Coating to increase friction on stainless steel 3

[nickjk]

I am looking for a high friction coating to be used on stainless steel (this is opposite for what most coatings are used for and I am having a hard time finding info.)

The requirements are as follows:

Coating to be applicable and applied to #17-4 stainless steel shaft.

The shaft travels through a 440C ground I.D. stainless steel sleeve.

From testing data I found the current coeficient of friction between the sleeve and shaft to be .15 when a calculated contact pressure of 18900 lbs/in sq. is applied to the shaft by squeezing the sleeve.

Because of limited space requirements I need to raise the coeficient to a minimum of .20 perfered .30 at 18900 lbs/in sq of contact pressure. I cannot raise the contact pressure.

Purpose of friction is to lock shaft to sleeve and prevent a vertical movement when sleeve is squeezed.

Note: Clearance between shaft and sleeve when sleeve is released is .0007 inch total. What would be the thickness of the coating and its abbility to hold tight tolerance.

The Load application will be applied approx. 250,000 cycles per lifetime.

Any help and sources would greatly be appreciated.

Thank You,

Nick

 

[metengr]

nickjk;
Why don't you simply increase surface roughness to increase the COF? This is relatively easy to do and avoids the use of any coatings that could delaminate in service.

 

[CoryPad]

Ekagrip from ESK:

http://www.esk.com/en/products-bran...rt-coating/electroless-nickel-composites.html

Coating thickness of 10 to 20 [µ]m (0.0004 to 0.0008 inches).

Regards,

Cory

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

 

[nickjk]

Because of the .0007 sliding fit and close tolerence of
+/-.0001 on the shaft and the grinding process to maintaine the fit, the current surface finish call out is between 15-35 micro inch. The inside of the sleeve is probably between 10-20 micro inch.

Please note: I am a design engineer and my experience relating actual surface finish to coeficient of friction numbers when dry, wet, static, kinetic is very limited.


During my research I found a article on a hub that uses a special coating to increase the coeficient of friction from .14 to .30. Although the product is completely different the solution is the same. I cannot increase the size to increase pressure - I need to increase friction.

see following link.

http://www.engineeringtalk.com/news/voi/voi105.html

What is that special coating to increase the friction value?

Thank you for your responce and any continued help would greatly be appreciated.

Nick

 

[mewhg]

How about using one of the Loctite cylindrical locking compounds like 680? I have used these in the past with very good results.

 

[EdStainless]

I would approach this as a surface texture issue. It isn't just a matter of making it rougher, but you need to control the texture. I would suggest that you try grinding the surfaces to a 15-20uinch and then using a fine abrasive blasting to add micro texture. This should be a very durable solution once you find the correct roughness.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[nickjk]

Thank you for your suggestions

I tried three different approaches to change and control the surface texture of the shaft without using a coating.

1) I ground a thread approx. .007 in. deep with a 20 TPI to help add a uniform texture.

2) I used a glass bead blast to provide a uniform texture.

3) I used an aluminum oxide blast to provide a rougher texture than the glass bead. During assembly I was not able to install the shaft into the sleeve because of its effect on the original .0007 clr.

Approach 1 and 2 did not change the testing results from the original shaft tested. The coefficient of friction remained unchanged.

I always thought that static frictional force was independent of surface area for calculation. Friction force = coefficient of friction x normal force.

I am starting to think that the 17-4 shaft does not have the physical strength in its surface to withstand the 18,900 lbs per in. sq. pressure and the applied load. I think maybe a fretting condition starts and then the sleeve allows the shaft to slip.

This is important because if I go to a coating to help solve the problem, do I need a coating to help with the strength characteristics of the shaft or do I need a coating to increase the surface texture and strength.

I will try to research this further.

Please any suggestions or ideas would greatly be appreciated.

NickJK

 

[steryyy]

if you want do a purchasing you can see coating firms at

http://www.finishing-blasting.com

 

[metengr]

Purpose of friction is to lock shaft to sleeve and prevent a vertical movement when sleeve is squeezed.

.... could you consider an interference fit joint between the shaft and sleeve to permanently lock it in place?

 

[MikeHalloran]

Guys, OP wants to lock|release _on_command_, not lock permanently.

Personally, I don't hold out a lot of hope for substantially increasing the friction coefficient. Try immersing it in Dexron ATF.. and that's probably about as high as it will get.



Mike Halloran
Pembroke Pines, FL, USA

 

[StoneCold]

Why not just cut a keyway in the shaft and be done with it?

Regards
Stonecold

 

[MikeHalloran]

I think OP wants to lock the shaft against axial motion, like a fair number of commercial hydraulic cylinder locks already do.

