Overtightening a bolt beyond its recommended bolt load. 1

[EngTal]

What would be the consequence of tightening A bolt beyound its recommended bolt load. The recommended operational bolt load is 430 kN and we are planning to tight the bolt via hydraulic bolt tensioner and approximately achieving more than 670 kN bolt load/bolt. 430kN bolt load is recommended by vendor of the equipment. The operational condition of the equipment is 140 kg/cm2 and 150 deg Celsius.

How far these recommended bolt loads are from yield point of bolt?
Specs of bolt: 2-1/2" UN8 B7 studs.

 

[dcascap]

Min Yield strength of this bolt is 720 MPa, tensile is 860 MPa.
Tensile area is A= 645.16 mm2
Therefore Min yield load is 464.8 kN
Max load is 554.8 kN

So the recommended preload is 90% of yield strength.
If you preload to 670kN you will probably break it before reaching the load.

Hope it helps!

 

[kingnero]

Additionally, at 150°C you're not going to have the mechanical properties at the same level as room temp.

 

[EngTal]

Thank you for your response. I think the tensile area used here is quite small. the studs diameter is 63.5 from which calculated tensile area should be 3165 mm2. using that min yield load is around 2000 kN.

 

[dcascap]

You are right, I miss copied the value in excel. The tensile area is 2854 mm2 ( a bit lower than you said due to the threads etc).

Yield 2054 kN
Tensile 2454 kN

Then if you preload 670 kN should be alright is around 30% yield.

You're in the spec service temperature range -100˚C to 565˚C so it should be alright.

check this specs also it's in imperial units.

 

[EngTal]

This spec sheet is really helpful. Can you please share its reference. And where are the spec service temps range and bolt material is mentioned?

 

[dcascap]

Here you are. Temperatures are in page 83, preoad page 100

https://new.bremick.com.au/images/site/PET_PC_web.pdf

 

[r6155]

Why not control the bolt elongation?

 

[CWB1]

The OP's questions cannot be answered with the info given. Only the supplier or testing can provide the proof load, and you need additional detail to complete a BJA to determine the first failure mode. Like any bolted joint you could have a thread failure, a shank failure, underhead/bearing area failure, or no failure.

Charts like the one shown are dangerously inaccurate, which is why competent engineers abhor them. We analyze every bolted joint and test all safety critical joints.

 

[PersonalProfile]

The following may be of interest, though is for structural fasteners:

Controlling tension by the turn-of-nut method is primarily a strain control. If
the elongation of the bolt remains within the elastic range, both the starting point
(i.e., snug tight) and the amount and accuracy of the nut rotation beyond snug tight
will be influential in determining the preload. However, in the inelastic region the
load versus elongation curve is relatively flat, with the consequence that variations
in the snug-tight condition result in only minor variations in the preload of the
installed bolt. This inelastic behavior will be a characteristic of practically all
installed bolts.

Calibration tests of A325 bolts with grips more than 4 diameters or 4 in. showed
that the one-half turn of the nut rotation produced consistent bolt tensions in the
inelastic range.4.2 These tests also showed a sufficient margin of safety against fracture
by excessive nut rotation.

Ex:

https://www.boltcouncil.org/files/2ndEditionGuide.pdf

Regards.

 

[r6155]

Charts like the one shown are dangerously inaccurate, which is why competent engineers abhor them. We analyze every bolted joint and test all safety critical joints.

I agree 100%. with CWB1

 

[dcascap]

The OP's questions cannot be answered with the info given. Only the supplier or testing can provide the proof load, and you need additional detail to complete a BJA to determine the first failure mode. Like any bolted joint you could have a thread failure, a shank failure, underhead/bearing area failure, or no failure.

Charts like the one shown are dangerously inaccurate, which is why competent engineers abhor them. We analyze every bolted joint and test all safety critical joints.

I agree with you, I assumed that they wanted to increase the preload due to the calculations they made. But it's true what you say and they should ask the supplier for more info.