Screw

                  A screw is one of the six classical simple machines, essentially an inclined plane wrapped helically around an axis for a number of turns (which may be less than one). A screw can convert a rotational movement to a linear movement, and a torque (rotational force) to a linear force. When the shaft of the screw is rotated relative to the stationary threads, the screw moves along its axis relative to the medium surrounding it; for example rotating a woodscrew forces it into wood; rotating a fixed Archimedean screw used to pump water moves the water.

                 The screw's lead, the distance the screw travels in one revolution , determines the mechanical advantage of the machine; the smaller the lead, the higher the mechanical advantage. Mechanical advantage can be estimated by dividing the circumference of the shaft by the distance between the threads.

                  Practical screw devices may or may not have a shaft around which the thread is wrapped; the propeller blade, for example, does not. Uses include:

• the bolt, used as a fastener together with a nut or tapped hole with mating thread
• the metal woodscrew, a fastener with a thread sharp enough to cut its way through wood, forming a thread in the wood, driving the screw in, and holding it in place
• the screw top, to hold the lid of a bottle or jar tightly in place
• the lathe screw, which uses rotation of a knob by hand to make much smaller, precisely controlled linear movements
• the similar worm gear, to drive a perpendicular gear with increased mechanical advantage
• the lead screw, ball screw and roller screw, which convert screw rotation to linear movement of a shaft
• the corkscrew
• the micrometer, essentially a calibrated, precise screw used for measuring linear distances
• the propeller blade to move a water- or aircraft, an example of a screw of less than one turn which is not required to move a shaft
• the electric fan blade, a fixed propeller which moves the air
• the helical twist drill bit, an Archimedean screw used to remove swarf from a hole being drilled
• the screw conveyor, closely related to the Archimedean screw