The screw jack is a device, for lifting heavy loads, by applying a comparatively smaller effort at its handle. The principle on which a screw jack works is similar to that of an inclined plane. The below fig. a common form of a screw jack, which consists of a square threaded rod is also called screw rod which fits into the inner threads of the nut. The load to be raised or lowered is placed on the head of the square threaded rod which is rotated by the application of an effort at the end of lever for lifting or lowering the load.
Torque Required to lift the load by a Screw Jack:
If one complete turn of a screw thread by imagined to be unwound from the body of the screw and developed, it will form an inclined plane as shown in below fig.
p = Pitch of the screw
d = Mean diameter of the screw
α = Helix angle
P = Effort applied at the circumference of the screw to lift the load
W = Load to be lifted
μ = Coefficient of friction between the screw and nut = tan Φ, where Φ is the friction angle.