How to Calculate a Motor Torque

Torque is a way of measuring how powerful a car's motor is. You may have heard of horsepower, but torque is important too. There are two common methods of calculating the torque of a motor. You can calculate torque or full-load torque. Full-load torque takes horsepower into account and assumes the engine is working at maximum power. Learn the formulas you'll need to calculate torque and full-load torque, and how to apply them to a motor.

Difficulty: Moderate

Instructions

Calculating Torque

1. Use the formula "T = F x D" where "T = torque" in ft-lb, which means the weight in pounds (lb.) being moved over the distance of one foot (ft.), "F = force" in pounds (lb.), and "D = distance" in feet (ft.)

2. Replace the letter "F" with the force being applied. Replace the letter "D" with the distance that must travel for one rotation. For example, 60 pounds of force pushing on a lever arm that is 3 feet long would result in the formula "T = 60 x 3."

3. Multiply "F" by "D" to get your answer. For this equation, the answer is "T = 180." The torque is 180 ft-lb. This means this particular device can move 180 pounds per foot.

Calculating Full-Load Torque

4. Use the formula "T = HP x 5252 divided by RPM" where "HP" = horsepower and "RPM" = revolutions per minute.

5. Replace "HP" with the horsepower of the motor and "RPM" with the rpm of the motor. For example, a 30 horsepower motor working at 1725 revolutions per minute gives you the formula "T = 30 x 5252 divided by 1725".

6. Multiply the horsepower by 5252, in this case "30 x 5252" and divide your answer by RPM. For this equation, "30 x 5252" divided by "1725" equals 91.34. Your answer is "T = 91.34 ft-lb". The full-load torque of the engine is 91.34 foot-pounds.

Tips & Warnings

• Foot-pounds may also be expressed as lb-ft instead of ft-lb.