Surface finishes

[Dougt115]

What is the best surface finish in terms of friction and fatigue?

 

[redpicker]

Depends.

If you are trying to increase fatigue and increase friction, the rougher, the better.

If you are trying to decrease fatigue and decrease friction, the smoother, the better (up to a certain point).

If you are trying to decrease fatigue and increase friction (or vice versa), well, you have some trade-offs to make.

rp

 

[EdStainless]

It does also matter what the material is and how you achieve the surface (grinding, lapping, honing).

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Plymouth Tube

 

[tbuelna]

What is the best surface finish in terms of friction and fatigue?

Surface finish or surface roughness? I would interpret "surface finish" to mean some sort of surface coating or treatment. While surface roughness would relate to the micro level surface texture.

There are many surface coatings (or finishes) that can reduce static friction, and there are surface treatments that can improve fatigue performance by producing a residual compressive stress layer in the outer surface of a metal component (shot peen or nitriding).

With regards to surface roughness (or texture) in terms of friction and fatigue, the answer is a bit more complicated. All other things being equal, a sliding contact interface between two similar metal components will have reduced friction characteristics with reduced surface roughness and a smoother texture. The smoother texture should produce lower contact pressures at the numerous tiny asperity tips, and thus a reduced tendency for the mechanical welding/bonding effect between tips that can increase friction. However, fatigue properties of metal components are usually not improved simply by having a less rough (or smoother) surface. Surface fractures typically originate at the numerous tiny stress concentration points located at the "valley" bottoms between surface asperities. So making a surface smoother simply by knocking off the tops of the asperities does not reduce the local stress concentration situation existing within the valleys, and would not improve the fatigue situation much.

Hope that helps.
Terry

 

[dgallup]

Lubricated or unlubricated?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Dougt115]

Sorry I should have been more specific.

The desire is to decrease friction and maintain endurance on a 17-4 stainless steel currently hardening to H900.

 

[EdStainless]

Friction against what?
Is there any lubrication? can there be?
How much load?
Do you care more about sliding friction or the initial break away force?

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Plymouth Tube

 

[Dougt115]

Sliding in a bushing and there is lubrication. I am having a heating issue and at 0.5Hz and a 0.5 in reciprocating travel I need to get the friction down.

 

[EdStainless]

I doubt that the surface finish of the 17-4 will have that much impact, since I presume that you have a fairly good finish on the part now. You could hone or lap it, but if you get too smooth you may loose the ability to carry lubrication.
The bushing material, clearance, and the lubricant will be more important.

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Plymouth Tube

 

[tbuelna]

Dougt115-

It sounds like you have a heat transfer issue rather than simply a friction issue. But you still need to provide a bit more information before we can diagnose the problem and suggest a remedy.

You say there is a "heating issue" with your 17-4 H900 cres bushing and the apparent source of the heat input is sliding friction resulting from "reciprocating travel". The mechanical properties of your 17-4 H900 bushing won't be significantly affected until the bushing gets close to its 900degF hardening temperature. You also mentioned there is some form of lubricant present at the sliding interface, but most conventional lubricants that I know of (oils or greases) are limited to operating temps of less than 500degF. Finally, since the friction heating in your bushing/shaft contact is the product of radial load and reciprocal axial sliding motion, there is not much improvement (in terms of friction heating) that will result from minor changes to surface texture/roughness.

Due to the fact that you have reciprocal sliding contact motion there is a brief increase in friction when the sliding stops and reverses direction during each cycle. There are some very hard surface coatings, such as thin-dense-chrome or DLC, that will reduce the friction loss at this condition. Another thing that you can do to minimize the increased friction at the point where sliding stops and reverses direction is to make sure your bushing/shaft fit has adequate L/D proportions and minimum radial clearance. This will minimize the tendency of the shaft/bushing to "cock" relative to each other and edge load when the direction of motion reverses. The increased radial edge loading effect at opposing ends of the bushing due to cocking can be quite significant.

Lastly, if your "heating issue" is mostly a result of limited heat transfer away from the sliding contact interface, then the easiest fixes are to increase the total surface area at the contact interface and/or change the bushing material to something with better thermal conductivity.

Hope that helps.
Terry

 

[Tmoose]

if the motion is sinusoidal the Peak velocity is less than 1 inch per second.
How much radial load is there?

Others' questions about "heating issue" interest me too.

 

[tbuelna]

Tmoose,

Given the .5" stroke @ 0.5Hz and assuming a conservative Mu value (.15?) at the lubricated sliding interface, all we would need is the normal (radial) force to make a rough estimate of the heat energy generated by friction.

We also know that the bushing material is 17-4 H900 cres which has relatively poor thermal conductivity (k=12.7 Btu/ft-hr-degF). If we knew the dimensions of the bushing (ID, OD & length) it would also be possible to do a quick evaluation of the heat transfer situation at the contact interface. If there are very high contact pressures at the bushing surface, due to limited bushing surface area, high normal forces, or a combination of the two, then it may be possible that there is insufficient heat transfer capability through the bushing wall to keep the sliding surface temperature within acceptable limits. The conductive heat transfer situation would be made worse if the mounting arrangement created some thermal isolation of the bushing.

Interesting problem. Maybe Dougt115 will provide some additional details.

 

[EdStainless]

You need to make sure that the surface finish on the hard component (17-4) was done in the direction of travel.
Yes it is possible to put a linear finish on a shaft.
I am most concerned about the bushing material, bushing length, and radial clearance.

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Plymouth Tube

 

[TVP]

Regarding improved surface roughness for fatigue, you may want to consider Isotropic Surface Finishing:

http://www.remchem.com/page3-31/ISFProcess

http://www.rosler.com/produkte/gleitschlifftechnik/isf_verfahren/

http://www.pfonline.com/articles/sm...nish-through-chemically-accelerated-vibration

 

[Dougt115]

Details of the part.

Shaft is 0.375 inches diameter, polished and chrome hardened.
Silicon lubrication
Bushing is .188 inches long, split, style 17A.
Load is axial with pivots to prevent loading. (Think of the old steam engine reciprocating drives.)

Temperature range is from room up to 160 degrees F where is seizes up.

The bushing is my leading concern right now. As I cannot get a complete datasheet for it.

 

[TVP]

Is the chrome surface galled or otherwise damaged? I would definietly investigate the bushing. Is it possible that bending is introduced, causing excessive contact stress along the bushing?

 

[tbuelna]

Dougt115-

Thanks for the reply. Unfortunately, your post brings up even more questions.

Can you provide a sketch of the mechanism in question, including dimensions and how forces are applied to the components? Your post states that the applied load is "axial" and acts through a pivot, so the only way the bushing would experience any forces capable of creating sliding friction with the shaft is if the pivot is offset and produces a "cocking" moment on the bushing. The bushing does have a low L/D (0.50) which would make it susceptible to cocking. If you know what the diametral clearance between the shaft and bushing is you can do a quick analysis to determine how much moment force is being applied at opposing bushing/shaft contact points due to the cocking. If there is excessive clearance between the bushing/shaft, then the cocking moment may produce sufficient normal force at the bushing sliding contact points to create a crude form of friction brake.

Regards,
Terry

 

[Dougt115]

Thank you for your help. I have further isolated the problem to a hydraulic fluid and not a friction issue.