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Screw Jacks

Screw jacks and effort force

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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)

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 = 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 Calculator - without Friction - Imperial Units

load (lb)

pitch - distance or lead of thread in one turn (in)

lever arm (in)

Screw Jack Calculator - without Friction - Metric Units

load (kg)

pitch - distance or lead of thread in one turn  (mm)

lever arm (mm)

Note! - there is a difference between mass and weight!

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

r = pitch radius of screw (in)

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)

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