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Torque vs. Tension in Bolts

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     I just had a similar question come in from our construction department 
     in regards to using a torque wrench for tightening bolts on a crane.  
     Here was my response:
     I am sure you have heard this before from Structural Engineering but 
     as you know using torque wrenches to determine tension in a bolt is 
     very suspect without calibration from say a skidmore test.  Roughly 
     50% of the torque input in tightening is wasted in overcoming the 
     friction at the bearing face of the turning member, roughly 40% of the 
     input torque is wasted in overcoming contact friction within the bolt 
     threads, and only about 10% of the input torque actually produces the 
     desired bolt tension.  Therefore, a relatively small change in either 
     the first two factors can drastically effect the third.
     With all that said there are formula for determining bolt tension 
     based on torque.  The standard equation is:
     T = KDW/12 
     T = Torque (ft lb)
     K = Torque coefficient (analogous to but not to be confused  
     with coefficient of friction)
     D = Nominal bolt size (in.)
     W = Bolt tension (lb)
     Typical values of "K" are:
     0.35  Dirty or rusty threads, extremely high friction 
     0.30  Heavy galvanized coatings
     0.25  Dry plated fasteners, poor lubricity 
     0.20  Normal as "received" fasteners
     0.15  Plated lubricated fasteners
     0.12  High lubricity using E.P. grease, oil, or wax
     You will have to apply the "K" value that you feel most closely        
     applies to your site condition.
     Thomas Hunt, S.E.
     Fluor Daniel, Aliso Viejo