How to torque a 2" grade 8 fastener 7

[richardl63]

I need to torque a 2"-4.5 pitch nut on a grade 8 threaded rod. I usually don't work with fasteners this large, so don't know what tools may be available. Advised Lubricated torque on a 2-4.5 fastener is 5,621 ft-lbs, and dry advised torque is 7,495 ft-lbs. I know the recommendations vary depending on the source, regardless, it's a lot of torque. I know there are some very expensive torque multipliers and hydraulic wrenches, but this is a one time job. Is there a low cost tool do to this. A 20' long cheater bar seems ridiculous, but I have not found an economical tool in my searches.

Richard Lee, PE
Lee Engineering Inc.
richardl@leeengineeringinc.com

http://www.leeengineeringinc.com

Richard Lee, PE
Lee Engineering Inc

http://www.leeengineeringinc.com

 

[DAVIDSTECKER]

Superbolt/Supernut are probably the best and easiest for one time and field applications.
Another way would be to calculate the amount of stretch required to create the stress that torque would develop then measure the free end of the bolt with a dial indicator as the nut is tighten with a Big cheater bar.
Yet another method would be to go with thermal expansion, heat the bolt and install it hot and let the contraction draw it tight.

 

[Agent666]

I'm curious for my own interest since several people have mentioned heating the bolts to achieve the tensioning, how hot would you need to get a bolt to achieve the suggested method of heating and letting it shrink to clamp the surfaces together?.

Wouldn't heating grade 8.8 bolts affect the final properties depending on the rate at which it cools (I'm thinking of those phase diagrams from 15 years ago in University), I'm imagining its needs to be pretty hot to expand the equivalent of the same length as the part turns, not withstanding that the yield strength goes down at higher temperatures and bolt may yield as well as clamping. They are heat treated to achieve the mechanical properties required after all (similar to how welding them is out of the question as it alters the structure and make them susceptible to hydrogen embrittlement)?

I'd stay away from heating them for this reason alone, but interested if people think this is an issue.

 

[DAVIDSTECKER]

As long as you stay below the Tempering temp you're good.
Generally 450-500F would be safe but check for your self based on the material.
The rest is math to figure out what amount of stretch is required to develop the tensile stress that torqueing would create.
Then using the thermal expansion rate calculate the temp required to grow the bolt to the desired length as long as you don't exceed the Tempering Temp.
I just found my copy of "Torque-Turn Tactics" which could bypass all of the rest of methods.

 

[EngineerTex]

Note that when you're preheating the stud, the stiffness of the joint being clamped should be taken into account as well. First, determine the spring rate of the fastener and the spring rate of the joint (The joint will easily be 5X+ as stiff as the fastener). Then put together the equation that will relate the two stiffnesses and from there, determine the required stretch and take that value into account when determining the temperature to preheat the fastener (hint: It will be a higher temperature than if you do not take the joint stiffness into account)

Engineering is not the science behind building. It is the science behind not building.

 

[enginesrus]

If you are good at fabrication, you could fabricate your own hydraulic wrench using an inexpensive portapower unit like from harbor freight. Knowing the arm and the piston area, and using a pressure gauge to set the proper torque.