Screw jack is used in applications where linear motion is required. A jack-screw is operated by turning a lead screw. It is a device which is used to lift heavy loads like foundation of a house and large vehicles. It works on the principle of inclined plane. The load usually acts centrally upon it. A horizontal rotational force is applied to the lever or handle provided at the top. This force will move the spindle upwards and hence the object will be lifted up. As the force on the handle is removed the object will not come down because of self locking. Self locking means when the rotational force on the screw is removed, it will remain motionless where it was left and will not rotate backwards, regardless of how much load it is supporting. Thus such jacks are safer than hydraulic jacks. The threads in a screw jack should be such which support heavy loads with minimum friction. It is achieved by using square or buttress threads. However, most screw jacks have large friction which increases the input applied force and the actual mechanical advantage is significantly reduced. The large area of sliding contact between the screw threads increases friction and lowers efficiency. So they are not often used for applications such as continuous transmission of high power, but more often in intermittent positioning applications. However Screw jacks with high friction are used in applications like actuators. In those applications Acme threads are used to sustain high friction. The capacity of screw jacks is between 5kN and 2000kN.
There are two types of screw jacks
Simple Screw Jack: It consists on one spindle externally threaded. In a single start thread, when we give one rotation to the lever, the spindle will move upward equal to one pitch of the thread. In double start, it will move twice the pitch.
Velocity ratio = VR = Distance moved by P/Distance moved by W = 2Πl/p
Where P = Horizontal force applied
W = Load on screw jack
l = distance from lever end to spindle axis
p = pitch
Differential Screw Jack: It consists of two spindles ‘A’ and ‘B’, ‘B’ externally threaded and ‘A’ both internally and externally threaded. The internal threads of spindle A meshes with internal threads of spindle B. Spindle A is screwed to fixed base. When the lever is rotated such that spindle A rises, spindle B also rotates and it will come down.
Velocity ratio = VR = Distance moved by the effort/Distance moved by the load