# Traction Limited, rather than torque

I’ve been reading all these posts on motors and gears (and been struggling myself with gearbox design) when a thought struck me - it is quite possible that our robots may be traction limited, rather than torque limited:

That is, assume we had an infinite torque drivetrain. What would be the limitation on the robot if it was chained to a wall? At some point, whatever we were using for wheels / treads etc. would break traction and slip. Therefore, that traction limit would define the maximum torque through the system and propelling our robots, yes?

So I offer that we should be mindful that given all the power of our new chippy motors, we might easily reach the limits of traction, unless we design the whole system (including traction devices), rather than just the drive system. What’s the point of designing a drive system that can handle 315 oz-in or more of input torque, when our wheels or treads slip at 150 oz-in?

Am I thinking this through correctly, or am I missing something??

My \$0.02.
Simon

You really don’t want unlimited traction, because it will lead to stalled motors & or damaged carpet. Neither of these are good things.

If you don’t worry about traction in your design, you will not be likely to realize the benefit of all that torque, but if your traction is too good, you’ll find yourself tripping breakers and shredding carpet.

Just something to keep in mind.

P.J.

Hmmm… really makes ya think, eh?

Greg

I AGREE! Our team had enough trobule transmitting the torque of two drill motors in low gear (2:1 to wheels, large sprocket on wheel) to the ground! I hate to see what it will be like this year…

BUT of course we have three 181lb objects we could pick up to increase our normal force. Now how can we do that? Hmmm…

how do you calculate the traction of your wheels/treads/drive train traction into in-lbs?

i’m curious to figure ours out.

*Originally posted by Anton Abaya *
**how do you calculate the traction of your wheels/treads/drive train traction into in-lbs?

i’m curious to figure ours out. **

Okay… well… I’m no engeneer (not even close) but this is how I wound up understanding:

Get a fish scale. Hook it to your robot parallel to the plane the center of your wheels are on. Increase power to motors until you loose traction. You can use the dashboard port to see the PWM

value you were at, and I think you could then extrapolate the torque/rpm of the motor at the time. Why the scale? The force you should read should be equal to your coefficent of friction times your normal force (weight). F=uM.

Since we know a goal takes 32-35lb of force to move, you can use this little test to see if you really can move four!

Like I said, I’m not the mechanics man, so can a more authoritative source back me up on this?

*Originally posted by Anton Abaya *
**how do you calculate the traction of your wheels/treads/drive train traction into in-lbs?

i’m curious to figure ours out. **

Using the friction-pull test above, you know the force; Force (in lbs from a spring scale) times radius of the drive wheels (in inches) will produce the torque at the wheel hub. All the rest is gear / sprocket tooth ratio.