Friction & Surface Area 2

[ciancon]

Hello,

I know that Friction is not affected by Surface Area. My question is why?

I've got it in my head that the Friction is actually linked to Pressure, and that as the Surface Area of a part increases, the Pressure decreases accordingly. As a result, the Friction balances out.

Does this make sense, or is this a complete misconception?

And if thats the case then why do some parts have small contact points in order to reduce the friction against other parts? Or am I getting confused with Surface Roughness?

 

[chicopee]

Stick to the classical equation for frictional force. Frictional force is directly proportional to the coefficient of friction and the normal force imposed on the item under frictional force. The increase in contact area may be required to reduce contact stress.

 

[JohnRBaker]

I've got it in my head that the Friction is actually linked to Pressure, and that as the Surface Area of a part increases, the Pressure decreases accordingly. As a result, the Friction balances out.

Your first assumption is correct, which means that while your conclusion may be accurate, it's only because the Pressure decreased, NOT because the surface Area increased.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[3DDave]

Small contacts typically are there to decrease the effective lever arm - hence small clocks have tiny axles in order that the friction has little leverage to create torque with. The other reason to decrease contact area is when there is lubricant that might be displaced; less area means less friction from fluid shear, but if the area is too small the fluid will be mostly squeezed out, replacing fluid shear with galling friction.

 

[GregLocock]

However many real world cases exist where the high school 'friction force is independent of area of contact' rule is not valid, in detail. Examples include wet or oily PU sliding on steel, tires on asphalt in the dry, tires on wet asphalt, and rubber, on many surfaces. The friction force is a combination of several mechanisms and they occur in different proportions for different materials and load regimes. For example a tire has a very high friction coefficient near zero vertical load, as the rough surface of the tire locks onto the other surface geometrically (like intermeshed gears).

Cheers

Greg Locock


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

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

 

[TimSchrader2]

Hello
Theoretically surface area is not a issue. But I have seen where it can make a difference in some things we make. Especially if there is some, even a little roughness to the surface, as mentioned above.

It can definetly make a difference on water slides. By arching the back and limiting contact area you can go quite a bit faster. The attendent of the slide did not count on this and I almost hurt someone real bad near the bottom of the slide. Thankfully I was able to avoid a direct hit. It could have been real bad.

 

[hydtools]

That had more to do with less fluid shear than friction on the slide.

Ted

 

[GregLocock]

Obviously more research is needed. "Optimal posture for maximum speeds on a global selection of waterslides". Grant please.

Cheers

Greg Locock


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

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

 

By arching the back and limiting contact area you can go quite a bit faster.I personally developed that technology years ago. I knew I should have patented it.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[BUGGAR]

By arching your back to limit your contact area, I assume you will be sliding on your shoulders and your butt? What about those that are butt challenged (plus or minus)? Is it true that if your butt is dimpled like a golf ball, you will slide faster?

 

[TimSchrader2]

Hello. It has been a few years, but as I recall the goal was to get as much wieght as possible on the heels of the feet to reduce the normal force over the wider Butt. A dimpled butt cover would require a proptotype for testing.

The shear flow of the water is another variable but also dependent on surface area to some extent. Not sure what formula could be used to define how much is friction to the slide or shear flow. Googled this a bit and could find nothing directly applicable. Neither Could I find water slides on the Moody chart.

 

[handleman]

Don't forget... the word that should always accompany "friction" in the OP's context is "dry". When you introduce lubricant it changes the whole game.

In the instance of waterslides, I'd posit that you're in a fully hydrodynamic situation, where resistance to motion is mostly going to be dependent on the viscosity of the lubricant and the surface area along which that viscosity or resistance to shear is acting. Therefore, if you minimize contact surface using heels and shoulders, you minimize resistance.

 

[itsmoked]

Yes, but if the normal force is great enough you'll punch thru the lubricant and lose the hydroplaning aspect.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[SwinnyGG]

If you did that, you would know- because you would experience a lot of heat and pain very rapidly.

 

[Lou Scannon]

...followed by loss of skin and other flesh
Until enough fluid would be released to restore hydrodynamic lubrication

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[handleman]

Yes, but if the normal force is great enough you'll punch thru the lubricant and lose the hydroplaning aspect.

In my not-insignificant experiential knowledge of skin/waterslide tribology, on a properly constructed and wetted slide the hydrodynamic pressure is sufficient to support the weight of a body even on the smallest patch[es] of human skin over which it is physiologically capable of spreading its normal force.

 

[iainuts]

Physics 101 always says normal force times coefficient of friction gives you the MAXIMUM amount of frictional force that can be developed. But if the shear stress on that frictional interface is greater than the shear strength of one of the two materials, then it seems we should predict failure of that material such that the frictional force produced can't exceed some shear strength times the contact area. We might even look to see if the weaker material might begin to loose structural strength over time if shear load at the interface comes close to some value, above which the material will begin to fail similar to the way metals fatigue. I have to believe there's research that's been done on this but I'm not familiar with it. Feel free to post links to the research.