Gearing for mecanum

Technical Technical Discussion
1

I have attempted to calculate the necessary gearing ratio for a mecanum drivetrain. The drivetrain will have 1 CIM per wheel (and no other wheels), and use 8-inch Andymark mecanums.

I calculated the gear ratio so as to be traction-limited under the following assumptions:

  • 150 pounds max robot weight
  • 0.7 max CoF for the wheels (reported by Andymark)
  • 40 amps max continuous current per motor

I came up with a desired ratio of about 16.25:1. This results in a theoretical free speed of 10 ft/s.

Last year’s robot used a kit frame. This had about an 8:1 gear ratio. Since this computes about double the required gear ratio for (theoretically) slicker wheels, I am skeptical of my results.

Does this seem correct?

2

I don’t know much about ratio however,

[R03]
The Robot weight may not exceed** 120 lbs**. When determining weight, the basic Robot structure and all elements of all additional Mechanisms that might be used in different configurations of the Robot shall be weighed together.

This excludes bumpers.

3

Taken care of. Note that the ratio was calculated for 150 lbs (robot+bumpers+battery).

4

… and batteries.

I used 150 pounds so I had a bit of a margin for error.

5

I figured that was the case, but I wasn’t sure.

6

We used stock 12:1 banebots planetaries with CIMs and 8" AM Mecanums several years ago and were relatively pleased with their performance.

I don’t remember having wheel slip aside from when one of the wheels was off the ground, and we did get in some pushing matches… so I can say that a 12:1 ratio does not exceed the ability of an AM mecanum to put torque to the ground.

Going to a slower, higher torque gearbox, like the 16:1 may have advantages in that a higher torque output can allow for greater acelleration and better low-speed handling.

But I think I’d stay away from the 8:1 ratio… that seems very “fast” to me. Although it is common to calculate for a a robot’s top speed, if you watch the game closely teams spend very little time at top speed, and a lot of time maneuvering, acellerating and decellerating.

Jason

7

Okay, that helps clear things up.

I think that in our case, we want to be traction-limited more than fast. Therefore, I’ll let the team know that we need a 16.25:1 ratio.

Thank you.

8

If you used 9:1 for 8" wheels, would you expect to see CIMs cut out at initial full acceleration? Not that I have any reason to ask :slight_smile:

9

We’ve used 9:1 on Andymark 6" HD mecanum for several years and are very happy with the results… That is at least close to a 12:1 on 8" wheels.

I’m not sure I’d want 8:1 on 8" wheels!

10

Last year we used the mecanum wheels directly out of AndyMark nanotube. The gearing on that was 12.75:1. We were pleased with the results.

11

We use the AndyMark nano transmissions but swapped gears to get 10.67 for 6" wheels - it works great. We figured quickness was paramount for this years game since the hump and bridges won’t allow for full court sprints.

HTH