1 Efficiency of Screw Jack

Screw jack:

It is a device which is used to lift load by applying less effort. Square-threaded screws are frequently used in jacks. Their analysis is similar to the analysis of a block sliding along an inclined plane. The screw carries a load W and is supported by the base of the jack. Contact between screw and base takes place along a portion of their threads. By applying a force P on the handle, the screw can be made to turn and to raise the load W.

 The thread of the base has been unwrapped and in straight line. The correct slope was obtained by plotting horizontally the product 2pr, where r is the mean radius of the thread, and vertically the lead L of the screw, i.e., the distance through which the screw advances in one turn. If the friction angle is larger than the lead angle, the screw is said to be self-locking; it will remain in place under the load. To lower the load, we must then apply the force. If friction angle is smaller than lead angle, the screw will unwind under the load; it is then necessary to apply the force to maintain equilibrium. The lead of a screw should not be confused with its pitch. The lead was defined as the distance through which the screw advances in one turn; the pitch is the distance measured between two consecutive threads. While lead and pitch are equal in the case of single- threaded screws, they are different in the case of multiple-threaded screws, i.e., screws having several independent threads. It is easily verified that for double-threaded screws, the lead is twice as large as the pitch; for triple-threaded screws, it is three times as large as the pitch.