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Engineering ToolBox > Screw Jacks
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)
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
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
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)