Stainless Bolts on Aluminum 3

[mit227]

Hey everyone,

Where i work im in charge of finding a retrofit for a design flaw. basically, it appears that some bad calculations were made and two M6 Stainless Steel fully threaded bolts ate through a bracket of AL 6061. I also noticed that the through holes were much too large (1/4" dia).

I went ahead and ordered 1/4" SS partially threaded bolts placed on all new systems and had the thickness of the AL bracket doubled; yet I am not completely convinced that this will keep the bolts from tearing at the aluminum once more. Another problem is that this system doesn't have regular stresses placed on them, instead they tend to be shocks and jerks.

i was wondering if we were to change the bolt and hole sizes for the future productions from 1/4" to m8 (we want to stay metric), if this would make a difference? and if anyone has any ideas on what else i may do to solve this problem?

WJM,

Miami, FL

 

[KENAT]

Stainless and Aluminium can have galvanic problem, is there any evidence of this?

KENAT, probably the least qualified checker you'll ever meet...

 

[CoryPad]

The description of "ate through" suggests corrosion. If so, then you should consider aluminium screws instead of stainless.

Regards,

Cory

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

 

[kenvlach]

"ate through" does indeed sound like corrosion.
Have the aluminum anodized.
Are you using washer-head capscrews?
It may help to add aluminum washers.

 

[mit227]

I apologize for my "ate through" language.

The problem isn't corrosion either, we took care of that during the design process. Instead, by ate through, i literally mean that the threads in the stainless steel screws tore through the aluminum block.

Thank you,

WJM

 

[KENAT]

That's what I initially thought Cory but then given the type of loads I'm wandering if the thread 'sawed' through it acting like a file.

In that case though making sure the pre-load was adequate should fix it.

Do you put a washer under the head to spread the load a little? Maybe use a bigger OD washer.

Of course trying to find the load/stress would be good.

KENAT, probably the least qualified checker you'll ever meet...

 

[KENAT]

Didn't see your posts ken & mit.

Are you saying the bracket is moving relative to the screw?

Is it designed to do this or is it a design flaw?

KENAT, probably the least qualified checker you'll ever meet...

 

[mit227]

I haven't thought of a bigger OD washer, we are currently using a spring lock washer. I was not here when this system was originally designed, but I do not think that the spring lock washer really had much of a purpose? I'll look into replacing it.

It is essentially the same idea of making the bolt slightly larger and I am making sure that the threads that pass through the aluminum block are minimal. Another idea was to place a stainless steel bushing/sleeve in the through holes to spread the load among the aluminum block.

WJM

 

[mit227]

The bracket and screws should move together. But bracket would receive a jerk randomly when in use. We believe these jerks eventually led to the screws tearing through the aluminum block.

Gauging the stresses during these jerks are difficult if possible to find.

 

[kenvlach]

mit227,
"The bracket and screws should move together."
-- move?

Might an accurate description be
1) the original SS capscrew threads 'chewed' the aluminum throughhole during installation, and
2) the aluminum bracket tore at the washer circumference under loading?

Smaller diameter capscrews & holes but larger flat washers may help (in addition to the doubling of aluminum already mentioned).

 

[KENAT]

If you have a big enough pre load then 'friction' will hold the bracket still relative to the screws.

If you can't get the pre-load big enough how about a pin(s) for location with the screws just holding it in place, although you've kind of achieved that with the plain shank on the screws.

KENAT, probably the least qualified checker you'll ever meet...

 

[mit227]

Thanks everyone, I will look into your suggestions