JCB 3CX, 4CX, 214e, 214, 215, 217 & VARIANTS Backhoe Loader. Service Manual

Section E

Hydraulics

Section E

7 - 1

Circuit Descriptions

7 - 1

Loader Valve - Variable Flow

Operation

Component Key:

The loader valve is mounted on the chassis frame, right

Auxiliary (optional) spool

hand side (when viewed from rear).

Shovel spool

Arms lift spool

It is a stack type, directional control valve that is configured

Pump inlet

for closed centre operation. The valve block comprises two

Tank port

or three service valve sections and a priority steer inlet

Load sense port (from valve blocks)

section.

Priority load sense port (from steer unit)

Priority work port (to steer unit)

When operated, the designated valve section controls the

Load sense carry over port (to backhoe valve)

volume and direction of oil by way of service ports A or B. A

Service ports

constant flow is maintained regardless of changing load

Auxiliary relief valve (rods side)

pressures.

Auxiliary relief valve (head side)

Priority relief valve †

While the spools are in neutral position, the primary shuttles

and secondary shuttles are vented to tank. When a spool is

† Note: Steer circuit pressure is controlled by a relief valve

operated, the load pressure sensed at the work port is

housed in the hydraulic steer unit (refer to Section H

directed via the primary and secondary shuttles to the

Steering). The priority relief valve housed in the loader valve

hydraulic pump, where the pump subsequently ‘swashes’ to

must be set at 2500 lb/in², this will ensure it does not

meet this load sense demand.

interfere with the operation of the relief valve housed in the

hydraulic steer unit.

3 21

A273670

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Issue 1

Section E

Hydraulics

Section E

7 - 2

Circuit Descriptions

7 - 2

Loader Valve - Variable Flow

Priority Inlet Section Operation

Component Key:

The priority inlet section houses the steering priority valve

Pump Inlet

and a relief valve assembly. Operation is as follows:

Tank

Load sense port (to pump)

Steering

PRLS

Priority Load Sense Port (from steer unit)

When the steering wheel is turned, a priority pressure

Priority Work Port (to steer unit)

demand signal is sent from the hydraulic steer unit and

received at the priority inlet section (port PRLS). The signal

oil flows through port A into chamber C via gallery B.

The combined force of spring D and signal oil pressure

moves spool E up. The position of the spool now allows oil

from the pump inlet port P to flow out to the hydraulic steer

unit via priority work port PR.

It must be noted that relief valve assembly J is redundant.

This relief valve is part of the inlet section, however the

hydraulic steer unit also houses a steer system relief valve.

To ensure the relief valve in the hydraulic steer unit controls

the steer system pressure, valve J is set abnormally high

(172 bar; 2500 lb/in²).

PRLS

EXHAUST

PUMP

LOAD SENSE

A273940

A273930

9803/3280

Issue 1

Section E

Hydraulics

Section E

7 - 3

Circuit Descriptions

7 - 3

Loader Valve - Variable Flow

Priority Inlet Section Operation

Component Key:

Service Selected (no steering demand)

Pump Inlet

If the steering is not selected, there will be no pressure

Tank

demand signal in chamber C. Pump pressure is ‘dead

Load sense port (to pump)

ended’ at the steer unit, this pressure is felt in chamber F via

PRLS

Priority Load Sense Port (from steer unit)

priority port PR and drilling G. The pressure is sufficient to

Priority Work Port (to steer unit)

force spool E down against the force of spring D. The

position of the spool now allows oil from the pump inlet port

P to flow into passage H and on to the selected service via

the valve block service ports.

PRLS

EXHAUST

PUMP

LOAD SENSE

A273970

A273980

9803/3280

Issue 1

Section E

Hydraulics

Section E

7 - 4

Circuit Descriptions

7 - 4

Loader Valve - Variable Flow

Auxiliary Spool

The auxiliary valve section comprises the spool assembly A,

load hold check valve assembly B, primary and secondary

shuttles, items L and K respectively.

While the spools are in neutral position, the primary shuttles

and secondary shuttles are vented to tank. When a spool is

operated, the load pressure sensed at the work port is

directed via the primary and secondary shuttles to the

hydraulic pump, where the pump subsequently ‘swashes’

and increases its output to meet this load sense demand.

