Eaton Electrical PVE12, PVE21, PVE19 User Manual

Vickers

®

Piston Pumps

Overhaul Manual

PVE Variable Pump
12–21 USgpm capacity at 1800 rpm

M-2854-SReleased 8/1/90

Table of Contents

Section Page

I. Introduction

A. Purpose of Manual 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. General Information (Related Publications/Model Code) 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

II. Description 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. General 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Application 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

III. Principles of Operation 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. Piston Pump 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Compensator (Flat Cut-Off Type - “C”) 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C. Compensator (Remote Control Type - “CG”) 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

D. Compensator (Load Sensing Type - “CV”) 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E. Compensator (Load Sensing with Pressure Limiting - “CVP”, “CVPC”, “CVPD”) 10. . . . . . . . . . . . . . . . . . . . . . . . . . . .

IV. Installation and Operating Instructions 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. Installation Drawings 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Mounting and Drive Connections 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C. Shaft Rotation 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

D. Piping and Tubing 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E. Hydraulic Fluid Recommendations 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F. Overload Protection 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

G. Start–up 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

V. Service and Maintenance 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. Service Tools 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Inspection 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C. Adding Fluid to the System 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

D. Adjustments 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E. Lubrication 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F. Replacement Parts 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

G. Troubleshooting 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VI.Overhaul 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. General 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Disassembly 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C. Inspection, Repair and Replacement 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

D. Assembly of Compensator 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E. Removal and Disassembly of Rotating Group 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F. Inspection, Repair and Replacement 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

G. Assembly of Housing Parts 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

H. Disassembly of Valve Block 28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I. Inspection, Repair and Replacement 28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

J. Assembly of Valve Block 28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

K. Shaft Bearing Preload Adjustment 29. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

L. Final Assembly of PVE Series Pump 29. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VII.Test Procedure 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. Test Conditions 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Preliminary Check 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C. Preliminary Set–Up 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

D. Performance Test 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E. Performance Test of Piston Pump with “C” Compensator Control 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F. Performance Test of PVE 12/19/21 Piston Pump with “CG” Compensator Control 31. . . . . . . . . . . . . . . . . . . . . . . . . .

G. Performance Test of PVE 12/19/21 Piston Pump with “CV”, “CVP”, “CVPC” and “CVPD” Compensator Control 32. . .

H. Load Sensing Control Test 32. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

Section I – Introduction

A. Purpose Of Manual

This manual describes operational characteristics and
overhaul information for the PVE12, 19(*) and the
PVE21(*)–** variable displacement piston pumps. The
information contained herein pertains to the latest design
series as listed in Table 1.

B. General Information

1. Related Publications - Service parts information
and installation dimensions are not contained in this
manual. The parts and installation drawings listed in
Table 1 are available from authorized distributors or sales
engineers.

34561

2

1

Pump, Variable Displacement,
Inline Piston, E-Series

2

Flow Rating
USgpm @ 1800 rpm

3

Shaft Rotation
(Viewed from shaft end)
R – Right hand (clockwise)
L – Left hand (counterclockwise)

4

Noise Level Rating
Blank – Standard Unit
Q – Industrial Quieter
1800 rpm @ 207 bar (3000 psi)

5

Mounting Flange
B – SAE B 2 bolt

6

Input Shaft Type
1 – SAE B Straight keyed
2 – SAE B Splined

7

Port Configuration
E – End ported, SAE O-ring
M – End ported, metric O-ring
(per ISO 6149)

Shaft Seal

8

S – Standard shaft seal
N – No shaft seal

Design

9

10

Control Options
(** = Pressure setting in tens of bars)

C** –

C**VP** –

C**VPC** –

C**VPD** –

Model
Series

PVE12
PVE19
PVE21

Parts

Drawing

M-2853-G
M-2841-S

2. Model Codes - Variations within each basic model
series are covered in the model code. Table 2 shows a
complete breakdown of the model codes covering these
units. Service inquiries should always include the complete
model code number as stamped on the mounting flange.

