Coefficients of Friction

[tcj103]

Our team went through this a couple years ago when we were picking wheel types.

While it's true that the coefficient of friction between two surfaces is based on what the materials are made from, there is still a macroscopic relationship that must be accounted for. There are tables of friction coefficients you can use as a start, but we have seen that things such as knobby tires of the same rubber compound will grip the carpet better. That's more because the knobs deform the carpet, so there is no longer a purely tangential force acting on the carpet; the tire has become something like a gear.

But I think the big question is really what kind of forces to design for in your gearbox, and friction coefficients are only one part of it. You know what your maximum torque on the motor is, and you should have some idea what your desired top speed for the robot will be as well.
Looking at those, you also want to figure where in the drive train the transmission will be; if the transmission will be stepping down the motor speed, and will go straight to the drive wheel, then it will have to handle the full torque you expect the wheels to see. However if the 'box will step down to a mid-level, then you're using chains and sprockets to step down the speed more, your max torque will not be as much at the gearbox. Remember also that the ultimate thing your transmitting from the motor to the wheel is power, which is RPM*Torque. Raise one, and you lower the other.

But I wouldn't design for max torque, and add a little, I would design for max torque, and multiply by 5 or 10. I don't know how many regionals your team will attend, or if you'll be at nationals, but add up all the matches you'll be in, plus practice matches, and practice time at home before it ships. Our team lost out winning the Philly regional because our drill motor transmission died in the finals. Very frustrating. But that was probably the robots 20th match, so it had a few miles on it. I can't stress (no pun intended) that enough.


Hope this helps.

Ted