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RE: Torque vs. Tension in Bolts

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The following statements are generally applicable to bolted connections. 
I'm not sure if the original question was specific to this application, 
however.

It has been known for quite some time now (40 years?! if not more) that the 
relationship between torque and tension is highly variable due primarily to 
differences in fit, finish, and surface condition of the mating parts 
(i.e., bolts, nuts, washers if used, material in the grip, etc.). This is 
clearly indicated in the RCSC Specification and Commentary (see 
www.boltcouncil.org) as well as in any modern bolting literature.

Side note: I understand that other industries, such as the automotive 
industry, commonly use torque for fastener installation. The application is 
quite different, however, because the work is done in a more controlled 
environment, with fasteners that have a finish, surface condition and 
assembly configuration that is well known. Plus, they don't pull the torque 
value out of a standard table...they test a lot of them and figure out the 
torque, not unlike we are all supposed to do for installations using a 
calibrated wrench. In contrast, we have exposed conditions that may be 
sunny, may be wet, may be dry, or may be snowy; fasteners that may be black 
or galvanized; surface conditions that may be mild steel (which is subject 
to galling) or hardened washer; fastener assemblies with lubrucation and 
fit-up between components that varies.

I don't mean to scare you with that last astatement. In case I did, let me 
assure you that the four bolt installation methods for pretensioned bolts 
provided in the RCSC Specification account for these concerns and work when 
applied properly.

I suppose the link that has been provided to a UK torque table/program 
might have some use, but for structural connections, particularly those 
involving ASTM A325 and A490 bolts in building and bridge structures, I 
think it is of little use. Again, follow one of the approved methods in the 
bolt spec and you'll be alright.

Charlie


-----Original Message-----
From:	Christopher Wright [SMTP:chrisw(--nospam--at)skypoint.com]
Sent:	Wednesday, July 14, 1999 8:50 AM
To:	SEAOC Newsletter
Subject:	Re: Torque vs. Tension in Bolts

>Can someone tell me a rule-of-thumb formula or another method to convert
>the torque value placed on a bolt to the tension value. ...
>I do read, however, in the AISC Connections book that any
>published method of conversion is not acurate.
So long as you're happy with a method of conversion that isn't accurate,
the relationship between torque and tension is T=kFd. T is the torque in
in-lb, F is the bolt tension in pounds and d is the nominal bolt diameter
in inches. The factor k is semi-empirical and commonly varies between
0.12 and 0.2 depending on friction and manufacturing allowances. The
higher value seems to be preferred in most of what I read. But as you
said, it isn't real accurate. It's also based on elastic behavior and may
not be suitable for LRFD adherents. ;->

In machinery, bolts are tightened to 50-75% of their proof load to avoid
fatigue failure. The corresponding tensile loads are high enough to crush
soft materials like wood and plastic unless you use smaller bolts and
consequently more of them.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw