Bolt Thread Pullout (failure) in alu 6063-T6

[Yevhenii]

TL;DR Summary

Thread in alu 6063-T6

I need advice how to calculate thread pull out in aluminium 6063-T6.
12 bolts screwed into an aluminium plate (thread depth 25 mm) must withstand a load of 350 kg.

Thanks in advance

 

[Baluncore]

What is the bolt material? What bolt diameter? Will the bolts gall in 6063-T2?
What thread lubrication? How many times will it be removed and replaced?
You might use a long stainless steel thread insert such as recoil or helicoil. That will increase the effective thread diameter in the 6063-T2.

 

[Yevhenii]

Will be screwed only one time. I want to know how it will work without helicoil threads. Bolt diameter M12x50 DIN 912, will be installed without grease.
I calculated before - the shear stresses in the bolt and aluminum plate are less than the maximum allowable stresses.
Also I heard, that if you want to install steel screws inside soft materials (aluminium) thread height should be not less than 2d of screw.
How I can make sure that these screws will work inside alu plate?
Thanks

 

[Baluncore]

If you tighten a fastener by turning the nut, then the fastener is called a bolt. If you turn the head and shank of the fastener, then it is called a screw. So you are using screws to attach the plate.

Also I heard, that if you want to install steel screws inside soft materials (aluminium) thread height should be not less than 2d of screw.

That should be the length of the engaged thread, not the height which better refers to the thread profile.

How I can make sure that these screws will work inside alu plate?

As the screw is turned and tightened, imperfections in the thread will deform and damage the threaded aluminium. A more reliable way would be to thread a stud into the aluminium, without axial load, then clamp the plate in place with an external nut. The thread in the aluminium should have a high profile, so it should be coarse. The nut end of the stud should have a fine thread.

A coarse thread helicoil, with a screw will not be as strong as the coarse stud with fine nut. A coarse screw threaded into aluminium will require a higher torque to reach the same tension as a fine nut on a stud.

 

[onatirec]

Would probably be fine. You are correct that the minimum thread engagement should be at least 25mm here.

The general formula for determining allowable stress is P=S*A, with P=load, S=tensile strength of Al, and A=tensile stress area.

For an internal thread this area is defined as:

with n=threads per inch, Le=length of engagement, Dsmin=minimum major diameter of the external thread, Enmax=maximum pitch diameter of internal thread.

350kg is a mass, not a load. Presumably this is pure tension with no shock loading, thermal loading, etc, so your load would be (9.81*350) N; divide by 12 to find your load per bolt and make sure that's less than the allowable load by looking up the yield strength of Al-6063, the necessary internal & external thread dimensions, and using the above equations. I'll leave it up to you to confirm.

This information can be found in Machinery's Handbook.