Torque Required to Strip Aluminum Thread 1

[Mandrill22]

I have a 440C SS bolt going into a 6061-T6 aluminum member. I'm trying to figure out if I can tighten the bolt with a torque of 16 N-m.

Threads: M6 x 1.0
Length of Engagement: 31 mm
"Hanging" axial load: 15.6 N
Applied torque (preload): 16 N-m
Bolt material: 440C SS
Female thread material: 6061-T6 Aluminum

 

[Mandrill22]

Oh, and it is dry.

 

[handleman]

NASA says you're cool. Based on a Google search that took less time than you typing the question.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170003491.pdf

 

[Mandrill22]

That isn't the right k value. It's 440C, which should be considerably higher. There's also no need to be rude. I tried Google and didn't find that.

 

[amtrak17]

I would approach this by the method outlined in MIL-HDBK-60 (Also found in Machinery's Handbook).
You can solve for your applied load based on torque of 16 N-m and an appropriate k value. Be sure to factor in error for load accuracy based upon which method you are using to torque. A torque wrench has +/- 25% accuracy when determining the actual applied load.
Is there any material being bolted between the bolt head and the aluminum? If there is, you will have to figure out how much applied load is taken by the bolt, and how much is taken by the clamped parts.
Since your female thread material is significantly weaker, the expected failure mode is internal thread stripping at the shear plane located at the root of the threads. You can find this shear area by applying the formula found here:

https://www.fastenal.com/content/feds/pdf/Article - Screw Threads Design.pdf.

Hope this helps.

 

[Mandrill22]

Thanks, that does help. I just need to find the k factor.

 

[amtrak17]

There are pretty good friction coefficient tables out there. If you actually have the 2 materials on hand you could simply measure the true friction coefficient by placing a block of one on top of a block of the other and seeing at what angle the top block slides. Static friction coefficient = tan(theta)

 

[Tmoose]

My concern with the inclined plane method of determining coefficient of friction is the block-to-plane interface probably has way lower contact pressure than screw threads experience.

 

[hydtools]

Friction coefficient is not defined by contact pressure or area of contact. The inclined plane method should be valid.

Ted

 

[dvd]

Why would a calculation be more appealing than testing?

 

[amtrak17]

Right, friction coefficient is based on material surface properties alone, nothing else. Also note that friction coefficient and K (torque coefficient) are not the same thing. You will have to calculate K from friction coefficient based on JIS B 1803 or similar formula. Also keep in mind that assuming a lower friction coefficient is more conservative for the case of thread stripping, because you are assuming it takes less torque to get to a given pre-load. I just worked your problem assuming friction coefficients of 0.61 between aluminum and steel, and got that a 16 N-m torque would give you about 3600 N of pre-load in the joint. If you're hanging the 15.6 N load directly on that bolt, that adds to the pre-load. If the shear area of your nut threads can handle that, it's likely safe. I would apply a safety factor of 2 if possible.

 

[MikeC88]

Hi,

As others have said identifying the K factor, or individual friction factors (thread, underhead etc) plays an important part in in determining the applied preload.

The attached shows the the rough method to determine thread shear, and some other items you may want to consider such as additional factors to consider when determining preload requirement of joint, and torque scatter due to tightening method.

Strongly recommend obtaining Bickfords An introduction to the design and behavior of bolted joints, if you are going to be dealing with many fasteners, or having a look at bolt science.

Attached are very rough, so you will need to check value of UTS etc - I have chosen first source so these are possibly not correct.

 

[amtrak17]

I agree with MikeC88's calcs and comments above. I changed my friction coefficients to give the same 0.52 K value and got the same. This is a more conservative assumption on the resulting pre-load due to torque. Ultimately the hardest part is going to be obtaining an appropriate K value that you trust for your exact materials. As others have mentioned, best practice would be to confirm this with a test or several tests if possible. You can order plenty of SS screws and Aluminum nuts off McMaster for really cheap.

 

[Tmoose]

Others said things like " Friction coefficient is not defined by contact pressure or area of contact. The inclined plane method should be valid. "

Perhaps, if the sliding weight was heavy enough so the contact pressure was in the same range as the installation contact pressure between the SS bolt threads and the aluminum threads. Also the under-head contact pressure between ditto.
And the siding test was repeated 8 or more times.

https://arp-bolts.com/i/UT/UT_ARP8pp.005.jpg