A pulley is basically a circular lever that consists of a grooved wheel rotating about a center hub. Pulleys are classified either as fixed or movable, depending on whether the hub remains at a fixed position or moves. A fixed pulley only changes the direction of effort, since it is a first-class lever with the fulcrum located at its center. A movable pulley is a second-class lever with an ideal mechanical advantage of 2. Several fixed and movable pulleys can be used together in a lifting device called a block and tackle, if larger mechanical advantages are desired.
Examples are screwdrivers, chain sprockets, and gears.
Gears Gears are toothed wheels that transmit force and motion from one rotating, geared shaft to another. Teeth can be formed on the outside as for external gears, or on the inside, as for internal gears. It is also possible to simply change the direction of motion. Torque is equal to force multiplied by the distance from the point of force application to the center of rotation. A small gear driving a large gear multiplies torque, while a large gear driving a small gear multiplies speed.