87 9 10 11

10

Control Options (Con’t)
(** = Pressure setting in tens of bars)

CC** -

Pressure compensator

CG**-

Max. setting 207 bar
(3000 psi)
Range 02-21 bar
Pressure & load sensing
Pressure compensating
(see C**)
Load sensing (see CV**)
Pressure and load sensing
Pressure compensating
(see C**)
Load setting 24 bar
(350 psi)

CD ­CCD -

11

Control Bleed Down
Blank – C, CC, CG, CD, CCD
B – Bleed down orifice (CVP & CVPC)
P – Plug (no orifice) (CVP & CVPC)

12

Control Design

Range 17-31 bar
(251-450 psi)
Pressure & load sensing
Pressure compensating
(see C**)
Load setting 41 bar
(600 psi)
Range 32-45 bar
(451-650 psi)
Same as C** except with
max. adj. stop.
Pressure compensation
with remote control
(see C**)
Electric dual range com­pensation
Same as CD except with
max. adj. stop

Installation

Drawing

_____

322C

Table 1.

12

Same as C** except with
max. adj. stop.
Pressure compensation with
remote control (see C**)
Electric dual range com­pensation
Same as CD except with
max. adj. stop

Table 2. Model Code Breakdown

3

361

2

4 5

7 8

1

Pump, Variable Displacement,
Inline Piston, E–Series

2

Flow Rating
USgpm @ 1800 rpm
19 – 19 USgpm
21 – 21 USgpm

3

Shaft Rotation
(Viewed from shaft end)
R – Right hand (clockwise)
L – Left hand (counterclockwise)

4

Input Shaft Type
1 – SAE B Straight keyed
2 – SAE B 15 tooth splined
9 – SAE B 13 tooth spline

5

Pump Design

6

Control Options
(** = Pressure setting in tens of bars)

C-10 –

Pressure compensated
(PVE19, 250-3000 psi)
(PVE21, 250-2700 psi)

CG-10 –

Remote control pressure
compensator adjustable
from 350-3000 psi using

and external relief valve.
CV-10 –
CVP-12 –

Load sensing PVE19/21

Load sensing (160 psid)

with pressure compensation

PVE 19/21
CVPC-12 –

Load sensing (350 psid)

with pressure compensation

PVE 19/21

Table 2. Model Code Breakdown (Con’t)

7

Control Design

Special Suffix

8

4

Shaft

Shaft
Seal

Housing

Yoke

Compensator

Drive
Shaft

Tapered
Roller
Bearing

Yoke

Bearing

PVE12 Section View

Wafer Plate

Piston

Valve Block

Compensator

Housing

Rotating Group

PVE19 Section View

Figure 1. Sectional Views of the PVE Pumps

5

Section II – Description

A. General

Assembly of a typical pump package is shown in Figure 1.
Six types of compensator subassemblies are used with the

PVE series pumps. Refer to Section III for principles of
operation. See Model Code for pressure settings.

1. Compensator (C), (Flat Cut-Off Type): A pump using
this compensator will maintain a constant load pressure for
all values of flow within the capacity of the pump.

2. Compensator (CG), (Remote Control Type): This
compensator is similar to the “C” type compensator except
the compensator is controlled by a remote hydraulic source
such as a relief valve.

3. Compensator (CV), (Load Sensing Type): A load
sensing compensator provides flow at a pressure equal to
that required by the load plus a constant value used as a
pressure drop across a metering valve. The pump will
change its flow with changes in size of the metering valve
orifice. The pump and compensator together provide a
constant flow source for the load, at a pressure established
by the requirements of the load, hence the title “Load
Sensing”.

