Minimum low loop pressure must be maintained under all operating conditions to avoid cavitation.
All pressure limits are differential pressures referenced to low loop (charge) pressure. Subtract low loop
pressure from gauge readings to compute the differential.
Servo Pressure
Servo pressure is the pressure in the Servo-system needed to position and hold the pump on stroke. It
depends on system pressure and speed.
At minimum servo pressure the pump will run at reduced stroke depending on speed and pressure.
Minimum servo pressure at corner power holds the pump on full stroke at max speed and max pressure.
Maximum servo pressure is the highest pressure typically given by the charge pressure setting.
Charge Pressure
An internal charge relief valve regulates charge pressure. Charge pressure supplies the control with
pressure to operate the swashplate and to maintain a minimum pressure in the low side of the
transmission loop. The charge pressure setting listed in the order code is the set pressure of the charge
relief valve with the pump in neutral, operating at 1800 min
-1
[rpm], and with a fluid viscosity of 32 mm
2
/s
[150 SUS]. Pumps configured with no charge pump (external charge supply) are set with a charge flow of
30 l/min. [7.93 US gal/min.] and a fluid viscosity of 32 mm2/s [150 SUS].
The charge pressure setting is referenced to case pressure. Charge pressure is the differential pressure
above case pressure.
Minimum charge pressure is the lowest pressure allowed to maintain a safe working condition in the
low side of the loop. Minimum control pressure requirements are a function of speed, pressure, and
swashplate angle, and may be higher than the minimum
charge pressure shown in the Operating parameters tables.
Maximum charge pressure is the highest charge pressure allowed by the charge relief adjustment, and
which provides normal component life. Elevated charge pressure can be used as a secondary means to
reduce the swashplate response time.
At normal operating temperature charge inlet pressure must not fall below rated charge inlet pressure
(vacuum).
Minimum charge inlet pressure is only allowed at cold start conditions. In some applications it is
recommended to warm up the fluid (e.g. in the tank) before starting the engine and then run the engine
at limited speed.
Maximum charge pump inlet pressure may be applied continuously.
Case Pressure
Under normal operating conditions, the rated case pressure must not be exceeded. During cold start
case pressure must be kept below maximum intermittent case pressure. Size drain plumbing accordingly.
Auxiliary Pad Mounted Pumps. The auxiliary pad cavity of S90 pumps configured without integral
charge pumps is referenced to case pressure. Units with integral charge pumps have auxiliary mounting
pad cavities referenced to charge inlet (vacuum).
Caution
Possible component damage or leakage
Operation with case pressure in excess of stated limits may damage seals, gaskets, and/or housings,
causing external leakage. Performance may also be affected since charge and system pressure are
additive to case pressure.
Technical Information
Series 90 Axial Piston Pumps
Operating Parameters
520L0603 • Rev 0804 • March 2016 11
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