When a service is selected (the spool moved up or down),

the waisted sections of the spool connect passage C to one

of the service ports D. Oil from the pump must overcome the

force of spring E to lift poppet F and make the connection

from the pump passage J to the service ports D via passage

In some instances there is a back pressure in the service line

(e.g. generated by the weight of a loaded shovel). This

pressure is felt in chamber G via drilling H in poppet F. The

back pressure combined with the force of spring E keeps

the load hold check valve firmly closed, preventing the load

from dropping.

The pressure in pump passage J will increase until it is

greater than that in the service line. At this point, the load

hold check valve will open, as previously described.

A276100

PUMP

OAD

ENSE

A276290

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Issue 1

Section E

Hydraulics

Section E

7 - 5

Circuit Descriptions

7 - 5

Loader Valve - Variable Flow

Loader Shovel Spool

The loader shovel spool operates in the same manner as the

auxiliary spool, refer to Auxiliary Spool.

The shovel valve section also houses auxiliary relief valves

(items 30 and 30A).

Under normal operating conditions, with the shovel in the

'carry' position, the mechanical linkage keeps the shovel

level as the arms are raised, to prevent spillage of the load.

If, as illustrated, the shovel is fully tipped when the arms are

being raised, the shovel is unable to tip further, producing

back pressure in the head side of the shovel rams and

cavitation in the rod side.

As the arms continue to rise, the back pressure increases

until it reaches the setting of A.R.V. 30A. This A.R.V. then

opens, allowing the excess back pressure to be dumped to

exhaust and prevent the rams and linkage from being

damaged.

The rod side A.R.V. 30 senses a higher pressure in the

exhaust gallery than in the service port and therefore opens

30A

to allow exhaust oil to overcome the cavitation.

The shovel service may also be fitted with a ‘return to dig’

detent solenoid 12.

A278920

PRLS

30A

A279300

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Issue 1

Section E

Hydraulics

Section E

7 - 6

Circuit Descriptions

7 - 6

Loader Valve - Variable Flow

Loader Lift Spool

The loader lift spool operates in the same manner as the

auxiliary spool, refer to Auxiliary Spool.

The lift service spool (item 18) has a ‘float’ detent position.

The float facility is provided to allow the arms to move up

and down so that the shovel can follow the surface contours

as the machine is driven over uneven ground.

This is achieved by moving the lift spool 18 down beyond

'arms lower' into the 'float' detent. Both service ports are

connected to exhaust via the spool waists A.

Oil can then be displaced from either end of the lift rams and

into the exhaust gallery, allowing the rams to open and close

as required.

A278910

A279430

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Issue 1

www.WorkshopManuals.co.uk_{Section E}

Section E

Hydraulics

7 - 7

Circuit Descriptions

7 - 7

Loader Valve - Variable Flow

Shuttle Valve Signal Network

Each control valve section has two shuttle valves. Each valve

compares two pressure signals. One of the shuttles is the

primary 18 and the other is the secondary 17.

The primary shuttle 18 compares the pressure signal

between the two service ports A and B in each valve section,

the head and the rod pressure of the cylinder. In the example

shown the boom service 37 port B pressure signal is greater

than port A. Primary shuttle 18 moves across, the pressure

signal is the highest shuttle pressure. This signal pressure

passes to the next shuttle.

The secondary shuttle operates the same as the primary

shuttle but compares pressure signals between valve

sections.

The signal network is arranged in series. It starts at loader

LS T

valve block 9 inlet section which is connected to the steer

valve 45. The last secondary shuttle in the loader valve block

is connected to the first secondary shuttle in the backhoe

valve block 25. The highest shuttle signal pressure from the

loader valve block goes to the backhoe valve block.

The highest shuttle signal pressure is felt at the hydraulic

pump which instructs the pump to vary the output to meet

the highest shuttle load requirement. The stand-by pressure

spool in the pump adds margin pressure to the load

requirement. The single highest shuttle signal pressure plus

margin pressure governs pump output.

The pump will not change output until the shuttle network

identifies a different shuttle signal.

