This document summarizes the operation of a variable displacement piston pump used in a hydraulic system. The pump contains bias and actuator pistons that control the angle of the swashplate to vary the pump's displacement. A compensator valve senses pressure requirements and system demands to control the pistons and maintain appropriate pressures. The pump operates in different modes like upstroking, destroking, and low pressure standby depending on hydraulic circuit activity and demands.

1. 7AR D6R TRACK-TYPE TRACTOR Systems Operation
Media Number SENR8359-01 Publication Date 1999/08/01 Update Date 2001/10/05
Hydraulic Pump - Piston
SMCS Code: 5070
Illustration 7
Variable displacement piston pump
(1) Spring
(2) Spring
(3) Compensator valve
(4) Actuator piston
(5) Spacer
(6) Shoe plate
(7) Pump
(8) Drive shaft
(9) Swashplate
(10) Pistons (nine)

2. (11) Bias piston
(12) Spring
(13) Barrel
(14) Pressure compensator spool
(15) Flow compensator spool
The pump for the hydraulic system is an automatically controlled variable displacement axial piston
pump. The pump contains two control pistons: bias piston (11) and actuator piston (4). The pump
also includes compensator valve (3) in order to limit system pressure. The compensator valve
senses both the pressure requirements and the system requirements.
While the engine operates, pump drive shaft (8) rotates barrel (13) with nine pistons (10). The ends
of the pistons are in the shape of a ball that fits into shoe plate (6). Shoe plate (6) slides on a thin
film of oil on swashplate (9). Swashplate (9) does not rotate. Shoe plate (6) is held in place by
spacer (5).
Note: The swashplate pivots, in order to increase or decrease the swashplate angle. A change in
the swashplate angle changes the pump displacement.
Pump (7) contains two control pistons: bias piston (11) and actuator piston (4). Bias piston (11) is
used in order to upstroke the pump. The bias piston is spring loaded. Also, pump pressure assists
the bias piston.
Actuator piston (4) is used in order to destroke the pump. Actuator piston (4) has a larger area
than the bias piston.
Flow compensator spool (15) and/or pressure compensator spool (14) regulates the pressure in
actuator piston (4) in order to change the pump displacement. The pressure in actuator piston (4)
is supplied by the pump discharge pressure. Compensator valve (3) applies pump pressure to
actuator piston (4). The larger actuator piston can override both bias piston (11) and spring (12) in
order to destroke the pump.
When drive shaft (8) turns barrel (13), pistons (10) move in and out of the barrel as shoe plate (6)
follows the angle of the swashplate. As piston (10) moves out of barrel (13), piston (10) draws oil
from the hydraulic tank, through the pump inlet, and into the piston cylinder. As the rotation of the
barrel continues, the piston moves into the barrel. The piston pushes the oil from the piston
cylinder and through the pump outlet.
Compensator valve (3) automatically maintains both the pump pressure and the flow in order to
fulfill the system requirements. When none of the hydraulic implement circuits are active, the pump
is at low pressure standby. Low pressure standby is approximately 2950 kPa (430 psi).
However, if one or more circuits are active, a resolver network compares the control valve work
port pressures. The highest resolved pressure that is felt flows through a signal line to the pump
compensator valve inlet. Then, compensator valve (3) maintains the system requirements. Usually,
the actual system pressure is approximately 2100 kPa (305 psi) higher than the highest work port
pressure that is required, unless the pump is at full stroke. The difference between the work port
requirement and the higher supply pressure is called margin pressure.

3. Also, the compensator valve limits pressure in order to prevent overloads of the pump and of the
system. When work port pressure rises higher than a set pump pressure of 20000 ± 350 kPa (2900
± 51 psi), the pressure limiting ability of the compensator overrides the load sensing part of the
compensator. Pump output is lowered. This function occurs at approximately 690 kPa (100 psi)
lower than the maximum pressure setting. The pressure limiting ability of the compensator protects
the hydraulic system from damage by high pressures.
The schematics in the next four sections illustrate the actions of the pump and of the compensator
valve during different conditions in the hydraulic system.
Upstroking
Illustration 8
Pump and compensator valve (upstroking)
(1) Spring
(4) Actuator piston
(9) Swashplate
(11) Bias piston
(12) Spring
(15) Flow compensator spool
(17) Signal passage

