Torque

Torque is the application of a force through a lever arm. It is a force that causes or tends to cause a twisting or rotary movement. In equation form:

Torque = Force x Lever arm length or:

where To = Torque (lb-ft or lb-in); F = Force (lb); LA = Lever arm length (ft or in). The lever arm is the distance from the point the force is applied to the center of revolution.

Because force is measured in pounds and length in feet or inches, the common units of torque are pound-feet (lb-ft) or pound-inches (lb-in).

Problem: How much torque is developed when a 50.0-lb force is applied at the end of a wrench that is 1.0 ft long?

Solution:

Notice that the answer has been written as "50 pound-feet." To distinguish torque from work, torque is written with units with the force unit first, "pound-feet," and the units for work is written with the distance unit first, "foot-pounds."

There is one additional difference between torque and work. We stated that unless there is movement, there is no work. Because torque is a force working through a lever arm, torque can exist without movement.

Problem: Which of the following will cause a greater torque to be exerted on a shaft: (1) 50.0 lb of force applied at the end of a 6.0-in (1/2-ft) wrench, or (2) 15.0 lb of force applied at the end of a 24.0-in (2-ft) wrench?

Solution:

Situation (2) will cause greater torque (twisting effect) on the shaft.

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