CAUTION

4. Compensator (CVP), (Load Sensing Pressure
Limiting Type): The CVP control is a combination of the
standard flat cut-off compensator (C) and the load sensing
compensator (CV). The load sensing compensator controls
flow to the load across an external valve orifice. If pressure
build–up exceeds the flat cut-off compensator setting, the flat
cut-off compensator overrides the load sensing compensator
and lowers the flow to prevent excessive pressure build-up
at the pump.

5. Compensator (CVPC), (Load Sensing Pressure
Limiting Type): This compensator is the same as the “CVP”
compensator except the load sensing spring is heavier. The
heavier spring provides a slightly higher pressure differential
(160nP vs. 350nP) across the external valve orifice. See
Figure 5.

6. Compensator (CVPD), (Load Sensing, Pressure
Limiting Type): Same as “CVPC” except with higher
pressures.

B. Application

Pump ratings in USgpm as shown in the model coding are at
1800 rpm. For ratings at other speeds, methods of installa­tion and other application information, contact an authorized
distributor or sales engineer.

A relief valve must be provided in the external circuit
to prevent excessive pressure build up at the pump.

Outlet Wafer Plate
Kidney Slot

Outlet Port

Inlet Port

Intake Kidney
Slot Area

Wafer Plate

Figure 2.

Cylinder
Block Bore

Piston

Shoe Plate

Yoke Face

Drive Shaft

6

Section III – Principles of Operation

A. Piston Pump

Rotation of the pump drive shaft causes the cylinder block,
shoe plate and pistons to rotate. See Figure 2. The piston
shoes are held against the yoke face by the shoe plate. The
angle of the yoke face imparts a reciprocating motion to
each piston within the cylinder block. Inlet and outlet ports
connect to a kidney slotted wafer plate. As the pistons move
out of the cylinder block, a vacuum is created and fluid is
forced into the void by atmospheric pressure. The fluid
moves with the cylinder block past the intake kidney slot to
the outlet (pressure) kidney slot. The motion of the piston
reverses and fluid is pushed out the cylinder block into the
outlet port.

B. Compensator (Flat Cut-Off Type)

A flat cut-off compensated pump will maintain a constant
load pressure for all values of flow within the capacity of the
pump providing the load is sufficient to build up pressure.

A step by step description of the flat cut-off type compensa­tor control follows. Refer to Figure 3 throughout this
discussion.

When a no load condition exists, the pump will deliver
maximum flow at zero pressure. As the actuator load
increases, pressure will rise; however, flow will remain at
maximum until pressure reaches the compensator spring
setting (cracking pressure). As a further increase in load
occurs, system pressure will cause the compensator spool to
move against the compensator spring, metering flow to the
yoke stroking piston. The yoke stroking piston then moves the
yoke to reduce flow. As flow is reduced, system pressure
reduces slightly causing the compensator spool to return to the
null position. At null, flow to the yoke stroking piston stops.
Movement of the yoke will stop and the flow will stabilize at a
reduce value. If the load were to continue to increase, the
pump flow will reduce to zero (0) and a deadhead pressure
condition would exist. The pressure differential needed to
cause the compensator spool to change from maximum flow
(cracking pressure) to zero flow (deadhead pressure) is
approximately 50 to 150 PSI.

Pump outlet flow is proportional to the control range from
cracking pressure to deadhead pressure. (i.e. If cracking
pressure is 2900 PSI (max. flow) and deadhead pressure is
3000 PSI (min. flow), a pressure of 2950 PSI would be equal
to 1/2 maximum flow.)

If the load decreases, pressure will decrease proportionally
and the compensator spring will move the spool down,
opening the yoke stroking piston to case drain. As fluid is
metered from the yoke stroking piston, the yoke spring will
stroke the yoke to increase flow. The increase in flow causes
a proportional increase in system pressure. The increase in
system pressure returns the compensator spool to a null
position and flow from the yoke stroking position will stop;
simultaneously, movement of the yoke will stop. The flow will
stay constant until another change of load occurs.