4. (18) Pump output passage
(19) Case drain passage
(20) Passage
(21) Passage
(22) Passage
(23) Passage
(24) Cavity
(BB) Pressure oil with the first pressure reduction
(EE) Signal oil
(LL) Tank oil
The pump maintains a constant flow up to the cutoff pressure for each system pressure and for
each flow demand. (The exception is the maximum displacement position of the swashplate, when
the pump output is a function of the engine speed.)
Note: Refer to "High Pressure Stall".
A need for more hydraulic horsepower (demand) due to an increased circuit load pressure is met
by an increased engine torque. (The increased circuit load pressure is not met by a change in the
swashplate angle or in the engine speed.)
Upstroking occurs when the pump displacement (output) increases.
Four conditions can result in upstroking:
·During the initial operation from low pressure standby, the load signal pressure increases
pump output for each system demand. See "Low Pressure Standby".
·For each system demand, the pump slightly upstrokes in order to compensate for a built-in
pump leakage flow.
·The system demand increases.
·Another hydraulic circuit is activated.
Signal pressure flows through signal passage (17) and signal oil fills cavity (24). The signal
pressure plus the force of spring (1) moves flow compensator spool (15) downward. (Refer to
Illustration 8.) The oil that is behind actuator piston (4) drains past flow compensator spool (15),

5. through passage (21), and to case drain passage (19). As the supply oil is momentarily cut off to
the actuator piston by flow compensator spool (15), the oil in passage (20) and in bias piston (11)
works with spring (12) in order to move the swashplate (9) toward the maximum angle (upstroke).
Pump output is increased.
The pump output pressure increases until the pressure in passage (22) moves spool (15) up to the
metering position. In the metering position, the pump pressure is initially greater than the combined
force of spring (1) and of the signal pressure in cavity (24). (Refer to Illustration 7.) Flow
compensator spool (15) moves upward. Now, pressure is sent through passage (23) to actuator
piston (4).
The area of actuator piston (4) is greater than the area of bias piston (11). Therefore, the force that
moves swashplate (9) toward the minimum angle (actuator piston) is greater than the force that
moves swashplate (9) toward the maximum angle (bias piston plus bias spring). The swashplate
angle decreases. Pump output decreases. When the pump pressure decreases enough, the
combined signal pressure and spring force in cavity (24) move flow compensator spool (15)
downward. (Refer to Illustration 8.) The oil behind actuator piston (4) flows to the case drain.
Illustration 9
Typical example of metering spool
Bias piston (11) and spring (12) force the angle of swashplate (9) to increase. This up and down
spool movement is called metering. Metering keeps the pressure equal on both ends of flow
compensator spool (15). Spring (1) is equal to 2100 kPa (305 psi). Therefore, pump pressure is
2100 kPa (305 psi) higher than the signal pressure. The difference is called margin pressure.
Destroking

6. Illustration 10
Pump and compensator valve (destroking)
(1) Spring
(4) Actuator piston
(9) Swashplate
(11) Bias piston
(12) Spring
(15) Flow compensator spool
(17) Signal passage
(19) Case drain passage
(21) Passage
(22) Passage
(23) Passage
(24) Cavity
(AA) High pressure oil

7. (DD) Signal oil
(LL) Tank oil
The pump maintains a constant flow for each system pressure and for each demand. (The
exception is the maximum displacement position of the swashplate, when the pump output is a
function of the engine speed.) When the demand decreases, the torque on the engine is decreased.
(The swashplate angle or the engine speed do not change.)
Destroking occurs when the pump displacement (output) decreases.
Four conditions can result in destroking:
·The system demand stops. For example, the control valve is moved to the HOLD position.
·The system demand is reduced.
·Any of the operating hydraulic circuits in a multiple circuit operation is in a standby mode
or in a reduced flow mode.
·When the highest operating pressure decreases slightly, the built-in pump leakage
decreases.
The lower signal pressure flows through signal passage (17) and signal oil fills cavity (24). Now,
the signal pressure plus the force of spring (1) in cavity (24) is less than the pump pressure in
passage (22). Flow compensator spool (15) is pushed upward. Oil behind actuator piston (4)
cannot flow through passage (21) to case drain passage (19). Pump oil now flows through passage
(22), past flow compensator spool (15), through passage (23), and into actuator piston (4). Pump
pressure behind actuator piston (4) is now higher than the combined force of bias piston (11) and
spring (12). The angle of swashplate (9) decreases. Pump output decreases until pump output is
not sufficient to maintain system pressure. System pressure decreases.
When system pressure approaches 2100 kPa (305 psi) (margin pressure) to fulfill a system
requirement, flow compensator spool (15) moves downward to the metering position. When all the
control valves are in the HOLD position and the system pressure approaches 2950 kPa (430 psi)
(low pressure standby), flow compensator spool (15) moves downward to the metering position.
The angle of swashplate (9) increases slightly so that pump output compensates for system
leakage. Also, the higher swashplate angle maintains the lower required system pressure. While
signal pressure remains constant, flow compensator spool (15) stays in the metering position. The
hydraulic system is now stabilized.
Note: For an explanation of metering, refer to "Upstroking".
High Pressure Stall

8. Illustration 11
Pump and compensator valve (high pressure stall)
(1) Spring
(2) Spring
(4) Actuator piston
(9) Swashplate
(14) Pressure compensator spool
(15) Flow compensator spool
(17) Signal passage
(22) Passage
(23) Passage
(24) Cavity
(AA) High pressure oil
(EE) Signal oil
(LL) Tank oil

