Machine, in engineering, device used to change the magnitude or direction of an applied force (see Force). The four simple machines are the lever, the pulley, the wheel and axle, and the inclined plane, a machine that consists of an inclined track or ramp. The screw and the wedge are also usually considered simple machines, but they are in actuality adaptations of the inclined plane.
The value of a simple machine is that it enables a person to exert a force greater than could be exerted by using muscles alone (as with a lever, a wheel and axle, or an inclined plane) or to apply force more efficiently, as with the pulley. The increase in force is usually at the expense of speed. The ratio between the force applied and the resistance offered by the load against which the force is acting is called the theoretical advantage of the machine. Because all machines must overcome some friction when performing their work, the actual

mechanical advantage of a machine is always less than the theoretical value. The efficiency with which a machine operates is the ratio between the amount of energy produced (the output) and the amount of energy expended (the input). The efficiency, which is expressed as a percentage, is always less than 100 percent.
Simple machines used in combination make up complex machines. Complex machines, in turn, make up the machine elements out of which the entire range of metalworking, woodworking, and other machinery is constructed (see Machine Tools).
Hydraulic machines transmit power by means of working fluid, such as oil, used to transfer forces over distances where mechanical linkages would be inappropriate or less effective. An example is the braking system of an automobile, where the force exerted on the brake pedal is transmitted through a hydraulic line to actuate the brake at the wheel.1