STEER VALVE

PUMP

9 LOADER VALVE

B A

A316510

25 BACKHOE VALVE

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Issue 1

Section E

Hydraulics

Section E

8 - 1

Circuit Descriptions

8 - 1

Excavator Valve

Manual Control - Neutral Circuit

Component Key: (JCB 'X' Control Pattern)

Oil from the loader valve enters the excavator valve at A

and flows through the neutral gallery and around the waists

A Inlet

of the solid spools. It also fills the parallel gallery B but is not

B Parallel Gallery

at a high enough pressure to open the load hold check

C Load Hold Check Valves

valves C.

4A Slew Spool

4B Boom Spool

4C Stabiliser Spool

4D Stabiliser Spool

4E Dipper Spool

4F Bucket Spool

Note: Machines with ISO control pattern have the boom and

dipper spools interchanged.

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Issue 1

Section E

Hydraulics

Section E

8 - 4

Circuit Descriptions

8 - 4

Excavator Valve - Load Hold Check Valves

Operation 1

Component Key:

B Parallel Gallery

Flow to the service via a typical solid spool 4B is controlled

D Service Line

by the load hold check valve 4S which is a spring-loaded

4B Spool

non-return valve operating across the pressure feed from the

4S Load Hold Check Valve

parallel gallery B. The valve prevents reverse flow from the

rams into the pressure feed line, so maintaining ram

pressure until exceeded by system pressure. The illustration

shows a service selected but back pressure D exceeds

system pressure which closes the load hold check valve 4S.

Operation 2

When pressure in the feed line exceeds back pressure, the

load check valve 4S opens and oil operates the ram. The

remaining load hold check valves are also opened by system

pressure but the galleries are dead-ended because the

spools are in neutral.

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Issue 1

Section E

Hydraulics

Section E

8 - 5

Circuit Descriptions

8 - 5

Excavator Valve

- One Way Restrictor Operation

Componet Key:

Because of its weight, the excavator end could take over

C Lower Service Port

control when boom lower was selected. The falling boom

D Upper Service Port

would tend to push oil out of the ram faster than the head

E One Way Restrictor

side was being filled. Consequently, when the excavator had

X Parallel Gallery

reached the ground, there would be a time lapse while the

4A Slew Spool

ram filled with oil before the service would operate again.

4B Boom Spool

4H ARV

To prevent this from happening, the boom service is fitted

4J ARV

with a one way restrictor. When the boom is being raised,

the poppet in restrictor E is held off its seat by the

pressurised oil flow, therefore oil delivery to the boom ram

Slew Ram End Damping

rod side is unrestricted.

When the boom is being lowered, the oil flow through the

As ram A nears the closed position, damping rod B seats in

one way restrictor E re-seats the poppet, therefore the flow

cone C, where it is held by spring D. Tapered flutes on the

end of the rod produce a restricting orifice, thus restricting

of oil is restricted through the small drilling in the base of the

the speed of the oil being exhausted from the ram. This

poppet. This slows the boom down to a controllable speed.

provides a cushioning effect between the piston and the

dump end of the ram, effectively damping out the shock

loads which would otherwise occur when the boom reaches

the end of its slewing arc.

Excavator Valve - Slew Operation

The illustration shows R.H. slew selected. The lower port C

has been pressurised by the spool. Oil flows from the

parallel gallery B, out past A.R.V. 4H to both the head side

of the L.H. slew ram and the rod side of the R.H. slew ram.

The boom therefore slews to the right hand side of the

machine. Displaced oil from the rod side of the L.H. slew

ram and from the head side of the R.H. slew ram flows back

through the upper service port D and back to tank.

A230420

A Ram

B Damping Rod

C Cone Seat

D Spring

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Issue 1

Section E

Hydraulics

Section E

8 - 8

Circuit Descriptions

8 - 8

Excavator Valve - Slew A.R.V. and Anti-

Cavitation Operation

The spool 4A is in neutral but the momentum of the slewing

Componet Key:

excavator end creates back pressure in the head side of the

R.H. slew cylinder and in the rod side of the L.H. slew

4A Slew Spool

cylinder. This opens A.R.V. 4J and dumps oil to exhaust. At

4E Dipper Spool

this point the cylinders cavitate and exhaust oil pressure

4F Bucket Spool

causes A.R.V. 4H to open, allowing oil from the exhaust

4H ARV

gallery to fill the L.H. cylinder.

