Copyright Goodheart-Willcox Co., Inc.
Chapter 18 Load-Sensing Pressure-Compensating (LSPC) Hydraulic Systems 443
If the PC spool is set at 2500 psi (172 bar), once the pump reaches that pres-
sure, the PC spool shifts down, directing oil to the pump’s control piston to
destroke the pump. The pump fl ow will be reduced to practically no fl ow but
will remain at the high stall pressure value until the DCV is returned to a neu-
tral position.
Another interesting point about stall mode on a variable-displacement
LSPC pump is that the fl ow control spool is technically shifted up, while
the pressure compensator spool is shifted down. How is this possible? The
bottom of the fl ow control spool has a 300 psi (21 bar) spring setting and
2500 psi (172 bar) of pump outlet pressure acting on it, while the top of the
spool only has pump outlet pressure, 2500 psi (172 bar), acting on it. As a
result, the fl ow control spool is shifted upward due to the higher combined
pressure value at the bottom of the spool. However, the pump outlet pres-
sure can still make its way around the top of the fl ow control spool so that it
can still act on the top of the PC spool, allowing the PC spool to shift down
and destroke the pump.
Main System Relief in Variable-Displacement LSPC Systems
Many agricultural variable-displacement LSPC systems do not use any
type of a main system relief valve, and rely solely on the pump’s PC valve
to protect the system during high-pressure conditions. Conversely, many con-
struction equipment machines contain a main system relief valve in addition
to the pump’s PC valve. If the machine has both a main system relief and a
pump PC, traditionally one of those components is used as a backup in case
the primary fails. This means that one of the components serves primarily as a
high-pressure relief while the other is not normally used but is set higher than
the primary component. Normally the main system relief is set higher than the
pump’s PC. However, in a few rare cases, a machine can have the pump com-
pensator set higher (for example, 4000 psi [276 bar]) than the main system relief
(for example, 3600 psi [248 bar]).
Most systems set the main system relief valve as the secondary safety
feature in the event that the pump PC is slow or is ineffective at destroking
the pump. A Caterpillar D8R dozer has a main system relief set at 3900 psi
(269 bar) and the pump pressure cutoff set at 3500 psi. In this example, if the
main system relief valve is inaccurately set below the pressure compensator
value, the LSPC system will be operating ineffi ciently. The ineffi ciency will be
evident during stall mode, when the pump is destroked. If the main system
relief valve takes command during high system pressures, the pump will not
only be running at high pressure, but will also be fl owing a lot of oil over the
main relief valve, consuming unnecessary energy.
Industry experts recommend that when using a main system relief in
conjunction with a variable-displacement pump, the main system relief value
should be set 10 to 15% above the pressure compensator value.