Screw Jacks
Screw jacks and 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, 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)
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)
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)
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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