A screw is a simple machine that provides mechanical advantage by amplifying and converting a small rotational force into a large linear force. A common screw consists of a threaded cylindrical shaft with the shape of the thread being a helix. The pitch of a screw is defined as the axial distance between two adjacent threads while the lead of a screw is defined as the axial distance traveled by the screw for one complete, 360°, rotation. For a common helical screw, the pitch is equal to the lead. The work required to turn the screw one complete rotation is:
where is the applied force and is the distance from the screw's axis to the point of application of the force. Taken together, constitutes the applied torque. The work done is represented by:
where is the axial output force applied by the shaft of the screw on a load. Neglecting friction, the work done by the machine is equal to the work done on the machine,
so that the mechanical advantage of the screw is the ratio of the circumference of the circle along which the torque is applied , to the lead of the screw, :
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