Drivetrains: Pushing Power?

[Kims Robot]

Im working with a Project Lead the Way EDD class that is working on different drivetrain designs (bunch of FIRST students experimenting!). I have them running several calculations on the drivetrains, speed, traction, etc.

We went through several of the drivetrain presentations I've found on the FIRST site, from conferences and here on CD, and have been unable to find a way to compare the "Pushing Power" of different drivetrain designs (theoretically). Im thinking it has to do with the stall torque of the motors, but how do you factor in when you have multiple motors? How about using the friction of different wheels? Is there a good way to calculate this and compare the drivetrains?

[MrForbes]

I like how 842 does it, they lean a bathroom scale up against a wall and see how many pounds the scale says as the robot pushes against it.

[Ben Piecuch]

Kim,

JVN's 2008 spreadsheet (found in the white pages) does that exact calculation for you. He compares the maximum torque from the gearbox and chain reduction to the actual frictional force from the wheels, and uses the lower of the two numbers to determine your maximum pushing force. The spreadsheet even has the ability to combine multiple motors.

The difficulty is determining the Co of Friction for the various wheels, surfaces, etc... That's where FRC842's bathroom scale and immovable object help out.

Good luck,

Bengineer

[Richard Wallace]

Kim, I attempted a simplified explanation of how torque relates to pushing power here.

I know you've already surveyed the White Papers here on CD. You might want to spend some time playing with JVN's 2008 mechanical design calculator.

[Kims Robot]

Thanks, I hadnt looked in detail at the design calculator, I will give that a shot. The biggest thing is that I want them to UNDERSTAND it, rather than just throwing some numbers in a spreadsheet... So I think I will try to borrow the formulas and explain it to them before I show them the sheet.

Richard - thanks for the reference to the post, I will see if that helps them make more sense of it!

And as for the bathroom scale, they have come up with ways of "testing" but Im trying to get them to do the true engineering... DESIGN it first, then see if your calculations match what the real measured values are... if not, why??

Thanks for the help so far!

[MrForbes]

Originally Posted by Kims Robot

And as for the bathroom scale, they have come up with ways of "testing" but Im trying to get them to do the true engineering... DESIGN it first, then see if your calculations match what the real measured values are... if not, why??

Yes, I was a bit hasty to suggest that first, you definitely should do the engineering first, then test it!

[AdamHeard]

Like mentioned before, it's based on the stall torque provided by the motors and the maximum traction force, whichever is lower.

For the force the motors can provide, calculate the torque at the wheel, and divide it by the radius of the wheel (T = F x D.... so F = T/D.... D is distance).

For the maximum traction force, you must lookup/experimentally determine/calculate the Coefficient of friction of the wheels and multiply that by the weight of the robot. Hopefully this number is the smaller of the two.

Now, that's just theory; I'd expect it to be off by a little bit when measured 842 style.