Why is it bad to direct-drive?

[ksafin]

Hi all.

I heard, on, I believe, multiple occasions that having motors attached directly to wheels is bad for the motors.

Could anyone tell me why?

IE if I had a CIM motor with a belt pulley on its shaft and had that connected to a pulley on one of my wheels, why would this be bad?

[nathan_hui]

Rarely is any setup in FRC high-speed and low torque enough to warrant not using a gearbox. Now if you use the pulley to set up a gearing reduction, that's not too bad.

Another thing is that all of the motors were designed for use with a gearbox. I believe someone pointed out that the CIM lacks real bearings around it's shaft, so it's not set up for taking dynamic load on the shaft. For the rest of the motors, it should be fairly obvious: you can't put much on the small shafts, which is why you put a gear on it and stick it in a gearbox.

[kevin.li.rit]

Originally Posted by ksafin

Hi all.

I heard, on, I believe, multiple occasions that having motors attached directly to wheels is bad for the motors.

Could anyone tell me why?

IE if I had a CIM motor with a belt pulley on its shaft and had that connected to a pulley on one of my wheels, why would this be bad?

You're describing 2 different scenarios between your first 2 statements.

Attaching a wheel directly to a motor could be bad but really it depends on your application. For example a CIM direct driven ball launcher with a KOP wheel is ok.

A CIM direct driving a 6 inch wheel to propel a 100 lb robot is bad.

This is because of the load on the motor in the second case is large. The motor must draw a large amount of current to move your robot. Likely it would be too much and your motor will either trip the breaker or burn out.

It's okay in the first example because the load, of the wheel and ball is low and only briefly high when shooting.

[Christopher149]

Motors themselves don't usually have that much torque. Gearboxes reduce your top (theoretical) speed and raise your acceleration to reasonable levels.

[chadr03]

I would suspect people are worried about radial loading on the motor. It is more of a question is the motor designed for a radial load or not. Many motors are only designed to transmit torque and the motor's internal bearings are not sized sufficiently to account for radial loads such as gears or belts. While I do not know the if there are any radial loading specs on the CIMs they usually have a shaft mounted pinion with a cantilevered setup. This will produce a pretty significant radial loading so I would suspect that you would not have any issues directly driving something that produced a moderate radial load.