This would be a good time to buy all the commercial rod locks you can find, and disassemble and reverse engineer them to figure out exactly how they work.



Mike Halloran
Pembroke Pines, FL, USA

 

[owenmaterials]

nickjk, I would suggest that you choose a coating material with low yield strength and high shear (tensile) strength.

 

[nickjk]

ownmaterials, One of the coatings suggested to me was a trade name Tikankote C6.
Process PVD
Composition TiAIN
Thickness 1.5-5.5 microns
Hardness 3000-3400 HV
Coefficient of friction .4
I do not know the yield and shear strength, but I will inquire right away.
Do you have any thoughts on the above coating or suggestions on another coating or process.

Thank you,
nickjk

 

[owenmaterials]

You do not have to know the actual yield and shear strength.
It seems that although a coefficient of friction of this AlTiN, .4, is not too impressive, it will serve your purpose well as you mentioned that you only need a coeeficient of .2 to .3.

 

[unclesyd]

You might give these people a call. The as treated surface of 17/4 has a high CoF in the unlubricated state. We have to insure that all parts that are LNC are well lubricated when in contact with other components.

http://www.nitromet.com/

If you have room you might look at a Spieth Sleeve for as you clamping device.

http://www.ame.com/line.cfm?id=comp-spiethsleeve

Also Industrial Hard Chrome plate unlubricated can have a very high CoF. We have rolls that are hard chrome plated to pull synthetic fibers for processing. The rolls are vapor honed to increase the friction. At this point there is no lubrication on the fibre that would lower the CoF between the roll and fiber.

 

[mewhg]

I second unclesyd suggestion of a clamping sleeve. There are several on the market that do a good job.

Why does the sleeve need to be released?

 

[nickjk]

For the application, the sleeve needs to release and cycle many times. I have used commercial locking sleeves in the past. This application does not have the space available, If I could increase the shaft diameter or increase the length of the sleeve this would not be an issue. I was hoping to get information on maximum normal pressure and sheer stress (for the coating) that a mating detail could apply.

Formulas related to frictional forces and coefficient of friction are independent of surface area. Two surfaces mated together will contact on their peeks and valleys or peeks and peeks. When I calculated the pressure applied I used the actual surface area but in all reality the contact area will be a great deal smaller making the pressure at the peeks a great deal larger. How much normal pressure or shear can those peeks and valleys take before the surface distorts and fails and will the hard coating process help?

I feel I am on the right track but do not have the data to get that warm fuzzy feeling.
Next week I am going to have the shafts PVD coated (TiAIN) as specified above.

If you believe I am all wet or that I might be on the right track, please any information would greatly be appreciated.

And thanks everyone for your suggestions and support.

Nickjk

 

[MikeHalloran]

I think you're in deep trouble.

It's time to re-work the application so there _is_ room for a commercial locking sleeve, which you already know how to apply.

The other rational choice is to get a commercial rod- lock producer involved in extending the envelope of what they produce in a direction that can fit in your product. They know where the corners of the solution space are, and how closely they can be approached in a practical and producible way. We, here, don't. Okay, _I_ don't.







Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi nickjk

Have a look at these sites regarding friction they may prove of interest:-

http://www.engin.brown.edu/courses/en3/Notes/Statics/friction/friction.htm

Its a good idea to read it all, I found it quite interesting
but the real interesting bit for me was:-
Scroll down on the above site untill you see this heading:-

12.4 Static and kinetic friction

At the bottom of this section it states that friction force can only be slightly influenced by surface roughness.
It also mentions about a coating on the material surface:-

"Each asperity tip is covered with a thin layer of oxide, adsorbed water, or grease. It’s possible to remove this film in a lab experiment – in which case friction behavior changes dramatically and no longer follows Coulomb/Amonton law – but for real engineering surfaces it’s always present."

It goes on to say about these layers being of low mechanical strength which deform and allow to slide past each other.

My point is that you haven't said how the materials are cleaned ie:- as there been any attempt to clean these surfaces of any layers as described above?
In addition if you only coat the shafts as you have stated
in your latest post then the sleeve asperities may still
allow sliding at the same force you have now.
Also I agree with you regarding the pressures you have calculated in that they will be infinitely higher due to
only having line contact.
I would have been tempted to use formula's for a roller
in contact with a concave surface similar to this site:-

http://www.mech.uwa.edu.au/DANotes/MST/contact/contact.html

however your case would involve two concave surfaces assuming the sleeve is split.

At the end of all this I have to agree Mike Halloran and others that using a off the shelf solution is your best way forward.
Here is another site regarding friction which may be of interest:-

http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm#Factors

regards

desertfox