Coefficients of Friction

[JoelP]

I'm in the process of designing a 2 speed transmission for team 1155 the Sciborgs. One thing I need in order to design a transmission with adequate torque and for other calculations is the coefficient of friction of the wheels. However, we unfortunately do not have the resources to purchase different sets of wheels and experimentally determine the coefficient of friction.

So I've come to ask assistance from the chiefdelphi community. Since contact area is independent of the coefficient of static friction, wheel size and number shouldn't matter correct?

What is the coefficient of friction for a 130lb robot (I hope they don't raise the weight limit next year.) on standard FIRST carpet (I hope they don't change that either.) with normal kit skyway wheels?
with Colson wheels?
with IFI traction wheels (roughtop/wedgetop)?
with polyurethane wheels (any specific durometer)?

Perhaps this can be turned into a sticky for other teams to reference if a good amount of data is collected?

Thanks for any help!!

[sanddrag]

With 1.125" wide wedgetop, 4 wheels contacting the ground, we obtained a COF of about 1.1 by using a cheap bathroom scale and pushing against a wall at full force on carpet quite similar but not identical. For Skyway beadlocks (8x2) turned down flat, we obtained approximately .9 from what I can remember, but that was with a (approximately) 90lb robot. The colson wheels have a lower COF than the wedgetop tread.

Width does have something to do with it on carpet. I saw teams with .75" wide wedgetop tread get pushed around. But I don't think you need to go to 2" wide to get more traction. I'd say 1.25" is about the absolute max you need to go with wedgetop. There is a region where more width will give you more traction but there is a point where all more width does is take up more space.

[Greg Needel]

Originally Posted by sanddrag

Width does have something to do with it on carpet. I saw teams with .75" wide wedgetop tread get pushed around. But I don't think you need to go to 2" wide to get more traction. I'd say 1.25" is about the absolute max you need to go with wedgetop. There is a region where more width will give you more traction but there is a point where all more width does is take up more space.

Friction force does not increase with a larger surface contact. The reasoning behind that is due to the force in between the surface and the wheel, in this case the robot weight. A larger area between the wheel and the carpet would create a larger source of frictional forces, but it reduces the pressure between them. The same force dissipated over a larger area with the same coefficient of friction is equivalent to a smaller area with less sources of friction forces.

Basically since your robot weight is a constant and your CG is basically the same with both smaller and larger wheels, your traction actually does not change based on wheel width, assuming the same traction material on narrow and wide wheels.

[sanddrag]

Originally Posted by Greg Needel

Basically since your robot weight is a constant and your CG is basically the same with both smaller and larger wheels, your traction actually does not change based on wheel width, assuming the same traction material on narrow and wide wheels.

That's what simple physics tells you, I know. But I have a feeling the interlocking action both on a macroscopic and microscopic scale between tread and carpet is anything but simple. Why do drag racing cars use such wide tires in the back?

[Billfred]

Originally Posted by sanddrag

That's what simple physics tells you, I know. But I have a feeling the interlocking action both on a macroscopic and microscopic scale between tread and carpet is anything but simple. Why do drag racing cars use such wide tires in the back?

Paul Copioli touched on that very question here.

<edit>There is a benefit to going wider in the form of slower wear on the tread surface. Perhaps not too useful for a one-regional team, but the long haulers might use that to their advantage.</edit>

[Jeff K.]

Here is something on calculating coefficient, just find the 2nd reply
http://www.chiefdelphi.com/forums/sh...ht=coefficient

Edit:Sanddrag beat me to posting so no need for the link.


From experience though, the IFI Traction Treads, roughtop/wedgetop are the way to go if you want traction. IFI did some math and found out Wedgetop has a coefficient of friction of 1.2 and the roughtop has a coefficient of friction of 1.3.

[JVN]

Originally Posted by JoelP

So I've come to ask assistance from the chiefdelphi community. Since contact area is independent of the coefficient of static friction, wheel size and number shouldn't matter correct?

What is the coefficient of friction for a 130lb robot (I hope they don't raise the weight limit next year.) on standard FIRST carpet (I hope they don't change that either.) with normal kit skyway wheels?
with Colson wheels?
with IFI traction wheels (roughtop/wedgetop)?
with polyurethane wheels (any specific durometer)?

Skyways - ~0.8
Colsons - ~1.0-1.2
Roughtop/Wedgetop - ~1.2-1.4

Those are the benchmarks I use.
JV

[Alekat]

Originally Posted by JVN

Skyways - ~0.8
Colsons - ~1.0-1.2
Roughtop/Wedgetop - ~1.2-1.4

Those are the benchmarks I use.
JV

Wait, I didn't think coefficients of friction got above one. How do you get more frictional force than normal force?

[Paul Copioli]

The coefficient of friction can theoretically reach infinity (think Spiderman). The coefficient of friction can be calculated by taking your floor material and putting it on a ramp. Be able to change the angle of the ramp with respect to the floor. Place your item that you want to test on the ramp. Change the angle until the item starts to slip. Measure the angle. The coefficient of friction equals tan(angle). As you can see, any angle that can get higher than 45 degrees with have a coefficient of friction higher than 1.

