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Torque vs. Tension in Bolts

• To: SEAINT(--nospam--at)seaint.org
• Subject: Torque vs. Tension in Bolts
• From: Tom.Hunt(--nospam--at)fluordaniel.com
• Date: Wed, 14 Jul 1999 07:27:11 -0400

```     Peder,

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

Where
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