COEFFICIENT OF FRICTION, AL. BUSH IN AL. PLATE

[StressMan2506]

Fellow Stress/Structural/Mechanical Engineers:

I logged on tonight looking for a value, or range of values, applicable to a freeze-fit Al. bush in Al. plate. Such info may be somewhere within Eng-Tips, but I think it'll take me a long time to find it, so can anyone help please?

I hit upon "How to analyze the stress caused by a Freeze plug" posted by USACompositeGuy. I cannot add to anything posted in response, but I noticed that MNLiaison appears to have experience in this area. Can you help me, MNLiaison?

I can calculate interference stresses, but do not have a referenced value of friction coefficient applicable to an eccentrically drilled bush. I need to check torque resistance.

Thanks in anticipation.

 

[desertfox]

Hi

try these links

http://www.tribology-abc.com/sub23.htm

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

regards

desertfox

 

[StressMan2506]

Thanks, desertfox. I've followed the links. Unfortunately they don't represent my case, but roymech could provide a path to the info I seek.

 

[RPstress]

Static Al on Al should be representative unless it was assembled with sealant.

Quantifying the radial forces might be trickier with an eccentric hole. I'd use a value for a central hole as an average. In extremis go for an FE model, but traditionally these sorts of calcs have been done simply by hand, ignoring things such as the proximity of the edge of the parent material (plate) and any eccentricities.

 

[StressMan2506]

Thanks RPStress. I've got the method for calculating interference pressures in a Mathcad and

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

as recommended by desertfox provides a dry al-to-al range which I have successfully made use of.

 

[Kwan]

I would not rely of friction of the plug to drive load through a joint. Vibration can seriously degrade your friction load path.

 

[StressMan2506]

Hi Kwan:

Thanks for your concern, but we commonly make use of freeze-fit bushes in concession conditions where an oversize fastener would be too large. If we didn't use bushes in such cases the part or assembly would be scrap. Where geometry forces us to drill the bush eccentrically, we need to check that the torque induced by the eccentric load can be resisted by the friction at the interface of bush and plate. We are talking here of forces which are ordinarily transferred by fastener shear and bearing, not by fastener tension. If there is such a load path, we introduce support washers. I am not aware of correctly sized and freeze-fitted bushes working loose in service although I appreciate it could happen through poor workmanship.

 

[Kwan]

I also work with bushings for concession conditions. I fully understand the situation. However, I don't accept the load transfer through an eccentric bushing (tension is ok but not shear). There are ways to create a good load path (doublers, etc).

 

[StressMan2506]

Hi Kwan:

Do you have details you can post somewhere?