stretch vs torque for Bolts
stretch vs torque for Bolts
(OP)
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
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





RE: stretch vs torque for Bolts
Have a read through this site:-
h
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
RE: stretch vs torque for Bolts
Stretch is more directly related to preload, but is harder and more expensive to measure than torque in most cases.
RE: stretch vs torque for Bolts
RE: stretch vs torque for Bolts
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
RE: stretch vs torque for Bolts
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
RE: stretch vs torque for Bolts
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.
RE: stretch vs torque for Bolts
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.
RE: stretch vs torque for Bolts