maximum fastener pretension
maximum fastener pretension
(OP)
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...
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...





RE: maximum fastener pretension
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: How do I calculate an assembly torque? 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
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RE: maximum fastener pretension
Good luck if you do!
RE: maximum fastener pretension
thread404-201427: Bolt Stretch Versus Clamping Force
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
RE: maximum fastener pretension
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