Screw Jacks

Calculate screw jacks effort force

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

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

F = Q p / 2 π R (1)

where

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 100000 lb, the lever-arm is 18 inches and the pitch equals 1/8 in., 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)

where

μ = friction coefficient

For motion in the opposite 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) (3)

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