The physics of pushing

[Ether]

Originally Posted by DonRotolo

OK, maybe this will help:

If each wheel gets 20 ft-lbs of torque, and the wheel is 6" in diameter, the force acting on the carpet* is 20 / 0.25 = 80 Lbf. (20 is the torque in ft-lbs, 0.25 is the radius of the wheel in ft, and Lbf is "pounds force" (different from pounds weight)).

*Theoretically. Remove maybe 5% for losses (as suggested above), you get 76 Lbf.

Four wheels, 76 * 4 = 304 pounds of force.

Assumes you don't break traction with the carpet.

Be aware that if you are powering 2 wheels with 1 motor, each wheel gets half the torque. Similarly, if you are powering 3 wheels with 2 motors, each wheel gets 1/3 the torque produced by adding the torques of the 2 motors together (and factoring that torque up by the gear ratio, of course). That's why I worded my post in terms of drive motors instead of wheels.

[JamesCH95]

Originally Posted by DonRotolo

OK, maybe this will help:

If each wheel gets 20 ft-lbs of torque, and the wheel is 6" in diameter, the force acting on the carpet* is 20 / 0.25 = 80 Lbf. (20 is the torque in ft-lbs, 0.25 is the radius of the wheel in ft, and Lbf is "pounds force" (different from pounds weight)).

*Theoretically. Remove maybe 5% for losses (as suggested above), you get 76 Lbf.

Four wheels, 76 * 4 = 304 pounds of force.

Assumes you don't break traction with the carpet.

Bardd: note that this would require a wheel tread with a coefficient of friction of around 2.0, much higher that commonly used tread materials. When you finish your calculation you should double-check it with other potentially limiting factors such as traction and component strength.

[IKE]

There is also a lot of good info in this thread on designing for pushing from the power electronics aspect:
http://www.chiefdelphi.com/forums/sh...hlight=40+amps

Just because the motors can provide the power doesn't mean the electronics can provide the power (for very long).

[Adam.garcia]

Originally Posted by bardd

I'm trying to decide on a good gear ratio for our drive system, but I'm kind of stuck on one thing - I can't figure out how to calculate the force the robot could inflict on another robot while pushing (not momentarily). I alraedy have the torqe of each wheel, I'm just missing the next step.
Would the number of wheels make a difference?

If you'd look at JVN's Mechanical Design Calculator, you can see the effects of gear ratio, # of wheels, # of motors, etc. on the effective pushing power of a robot. It allows for easy "prototyping", as the numbers are as simple to change as the push of a button.

I would defer the mathematical explanation to somebody of more knowledge, but for starters you can look at the formulas that he uses in this spreadsheet. JVN's Design Calculator may be found here:

http://www.chiefdelphi.com/media/papers/2059