Copyright Goodheart-Willcox Co., Inc.
442 Hydraulic Systems for Mobile Equipment
Note
It is important to realize that a low pressure standby value might be
influenced by more than just a pump’s margin spring. Some LSPC
systems have a warm-up signal pressure created in the steering
circuit. Some manufacturers take that steering signal pressure
into consideration, eliminating it or isolating it to achieve a true
low-pressure standby value. Chapter 25 explains warm-up signal
pressures used in steering circuits.
Stall Mode
When a cylinder reaches its end of travel, the pump begins to stall, also
known as pump cut off. The stall mode pressure is set by the PC spool spring.
This is the same PC spool that was explained in Chapter 17. The PC acts sim-
ilar to a main system relief valve. Anytime the system pressure deadheads,
the PC spool shifts, allowing oil to be sent to the control piston to destroke the
pump. See Figure 18-19. The PC spool does not need to sense signal pressure.
Figure 18-19. When a cylinder reaches its end of travel (stall mode), the axial piston pump will be destroked by the
pressure compensator spool. Once the high pressure is directed to the control piston, the control piston’s force is enough
to move the swash plate parallel to the displacement pistons’ axis of rotation. This reduces pump flow to nearly zero.
Bias spring
Swash
plate
DCV
Control piston
Axial piston
Axial piston
Flow control
spool (300 psi)
Pressure
compensator
spool (2500 psi)
Shuttle valve