ASME 30,000 Psi Pre-Stress Studs

[rjs286]

Well guys, Ive been thinking about this for awhile now, and have mixed feelings about why ASME wants only 30,000 Psi pre-stress on their studs. We use primarily ASTM A193 B7 studs. I can see it as a safety factor in the stud, but I have a couple of reasons against it. In every handbook or engineering text book I have looked at, to achieve proper clamping load on a joint, should be about 75% proof stress. This would also ensure that the bolt or stud would be properly stretched, and also would reduce fatigue in the stud from cycling of pressures, compared to a lower torque value. Also have to consider soft joints with gaskets.

All my calculations show a torque at least twice as much as what ASME wants for a 30,000 psi pre-stress. Even if I take into account the loading from the external pressure, the stud does not fail. (Only 1.15 Safety Factor at this point though)

On occasion I receive calls from customers wanting to know torque values on our products. Its a mixed feeling on what to tell them the exact correct value is.

Any thoughts?

 

[jte]

You don't state which ASME code or standard you are working with; I'll assume you are using Section VIII Div. 1.

Read Appendix S. Simply put, the allowable stress is to be used for Appendix 2 calc's. It is expected that the allowable stress will be routinely exceeded upon actual torquing of the nuts.

See also the Post Construction Committee's latest publication, PCC-1 2010.

http://catalog.asme.org/Codes/PrintBook/PCC1_2010_Guidelines_Pressure.cfm

A search in this web site for Appendix S yields lots of previous discussion on this issue as well.

Finally... there is no such thing as an "exact correct value." Waaaaay to many variables for anyone to claim that torque calc's are anything but an approximation.

jt

 

[rjs286]

Yes, I agree there is no exact number, way to many variables, wet dry lub etc.

 

[desertfox]

Hi rjs286

There are other things to consider when making a bolted joint, your studs or bolts may stand up to a load of 75% proof stress but the materials clamped between they might not.
Say your making a joint between two pieces of copper with grade 8 steel bolts, the preload generated in the bolt at 75% proof stress might be sufficient to cause the copper to yield on tightening.