The charge pump is a fixed-displacement, gerotor type pump. The main pump drives the charge pump
off the main shaft. A spring and poppet style relief valve limits charge pressure.
The standard charge pump is satisfactory for most applications; however, other displacements are
available. A gear pump, mounted to the auxiliary pad, may augment charge flow if additional volume is
required.
Charge relief valve
The charge relief valve maintains charge pressure at a designated level. The charge relief valve is a direct
acting poppet valve which opens and discharges fluid to the pump case when pressure reaches that
level. It is nominally set with the pump running at 1800 rpm. In forward or reverse, charge pressure is
slightly lower than when in neutral position. The model code of the pump specifies the charge relief valve
setting.
Loop flushing valve
A loop flushing valve dumps flow from the low side of the main loop, removing heat and contaminants.
Pumps equipped with an integral loop flushing valve also include a loop flushing relief valve. The loop
flushing relief valve poppet includes an orifice that controls flushing flow under most conditions. A
combination of orifice size and charge pressure relief setting produces a specific flushing flow.
Service Manual
Series 42 Axial Piston Closed Circuit Pumps
Functional Description
520L0638 • Rev 0300 • July 2015 11
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