Friction Modeling Question Has Been Driving Me Nuts

[mxv]

Hi everyone,

I have a question that general physics doesn't seem to help with and I haven't been able to solve it by searching Google or this Forum.

I'm trying to figure out a more specific way to estimate frictional forces than what we all learned in school. I know we are all familiar with Ff=uFn where Ff=frictional force, u=coefficient of friction, and Fn=Normal force. With this equation a brick on its broad side being pulled will have as much friction as the same brick resting on its skinny side. I get this in theory using the equation, but this is too ideal. We all know that contact patch does have a lot to do with friction force or race car tires would be just as skinny as a bicycles to reduce weight. ( I know tire deflection and heat have a lot to do with that, but you get the idea. Every car suspension and drive train designer knows contact patch matters for determining how much torque the car can put down to the ground.)

I couldn't find any equations that estimate real life friction forces based off of surface roughness and some sort of value for local deformation. So my question is this. I know I'm not the first engineer to try and estimate or model friction forces. How have other done this in the past? I know real life testing is the only way to get concrete answers, but I'm hoping to find an equation that will at least help me narrow down my solutions so I'm not testing 100 different geometries and materials.

Thank you in advance for any and all help with this, and let me know if i need to clarify the question in any way to help you help all of us with this answer.

 

[GregLocock]

For rubber on asphalt a brush model produces directionally correct but inaccurate estimates of friction. There is matlab code for a brush model in the back of this book

https://books.google.com.au/books/a...p_read_button&redir_esc=y#v=onepage&q&f=false

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[mxv]

Thanks for the info on rubber @GregLocock! I'l have to store that one away for the next time i'm working with that material. Also I really appreciate the book link. It might have more info along the lines of what i am looking for.

I realize I forgot to mention the friction I am trying to estimate is plastic on steel. I am trying to figure out what would be the best geometry to use to design a custom bearing made form IGUS material that will rub on smooth (~8uin Ra) steel parts.

 

[handleman]

Ummm.... Why not talk to Igus? I hear they have some sort of vested interest in this kind of thing. My gut tells me that they have probably spent some time and possibly research trying to figure that stuff out.

 

[mxv]

You're not wrong handleman. I was exploring some other materials as well which is why I didn't want to bother them too much before I knew for sure I'd be using them as a supplier.

 

[FACS]

If you want to do an experiment, and measure the friction yourself; Use the Incline Plane method for measuring friction between two materials. This is easily Googleable so I'm surprised you didn't see it.

It's very easy and I've used it several times with good success.

 

[mxv]

Thanks for your input FACS! I don't have any of the material laying around to do this test, or else I would. Good call though. If I get desperate enough I might have to pony up and buy some stock and measure it that way.

 

[BUGGAR]

I think your are confusing traction (drag slicks) with friction (ice blocks in the lab). Traction is more of a geared tooth idea.