A lead screw, or power screw as it is also known, is a threaded bar or rod that converts radial or rotational motion into linear or straight line motion. This is achieved by passing the threaded rod through a corresponding opposite thread cut into a movable work piece as is seen in bench vices and car jacks. When the lead screw is turned, the work piece will move up or down the threads depending on the direction of the screw. The main difference between a lead screw and a regular fastener such as a nut and bolt is that a lead screw’s thread is far coarser and typically square in profile to reduce friction.
This conversion of motion is simply demonstrated by screwing a cap onto a bottle. When tightened, the cap moves down the thread on the bottle until it is tight and then, when loosened, moves up the threads until it is removed. A bench vice, for example, consists of one movable and one static jaw. The moving jaw is equipped with a threaded hole through which the lead screw passes. The lead screw is then locked into place with a circlip on the static jaw or frame of the vice.
When the power screw is turned, the moving jaw will be advanced up the threaded bar until it is locked against the static jaw. When the screw is turned in the opposite direction the moving jaw will move in the opposite direction, opening the vice again. A "C" clamp works on the same principle except it is then the lead screw itself that moves backward and forwards to clamp a work piece. Either way, the lead screw is converting its axial rotation into a linear or back and forth motion.
Although a power screw works on the same principle as a simple nut and bolt, the threads used in both applications differ considerably. The screw thread on a bolt, for instance, is of a V profile and is cut with a fairly fine or dense thread pitch, i.e., number of threads per inch of bolt length. This arrangement increases the friction caused when the nut is turned along the bolt and aids in creating a very secure locking effect when the nut is fully tightened. The thread pitch on a lead screw is far coarser, i.e., less threads per inch of screw length, which creates less friction and allows for easier and quicker turning of the screw while still producing a secure grip when tightened. The thread profile of a power screw is also typically square which, although more expensive to machine, produces the highest efficiency level.