Foot-pounds or pound-feet?
Use the terms interchangeably and people may question your technical competence.
WHAT'S THE DIFFERENCE between foot-pounds and pound-feet? Both consist of the same units-a force and a distance-multiplied together. The physical quantities being expressed, however, are quite different. Yet applying the terms interchangeably is one of the most common misuses of technical language.
The product of a force times a distance, as in "pound-feet" ("Newton-meters," for those metrically inclined), is torque. In electric machinery, we all know what torque means. It's the turning effort developed by a motor that causes a shaft to rotate. Based on the simple principle of the lever, torque is increased by either increasing the force itself (as by strengthening a magnetic field) or by increasing the distance-the lever arm -between the point efforce application and the axis of rotation. (Anyone using a wrench is familiar with that concept.)
Any physics text or engineering handbook will make the principle clear and will define force times distance - pounds times feet, or ounces times inches - as torque. To "say it right," remember the universal standard convention that the force comes first; the distance second.
So, is "foot-pounds" meaningless, then? Not at all. When the distance is named first, and the force second, the product represents something equally real but quite different: work.
Work measures the expenditure of energy. Thus, when a mass is lifted through a distance (against the force of gravity), or moved horizontally against a friction force, the amount of work done is measured by the product of the force involved and the distance moved. Increasing either one increases the amount of work.
How fast that work is done is a measure of power (horsepower or watts). For example, if a 100-pound weight is lifted 5.5 feet in one second, the work done is 550 foot-pounds, and the power involved is 550 foot-pounds per second-which equals one horsepower (746 watts).
Whether speaking of torque, then, as related to fastener tightening or motor capability, use the term putting force first (as in pound-feet). Putting the distance first (as in foot-pounds) is not equivalent; it represents something not at all related to torque.
And as we've said elsewhere and often, using technical terms incorrectly invites skepticism concerning your knowledge of technology.
By Richard L. Nailen, P.E., EA Engineering Editor
