Coefficients of Friction

[JVN]

Originally Posted by JoelP

For example, if my robot was in a pushing match with another robot but unable to move it, the transmission would need to put out enough torque to cause the wheels to spin before the fuses blew. If the weight limit was increased, creating a larger frictional force which would have to be overcome by a transmission that wasn't designed to put out that much torque.

This is absolutely right.
If the weight limit goes up, suddenly a transmission that would have drawn 30 amps/motor will now draw 60 amps per motor, and you're in trouble.

Originally Posted by JoelP

Of course this could be easily fixed by changing the gearing in the transmission, but I'm not looking forward to making changes and testing a new transmission in the constrained conditions of the 6 week build period, especially when my team doesn't have the resources to quickly create parts for a new transmission.

So design a transmission that leaves you some breathing room...
I always try to leave some adjustability in the final sprocket reduction between the transmission and the wheel:

• If I find the robot is going too fast, I can increase this sprocket ratio, to slow it down.
• If I find the robot is going too slow, I can decrease this sprocket ratio to speed it up.
• If I see FIRST has increased the weight limit by 30 lbs, I can increase this sprocket ratio to bring the motor-load back down to acceptable levels.

This method would allow you to design your transmission, and just tweak your final chain/sprocket reduction when you see the game.

JV

[=Martin=Taylor=]

What about gum-rubber?

Anyone know the coefficient of friction for that?
(on a standard 120lb robot of course)

I'm just curious.

[Donut]

Our team hopes to do a traction test over this summer with varying materials. We'll post the results we get on here. Alot of what we test depends on the price of it and how difficult it is to attain, but these are some of the things I have in mind right now...

Standard Skyway Wheels (both 6" and 8" to settle the debate once and for all on whether bigger diameter wheels get better traction)
Pneumatic Skyway Wheels
Roughtop Tread
Wedgetop Tread
Linatrile Tread from Brecoflex
Correx Gum Tread from Brecoflex
Nitrile Tread from Brecoflex
Gum Rubber
Tread on Bottom of a Shoe
Anti-Slip Matting
Go-Kart or Mini-Bike Pneumatic Wheels
Colson Wheels

Some of these things we already have lying around, the others we'll have to figure out. Any other suggestions are welcome, and it can be ANYTHING (if you didn't gather that from the fact that we're going to test a shoe).

[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

[Dick Linn]

We've used the 8 inch (nominal) Skyway pneumatic wheels for a couple of years. The ones that are available with the 5/8 inch bore and keyway. They are a bit taller than 8 inches. We generally end up with more traction than we really need, at least on carpet. I'd prefer 6 inchers, but these are easy to work with and fairly inexpensive. They do tend to bounce around, though. We were using 4 CIMs and the kit gearboxes with about a 1:2 further reduction to the wheels.