM Dogshifter that we built in a previous year, so I’ve become familiar with its capabilities. But for an offseason gearbox/drivetrain that is being used for teaching and entry point into WCD, single speed was sufficient.

6

This gearing is rather fast for most of the recent FRC games; implemented on the recent control system, I would definitely schedule some tests to ensure that you will not have brownouts at startup or (more likely) during pushing contests due to low voltage. Your optimal speed will, of course, be highly dependent on your game strategy, which cannot possibly be known until January; just intended as a “what you really need to check” between making a game strategy and selecting this as a drive train.

7

Sounds fair! Good reasoning for using the shorter shafts, sounds like you’ll save a lot of money/time if you make this.
I assumed you made the pates so large (the size of the gears OD) so that you could protect it or something. If you’re not interested in using a cover, you can bring the edges of your gearbox to the bearings rather than the OD of the gears, unless it’s an aesthetic choice.

EDIT: 16ft/s free speed isn’t too fast IMO, especially if you run 6 CIMs. On a 4 CIM drive your acceleration suffers, but 6 CIMs will make the difference.

8

Gearing our 4cim 2 mini cim drive at 10.04:1 this year with 8" wheels made us suffer bad brownout problems. Our batteries would barely get us through a match. We bumped our gear ratio up to about 12:1 or something, this off season and have had much better results. I really suggest you test your gearing with 6cims before building this during a season, you might not move at all due to browning out. Wheel diameter will be a big factor too, so smaller wheels could help with this.

9

What free speed were you looking at before and after the change? Do you think that removing the MiniCIMs could have helped/ did help?

10

The most effective 6-cim single speed drivetrain I’ve ever seen was geared to 10fps adjusted. 16fps is in the upper end of high gear for most 2-speed drivetrains I’ve seen (unless you’re from CA, where time must move slower or something).

You might get away with 16fps in a game without much defense, but as soon as you try to drive through another robot you’ll regret it, I think. A simple way to test this is to design your gearbox with a “VEX reduction”, or an 84-tooth ratio. That way you can swap between ratios to find what you like best.

11

  1. I designed the gearbox to utilize the same 2nd stage reductions as the WCP SS gearbox, so swapping in a different 2nd stage would be relatively easy

  2. Depending on the game, I think this could be adapted to a dogshifter config. But as I voiced above about spacer clearance relatively to the cim near the mount, it might not be possible.

12

Is that number accounting for mechanical efficiency losses or assuming that it’s 100%?

If the former, looks good.
If the latter, you may want to tweak it depending on if you value speed or pushing more.

13

I utilized the drivetrain calculator on WCProducts website and the efficiency was at the default 90%

14