Hydrstatic systems all have a Charge Pump and its main purpose is to supply makeup and flushing oil to the closed loop flow between the pump and actuator, usually a hydraulic motor.
The other function of the Charge Pump is to supply Pilot Oil to the Pump and/or Motor control circuits to change flow volume and/or direction.
On some circuits the Charge Pump is also used to power auxilliary equipment such as the Mower Deck lift on my Lawn Tractor.
Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com
Not sure of your familiarity with hst terms, hope this makes sense also.
The charge pump also provides positive pressure, 250-400 psi, to force fluid into the inlet passages of the valve block (into the pump) and prevent cavitation in the loop. The flow through hst pump porting is much higher than could be achieved with a common 15 psi atmospheric inlet open loop pump.
Hst pumps must also have this postive pressure in the pistons to stay in 'compression' as it were, to keep the slippers and shoes all down against the swash plate as they rotate. If they go into suction the joints become in 'tension' through part of the revolution and the pump mechanically destroys itself fairly quickly. Most hst pumps are designed so the servo system loses control of the pistons (i.e. the pump goes to no flow output) before the charge pressure gets so low it damages the pump.