4J ARV

4M ARV

4N ARV

4P ARV

4Q ARV (option)

Excavator Valve - A.R.V. Operation

The illustration shows the bucket spool 4F selected to

operate the service against an immovable object. This forces

the dipper away from the obstruction and pressurises the

head side of the dipper ram.

When this pressure reaches the setting of A.R.V. 4M, this

valve opens, relieving the pressure into the exhaust gallery.

Cavitation occurs in the rod side of the dipper service until

A.R.V 4N opens, allowing the higher pressure in the exhaust

gallery to supplement that in the service line.

Note: A.R.V. 4Q is only fitted to machines with a

Rockbreaker.

For a further detailed description, refer also to Pilot -

Operated Pressure Relief Valve Operation.

9803/3280

Issue 1

Section E

Hydraulics

Section E

8 - 9

Circuit Descriptions

8 - 9

Pilot - Operated Pressure Relief Valve

Operation

Component Key:

Valve at Rest

A Service Pressure

A.R.V's are positioned in the excavator valve block in order

B Poppet

to relieve excessive pressure in the services as described in

C Piston

Excavator Valve - ARV Operation.

D Spring

E Poppet

When the service is in neutral and there are no excessive

F Spring

forces acting on the equipment, service pressure at A will be

G Locknut

acting on the lower face of poppet B and will also be felt

H Cavity

inside the valve via hollow piston C.

J Sleeve

K Exhaust Cavity

The force of springs D and F, combined with the service

L Cavity

pressure acting on the upper faces of poppet B and piston

C, keeps poppets B and E tightly seated.

The force of spring F is adjustable to suit the relevant service

by means of adjuster screw and locknut G.

Pilot Valve Opens

As service pressure reaches the pilot setting of the valve,

pilot poppet E lifts, allowing oil to escape into cavity H and

pass down the sides of sleeve J into the exhaust gallery K.

Main Poppet Opens

As service pressure continues to rise and oil escapes from

cavity L, the pressure differential between the upper and

lower surfaces of piston C causes this piston to rise and

seat on the point of pilot poppet E.

Oil continues to escape from cavity L but the incoming flow

to the cavity has been cut off. This produces a pressure

drop above poppet B, causing the poppet to lift and release

service pressure into exhaust gallery K.

9803/3280

Issue 1

Section E

Hydraulics

Section E

8 - 11

Circuit Descriptions

8 - 11

*_{Excavator Valve - Manual Control}

Hydraclamp Valve Operation

(Sideshift Machines)

When the backhoe is being used for excavating duties, the

kingpost assembly must be 'clamped' to the sideshift

2 - Clamps Released - Not Precision Control Machines

rearframe.

When the hydraclamp switch A is in the ON position

(pressed down), the solenoid valve E is energised. The

The hydraclamp control valve assembly (items G, D and C) is

solenoid valve in this energised state now allows a

positioned in the inlet end of the excavator valve block and is

connection to be made from the parallel gallery B to the

connected directly to the parallel gallery B. The valve

drain port (tank) via gallery F.

operates in conjunction with solenoid valve E housed in the

side of the excavator valve block.

Oil from the hydra-clamps is vented through the clamp valve

and gallery F to the drain port (tank).

1 - Clamps Pressurised

When the hydraclamp switch A is in the OFF position (not

Also, pressure resulting from the operation of an excavator

pressed), the solenoid valve E is de-energised. The solenoid

service passes by poppet D as before but the oil takes the

valve in this de-energised state blocks the exhaust path from

path of least resistance and is vented via gallery F to the

the parallel gallery B to the drain port (tank).

drain port (tank).

When an excavator service is operated, pressure is

Restrictor H ensures that not all oil in gallery B is dumped

generated in parallel gallery B, this pressurised oil enters the

back to tank.

clamp valve C and lifts poppet D off its seat against the force

of spring G. The oil flows past the poppet and out to the

3 - Clamps Locked Up

hydra-clamps 62.

If no service is being operated, pressure in the parallel gallery

falls to that of the neutral circuit and the force of spring G is

sufficient to keep the poppet seated.

Pressure is therefore trapped in the line to the clamps,

maintaining the excavator end in a securely clamped

condition.

