First, measure your torque wrench according to the diagram above. (engraving it into the handle for future reference wouldn't hurt) . Next measure the extensions you will be using. We will call them:
E -- Effective length of extension -
measured along the centerline of the torque wrench
L -- Lever length of the wrench -- center of grip to center of drive
So the objective is to find how our extension effects the reading
on the torque wrench. Lets call the setting:
T(W)-- Torque set on the wrench
And the resulting actual torque applied to the fastener,
thru the extension:
T(E) -- Torque applied by the extension to the fastener
This is the formula needed to correctly set our T(W).
T(W) = T(E) x (L ÷ L+E)
T(E) = T(W) x (L+E ÷ L)
Example: Your extension is 3 inches long(E)
Your torque wrench is 18 inches long(L)
T(W) = T(E) x (18 ÷ 18+3) = .86
T(E) = T(W) x (18+3 ÷ 18) = 1.17
Which, for this wrench and extension set, translates to:
T(W) = T(E) x .86
T(E) = T(W) x 1.17
Next: Your fastener requires 20 ft. pounds of torque,
T(W) = 20 x (18 ÷ 18+3) = 17.20
T(E) = 17.20 x (18+3 ÷ 18) = 20.12
So you can see if you set your 18" torque wrench to
17. 20 ft. pounds, you will be applying 20.12 ft. pounds
of torque to the fastener, when you use a 3" extension.
Regular socket extensions which extend directly under the
drive head along the axis of rotation of the ratchet do not
affect the calibration of the Torque Wrench.
Handle extensions (a piece of pipe put onto the wrench in order to make torqueing easier) SHOULD NOT BE USED under any circumstances. Their use will result in erroneous torque readings, and may damage the grip or the adjusting mechanism. While applying torque, the wrench should be-held ONLY BY THE GRIP. At high torque readings; if both hands are necessary to apply enough pressure to operate the wrench, hold the grip in one hand, and put your other hand on the top of the first hand, never on the wrench body.