How does a Screw Jack Work?

            A jack is essential equipment for raising heavy objects off the ground, usually used to remove or adjust wheels of automobiles. Almost all drivers of heavy or light vehicles always carry a screw jack with them in their vehicle, for the simple reason that it alone performs the role of several people in time of emergency. The screw jack is a simple machine like pulleys or levers and is used for raising large loads with a little human effort.

Cross-Section of Screw Jack

            A simple screw jack comprises a thick solid rod in which a screw thread has been cut, a base plate in which it can rotate, and a block or nut through which the screw threads run. The block is shaped in such a manner that it hooks on to the underside of the car. When the screw is rotated with the help of Tommy bar, which passes through a hole in the screw, the block is slowly raised or lowered.

            A screw thread is a spiral cut made in the rod so that the screw may be regarded as a spiral inclined plane. For each complete turn of the screw, it advances by a distance equal to the pitch of the screw. Pitch of the screw is the distance between any tow adjacent ridges of the thread. The screw of a screw jack whose diameter is ¾ inch may have six threads per inch, so the pitch of the thread is 1/6 inch. Thus, for every one complete revolution of the Tommy bar, the lifting block moves 1/6 of an inch up or down.

            The velocity ratio of any machine is found by dividing the distance ‘e’ through which the effort moves by the distance ‘I’ through which the load is raised. If the Tommy bar is a little more than 6 inches in length then it will trace out a circle of radius of 6 inches for each complete revolution. The circumference of this circle is about 37.7 inches so that the load is raised by 1/6 inch for every 37.7 inches that the effort moves.

            Even if the thread of the screw is kept clean and well greased, there is still likely to be considerable friction between the screw and the lifting block as well as the base plate. Thus a load of 225 kilograms, which is the weight, supported by one of the four wheels of an average family car; it could be lifted by a 1.25 kilograms effort if there were no friction. But in practice, the effort would be between 3.4 kilograms to 4.5 kilograms. In other words, it is an ideal machine, which requires no energy to move its component parts, as the velocity ratio is equal to the mechanical advantage.

            On account of the large frictional forces between the screw and the block the load cannot unscrew it self and run back under its own weight.