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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.

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, N)

Q = weight or load (lb, N)

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

R = lever-arm radius (in, m)

The torque acting on the screw can be calculated as

T = F R                                 (2)

where

T = torque (lb in, Nm)

Example - Screw jack without friction

If the load on a screw jack is 50000 lb, the lever-arm is 20 inches and the pitch equals 1/8 inches - the effort force can be estimated to

F = ((50000 lb) (1/8 in)) / (2 π (20 in))

    = 49.8 lb

The torque acting on the screw can be calculated

T = (49.8 lb) (20 in)

    = 995 in lb

    = 82.9 ft lb

Example - Metric Screw Lift

A M24 metric screw with pitch 3 mm is used in a mechanical screw lift with load 225 kN. The lever arm is 0.5 m. The effort force required can be calculated as 

F = ((225000 N) (0.003 m)) / (2 π (0.5 m))

    = 215 N

The torque acting on the screw can be calculated

T = (215 N) (0.5 m)

    = 107 Nm

Screw Jack Calculator - without Friction - Imperial Units

Calculate torque and effort force acting on the lever arm.

load (lb)

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

lever arm (in)


Load Calculator!

Screw Jack Calculator - without Friction - Metric Units

Calculate torque and effort force acting on the lever arm.

load (N)

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

lever arm (m)


Load Calculator!

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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