If the load continues to decrease, pump flow will continue too
increase, holding the outlet at compensator cracking
pressure. When maximum flow is reached (max. stroke), a
maximum flow and a maximum pressure condition exists. A
further decrease in load will lower the outlet pressure until a
final theoretical condition of maximum flow and zero
pressure is obtained.

C. Compensator (Remote Control - “CG”)

This compensator allows the operator to change the
pressure setting through the use of a remote control valve.
The “CG” compensator has the same performance
characteristics as the “C” type compensator.

7

Compensator Spring

Compensator Spool

Outlet

Inlet

Cross Hole (Open to
Spring Area)

Pump Load

Yoke Stroking Piston

Drain

Yoke Spring

Rotating Group

Figure 3. Flat Cut-Off Compensator

D. Compensator

(Load Sensing Type - “CV”)
Application

A frequent application of pressure compensator pumps is to
supply sevo valves or mechanically operated metering
valves, whose function is to control flow to a hydraulic
actuator (cylinder or motor).

In such circuits it is often desirable that flow be proportional
only to an external valve spool position. This requires a
constant pressure drop across the external valve. (NOTE:
Flow through a valve varies with pressure drop as well as
with valve spool position.)

Pumps incorporating the load sensing feature have a
constant flow characteristic: Flow is constant regardless of
the load pressure.

CAUTION

A relief valve must be used to prevent outlet pres­sure from exceeding pump ratings if the load is
excessive.

Minimum Pump Pressure

The minimum outlet pressure developed by the pump (no
load) is a function of the compensator spring force versus
the yoke spring force, whichever is greater. (Please note that
external valve spool position (orifice size), has nothing to do
with the minimum outlet pressure of the pump). The orifice
size controls pump rate of flow only. The minimum outlet
pressure will be constant for all settings of the valve spool
orifice and is considered the pressure drop across the orifice
(nP).

8

Actuator
Load

Compensator Spring

Compensator Spool

Outlet

Relief
Valve

Inlet

External Valve
Spool Orifice
(nP-Pressure Drop)

Yoke Stroking Piston

Drain

Yoke Spring

Rotating Group

Figure 4. Load Sensing Compensator (CV).

Circuit Operation At Minimum Pressure

Refer to Figure 4 during the following description. Assume a
no load condition.

The pump load consists of the pressure drop across the
valve spool orifice (nP), plus the pressure developed by the
work being performed at the actuator. (In this case the
actuator is unloaded and only the pressure drop across the
valve spool orifice (nP) will be considered.)

Flow is restricted through the valve spool orifice and
develops a pressure at the outlet of the pump. This pressure
is applied to the lower end of the compensator spool. Initially,
the compensator spring is holding the spool in the down
position and the yoke is at maximum delivery position. When
the pump is started, the increasing flow increases pressure
at the lower end of the compensator spool and the
compensator spool opens pressure to the yoke stroking
piston. The yoke then strokes to a lower flow, lowering the
pressure drop across the external valve orifice. When
pressure reaches nP, the compensator spool will null. At this
time, the stroking piston will remain stable until the external
valve spool orifice is changed. (See Figure 4.)

External Valve Spool Orifice Size Reduced

If the external valve spool orifice is reduced in size, pressure
at the pump outlet will rise proportionally causing the
compensator spool to move against the compensator spring.
When the compensator spool moves far enough to open the
yoke stroking piston to pump outlet pressure, the yoke
stroking piston will move the yoke to a lower flow setting.

The compensator senses pressure at the downstream side
of the external valve spool orifice and compares this
pressure to the pump outlet pressure. The compensator then
adjusts the yoke to a flow which holds a constant pressure
drop (nP) across the external valve spool orifice.

The pressure developed at the pump outlet is a summation
of the pressure drop across the external valve spool orifice
and the actuator load pressure. As the actuator load
pressure increases, the increase is reflected directly back to
the pump outlet. Since the compensator monitors the
difference between pump outlet pressure and actuator load
pressure, and this difference (nP) does not change with load
variations, flow from the pump will stay constant.

