Bolt tension as a function of tensile strength 5

[Gene1964]

For determining bolt torque values, is there a thumb rule to use such as "50% of tensile strength?"

My application would mainly be flange bolts (B7).

 

[Pressed]

I have always relied on the source of the bolt to find what the torque is; as things change. This link has a formula.

[URL unfurl="true"]http://www.zerofast.com/torque.htm[/URL]

[URL unfurl="true"]http://www.clevelandvibrator.com/NewPDF/BOLT%20TORQUE%202%20pages.pdf[/URL]

_______________________________________
Feeling frisky.........

http://www.tailofthedragon.com

 

[MintJulep]

You should not use rules of thumb in place of proper engineering.

Understand your joint.

Determine what pre-load your joint requires, and how much variation in pre-load you can tolerate.

Determine if torque control can attain that level of tolerance, or select a better method of pre-load control.

Select fasteners.

 

[CoryPad]

Well said MintJulep.

My experience with bolted joints shows many rules-of-thumb:

90% of yield
90% of proof
75% of tensile
etc.

Rules of thumb really aren't useful.

Regards,

Cory

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

 

[Gene1964]

Thanks for the information. I'll probably go with 75% of tensile strength. That number was also referenced on the zerofast website (Thanks, Pressed!)

 

[arto]

ASME PCC-1 uses 50ksi for A-193-B7

 

[ajh1]

For structural applications, check out the RCSC bolt installation spec. You can download from

http://www.boltcouncil.org

 

[TVP]

Gene1964,

The zerofast website article by Joe Greenslade shows values based on 75% of the YIELD strength, not 75% of the tensile strength.

 

[Gene1964]

Good catch, TVP! Thanks!

 

[yates]

Rules of thumb can be dangerous when considering fasteners. Don't forget, they're what holds everything together. I am horrified, as a fastener manufacturer, when my customer asks 'you make these, you tell me what torque to tighten them with' What can I give him as an answer ?, the only decent answer is the question 'how is your joint loaded, shear, tension, vibrating or not ? what are the materials being fastened ? any expansion due to heat ? fasteners lubricated or not ? washers or not ? bolt screw into a locknut or a tapped hole ? do you want to be able to undo the joint often ? and so on.
Mint julep has it right, attack things in this order and you'll be OK

 

[wsmith22]

Yes, rules of thumb are dangerous. However, ASME Section VIII, Div.1 added a bolt torque procedure as a non-mandatory element of Appendix 2. What are we supposed to do when we hydrotest many different flange designs (designed per App.2) with different bolt materials? (449, B7, B8M Cl.1, B8M Cl.2, etc.) You need a basis rule of thumb to come up with a bolt torque value for the assembly personnel. (I'm only talking about self enerigizing gasket flanges)

Also, what is the feeling on bolt preload tension for a self-energizing gasket such as an o-ring? For this type of flange design, I feel a bolt preload tension greater than the bolt design allowable at temperature would suffice. i.e. use a 30 ksi preload tension for SA-193 B8M Cl,2 bolts with an allowable of 22 ksi at 250 deg F. Any feedback would be greatly appreciated.

 

[Screwman]

Yates,
Excellent response. I get the same question as you all the time and feel the same frustration. We have shown that over 100 variables effect the end clamp load in a joint and customers expect us to be able to give them a torque recommendation when they cannot even provide us with a target clamp load!
In fastening, the rules of thumb have caused many major failures becuae they fail to take into account any individual considerations present in a particular situation.

Tighter is better, but only until it causes embedment, joint collapse and flange bending......

Dick

 

[eng1234]

Screwman,

This post is dating back a bit and understand your frustration. I infact am guilty of asking our bolt manufacturer to help determine proper bolt torque tightening for each fastener we purchase from them.

Using T=KFD, we should be able to calculate all we need. However how do we know what target clamp load we want?

 

[TVP]

eng1234,

The target clamp load should be sufficient to resist the forces applied to the joint (tension, shear, bending, torsion, etc.). Do you know what forces are applied to your joint(s)?