Motors - changing HS / Torque vs changing gear ratios

  1. Is it better to have HS motors with a torque gear ratio or Torque motors with a high speed gear ratio? What balance have you found to be best and for what purposes?

We’re considering adjusting the drivetrain but don’t have much time before our next competition to play around with various combinations, so we were hoping for insight from people-in-the-know. The problem is that under regular operation the drivetrain has decent speed to torque (can pick up and carry mobile goals, score in 10 point and 20 point zones, and is of reasonable speed (not super slow - but not as fast as we’d like!). In today’s competition, however, we had a few times where hits from another robot caused the drivetrain to stall.

We currently have 4 motors (2 each side) set to high speed with a 3:2 gear ratio each running an individual omni wheel. We definitely didn’t want to lose speed if it was avoidable (if anything, we’d very much like to get more!), but we are concerned about the stalling even though it was only an occasional problem.

Also, while we’re asking motor questions:
2) Is there an easy way to change the motor gears without having to completely remove the motors if you haven’t already pre-zip-tied them?
3) Would it offer any benefits (or just cause painful problems) to run different motor speeds on your drivetrain front and back?
–For instance, what if the 2 front motors were Torque and the 2 rear motors were HS?
–Would the answer change if the front and back were connected with a chain?
–Do the 2 motors going through motor controllers have a loss of torque (so maybe putting 2 HS motors directly to the 1 & 10 ports and 2 T motors to MC)?

Team TX those are good questions.
1 Our team thinks its better to have a 4 motor (HS) without a gear ratio (just connected to the wheel).
2 Their is no easy way to change the gears inside a motor with out zip tying (vex doesn’t like).
3Doing front and back motors different gear ratio inside is bad because the torque motors would brake/ burn out.
4 don’t use ports 1 and 10 they can easily fry your cortex. Only use 1 and 10 for low stress motors.

Here’s some of my thoughts:
Firstly, the size of the omni that you’re using will largely impact what gear ratio you can use. If you use the small omni’s, you could probably do a direct-drive 4-motor turbo, but if you’re using larger ones then I’d only recommend going with high strength if it’s a 4-motor drive.

A note on your gear ratio: If you’re using a high speed motor geared externally for torque at 3:2 (like you said), you have an output to the omni of 106 rpm, which is only 6 more than a high strength motor. If you mis-typed and meant 2:3, then that’s why you’re burning out. Generally, I’d recommend not externally gearing the motors when possible to save on friction, since this can decrease performance and cause you to burn out more often.

Likewise, if you’re burning out often I recommend doing a thorough check of the drive to make sure there aren’t any joints with a lot of friction in them. To do this, simply pull the axles out of the motor (but so they’re still in the bushings) and see how freely the wheel can spin. Alternatively, you can remove one of the pancake gears out of the back of the motor if it’s more accessible in order to disengage the motor. Ideally, the drive wheel should coast for a few seconds, but that’s only in a perfect world. Try to limit friction as much as possible.

Do not use different speed motors on the front/back of your drive. It will just cause you to burn out more often when they fight each other. If you connected the front and rear wheels with a chain while they were different speeds, it would only make the problem worse.

If you want more speed out of your drive, I recommend going with a 6-motor drive. Not sure what your motor map looks like for your robot, but you can probably spare 2 extras somewhere.

Like @Cam said, zipties are more or less your only option UNLESS you devise a quick-mount system for your motors. I can’t find a video atm, but you can probably find one if you search around for a bit. Usually, they consist of mounting your motor on a separate c-channel (a few holes longer on either side of the motor to access screws through the c-channel) from the one it was on previously, and then screw the two c-channels together with standoffs. This was, the screws into the standoff are easily accessible from the back, and thus making your motor easy to detach.

I also engineered a rather innovative or sketchy (depends on the way you look at it; I saw it as innovative, the rest of the members of my club thought it was sketchy. For the record, it was incredibly effect) solution on my DR4B last year. I had 5-wide c-channels that I bolted standoffs into, which lined up with the holes on the side of the c-channel. I then utilized plates and “slotted angle” metal (to account for holes not lining up) which I mounted motors on to. The plate/slotted angle assembly then mounted on to the c-channel using thumb screws. It was completely tooless (save for removing motors) and I never had a single problem with durability. If you want pictures, I can mock something up when I’m at VEX tomorrow and take some for you.

Sorry for the lengthy reply, but hopefully it made sense. Let me know if you have any other questions, I’d be happy to help!

Thank you both!

We’ll avoid using a mix of high speed and torque so we don’t burn anything out.

Apparently the consensus is that we really are only rarely stalling; what seemed to be a major stall after some aggressive robot play at our latest competition turned out to be a drive motor coming loose and a shaft coming out of the motor (we got hit pretty hard, among other things, although it likely had to have been a bit loose to start after a long day of hard use).

@Cam - we hadn’t heard that ports 1 and 10 could be an issue! Is that something you’ve encountered a lot of?

@536 Xavier Robotics

  • It is HS motors with external gearing for more torque (because we were having a bit of trouble getting over the low bar). Could you please explain the 106 rpm output and what that means / how it is calculated? Does that mean that we’re not getting very much extra torque from the set-up? (Apparently we just needed the little bit, though, as we can go over the bar currently.)
  • Would a Torque geared motor with a 1:1 gear ratio be about the same? Slower but stronger? Faster but weaker?
  • The front wheels need to be geared since we can’t put the motor up there due to the mobile goal grabber. Should we put the back directly on the wheel but gear the front?
  • We are tapped out on motors (using all 12, with 4 on the drivetrain) and wouldn’t be able to spare any, unfortunately. We can currently stack up to 15 on a MG and score in all areas. We were just hoping to squeeze out a bit more speed (but don’t want to lose our 20 pt ability)!
  • Those ideas for easier access sound very interesting; a bit late for this season, but we’re totally interested in such innovative ideas for future seasons. Pictures would be great!

Zip tying was ruled legal in a Q&A last year. Since it applies to a robot construction rule and hasn’t been countermanded by the new game manual or a more recent Q&A, it’s still legal. If you have any doubt (I don’t, but I understand you might) then you can ask a new question about it this year.

Here’s the Q&A from last year: