THEORY OF OPERATION - CONTINUED
HYDRAULIC SYSTEM - CONTINUED
High Pressure Stall.
Pressure compensator spool is parallel with flow compensator spool. Pressure compensator spool is forced
right by cutoff spring during normal operation.
If piston pump is at maximum system pressure, maximum system pressure is equal to hydraulic oil pres-
sure from signal limiter valve plus margin pressure. Signal limiter valve in main control valve limits signal
oil pressure to 3,450 ± 50 psi (25,855 ± 345 kPa). Pump cutoff pressure is 3,750 ± 50 psi (23,787 ± 345
kPa). Pump cutoff pressure is controlled by pressure compensator spool. Pump cutoff pressure serves as a
backup to oil pressure from signal limiter valve plus margin pressure.
When a circuit is stalled, signal limiter valve opens to limit maximum system pressure.
Combined pressure of load sensing signal and margin spring forces flow compensator spool right. Com-
bined forces of signal pressure and margin pressure are less than pump output pressure. System needs very
little flow when a circuit is stalled and pressure is high. Margin spool moves to a metering position to
maintain desired minimum flow rate. In metering position, margin spring and signal limiter valve equal
pump output pressure.
Activate a second circuit while one circuit is in a stall condition. Pump will upstroke to meet new flow
demand. Without a signal limiter valve in PPPC system, if a single circuit is stalled, no other circuit will