Screw Jacks

Screw jacks and effort force

A screw jack is a mechanical device that can increase the magnitude of an effort force.

Screw jack - load and effort force

The effort force for a screw jack when neglecting friction can be expressed as

F = (Q p) / (2 π R)                                 (1)


F = effort force at the end of the arm or handle (lb)

Q = weight or load (lb)

p = pitch distance or lead of thread in one turn  (in)

r = pitch radius of screw (in)

R = lever-arm radius (in)

Example - Screw jack without friction

If the load on a screw jack is 200000 lb, the lever-arm is 20 inches and the pitch equals 1/8 inches, the effort force can be estimated as

F = ((200000 lb) (1/8 in)) / (2 π (20 in))

    = 199 lb

Screw Jack with Friction

For motion in the same direction as the load (load assist the screw jack), the effort force can expressed as

F = Q ( (2 π μ r - p) / (2 π r + μ p) ) (r / R)                                   (2)


μ = friction coefficient

For motion in the opposite direction as the load (load resists the screw jack), the effort force can expressed as

F = Q ( (2 π μ r + p) / (2 π r - μ p) ) (r / R)                                 (3)

Related Topics

  • Mechanics - Forces, acceleration, displacement, vectors, motion, momentum, energy of objects and more
  • Statics - Loads - force and torque, beams and columns

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