Pound-foot (torque)

A pound-foot (lbf⋅ft) is a unit of torque (a pseudovector). One pound-foot is the torque created by one pound of force acting at a perpendicular distance of one foot from a pivot point.[2] Conversely one pound-foot is the moment about an axis that applies one pound-force at a radius of one foot.

Not to be confused with Foot-pound (energy).
pound-foot
Unit systemBritish Gravitational System, English Engineering Units
Unit ofTorque
Symbollbf⋅ft or lb-ft
Conversions
1 lbf⋅ft in ...... is equal to ...
   SI units   ≈ 1.355818 N⋅m[1]
   Gravitational metric system   ≈ 0.1382550 kgf⋅m

The value in SI units is given by multiplying the following approximate factors:

One pound (force) = 4.448 222 newtons[3][4]
One foot = 0.3048 m[5]

This gives the conversion factor:

One pound-foot = 1.35582 newton metres.

The name "pound-foot", intended to minimize confusion with the foot-pound as a unit of work, was apparently first proposed by British physicist Arthur Mason Worthington.[6]

Despite this, in practice torque units are commonly called the foot-pound (denoted as either lb-ft or ft-lb) or the inch-pound (denoted as in-lb).[7][8] Practitioners depend on context and the hyphenated abbreviations to know that these refer to neither energy nor moment of mass (as the symbol ft-lb rather than lbf-ft would imply).

Similarly, an inch-pound (or pound-inch) is the torque of one pound of force applied to one inch of distance from the pivot, and is equal to 1⁄12 lbf⋅ft (0.1129848 N⋅m). It is commonly used on torque wrenches and torque screwdrivers for setting specific fastener tension.

References
  1. "Appendix B.9: Factors for units listed by kind of quantity or field of science". NIST Guide to the SI. National Institute of Standards and Technology. September 7, 2016. Retrieved 2018-07-09.
  2. Pickerill, Ken (2009). Today's Technician: Automotive Engine Performance Classroom Manual and Shop Manual (5th ed.). Cengage Learning. pp. 50–51. ISBN 978-1111782382.
  3. United States National Bureau of Standards (1959-06-25). "Notices "Refinement of values for the yard and the pound"" (PDF). Retrieved 2006-08-12.
  4. Howard Ludwig (Mar 3, 2017). "What is the relation between pounds of force and pounds as a measurement of mass?".
  5. Collins, Joseph B. (2009), "OpenMath Context Dictionaries for SI Quantities and Units", in Carette, Jacques; Dixon, Lucas; Coen, Claudio Sacerdoti; Watt, Stephen (eds.), Intelligent Computer Mathematics: 16th Symposium, Calculemus 2009, 8th International Conference, MKM 2009, Grand Bend, Canada, July 6-12, 2009 Proceedings, 5625, Springer Science & Business Media, p. 260, ISBN 978-3642026140
  6. Arthur Mason Worthington (1900). Dynamics of rotation : an elementary introduction to rigid dynamics (3rd ed.). Longmans, Green, and Co. p. 9.
  7. "Dial Torque Wrenches from Grainger". Grainger. 2020. In most US industrial settings, the torque ranges are given in ft-lb rather than lbf-ft.
  8. Erjavec, Jack (22 January 2010). Manual Transmissions & Transaxles: Classroom manual. p. 38. ISBN 978-1-4354-3933-7.


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