power
A measure of the effectiveness that a force producing a physical effect has over time. If linear, the quotient of: (force multiplied by the displacement of or in an object) ÷ time. If rotational, the quotient of: (force multiplied by the angle of displacement) ÷ time.
power: energy delivered per unit time
In mechanical engineering, power is the rate at which work is done or energy is transferred. A machine that lifts 100 kilograms one meter in 10 seconds delivers more power than one that takes 100 seconds to do the same job, even though both perform identical work. Power is measured in watts (one joule per second) or, in older imperial practice, horsepower (746 watts). The distinction matters: a small motor spinning fast can equal the power output of a large motor spinning slowly.
Linear power, used in presses and hydraulic cylinders, equals force times velocity: a hydraulic ram pushing with 5000 newtons while extending at 0.5 meters per second delivers 2500 watts. Rotational power, used in motors and turbines, equals torque times angular velocity: a shaft rotating at 1500 revolutions per minute while transmitting 10 newton-meters of torque produces roughly 1571 watts. The units differ, but the principle is identical: more force or more speed means more power at the load.
Losses and practical limits
Rated power is what a machine is designed to sustain without damage. Peak power, which motors can produce for seconds or minutes, exceeds this rating. Continuous operation at rated power generates heat through friction, bearing drag, and material deformation. Exceed the rating and components overheat, bearings seize, seals fail. Conversely, running a motor below its rated power wastes capital investment and often runs less efficiently than at nameplate conditions.
Input power and output power differ by efficiency, which depends on design and operating point. A hydraulic system that receives 50 kilowatts of pump power might deliver only 40 kilowatts at the actuator because of leakage, pressure drop, and heat generation. Industrial gearboxes typically run 90 to 98 percent efficient depending on type; belt drives run 92 to 97 percent. These losses compound in machinery with many stages, which is why direct-drive designs are preferred when torque requirements permit.
Power factor in electrical systems complicates the picture further. A three-phase motor drawing 100 amperes at 480 volts with a power factor of 0.85 consumes roughly 70.5 kilowatts of real power; the remainder is reactive power that does no useful work. Facility design and billing often penalize poor power factors, making motor selection and drive quality economically significant beyond the nameplate kilowatt rating alone.