9. A high pressure stall occurs when the hydraulic system stalls under a load or the cylinders reach
the end of the stroke. A stall occurs when pump output reaches 20000 kPa (2900 psi). The signal
pressure in signal passage (17) and in cavity (24) now equals the pump output pressure. Spring (1)
keeps flow compensator spool (15) moved downward.
When the system pressure reaches 20000 kPa (2900 psi) in passage (22), the force on pressure
compensator spool (14) compresses spring (2). Pressure compensator spool (14) moves upward.
Supply oil flows through passage (23) to actuator piston (4). The pressure that is felt on the
actuator piston destrokes the pump. Pump output decreases while the system pressure stays at
20000 kPa (2900 psi).
If the control lever is moved to the HOLD position during a high pressure stall, the signal pressure
in cavity (24) flows back through signal passage (17), through the resolver network, and to the
control valve. Then, the signal oil returns to the tank.
The system pressure decreases. At approximately 19650 kPa (2850 psi), spring (2) moves pressure
compensator spool (14) downward. The system pressure in passage (22) acts against the force of
spring (1) in order to move flow compensator spool (15) upward.
The supply oil flows past flow compensator spool (15), past pressure compensator spool (14),
through passage (23), and to actuator piston (4). Actuator piston (4) decreases the angle of
swashplate (9) until the system pressure decreases. As system pressure decreases, flow
compensator spool (15) moves downward to the metering position. Swashplate (9) maintains a
slight angle in order to compensate for system leakage. Also, the swashplate angle provides the
lower required pressure. Pump output is maintained at about 2950 kPa (430 psi). This condition is
called low pressure standby.
Low Pressure Standby

10. Illustration 12
Pump and compensator valve (low pressure standby)
(1) Spring
(4) Actuator piston
(9) Swashplate
(11) Bias piston
(12) Spring
(14) Pressure compensator spool
(15) Flow compensator spool
(17) Signal passage
(22) Passage
(23) Passage
(24) Cavity
(25) Cross-drilled hole
(BB) Pressure oil with the first pressure reduction
(LL) Tank oil
A low pressure standby occurs while the engine operates with the control levers in the HOLD. The
demand on the pump is zero. Therefore, the signal pressure in signal passage (17) is zero.
Before the engine is started, bias spring (12) maintains swashplate (9) at the maximum angle. As
the pump turns,the closed centered implement valve builds up pressure in the system. The pressure
in passage (22) is transmitted to the bottom of both pressure compensator spool (14) (pressure
limiter) and flow compensator spool (15). As the pressure increases, the pressure pushes the flow
compensator spool against spring (1). When system pressure is higher than 2950 kPa (430 psi),
flow compensator spool (15) moves upward. Flow compensator spool (15) moves sufficiently in
order to open a passage for pressure oil to the back of actuator piston (4). The actuator piston
moves to the right. Bias spring (12) is compressed. The swashplate moves toward the minimum
angle. The actuator piston continues to move to the right until the actuator piston uncovers cross-
drilled passage (25) of the actuator piston rod. Oil drains to the case.
Pump output is not sufficient to compensate for normal system leakage. The additional leakage
through cross-drilled hole (25) decreases the oil pressure behind the actuator piston. The decrease
in the oil pressure limits the movement of the piston. Now, the piston moves slightly to the left
until only a part of cross-drilled hole (25) is open to the case. The pump produces sufficient flow in
order to compensate for system leakage and for leakage to the pump case through the cross-drilled
hole. Also, this pump flow is sufficient to maintain system pressure at 2950 kPa (420 psi).

11. The pump is at low pressure standby. This pressure is different from margin pressure due to system
leakage and due to cross-drilled hole (25) in the actuator piston rod. The flow compensator spool
does not meter the oil. Instead, the flow compensator spool remains open. The spool moves
upward against spring (1). Oil flows to the back side of the actuator piston in order to compensate
for leakage through the cross-drilled hole. The flow maintains the pressure that is required at the
back of the piston in order to overcome the bias spring.
System pressure must be 1230 kPa (180 psi) higher than the margin pressure in order to move the
spool upward against spring (1).
Oil pressure behind the actuator piston is lower than system pressure due to the pressure drop
across the flow compensator spool.
Note: Low pressure standby is not adjustable. Low pressure standby varies between different
machines. Also, low pressure standby varies in the same pump with an increase in system leakage
or in pump leakage. As leakage increases, the pump upstrokes slightly in order to compensate for
leakage. The actuator piston covers more of the cross-drilled hole. Low pressure standby drops
toward margin pressure. When the actuator piston completely covers the cross-drilled hole, low
pressure standby equals margin pressure.
Copyright 1991, 2003 Caterpillar Inc.
All Rights Reserved.