Copyright Goodheart-Willcox Co., Inc.
428 Hydraulic Systems for Mobile Equipment
• PFC—Pressure and fl ow compensated system (Case IH, Eaton, and
John Deere’s agricultural division).
• CCLS—Closed-center load-sensing system (New Holland Agriculture
and AGCO).
The most problematic of the fi ve descriptors is PFC. Although Case IH,
John Deere agricultural equipment, and Eaton have all used PFC to describe
a load-sensing pressure-compensating hydraulic system, there is another type
of PFC hydraulic system known as positive fl ow control which is completely
different than an LSPC system. Positive fl ow control hydraulic systems will be
explained in Chapter 20. LSPC is the term adopted for this textbook. The term
pressure compensation, in the context of an LSPC system, can have two different
meanings.
Pressure-Compensating Hydraulic Pump
The fi rst PC description is based on the pressure compensator valve located
inside a variable-displacement pump, which was the focus of Chapter 17. The
pump’s PC spool operates in the same manner as described in Chapter 17.
Within an LSPC variable-displacement pump, the PC spool has the responsi-
bility to destroke the pump when a hydraulic cylinder reaches its end of travel.
The PC spool maintains that constant high pressure until the DCV is returned
to the neutral position.
Pressure-Compensated DCV
The second description for pressure compensation in an LSPC system
focuses on a pressure compensator valve inside the DCV, which was briefl y
mentioned in Chapter 8. The valve is also called a fl ow compensator valve. It is
essentially a pressure-reducing valve that senses working pressure. The valve
uses this signal pressure to compensate the DCV’s fl ow.
Without the compensator valve, anytime the system’s fl ow or pressure
changes, the operator would have to readjust the position of the DCV spool to
attempt to maintain the same implement speed. In this context, pressure com-
pensation is used to maintain a constant actuator speed (cylinder or hydraulic
motor) based on a fi xed position (opening) of the DCV spool.
The PC valve can be placed before the DCV spool, which is known as
pre-spool compensation or upstream compensation. Pre-spool compensation is
the focus of this chapter. When the pressure compensator valve is placed after
the DCV spool, it can be called many different terms. Two of the most common
terms are post-spool compensation or downstream compensation, which is the
focus of Chapter 19.
Consider an agricultural tractor that is using a DCV for operating a plant-
er’s fan motor. The hydraulic motor is used to develop either a precise positive
air pressure or a precise vacuum pressure. The farmer must dial-in the speed
of the hydraulic motor to achieve a specifi c air pressure or vacuum, otherwise
the seeding mechanism will not plant accurately.
In this example, assume that planter motor operation needs precisely 4 gpm
(15 lpm) at 1000 psi (69 bar). The challenge is that, as the operator uses other
hydraulic functions, such as raising or lowering the three-point hitch for draft