With all of the discussion about 3 minicim drivetrains, I decided to try my hand at designing my own. Here are some pictures (and STEP/Onshape links) of a 3 flipped minicim gearbox.
Onshape link:
STEP Download:
This gearbox is geared at 17.52 ft/s free or 14.19 ft/s adjusted and is designed as a drop in for my offseason bot. This gearbox will likely be made as practice for new mech students and installed to test the pros and cons of 3 minicims in a drive gearbox.
As always, your questions and feedback are much appreciated!
Are you planning on testing a 4-minicim configuration as well? What drove your decision to have a 17.5fps free speed over something higher/lower?
Could you remove the plate attached to the 2x1, or does it serve a purpose I’m not seeing?
I had no plans to test a 4 minicim configuration but if you or others want specific things tested I’m sure I can make some time for that (just unmount one of the motors i guess). I definitely could remove the plate attached to the 2 by 1, it’s more of a convenience thing. My team uses a standard hole pattern and especially because this is a drop-in replacement I made sure to use that same mounting pattern. To do this and make sure bolts/spacers cleared chain I needed some kind of adapter and this is a nice way to do it. I picked that free speed primarily considering convenience and prior experience. That was the highest reduction I could use WCP pocketed gears for and felt comfortable running concentric to the wheel. With just one stage and an idler, this gearbox shape/form is super simple and is just about 1.5 fps faster than my original offseason bot gearbox which my driver felt was slow.
Thanks for your help with the design and your feedback!
The high speed gearing (this is really a single stage gearbox) is somewhat disconcerting; unless the robot is unusually light, I expect that it would be difficult to achieve your calculated speeds on a 54 ft long uninterrupted field, much less the fields which the GDC has become adept at providing us.
Why did you decide to put the encoder on the idle gear shaft rather than the output gear shaft? It appears to be a software headache without a hardware advantage.
~14 ft/sec doesn’t seem terribly fast…It seems like a decent compromise with a single speed gearbox. We generally try to have our low gear ~8 ft/sec, and last year, our high speed was in the 18 ft/sec (simulated) speed ballpark.
As a software person - it’s a simple scalar for single speed gearboxes. It’s a bit more of a pain when your encoder is pre the shifting stages but even that’s manageable.
If it makes packaging the encoder easier I’ll take the minor annoyance of scaling. It really is a minor thing.
I like to have snap rings on our main drive shafts and a bolt with washer as a failsafe. Rather than give up that failsafe on our center drive shaft so an encoder can fit in, I thought it would be easy enough to multiply the encoder value and have the magnet in the idler shaft which is retained between 2 bearing flanges and sees much less load/failure risk.
As for the speed, our team typically tends to build pretty light robots (lots of wood and hole patterned tubing) and while this is definitely on the higher end of single speed drive gearboxes I’m pretty sure my drive team can handle it (only a bit faster than our offseason bot). We like to get a ton of drive practice but if this gearbox really is too fast for the game in case of obstacles or other things we will default to shifting or use a different gearbox. All depends on the game!
Putting an encoder on a stage nearer to the motors has the hardware advantage of less backlash between the motor shaft and encoder shaft, which can help with the control loops.
