Coefficient of friction problems 2

[nickjk]

I am currently testing a project that uses a stainless steel sleeve made out of 440c to lock onto a stainless steel shaft made out of 17-4 by subjecting a uniform external pressure to the sleeve.

My calculations show that the resultant pressure applied to the shaft by the sleeve after removal of clearance to be 18,800 psi. The effective area of the I.D. of the shaft to be .830 sq in. (Contact force applied to the shaft by the sleeve = 18,800 psi x .830 sq. in = 15,604 lbs

Testing showed the shaft to slip when 2300 lbs was applied axially.

2300 lbs = coefficient of friction x 15,604 lbs

Coefficient of friction = .147

My goal was to be able to apply 2750 lbs axial to the shaft. Therefore my first thought was to raise the coefficient of friction by changing the surface texture / finish of the shaft. One of the shafts I had a .007 deep x 20 TPI thread ground on the outside diameter. On another shaft I had the outside diameter glass beaded.

Test results from these two shaft came up the same.
At approx 2300 lbs the shafts would slip.

After this test I no longer feel it is the surface finish limiting the calculated coefficient of friction. I believe it has to do with the strength of the 17-4 shaft and peeks and valleys. I feel that with the 18,800 psi pressure applied and the high axial load the material starts to gall causing a slipping motion.

My experience in this area is very limited.

Anyone's help to prove or disprove this theory and possible solutions would greatly be appreciated.

Thank You,

Nickjk

 

[coatingengineer]

Hi,

Texturing, I would agree, will offer you only short-term success. At these pressures, your 'peaks' have no structural support and will lead to adhesive-wear. Hence, the cold-weld phenomenon you've seen.

Consider Aamco-Bronze. That way, you'll maintain a mated surface with comparable hardness, but with some intrinsic lubrication -- something to consider in applications involving tribological wear.

Good luck!

William Gunnar



William Gunnar

http://www.IndustrialCoatingsWorld.com

 

[salmon2]

I had my dissertation on tribology, still I feel I can be wrong on my answer.

nickjk, I think you are walking on the correct direction, it has something to do with the flow strength of material and surface textures.

I do not exactly know what you have done in your last trial, glass beaded, etc. But reducing surface roughness is one way to go, this way the real contact area is bigger and you increase the slip resistance given the same material strength and friction coefficient. FYI, the real contact area is much much smaller than the apparent contact area usually.

 

[nickjk]

I am currently doing some testing with very interesting results. Waiting for all testing to be complete before I discuss further.
Thank you both for your time.

Nickjk

 

[salmon2]

I am interested to see how it go, please update.