Pound-foot (torque)

Not to be confused with Foot-pound (energy).
pound-foot
Unit systemBritish Gravitational System, English Engineering Units
Unit ofTorque
Symbollbf⋅ft, 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 (lb⋅ft), abbreviated from pound-force 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 foot pound-force (ft · lbf) is the moment about an axis that applies one pound-force at a radius of one foot.

Unit

The value in Système International (SI) units is given by multiplying the following exact factors:

One pound (mass) = 0.45359237 kilograms[1]
Standard gravity = 9.80665 m/s2[1]
One foot = 0.3048 m[1]

This gives the exact conversion factor:

One pound-foot = 1.3558179483314004 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.[3]

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).[4][5] 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.

See also

References

  1. ^ a b c d Butcher, Kenneth; Crown, Linda; Gentry, Elizabeth J. (May 2006), "The International System of Units (SI) – Conversion Factors for General Use" (PDF), NIST Special Publication 1038, archived (PDF) from the original on 2023-05-30
  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. ^ Arthur Mason Worthington (1900). Dynamics of rotation : an elementary introduction to rigid dynamics (3rd ed.). Longmans, Green, and Co. p. 9.
  4. ^ "Dial Torque Wrenches from Grainger". Grainger. 2020. In most US industrial settings, the torque ranges are given in ft-lb rather than lbf-ft.
  5. ^ Erjavec, Jack (22 January 2010). Manual Transmissions & Transaxles: Classroom manual. Cengage Learning. p. 38. ISBN 978-1-4354-3933-7.