Motors/Gearboxes required to move a 20 kg/44 lb robot

[Nathan Streeter]

What will this encounter for high-torque situations? Pushing/Pulling? Driving up an incline? What other things does this drivetrain need to do well? Maneuver well? A lot of this will determine how much torque you’ll need and what wheels you want to use. Also, the surface it’ll be driving on is important for designing the frame and choosing wheels… will it be a hard, smooth surface? Tight carpet like an FRC field?

Since you have a pretty slow speed in mind, driven by a very fast motor I’d recommend going with a smaller wheel (4”, probably) so that you need less of a gear reduction. Also, it seems like a robot as small as 15”x15” would want to use smaller wheels.

If you have 4 relatively grippy wheels on the ground, in a semi-square layout, you’re probably going to have a very difficult time turning. I’d strongly recommend using either a low traction wheel at the non-driven end or an omni wheel.

So, if you’re using a 4” wheel to go 6 ft/s: (6ft/1s) * (12in/1ft) * (1rev/4*pi in) * (60s/1min) = 344 rpm
That means in order to go 6 ft/s using 4” wheels you need to spin them at 344rpm.

Your RS775-18 run at 12V should have a free speed of 13000rpm. When making a first pass for a top speed calculation of a drive train with low-to-moderate drag, using the max efficiency speed isn’t a bad idea, so I’ll use that. So, assume when you’re at top speed the motors are spinning at 11300rpm (per Ether’s helpful little .exe).

That means the ratio you need for the reduction is (11300rev/1min) * (1min/344rev) = 32.8:1

Go ahead and run these calculations by yourself using a different size wheel, if you want, but it certainly seems like a 50:1 would be ample reduction for 5” on a robot that weighs this little. Also, I doubt the RS775 with a 30+ reduction and 5” or 4” wheels on a <50 pound robot will have any trouble accelerating. I’d be happy to help you with those calculations if you’d like though.

How this drivetrain is being used is my big concern though… RS motors (and pretty much all brushed, fan-cooled motors) should hardly ever be stalled, as they’re very, very susceptible to burning out.

Also, if this is all pretty light-duty, I’d say you’re actually fine to leave the gearbox unsupported, as long as you put the wheel right against the motor.

So, please respond, answering some of the questions throughout my post and some of the above posts… about how the drivetrain is being used (and on what surfaces) and what wheels you’re thinking of using.