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Re: BOLT TORQUE/TENSION RELATIONSHIP

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Robert Shaw's "Structural Bolting Handbook" (an
inexpensive and concise publication that should be
required reading for every engineer who has anything
to do with high-strength bolting) has a chart that
show's the relationship (actually it shows the LACK of
a relationship) between torque and tension.  The chart
indicates that a 28 kip pre-tension force in a 3/4"
diameter A325 bolt requires anywhere from 200 ft-lb to
500 ft-lbs of torque.  That's enough variation to tell
me that I don't want to rely on torque as a measure of
pretension.  I think I read somewhere that 90% of the
energy put into torquing the nut onto a bolt is
"spent" overcoming friction.  In other words, for a
given torque value, slight variations in bolt
lubricity will result in BIG differences in bolt
pre-tension.  That chart was a real "eye-opener"

Cliff Schwinger


--- THunt(--nospam--at)absconsulting.com wrote:
> Savksa,
> 
> If this is a real job site question then you have
> been given good advise 
> in that relying on torque to achieve a given tension
> in a bolt is highly 
> unreliable.  If this is the case then I would go to
> the AISC website, 
> download the Research Council's bolt spec, and send
> it to whoever is 
> asking the question.  In the spec you will find the
> required bolt tension 
> and several approved methods for achieving this
> tension.  As mentioned 
> previously, no matter which method is used they need
> to verify the bolt 
> assembly tension in a pre-installation test which is
> normally done with a 
> Skidmore-Wilhelm device.  All of this is clearly
> explained in the spec and 
> further detailed in the commentary.
> 
> If this is more of a research or general question
> then there are several 
> formula for determining torque vs tension values in
> bolts.  Some formula, 
> especially from the machine handbooks, can be quite
> involved and would 
> take a spreadsheet to solve.  Below is a more basic
> formula that has been 
> around for many years.  I have had to use this in
> certain anchor bolt 
> pretension cases but again I would caution against
> using any such formula 
> for actual structural steel applications.
> 
> T = KDW/12
> 
> T = Torque (ft lb)
> K = Torque coefficient (similar to but different
> from coefficient of 
> friction)
> D = Nominal bolt diameter (inch)
> W = Bolt tension (lb)
> 
> K Values
> 
> 0.35  Dirty or rusty threads
> 0.30  Heavy galvanized coatings
> 0.25  Dry plated fasteners
> 0.20  Normal as "received" fasteners
> 0.15  Plated lubricated fasteners
> 0.12  High lubricity using grease, oil, or wax
> 
> Thomas Hunt, S.E.
> ABS Consulting
> 
> 
> 
> 
> 
> 
> savksa savksa <savksa(--nospam--at)yahoo.com>
> 06/15/2003 08:22 AM
> Please respond to seaint
> 
>  
>         To:     seaint(--nospam--at)seaint.org
>         cc: 
>         Subject:        BOLT TORQUE/TENSION
> RELATIONSHIP
> 
> 
> Hi!
>  
> 1.  I need to tension an ASTM A490 bolt to a certain
> value (somewhat of a 
> hypothetical question).  I need to calculate the
> torque required to 
> achieve this tension.  Could someone tell me how to
> calculate the torque? 
>  
> 2.  Do I need to worry about pre-tension?  If yes,
> will pre-tension need 
> to be considered in the equation in item 1 above for
> calculating the 
> torque?
>  
> Thanks for your help!
>  
>  
> Do you Yahoo!?
> SBC Yahoo! DSL - Now only $29.95 per month!
> 
> 


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