Hydraulic pumps convert the mechanical energy transmitted by a prime mover (e.g., electric motor, internal combustion engine) into hydraulic working energy or hydraulic horsepower. When a hydraulic pump is operated, its mechanical action creates a partial vacuum at the pump inlet that enables atmospheric pressure in the reservoir to force liquid through the inlet line into the pump. There are many different types of pumps used in hydraulic systems. The three basic designs used are: gear pumps, vane pumps, and piston pumps. A hydraulic system may use one of these pumps, or it may use two or more in combination. There are two types of gear pumps: external gear and internal gear. The gears rotate in opposite directions and mesh at a point in the housing between the outlet and inlet ports. A vane pump makes use of centrifugal force and it makes use of variable-volume chambers.