Pilot Control
In most case, the engines in hydraulic excavators drive three large hydraulic pumps. The two main pumps are for supplying oil at high pressure (up to 5000 psi @ 100 gpm) for the arms, swing motor, track motors, and accessories, and the third pump is a lower pressure (~700 psi) unit for pilot control. This circuit is used to control the spool valves so that less effort is required to operate the controls. Generally, the three pumps used in excavators consist of two variable displacement piston pumps and a gear pump, for example Concentric's high performance W900.