9

External Valve Spool Orifice Size Increased

If the external valve spool orifice size is increased, pump
outlet pressure will decrease, lowering force against the
compensator spool. (See Figure 4.) The compensator spring
causes the spool to move, opening the yoke stroking piston
to case drain. As fluid is metered from the yoke stroking
piston, the yoke spring force strokes the yoke to a higher
flow. The increase in flow through the external valve spool
orifice establishes once again the constant pressure drop
(nP). With differential pressure (nP) across the external
valve orifice, the compensator spool nulls. Flow from the
yoke stroking piston stops, and the pump flow rate stabilizes
at a higher value.

Operation of the load sensing compensator is such that as
the load pressure varies, the pump outlet pressure will follow
the variations, holding a constant pressure drop (nP) across
the external valve spool orifice, and a constant flow through
the external valve and actuator. Pump flow will change only
with changes in external valve spool orifice size.

E. Compensator (Load Sensing with Pressure

Limiting “CVP”, “CVPC”, “CVPD”)

As expected from the above title, these units are a combina­tion of the flat cut-off and load sensing compensators.

The load sensing portion functions at pressures below the
flat cut-off compensator setting and provides a constant flow
characteristic. If pressure exceeds the flat cut-off compensa­tor setting, the yoke will stroke to zero flow at maximum
pressure lowering the horsepower requirements for holding
circuits and protecting the pump. Refer to Figure 5
throughout the following circuit explanation.

Assume an actuator load that is increasing gradually. Also,
assume the pump outlet pressure is lower than the flat
cut-off compensator cracking pressure. As actuator load
pressure increases, the load sensing compensator spool
senses the difference between pump outlet pressure and
actuator load pressure. As long as the difference between
the pump outlet pressure and the actuator load pressure
(nP) is constant, flow to the load will stay constant. As
pressure rises across the load, leakage will increase in the
pump and load. The load sensing portion of the compensator
adjusts pump outlet flow to compensate for leakage while
providing a constant flow through the valve spool orifice.

The pump outlet pressure continues to increase until the flat
cut-off compensator spool reaches cracking pressure. The
flat cut-off compensator spool then meters flow to the yoke
stroking piston. The yoke stroking piston starts moving the
yoke to reduce flow while holding the outlet pressure at
compensator cracking pressure. This action continues until
the pump is fully compensated (zero flow and maximum
pressure).

Standby Operation Feature

Standby defined: When the external valve spool is shifted to
zero flow, the circuit is placed in standby.

The small fixed orifice located in the compensator body
provides a decompression feature for the load circuit during
standby operation. The decompression feature allows the
pump to stroke to zero flow and minimum pressure (nP), if
the load is blocked and the external valve spool orifice is
closed. (Refer to Figure 5.)

The circuit functions as follows:
Assume the pump is at zero flow with maximum pressure to

the load. The flat cut-off compensator spool will be in the up
position (compressing the spring) and the load sensing spool
will be in the down position due to actuator load pressure
plus the spring force. If the external valve spool orifice is
closed at this time, fluid under pressure will be trapped in the
load circuit and will hold the load sensing spool in the down
position. This will keep the pump outlet pressure at flat
cut–off compensator cracking pressure (a power loss since
no work is being performed at this time). To prevent this
condition from continuing, the small orifice meters the fluid
trapped in the load back through the flat cut-off compensator
spool to case drain. The actuator load pressure will decrease
gradually causing the load sensing spool to open pressure to
the yoke stroking piston, bypassing the flat cut-off compen­sator. As the actuator load pressure reduces, the pump
outlet pressure will reduce until minimum pump pressure is
obtained. When the minimum flow/minimum pressure
condition occurs, the pump is considered to be in standby.
During standby, the CVP(C) control reduces the input power
well below that of a standard “C” type compensator. This
provides an increase in system efficiency and reduces the
cost of operation.

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