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Torque Wrench:

A torque wrench is a precision measuring instrument. It measures the force required to turn a 
nut or bolt. 
Many different styles and sizes of torque wrenches are available. This information will pertain to the common
hand held torque wrench.

Torque wrenches
Top down (1/2" drive reversible ratchet head,  3/8" drive fixed head,  1/4"  drive Beam Type)

They consists of only a few parts. The head, beam, lock ring, grip, and a blade.
They should be maintained and calibrated per the manufactures instructions.
The torque settings are stamped into the beam. On the Beam 
Type wrench a scale is attached to the beam. 
The increments can vary form inch pounds (CM-KGS) to foot
pounds (NM) depending on the size of the wrench.
Torque wrench scale
Torque wrench beam scale
On the barrel type wrench you preset the
torque you want by releasing the lock ring
and rotating the grip to the specific value.

Once that value is reached the torque
wrench will over center and "click".
Informing you that the bolt or nut has
reached your preset value. Remove the 
wrench and move to the next one.
It's that simple to operate.
With the Beam Type wrench you have to pull or push the
handle and watch for the needle deflection. There's no tell tale
The torque wrench is calibrated from the center point of the
drive to the center of the grip.
While holding the wrench by the grip only you should apply a 
slow and steady force in the direction you're tightening.

Sometimes two hands are needed for heavy torque values. 
Place one on the grip and the other on top. Never grab the
wrench in any other place. 
Hand position
Hand position
Some times it might be necessary to hold
the head steady. Just apply enough 
downward pressure on the
 head to keep the socket

You should never leave a torque wrench
preset to any value other than zero
when not in use.
(Avoid premature and accuracy failure)
When the scale on the wrench is not in the increment you need, you can calculate the equivalent.

In-lbs to Ft-lbs      (In-lbs / 12= Ft-lbs)
In-lbs to Cm-kgs      (In-lbs x 1.152=Cm-kgs)
In-lbs to Nm      (In-lbs x .1130=NM)
Ft-lbs to In-lbs      (Ft-lbs x 12= In-lbs)
Ft-lbs to Cm-kgs      (Ft-lbs x 13.83=Cm-kgs)
 Ft-lbs to NM      (Ft-lbs x 1.356=NM)
Cm-kgs to In-lbs      (Cm-kgs x .8680=In-lbs)
Cm-kgs to Ft-lbs      (Cm-kgs x .07233=Ft-lbs)
Cm-kgs to NM      (Cm-kgs x .09806=NM)
NM to In-lbs      (NM x 8.851=In-lbs)
NM to Ft-lbs      (NM x .7376=Ft-lbs)
NM to Cm-kgs      (NM x 10.20=Cm-kgs)
Torque wrench adapter
In places where access to the nut or bolt are
limited you might need an adapter. This 
changes the torque value since you're
changing the length and or angle
of the wrench.

The corrected values can be calculated.
That's right more math! Really hard math!
Example I
Image drawing
Inline adapter
T1 = (TA)/(A+B)

A  = Torque wrench length
B  =  Adapter length
T  =  Actual torque value
T1 = Corrected torque value

41.25 ftlbs=(55 ftlbsx18")/(18"+6")
Example II
Image drawing
Using a handle extension no correction needed
Example III
Image drawing
Inline Adapter with extension
T1 = (TA) / (A+B)
Example IV
Image drawing
Both handle and adapter
T1 = (TA) / (A+B)
Example V
Image drawing
An angle other than 90 degrees. "B" should never exceed the 
length of "A".
This example should be avoided when every possible.
T1 = (TA)/(A+B)

A  = Torque wrench length
B  =  Adapter length
T  =  Actual torque value
T1 = Corrected torque value
It's recommended that a stirrup-type handle with
a pointer indicating angle of loading be used
to ensure the correct angle.
Example VI
Image drawing
No correction needed
You are now ready to tighten up the world!

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