Hi. Could anyone knowledgeable enough explain one bit about torque with multi-speed transmissions systems to me.
First, I fully understand why a transmission designed like a planetary gear by wrapping a chain around in place of the orbit gear doesn’t really work. Here is a nice animation by the awesome jpearman:
I’ll label the high-speed version as T=1, v=1. The high-torque version ends up as T=1 and v<1 instead of getting the trade-off you’re looking for. This is due to back-driving a motor because of nothing locking the gears in place. Although I expect the torque might be minimally above 1 if some friction is able to help against back-driving a motor, it’s certainly nowhere near what you’d want from the loss in speed and application of two motors for torque.
Second, I fully understand drive-line windup such as on H-drives and how a differential can drive two sides at different rates to avoid drive-line windup (which I’m sure we don’t really care about here like we do with military vehicles). So, yes, I understand the relative motions of the gears within a differential.
Onto my problem, which is trying to understand how a bunch of the transmission systems I’ve seen people design with differentials don’t suffer from the same back-driving issue as the planetary gear/chain models I’ve seen people design. As far as I’ve been able to tell, you have the same torque limit in the high-torque mode as in the high-speed mode due to back-driving a motor. Here is one of several examples I’ve looked at between the VEX EDR and VEX IQ forums:
Here is a different design, and I totally get why this works:
Here we have a worm gear, which is usually self-locking and so quite hard to back-drive. Either in this video or on the discussion somewhere on this forum they mention too much lubrication stopping the self-locking becoming an issue, just as I would expect.
So what is it about the version without the worm gear that actually makes it anything more than a fancy way to do less when you move to high-torque mode? Why is back-driving the motors not an issue? The comments on the worm-gear version being over-lubricated match what I expect so well, so maybe I actually do understand correctly?
Thanks for your time!