[Not2B]

Originally Posted by JoelP

What is the coefficient of friction for a 130lb robot (I hope they don't raise the weight limit next year.)

Don't worry about the weight. It has no effect on the coefficient of friction. It DOES, however, effect the force from friction. Force of friction = Coef of Friction * Normal Force. The weight is what we would call the "Normal" Force. At least, it is on a flat surface. (The normal force is really perpendicular to the surface you are thinking about, so a robot on a ramp has a force down based on weight, and a portion of that weight as a normal force.)

I just looked that over - if I confussed anyone... I'm sorry. I should look for a picture to explain that.

But I think your question has been answered. If you want to test the coef of friction of materials, let us know - I know there are several simple test rigs you could use. (But be careful... there is Static Friction, Dynamic Friction, Rolling Friction, and others, etc... make sure you learn the difference and check for the right one.)

[JoelP]

Originally Posted by Not2B

Don't worry about the weight. It has no effect on the coefficient of friction. It DOES, however, effect the force from friction. Force of friction = Coef of Friction * Normal Force. The weight is what we would call the "Normal" Force. At least, it is on a flat surface. (The normal force is really perpendicular to the surface you are thinking about, so a robot on a ramp has a force down based on weight, and a portion of that weight as a normal force.)

I just looked that over - if I confussed anyone... I'm sorry. I should look for a picture to explain that.

But I think your question has been answered. If you want to test the coef of friction of materials, let us know - I know there are several simple test rigs you could use. (But be careful... there is Static Friction, Dynamic Friction, Rolling Friction, and others, etc... make sure you learn the difference and check for the right one.)

I was only worried about that because the larger weight limit would create a larger normal force with the same coeff. of friction which would result in a larger frictional force than I had designed the transmission for.

[lukevanoort]

Originally Posted by JoelP

I was only worried about that because the larger weight limit would create a larger normal force with the same coeff. of friction which would result in a larger frictional force than I had designed the transmission for.

What is your particular worry about having more frictional force? If it has to do with turning, using a six wheel drive should work with any coeff.

[JoelP]

Originally Posted by lukevanoort

What is your particular worry about having more frictional force? If it has to do with turning, using a six wheel drive should work with any coeff.

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.

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.

[DonRotolo]

I think you might be asking the wrong question. Instead, it seems like you want to know "What's the maximum torque I can expect to have to transmit to the wheels?" - am I correct?

If so, the amount of frictional force the wheel can see is important, but if we assume the wheel is never going to slide - like a gear on a rack - then it becomes a question of how much torque you can generate with your motor(s), and considering your gear ratios. This simplifies the problem, and makes knowing the wheel configuration irrelevant.

If I'm wrong - I frequently am - please also remember to think about shock loads, which can be easily 2 to 4 times the regular load. Then a factor of safety, at least 2. Then remember to de-rate all the components that are not perfect shapes - for example, a hole or keyway in a shaft has a known and calculable effect upon it's strength and load capacity.

Another thing to consider would be to make a "fuse" - a part that will be the first to break, if necessary, but is easy to replace in a big hurry*, and of which you have plenty of spares. Like a shaft with a purpose-cut weakening groove.

Sounds like a very interesting project, good luck.

Don


*5 minutes is far too long. Ask Team 11 about their experience at Monty Madness, where they changed out a broken shaft that was about 30 minutes inside their robot in an absolutely amazing 7 minutes (I timed it). Unfortunately, the match started after 5 minutes, and so they couldn't get onto the field in time.

[JoelP]

Originally Posted by Don Rotolo

I think you might be asking the wrong question. Instead, it seems like you want to know "What's the maximum torque I can expect to have to transmit to the wheels?" - am I correct?

Yes, that is the question I ultimately wanted to answer, but I have all the information to do the calculations and come to that answer myself, except for the frictional force the transmission will have to overcome, which is why I needed the Coeff. of Friction of different wheels my team was considering.

If so, the amount of frictional force the wheel can see is important, but if we assume the wheel is never going to slide - like a gear on a rack - then it becomes a question of how much torque you can generate with your motor(s), and considering your gear ratios. This simplifies the problem, and makes knowing the wheel configuration irrelevant.

If I'm wrong - I frequently am - please also remember to think about shock loads, which can be easily 2 to 4 times the regular load. Then a factor of safety, at least 2. Then remember to de-rate all the components that are not perfect shapes - for example, a hole or keyway in a shaft has a known and calculable effect upon it's strength and load capacity.

Another thing to consider would be to make a "fuse" - a part that will be the first to break, if necessary, but is easy to replace in a big hurry*, and of which you have plenty of spares. Like a shaft with a purpose-cut weakening groove.

Sounds like a very interesting project, good luck.

Don


*5 minutes is far too long. Ask Team 11 about their experience at Monty Madness, where they changed out a broken shaft that was about 30 minutes inside their robot in an absolutely amazing 7 minutes (I timed it). Unfortunately, the match started after 5 minutes, and so they couldn't get onto the field in time.

Thanks, I'll be sure to take those in to account. I've been told to design the transmission and drivetrain to be practically bulletproof.