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Re: BOLT TORQUE/TENSION RELATIONSHIP

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There is NO direct correlation between the torque applied to tighten a fastener and the tension generated in the fastener (or the clamp load generated in the connection.) 
 
Another source of information is John H. Bickford's An Introduction to the Design and Behavior of Bolted Joints.  I believe this book is still available from the Industrial Fasteners Institute, Cleveland, OH.  216-241-1482.  Chapter 5 covers the use of torque-based installation techniques.  Using the same torque on fasteners from the same lot in essentially the same condition one can expect that using the same exact torque on each will generate tensions which vary plus or minus 30% from a targeted value.
 
By coincidence, I spent most of Monday in the section of our laboratory where we calibrate very expensive torque wrenches.  These are the types of wrenches used by inspectors to check previously installed fasteners.  (The accuracy of such wrenches in terms of their ability to measure torque is about plus or minus 2%.  The wrenches are calibrated in a special rig which holds the wrench in a fixed position over a torque transducer. 
 
Next, a mechanical device positioned at precisely the calibrated lever-arm distance is directed against the handle at a very precise angle.  For sake of interest, I asked one of our technicians to see what happened to the accuracy of the readings if one were to have the wrench not so precisely fixtured.  With just under 3 degrees of non-linearity we recorded a loss of accuracy in the torque measuring capability of the wrenches of about 7%.  I find that useful to know when I consider how bolts are installed in 'the real world' --- and how torque wrenches are used as well.
 
Another source of information on torque formulas is SAE J1701.
 
David Sharp
TurnaSure LLC
 
 
 
 
----- Original Message -----
Sent: Sunday, June 15, 2003 4:07 PM
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!


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