Threaded hole strength in 6061 aluminum

[shorton2]

It's been a long time since I had to do anything like this and my old ME training is rusty. I need to determine the tensile strength for a threaded hole. As the hole is under the normal rule-of-thumb depths for full bolt strength, AND the material is weaker than the bolt, I figure the weak link will be the threaded hole. I need to determine how much load this can handle.

Parameters and constraints:
- Standard steel M3 socket screw
- Threaded hole into a 6061 aluminum part.
- I'd plan to try to get a formed thread but to be conservative consider a medium fit cut-tap thread.
- Thread depth is 4.5mm max, I'd say allow .5mm for chamfer, etc., and call it 4mm of thread engagement possible. I can't make the hole any deeper.

Can someone help me calculate the tensile strength of this connection?

 

[XR250]

Sounds like it is time to pull out your old Machine Design textbook.

 

[JAE]

Won't the aluminum and steel start to rust when in contact?

 

[shorton2]

None of my books address it. Everything wants to default to the rules of thumb i.e. 2xD for dissimilar materials to achieve tensile strength of the screw. Don't have that luxury, I'm limited on thread engagement.

 

[TehMightyEngineer]

What JAE said. Stainless steel will help with the corrosion if I recall correctly.

Maine EIT, Civil/Structural.

 

[shorton2]

I'm OK with stainless, but it's not as strong. If I can determine the strength of the threaded hole and it's less than a SS fastener, then I'm all for it. FWIW these are not in a corrosive environment and I can use some anti-corrosive measures on the steel if necessary. Unless it's some sort of galvanic reaction.

But cart before the horse, I need to determine the threaded hole strength.

 

[ztengguy]

Tell us this, how much capacity do you need? From there cant you compare to an aluminum bolt of same size?

 

[shorton2]

I'd like as much safety factor as I can get. So I'd prefer to work it the other way. i.e. How much strength do I have then determine how much safety factor I have based on what would be applied.

 

[ztengguy]

This might help. We are just a bunch of stupid structural engineers, so we generally dont get too involved in this type of calc. Might ask this on a mechanical forum.

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

 

[shorton2]

Thank you. I've looked at that, but about half way through it it goes to being sure the screw fails first:

"To ensure that the screw fails before the thread strips it is necessary the the shear area is at least 2 times the tensile area."

Stupid me, I thought I WAS in the ME area, not sure how I made that mistake, thanks for pointing it out. I'll repost there.

 

[shorton2]

Branching this thread to the ME area, where I meant to put it in the first place (Still no sure how it ended up here, oh well...)

Moved to:

http://www.eng-tips.com/viewthread.cfm?qid=362988

 

[TehMightyEngineer]

Just to lastly touch on your corrosion issue for when you get to it; yes it is a galvanic reaction between aluminum and steel that will, over time, eat away at the aluminum section if I recall correctly.

Maine EIT, Civil/Structural.

 

[shorton2]

hmmm, that's not good, I'll do some research on that. Thanks.

 

[rothers]

I've used BS 3580 in the past which goes into great detail on the thread engagement required for fasteners in various materials.

 

[TehMightyEngineer]

Looks like I may have had it backwards, stainless steel + aluminum = heavy corrosion in the aluminum. Low alloy steel + aluminum = milder corrosion in the aluminum. These are made MUCH worse in the presence of an electrolyte (i.e. salt water).

Maine EIT, Civil/Structural.

 

[shorton2]

Thanks, worrying I did go look and cam to the same conclusion. Environment will be residential, dry, so no electrolyte.

 

[XR250]

Put some anti seize compound on the threads and you won't have a problem.

 

[TehMightyEngineer]

That will help but wont stop the metal on metal contact and will just concentrate the galvanic action. For example, if you replace your car's spark plugs you might put dielectric grease on the plug terminals. This helps to prevent arcing and corrosion of the terminals but obviously doesn't stop the electricity flowing from the wires into the plug.

Maine EIT, Civil/Structural.

 

[XR250]

I have done it over the last 20 years on motorcycles without an issue.

 

[TehMightyEngineer]

It probably does a good job sealing out moisture and that may be the benefit you're seeing.

Maine EIT, Civil/Structural.