Automatic valve with spool 3 controls the operation of two hydraulic pumps, 1 and 2, so that they operate in parallel up to a definite pressure, and in series at higher pressures. Up to a definite pressure, set by adjusting spring 5, both pumps deliver fluid to pipeline 7 and their outputs are added together. At this, valve spool 3 is in the position shown. When the pressure increases above the preset value, spool 3 is raised so that fluid is delivered under pressure from pump 2 through the valve and pump 1 along pipes 9 and 10. At this, check valve 8 closes off fluid supply from the tank to pump 1. Pump 1, drawing in fluid under pressure, raises its pressure so that the pressures are added together when the pumps operate in series. In this case, valve 4 closes because the pressure in pipeline 7 is higher than in pipe 9.
$4168$CHP,Dr$