# 6 motor drivetrain gearboxes

Just from my own curiousness, how do teams that use 6 motors on their drivetrains get the gearboxes right, since the RS-775 turns at 19500 RPM and the cim only turns at around 5500 RPM. I assume that the motors need to be turning at the exact same speed so the motors aren’t trying to turn a wheel at two different speeds. If someone could explain how the gearboxes are designed, or better yet show me a CAD file of one of the gearboxes, that would be great.

I imagine that an additional stage of reduction is used on the RS-775. It doesn’t have to spin at exactly the CIM’s free speed; as long as you get it within a couple hundred RPM, you should be fine. Multi-motor gearboxes deal with unevenness like this all the time; not all CIMs are identical.

The RS775 that you were provided in the KoP has a rated free speed of 13000rpm at 12volts, not 19500. The CIM’s rated free speed is 5310rpm not 5500. The ratio 13000/5310 = 2.45 so you would gear the RS775 down accordingly. That way, the CIM and the RS775 will operate at the same percent of their respective free speeds.

I assume that the motors need to be turning at the exact same speed so the motors aren’t trying to turn a wheel at two different speeds.

As explained above, you don’t want the CIM and the RS775 operating at the same rpm. You want them operating at the same percent of their respective free speeds. You do this by gearing them differently.

If you don’t get the gearing exactly right, the motors don’t “fight” each other: they both are powering their respective wheels. But one will be working “harder” than the other because it will be at a different operating point on its power curve.

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For maximum power, I would think you would want to match the torque curves of the 775 to the CIMs for efficiency. But, in my opinion, the extra power is wasted, as you can only get so much to the floor. On the other side of the coin, if you are looking for more speed with lower amp draw to each motor to prevent the circuit breakers from tripping, then the extra power will help to do that. You will find that there is a maximum speed that you will reach regarding your drive team’s ability to control the robot.