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:
${W}_{i}equals;2\mathrm{pi;}r{F}_{i}$
where ${F}_{i}$ is the applied force and $r$ is the distance from the screw's axis to the point of application of the force. Taken together, $r{F}_{i}$ constitutes the applied torque. The work done is represented by:
${W}_{o}\=l{F}_{o}$
where ${F}_{o}$ 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,
${W}_{o}\={W}_{i}$
so that the mechanical advantage of the screw is the ratio of the circumference of the circle along which the torque is applied $2\mathrm{pi;}r$, to the lead of the screw, $l$:
$\mathrm{MA}equals;\frac{{F}_{o}}{{F}_{i}}equals;\frac{2\mathrm{pi;}r}{l}$.