A lead screw converts rotary motion to linear motion by passing a threaded rod through a corresponding opposing thread cut into a moving piece. The thread is coarser and has a square profile to reduce friction. Examples include bench vices and car jacks.
A lead screw, or feed screw as it is also known, is a threaded rod or rod that converts radial or rotary motion into linear or rectilinear motion. This is accomplished by passing the threaded rod through a corresponding opposing thread cut into a moving piece as 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 the thread of a lead screw is much coarser and usually has a square profile to reduce friction.
This conversion of motion is simply demonstrated by screwing a cap onto a bottle. Once tightened, the cork travels down the threads of the bottle until it is tight and then, when loosened, it moves up the threads until it is removed. A bench vise, for example, consists of a movable and a static jaw. The mobile jaw has a threaded hole through which the nut passes. The nut is then locked in place with a retaining ring on the static jaw or vise frame.
When turning the feed screw, the moving jaw will be advanced along the threaded rod until it locks against the static jaw. When the screw is turned in the opposite direction, the movable jaw will move in the opposite direction, opening the vise again. A “C” clamp works on the same principle, except that it is the lead screw itself that moves back and forth to clamp a work piece. In both cases, the nut is converting its axial rotation into linear or back-and-forth motion.
Although a feed screw works on the same principle as a simple nut and bolt, the threads used in both applications differ significantly. For example, a bolt thread has a V-shaped profile and is cut with a fairly fine or dense thread pitch, which is the number of threads per inch of bolt length. This arrangement increases the friction caused when the nut is rotated along the bolt and helps create a very secure locking effect when the nut is fully tightened. The thread pitch on a lead screw is much coarser, i.e. fewer threads per inch of screw length, which creates less friction and allows for easier and faster turning of the screw while still producing a secure grip when tightened. The thread profile of a power screw is also typically square which, while more expensive to machine, produces the highest level of efficiency.
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