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mfgenggear (Aerospace)
16 Nov 09 16:26
there has been good discussions on Torque values on bolts.

I have a generic question, which is better & why?
or is it the application.

stretch the bolt to a calculated length!
& how is this length calculated?

vs the calculated torque required.

also specified vs prevaling torque,
This was depicted on old Atlas/Centaur drawing.
any one have experience with the above.

Thanks Guys
Helpful Member!  desertfox (Mechanical)
16 Nov 09 17:04
Hi mfgenggear

Have a read through this site:-

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

then at the bottom of this page there are various links here is just one:-

http://www.superbolt.com/

The way the bolt stretch is calculated is based on Hookes law ie E= stress/strain.
Prevailing torque is just the torque required to run the nut down the thread before tightening.

desertfox
Helpful Member!  SWComposites (Aerospace)
16 Nov 09 18:03
It may depend: what specific application and what specific bolts / fastener system?

Stretch is more directly related to preload, but is harder and more expensive to measure than torque in most cases.
Helpful Member!  rb1957 (Aerospace)
16 Nov 09 18:27
bolt stretch is a reason, though complicated/difficult way to control preload ... but it gives a tight control on the preload, as the geometry of the bolt is well controlled.
mfgenggear (Aerospace)
16 Nov 09 18:40
DesertFox

Thanks for the reply.
nice links & explanation
why even call out prevailing torque?
there must have been slight press involved.
in Automotive applications, it is not specified
from my Experience.
I have done a lot of mechanical assembly.

SWComposites

my experience has been mainly with torque
specified on Engineering Drawings.

my question is basicly generic.
looking for examples why you would use stretch vs torque.
I believe desert fox basicly answered the question
it's more accurate according to the link he supplied.

when is it deemed necessary to hold preload at greater accuracy?
if a stretch is used, then is loctite allowed?
or other words should it (loctite) even be specified.
sorry for the NB questions.

rb1957

nice explanation

it is possible to teach an old dog new tricks

Thanks all
desertfox (Mechanical)
16 Nov 09 19:08
Hi mfgenggear

Thanks for your response, in some applications the prevailing torque is is allowed for in the calculation of pre-load, usually for very critical joints but I haven't come across it yet in my work.
If your working on very large steel plant for instance like rolling mills on the very large joints ie the mill framework they tend to use bolt stretch because of the very large external forces the joint will see in service.

regards

desertfox
Helpful Member!  BobMeister (Mechanical)
25 Nov 09 10:20
I've seen bolt stretch used in automotive assembly to compensate for the inherent creep of non-ferrous engine part mounting pads. The specification of torque at assembly allows for the stretch and so creates a constant load on the mounting pad.

I've also seen bolt stretch/preload used extensively in overhead steel support beams for conveyors, platforms, etc. The difficulty in this application is that the installers (usually union ironworkers) use a spud rather than a torque wrench so a greater allowance for variation must be included without value specification. Specified torque values require spot checking or random tests after installation: installations use hundreds and thousands of bolts.
Helpful Member!  RPstress (Aerospace)
25 Nov 09 13:44
We have sometimes used bolt stretch (usually just by measuring the external length of the bolt head to tail) if the preload is particularly critical. The effective area depends on the length of thread in tension vs. the length of plain (unthreaded shank).

The use of torque to more-or-less control preload is a bit rough and ready, largely due to the variables like lubrication, wet vs. dry assembly, torquing from the head or nut, materials in contact (friction), etc.

A large, well lubricated nut in a fairly precise assembly like a rotorhead might have a preload given by anything up to 8 (maybe even 10) T/D, whereas a quarter bolt with a stiffnut and questionable lubirication might come out to around 3 to 5 T/D.

Quite a bit depends on the run-up torque being allowed for. Some specs require the nut to be run up the thread multiple times with the torque required being measured each time. This is a real pain for the mechanics and costs money.
 
Helpful Member!  metaltometal (Mechanical)
4 Dec 09 10:15
Nearly every response is right on the money.  Depending upon your specific application, elongation measurement is the best means of ensuring the correct and evenly applied preload to a fastener.  The best resource I have every used, in nearly 25 years of tightening experience has been John Bickford's "An Introduction to the Behavior of a Bolted Joint".  Expensive, but worth the money - about $150 or so.

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