Bolt Pre-stress 1

[SloFlo]

Does anyone know the equation to use to calculate the required torque, elongation, number of nut turns required to generate a 45,000 psi pre-stress at the root of the threads? The nominal bolt diameter is 2 inches.
Thanks in advance

 

[KENAT]

Take a look around, this or similar comes up pretty often, there may even be a FAQ about it if memory serves, if not here then forum725.

http://www.boltplanet.com

has a free calculator you may be able to use to validate your results, however don't rely on it as being free you get what you pay for and you need to understand the theory behind it etc.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[racookpe1978]

What are the temperatures of the bolt: at the start of torquing, under operation, after operation?

What lubrication? How much of a chance of galling? What materials?

What you are gambling is that the "turn-of-nut" type of calculation yields results that are predictable and proportional to "elongation of stud" which are proportional to "tension in stud" which become proportional to "amount of force remaining in stud under the final operating conditions". Then you further make an assumption that all of those ideal assumptions are going to be valid under your actual case.
Keep your bolts clean, your threads perfect, your lubrication evenly and uniformly applied, ..... your tension smoothly and exactly applied on centerline, etc. You will (probably) come close, but verify. Keep a margin in your back pocket.

Check your millwrights. Inspect.

 

[TheTick]

I've seen various threaded fasteners that have built-in tension indicators. Here is a link to one: <

http://news.thomasnet.com/fullstory/542797>.

I have seen others at trade shows. One I saw at a show has a cap on the head that turns freely until it reaches a set tension.

 

[desertfox]

Hi Sloflo

How accurate do you want your preload to be?
The method you choose should suit the accuracy you require.
Have a look at this site:-

http://www.roymech.co.uk/Useful_Tables/Screws/index_screws.htm

desertfox

 

[CoryPad]

Equations for inch fasteners are available in MIL-HDBK-60, available here:

http://assist.daps.dla.mil/quicksearch/

 

[Ron]

Use a Skidmore-Wilhelm device to determine that actual torque required to produce the desired tension. This is an accurate method.

 

[Screwman]

Sorry to say, but Skidmore devices are only considered accurate if the spring rate of the assembly and gage as taeted is the same as in the actual joint as assembled, and it usually isn't even close. A load cell sandwiched into a milled out area of the actual assembly (replaced by a hard steel spacer block in the final assembly) can give you some pretty decent ideas the results of your tightening.

For large assembly the turn-of-the-nut is often used.

As was mentioned earlier, how much accuracy you need should determine how much effort and cost you put into the testing (the only way to really determine an accurate tightening strategy) and calculation to do. Cost and time = accuracy.

 

[SloFlo]

Thanks for your responses. I'm getting closer to where I want to be. At this point I need to figure out how to determine the torque or force required to generate a 45,000 psi stress at the thread root. The bolt information is: 2.00- 8 UN- 2A. Steel, well lubricated.
Any suggestions?
Thanks

 

[rb1957]

generally T = Pd*0.2 = 45000*pi*4/4*2"*0.2 = 18000*pi in.lbs = 56550 in.lbs = 4712 ft.lbs.

Note, this is Very approximate ... the equation is approximate, the physical application is approximate (depending on the means of preload control used).