Friction coefficient of oak on steel 2

[tnteng]

Can anyone give some feedback on what pressure is assumed when a coefficient of friction is listed for oak (or wood) on steel in various tables that are available. The friction coeficients that I have found for oak (or wood) on steel have all been in the range of 0.2 to 0.6. For my current problem I have selected .4 for the friction coefficient. If the bearing pressure between the steel and wood increases beyond a certain limit, the friction coefficient would likely shoot up to some higher value due to compression of the wood fibers.

Thanks in advance for any help on this matter.

Tony Billeaud
Mechanical Engineer
Franks Casing Crew

 

[CountOlaf]

I would think the friction coeff. stays the same, but the resulting resistive force of friction which is linearly dependent on the normal force and that "constant" coefficient will go up if say the "bearing pressure" is increased. The range of values for the coeff. may be due to different coarseness of the oak tested or the steel finish as opposed to varying weights or bearing pressures of the materials in question.

 

[JStephen]

Coefficient of friction is real easy to measure if you have sample materials. Set a block of wood on a piece of steel (or vice versa), and tilt it until the block slides. Measure the angle. Sum the forces acting on the block and solve for coefficient of static friction.

If you want somewhat increased stress, you could put more weight on the block, and/or reduce the size of the wood part that touches the steel.

 

[ornerynorsk]

Burnishing over time also reduces coefficient. Back in the day, oak was sometimes used as a bearing block, as well as lignum vitae.

 

[unclesyd]

Well seasoned oak is a very good bearing if use Tallow (beef fat) as the lubricant.

 

[ccw]

Hi tnteng,
Several years back, a company that I worked for substituted UHMW polymer for long used oak slide bearings used on solid waste packer slides. My investigation of this material showed that the coef. of friction between UHMW and steel actually went down with increase in bearing pressure, at least to the point where the pressure was slightly less than yield (for the purist I recognize that plastic is not elastic).

Jstephens test is a good way to zero in on the right combination of materials, loads, and performance. Also the Ornerynorks burnishing tip should not be overlooked, as it otherwise makes the coef. friction appear to be time dependent.

CCW

 

[vonlueke]

JStephen: Excellent post, except I didn't understand the last sentence of your first paragraph. The coefficient of friction (COF) of the tilted surface is merely COF = tan(theta) = slope = rise/run, which means the angle of repose is theta = atan(rise/run).

Static COF is the slope (rise/run) at which the force of gravity overcomes static friction and the object first starts to move downhill. Kinetic COF is the slope at which the object continues sliding downhill when already in motion, which is always less than static COF.

So once the object starts sliding downhill, you then reduce the tilt of the surface until the object just stops sliding, to measure kinetic COF.

I just used the above method to measure my mouse on a fabric mouse pad, on a tilted board, and found its static COF = (49 mm)/(320 mm) = 0.153, which means the angle of repose was theta = atan(0.153) = 8.7 deg. I believe you'll find this to be rather low among mice (very smooth starting/stopping action).

 

[JStephen]

What I meant by the last sentence:

Normally, in physics problems and the like, you assume that coefficent of friction is independent of the pressure. And in this case, it doesn't matter what the size or shape is of the block of steel or oak.

If you wanted to see what the effect of increased pressure was, you could add weights to the block and retest it. If coefficient of friction is independent of pressure, then it won't change, and it will slide at exactly the same angle with or without the weights. You could also reduce the contact area of the block, or do both, and within limits, could make the contact pressure as high as you wanted to.

 

[metman]

Machinery's Handbook has a very nice aricle on COF. COF remains constant for the two materials in question for given surface conditions which of course includes dry or wet with whatever fluid and regardless of pressure except when the pressure is increased above some relatively high threshold level.

I once used vonlueke's COF = tangent(theta) in a dramatically simple example to show that our underground coal mining machine as applied in a Progamesia, Spain Bauxite mine needed bogey wheels. Can you imagine the difference in friction for steel sliding against steel for coal(carbon) vs Bauxite(Al2O3 = Aluminum Oxide)?

 

[007Driver]

Hello,

Maybe Im out in left field here, but I thought to measure Kinetic COF you tilt the base plate so the object sliding down the hill does not accelerate. If the item comes to a stop, you will have to know the speed of the item before you tilted the base and that would get tricky to calculate.