Differential transmission... with a twist

We have a passive differential transmission in our sites. We are, in fact, bound and determined to get it done. However, we’re looking for a few tips. This isn’t our first transmission, but we hope it’s our best.The goal is to have 4 total differentials on the drive train. On each side, there is one set of 2 differentials. Both motors (4 motor drive; 2 per side) will go straight into one differential. One motor will go into another differential via gearing, the other via chain. This accomplishes a relatively simple situation where if motors are turning in the same direction, one differential casing is active and the other is stationary. If the opposite is true, the secondary casing is active and the primary is still. One differential will be geared to the side of the drive train that it is on, the other to a strafing wheel. The result is a 4-motor, semi-synchronous, medium friction H-drive. Perhaps far in the future we will add a third set of differentials, allowing for either driving, strafing, or a 12 motor lift for that glorious >1 second hang. [this is the max number of functions with 4 motors while still being able to turn normally] What’s your guys’ recommendations, tips, etc?

Is the extra weight and friction bigger than using mecanum wheels?

Mecanum wheels have center of gravity problems. If we put them on our bot, it wouldn’t drive; the front wheels would have very little traction. Also, we could strafe at a reasonable speed with omni wheels. [mecanum strafe is 1/4 of forward speed]
Although, I’ve ran the numbers and the weight isn’t too different, It’s been a while since I’ve taken physics, so I am unsure about friction, but I know the vex mecanum wheels aren’t… perfect.
Thanks though for pointing this out.

That’s very cool. I’d love to see a prototype of it.

Edit: Also, you would need encoders for every motor & a bunch of PIDs.


I can’t believe that you came up with this. I came up with this exact same transmission 5 or 6 months ago. Here is a picture of a prototype that we made at the beginning of the season.

Also, we put this transmission on our robot, with all differentials made by vex, and we had a lot of trouble. The vex differentials are not made very well and can’t handle a lot of torque, we have had a lot of jumping gears both inside the differentials, and outside, (it is hard to hold the differentials steady because they are mounted by 2 axles. One thing that helped was to gear the motors going into the transmission for high speed, reducing the torque on the differential system, but we still had jumping gears and we panicked and took them off because we had a competition 3 days later.

If you go this route I cannot say it enough, build quality, also reduce the torque on the differentials. If you do get a working drive train with this, I would love to see it, we may go back to the transmissions later in the season.
dual diff.jpg

Thanks. I will show. Although, we are using a variation of the differential in 4194B’s setup. Our version is lighter and a good bit stronger.