"Coeff of Friction" for slips on a tube

[cnuk]

I have an application where I need to grip onto the OD of a tube with jaws/slips to resist an axial load that is applied to the tube. There will be three or five jaws equally spaced around the OD to clamp. In order to size the hydraulic cylinders that will provide the clamping force I need to know the "coefficient of friction" I can expect to achieve between the carbide slips and the 4140 tube. I say "coeff of friction" because we plastically deform the tube OD with the slips so it is not really a true friction problem. I have read that numbers up to 1.6 are achievable for some slip designs but I don't know if that is a realistic value or an extreme value. I suppose it depends how much you plastically deform the tube. It's a difficult number to calculate so that's not an option for me. Are there any vendors or rules of thumb out there for slips grabbing onto tubes? Seems like there would be from the machine chuck and pipe handling industries but I don't know them.

Thank You

 

[desertfox]

hi cnuk

Try this link and scroll down till you get to tungsten carbide on steel:-

http://www.engineershandbook.com/Tables/frictioncoefficients.htm

desertfox

 

[boo1]

Since you clamping force is taking the tube to the plastic deformation, your are not appling just friction. You have exceeded the hoop strength on the material.

 

[GregLocock]

A test will take a couple of days to organise and will tell you far more than a book number. I suggest you also try it in ideal conditions, and then oily, and wet, and so on.

If you are cosnidering different teeth profiles etc it actually sounds like a taguchi approach might be appropriate.

Time to throw the stress books away and have a play in the workshop. You'll need a mass, a source of gravity, and a force gauge. All 3 are fairly easy to come by.







Cheers

Greg Locock

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

 

[tbuelna]

cnuk,

If your clamps plastically deform the tube, then the interface may have no resistance to axial force whatsoever. Friction force is simply Fn x Mu. Clean, dry steel on clean, dry steel may have a Mu of 0.30. Oily steel on oily steel may be as low as 0.03. But if your Fn is zero, due to exceeding the elastic limit of the tube material, then the resulting axial friction resistive force would also be zero, by definition.

To calculate the compressive hoop strength of your tube, refer to Roark's 6th edition, ch.12.

Good luck.
Terry

 

[MikeHalloran]

Carbide is pretty brittle, so may be not such a good choice for what amounts to a radial swaging die.



Mike Halloran
Pembroke Pines, FL, USA