maximum fastener pretension 2

[mason82]

I'm trying to find a reference that will list the maximum pretension in a cap screw connection allowed before the fastener would fail in torsion.

My application is one where I need to achieve a specified friction between my mating parts and a third "sandwiched" part between the two. I want to do this by screwing the sandwich together without screwing through the center part. I also want to size the fastener to be as small as possible.

The problem is, I'm screwing the head into a tapped hole, so the smaller I go, the more susceptible the screw is to the head twisting off.

thanks...

 

[CoryPad]

When you say "fail in torsion", do you mean shank fracture due to applied torque? The shank fracture will be due to tension overstress (due to the lead angle).

If you use dimensions established in standards for high-strength fasteners, the drive feature should not fail before the shank does.

You can calculate this yourself using equations found in several places, including Machinery's Handbook. faq725-536 shows you how to calculate a preload based on screw yield stress. If you substitute ultimate tensile strength for yield strength, you could get what you want.

Regards,

Cory

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

 

[dvd]

Do you mean that the outer plates are both threaded and the middle plate has a clearance hole?

Good luck if you do!

 

[Gymmeh]

His is a related thread.

thread404-201427

You should be able to calculate required pre-load and write an exel program to make it repeatable, but it may save time, if you have a "company supplier" to get an engineers manual for their fasteners, this should have all the preload info and more.

Regards

 

[cbrn]

Hi,
there are several Norms dealing with this. One of the most complete is, imho, VDI-2230. It's not expensive to buy, if you need it.

Regards