Friction coefficient for steel on steel with a water film between

[Nukie]

Hi there,

I'm looking to find the friction coefficient for steel on steel with a water film between .

Any references ?

Thanks

 

[profengmen]

That will depend on the surface finish and the force on the surfaces.

Look up the "Inclined Plane Method" of measuring frictional coefficients in a statics reference book.

 

[amorrison]

Note that when the steel/water heats up to 100C the friction coefficient will "change".

Some variables
1. smoothness(grove pattern from previous wear)
2. water content in steel wear groves
3. water viscosity - function of temperature of steel which will be a function of the energy in the friction which has already occured (time dependent).
4. steel to steel pressure

When automobile brakes are wet there is not much stopping power at all even with hydraulic assist.

Maybe the guys at

http://www.cfd-online.com

and the fluids group at eng-tips could help you further.

 

[TheTick]

That will depend on the surface finish and the force on the surfaces.

Actually, the coefficient is independent of force. The very nature of the inclined plane method shows this.

 

[CRG]

Do you mean static friction? Steel does not float and as such, will only be partially supported by the film strength, capillary action, and buoyancy of the water. Hence, this may be difficult to predict without experimentation that resembles the same surface conditions. If this is not in the horizontal plane, you can ignore the buoyancy.

Do you mean dynamic friction? Seems like you could look at shear forces on water to determine the friction. Are you making water lubricated bearings?

 

[profengmen]

thetick,
oh yeah... I do that all the time...second time in here actually.

 

[Nukie]

CRG,

I'm asking for a co-worker and he wants this to calculate the force to move a piece of equipment made of steel piled on top of a similar piece of equipement stored underwater.

We understand that the steel/steel coefficient of friction will be different underwater because of the water film who act as a lubricant.

But we want to know how much it will affect !

 

[sailoday28]

The theory of hydrostatic or hydrodynamic lubrication relates to a film between the two moving surfaces. The reduction in force necessary to move the surfaces is extremely dependent upon the film thickness.
Other than pumping a liquid film (hydrostatic similar to an air cushion vehicle)for this case
or depending on the geometery between the surfaces as in some bearings (where the rotating surface acts as a pump to maintain the film---
I don't believe you will be able to calculate that film thickness.

 

[EdDanzer]

The maximum force will be for steel dry. The coefficient of friction is .74 for static dry mild steel no mild steel. For items horizontal, his should provide the maximum force:
Force to move = .75 x weight of the top part.
This is form Standard Handbook for Mechanical Engineers, seventh edition, Baumeister & Marks

 

[rnd2]

For a properly engineered water lubricated sleeve bearing the shaft is indeed supported purely by a film of water (fluid film state).
How thin is the film? Less than 0.05mm. Once fluid film state is reached, the friction approaches that achieved with ball bearings.

 

[CoryPad]

[blue]EdDanzer[/blue],

Your data point is for parts in vacuum, which is not applicable here. I question using a single data point with two decimal precision when there is no supporting specifications, testing, etc. for this precise condition.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[arto]

Is rust a factor?

 

[EdDanzer]

CoryPad,
Friction is a very subject value. There is an error: Force to move =.74 x weight. In many cases when two item set any film between them migrates out leaving dry metal contact. We used this calculation with two decimal point precision when designing a 50,000 ft lb brake and were within 5% of design torque. This brake has aluminum bronze running against 4150 in an oil filled housing. The oil is forced out with the clamping pressure so a dry value was used.

 

[amorrison]

From Machinery's Handbook 19th edition p543
Coef. of static friction

Iron iron - clean 1.0
- lubricated 0.08
Brake material on cast iron dry 0.4
wet 0.2
which references
The "Friction and Lubrication of Solids",Vol. I,by Bowden and Tabor, Clarendon Press,Oxford, 1950

 

[chicopee]

since I dont believe that a water film can be maintained before movement assume the static coeffecient of friction to be around .15 to get things moving. during motion I'll guess that value will drop at least 50%

 

[harrisj]

Do some tests.

A few square inches of steel sections, with whatever surface treatment (or corrosion) is on the prototype, and a scaled load. A piece of string and a spring balance - voila!

Worth any amount of theoretical conjecture when there isn't enough data or the working environment is indeterminate.

Your wife may object to bits of rusty metal in the bath, though. (Sorry, that's not very PC)

Cheers - John