The Effect of Using Extensions on Torque Wrenches
Torque is a measure of rotational force, or how hard something is being twisted. It can be calculated by multiplying the length of the wrench by the force applied at the end. For instance, pressing down on the end of a foot-long wrench with 100 pounds of force results in 100 foot-pounds.
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Why It Matters
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Twisting bolts to the proper torque when installing them is important when working on machines for gasket sealing, bolt stretch, and compression of the bolted parts. A torque wrench measures this torque and allows the bolt to be installed correctly. Often torque specifications for bolts are quite high, in some cases higher than a human can easily achieve. An old and highly effective trick for increasing torque is to put an extension, often called a cheater-bar, on the end of the wrench.
Using a Cheater Bar On a Torque Wrench
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The question is, will a torque wrench still measure accurately if a cheater bar is used on the end? The answer is yes. All this configuration will do is increase the force on the end of the torque wrench. The torque measured by the wrench doesn't care if there is a hand pressing down on the wrench or if a cheater-bar is supplying the force.
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Why It Works
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Suppose a torque spec is 200 foot-pounds and all you have is a 1 foot wrench. To get 200 foot-pounds you'd have to press down on the wrench with 200 pounds of force. This might be difficult, especially if you only weigh 100 pounds.
But if you attach a 1 foot extension, the total length is now 2 feet. A force of 100 pounds times a length of 2 feet gives 200 foot-pounds.
The torque is 200 foot-pounds measured anywhere along the bar. So what is the force on the end of the wrench--after all, force is what the wrench actually measures. Torque is just force in a circle. The wrench is calibrated for its 1 foot length. Reversing the equation, you can see that force is torque divided by length. So if the torque is 200 foot-pounds, and the wrench is 1 foot, then the force on the end of the wrench is 200 pounds. The wrench will measure accurately.
The Physics
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The equation for torque is:
Torque = length x force
Now if the length = 2 ft, and force = 100 lbs, then
Torque = 2 ft x 100 lbs = 200 ft-lbs.
This is true anywhere along the torque wrench and its extension.
Now, applying simple algebra to the torque equation gives
force = Torque / length.
But now length = 1 ft. So,
force = 200 ft-lb / 1 ft = 200 lbs,
which is exactly what it should measure.
Conclusion
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It is okay to use an extension or cheater bar with a torque wrench. Just make sure it is a stout pipe or else it will bend or break.
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Comments
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I woudl also like to add, a lot of people see extensions as socket extensions. Those have adverse reactions to torque output.
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This is inaccurate information. The 1" drive torque wrenches manufactured with a maximum torque of between 1000 and 2000 ft/lbs, are designed and calibrated with the extension that is provided in the case with them at the time of purchase. Tha calibrating facility that we use at my place of employment to calibrate our torque wrenches showed me the difference both without and with the extension handle and the reading is different when the extension handle is removed vs when it is used. This was shown on a digital readout on a calibrated unit that is the indutry standard for calibrating torque wrenches.
