Thread Stripping in AL 6061 T6511

[goalieman01]

Hey,

I am still young engineer and need some help.

I received a customer product the other day where the threads in an Aluminum 6061 T6511 plate had been stripped out. This product has been around awhile and this is the first occurrence. There are some other issues I am also dealing with but this is the only one they complained about.

So the issue I am trying to solve is why and how the threads were stripped. My initial thought and gut feeling is that they where simply over torqued, the thread size is M6 x1.0, and there is roughly 4 threads of engagement. The cap screws used to tighten the assembly together are made of Alloy steel grade 12.9.

The second thought was looking at the material properties of Al 6061 T6511. The assembly is rated for and often used at 204 C (400 F). It is a small actuator rated for 4 – 8 bar (60-120 psi).

Any help in how to figure it out or where to find the properties of Al and 400 F would help. I have been looking to find them but am coming up empty handed. My next step is going to be testing some units off the line and seeing how much torque is required to strip the threads. Thanks for the help

 

[swall]

You have a mismatch using a 12.9 fastener in aluminum threads. An 8.8/9.8 would be more appropriate. 6061-T6 is aged at 320F, so exposure to 400F will diminish the strength over time.

 

[BobM3]

Only 4 threads engaged? In any aluminum, I try to get at least 1.5 thread diameters of engagement. Have the screws been removed and inserted more than once?

 

[goalieman01]

swall - thank you, I will look into why the 12.9 fastener is called out on the assembly drawing. Why would and 8.8 or 9.9 be more appropriate, and how can I find this information out in the future(I was not he designer or engineer on the project)? I would guess a Machinery Handbook?

BobM3 - I went through ProE and took some measurements and there should be roughly 5.6 threads of engagement. I am unsure at the moment if the screws have been removed previously but I think they have. They had some air leaking from one of the seals (different problem). I will check and find out soon.

Thanks for the quick responses.

 

[BobM3]

Aluminum doesn't take repeated tightenings and loosenings very well. Use a thread lubricant on the threads if they have to be loosened/unloosened. Make sure the chamfer on the threaded hole isn't so deep as to be robbing you of part of a thread engagement. Consider helicoils or threaded inserts in the aluminum if you can't go deeper on the threads.

 

[CoryPad]

Bolted joints work best when the strengths of the mating threads are close to each other. For your joint, the bolt is 4 times stronger than the mating part, which makes it sensitive to overload. An 8.8 bolt is only ~ 2.7 times stronger than the Al, which makes it less sensitive to overload.

Machinery's Handbook does have quite a bit of information on threaded fasteners.

Here are some helpful Internet sites:

http://www.boltscience.com

http://www.archetypejoint.com

http://roymech.co.uk/Useful_Tables/Screws/index_screws.htm

http://www.bossard.com/index.cfm?app_page_id=3075,1

If you are serious about learning about bolted joints, then look at these references: faq725-600

 

[goalieman01]

Thanks for all the help, From what I know they the part didn't take repeated tightening, but I don't trust them enough to say they never or someone never tried.

Cory - I appreciate the links, I am talking with Boltscience about their software and training for our company. I will be looking over the other links for find more information for future design and analysis work.

I will post and update when I reach a solution if anyone is interested.