Pound-foot (torque)

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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

A pound-foot (lbf⋅ft) is a unit of torque representing 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.

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 lb-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, vol. 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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