DRAIN

CLAMP

PORT

S270680

A270690

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Issue 2*

Section E

Hydraulics

Section E

8 - 13

Circuit Descriptions

8 - 13

Excavator Valve

Precision Control (Servo)

(Machines up to serial no. 931159)

Component Key:

A Inlet Port

B Outlet Port

C Centering Spring

D Integral Load Sensing Valves (Isolator Compensator

Spools)

E Bypass Compensator Valve

F Load Sense Drain Regulator and Relief Valve

G Hydra-clamp Solenoid Valve

H Pilot Operated Check Valve

J Load Sense Drain Port

4A Slew Spool

4B Boom Spool

4C Stabiliser Spool

4D Stabiliser Spool

4E Dipper Spool

4F Bucket Spool

4G Aux. Spool (not used on some machine variants)

A402510

9803/3280

Issue 2*

Section E

Hydraulics

Section E

8 - 14

Circuit Descriptions

8 - 14

Excavator Valve

Precision Control (Servo) (continued)

(Machines up to serial no. 931159)

In the interest of machine efficiency and consistent backhoe

When a Service is Selected

operation the excavator valve is of a closed centre design

and incorporates load sensing valves D, for each service.

When one or more services are selected the load sense

These features enable the valve to operate as follows:

valves detect the service generating the highest pressure.

This causes the bypass compensator valve E to close and

No service is selected (all spools in neutral)

allow pump flow to the spools and service ports at sufficient

pressure to move the load. The load sense valves D also

Oil from the loader valve high pressure carry over enters the

incorporate check valves. These valves prevent the load

excavator valve at the inlet port A. Oil flows across the

falling back if the load pressure is greater than the pump

bypass compensator spool assembly E and returns to tank

pressure. The closed centre design ensures consistent

from the outlet port B.

service operation regardless of load.

Servo Spool Actuation

For a full description of spool and load sense valve operation

see Load Sense (Isolator Compensator) Valve Operation

When a service is operated servo pressure from the joystick

on the next page.

control valve acts on the end of the relevant service spool

Eg. 4A and displaces the spool proportionally against the

force of the centering spring C, allowing the oil to flow to

move the required service. When the servo pressure on the

end of the service spool is reduced the force of the centering

spring overcomes the servo pressure and the service spool

will move back proportionaly to the central neutral position.

Note that spools 4G, 4D and 4C are manually actuated.

A402500

9803/3280

Issue 2*

Section E

Hydraulics

Section E

8 - 16

Circuit Descriptions

8 - 16

Excavator Valve

Precision Control (Servo) (continued)

(Machines up to serial no. 931159)

Load Sense (Isolator Compensator) Valve

Operation

No service is selected (all spools in neutral)

With the spools in the neutral position oil from the pump P

Any load on the service ram R is held by the main spool S.

flows to each spool and is dead ended. This generates

Oil is also vented to tank from the load sense gallery 2 via

sufficient pressure (about 20bar) to overcome the spring E1

the load sense drain regulator/relief valve F (see below).

and oil flows across the bypass compensator spool

assembly E to tank. Note that oil from the pump also flows

into the ‘auxiliary pump gallery’ 1. Oil from the ‘bridge

galleries’ 3 is vented to tank via a groove in the main spool S.

Spool at point of operating a service

Pressure from the load on ram R enters the bridge gallery 3

bleeds to tank, again causing pressure imbalance and the

via the service port 4. Compensator check valve H is held

isolator spool to move, allowing the pressure in the gallery to

down on its seat preventing the load moving. At the same

be ‘topped up’. This ensures that ‘pressure balance’ is

time the isolator spool U lifts and allows oil from the auxiliary

always maintained.

pump gallery 1 to flow into the load sense gallery 2. Oil

It must be noted that the load sense gallery 2 is connected to

pressure builds in gallery 1 and the bypass compensator

all load sense valves (one for each service). The pressure in

spool E begins to close under the action of the oil and spring

the gallery 2 will always be equal to the highest load from

(E1) pressure. This causes the pump pressure to rise to a

any of the backhoe services. The maximum load sense

new value as follows:

pressure is regulated by the load sense drain regulator/relief

Pump pressure = Pressure in load sense gallery 2 + pressure

valve F (see below).

from spring E1.

When the pressure in load sense gallery 2 reaches the load

pressure, isolator spool U moves down and closes off the

auxiliary pump gallery 1 from load sense gallery 2. If the load

pressure subsequently rises or falls, the isolator spool moves

due to pressure imbalance and maintains the pressure in the

load sense gallery 2 at the same pressure as the load

pressure. The pressure in the load sense gallery 2 always

Spool fully operating a service

As the spool S is moved further oil from the pump is diverted

into the central gallery 5. If the pump pressure has risen

sufficiently (to a pressure equal to the load + spring pressure

E1) then check valve H moves up off its seat and allows oil

to flow into bridge gallery 3, and out to the ram R via service

port 4. Exhaust oil from the other side of ram R is diverted to

tank.

9803/3280

Issue 2*

Section E

Hydraulics

Section E

8 - 17

Circuit Descriptions

8 - 17

Excavator Valve

Precision Control (Servo)

2 - Clamps Released - Precision Control Machines Only

(Machines up to serial no. 931159)

To enable clamps release, the hydraclamp switch A must be

in the ON position and an excavator service must be

Hydraclamp Valve Operation

operated.

When the backhoe is being used for excavating duties, the

With the hydraclamp switch A in the ON position (pressed

kingpost assembly must be 'clamped' to the sideshift

down), the solenoid valve E is energised. Pressure resulting

rearframe.

from the operation of an excavator service moves piston C

which pushes ball D off its seat.

The hydraclamp control valve assembly (items G, D and C) is

positioned in the front face of the excavator valve block and

With ball D lifted and the solenoid valve in its energised

is connected directly to the parallel gallery B. The valve

state, oil from the hydra-clamps is vented through the clamp

operates in conjunction with solenoid valve E located in the

valve and gallery F to the drain port T (tank).

bottom of the excavator valve block.

Also, pressure resulting from the operation of an excavator

1 - Clamps Pressurised

service passes through restrictor H. The oil takes the path of

When the hydraclamp switch A is in the OFF position (not

least resistance and is vented via gallery F to the drain port T

pressed), the solenoid valve E is de-energised. The solenoid

(tank).

valve in this de-energised state blocks the exhaust path from

the galleries B and F to the drain port T (tank).

Restrictor H ensures that only a small amount of oil from

gallery B is dumped back to tank.

When an excavator service is operated, pressure is

generated in parallel gallery B, this pressurised oil moves

3 - Clamps Locked Up

piston C which pushes ball D off its seat against the force of

If no service is being operated, pressure in the parallel gallery

spring G. The oil flows through restrictor H, past the ball and

falls to that of the neutral circuit and the force of spring G is

out to the hydra-clamps 62.

sufficient to keep the ball D seated. Pressure is therefore

trapped in the line to the clamps, maintaining the excavator

end in a securely clamped condition.

HC B

A402930

9803/3280

Issue 2*

Section E

Hydraulics

Section E

8 - 18

Circuit Descriptions

8 - 18

Excavator Valve

Precision Control (Servo) (continued)

(Machines from January 2003)

In the interest of machine efficiency and consistent backhoe

operation the excavator valve is of a closed centre design

and incorporates pressure compensating valves F, for each

service.

These features enable the valve to operate as follows:

No service selected (all spools in neutral)

Oil from the loader valve high pressure carry over enters the

excavator valve at the inlet port A. Oil flows across the

flushing valve spool assembly B and returns to tank from the

outlet port C.

Servo Spool Actuation

When a service is operated servo pressure from the joystick

control valve acts on the end of the relevant service spool D

and displaces the spool proportionally against the force of

the centering spring E, allowing the oil to flow to move the

required service. When the servo pressure on the end of the

service spool is reduced the force of the centering spring

overcomes the servo pressure and the service spool will

move back proportionaly to the central neutral position.

When a Service is Selected

When one or more services are selected the pressure

compensator valves F detect the service generating the

highest pressure. This causes the flushing valve spool B to

close and allow pump flow to the spools and service ports at

sufficient pressure to move the load. Load hold check valves

G prevent the load falling back if the load pressure is greater

than the pump pressure. The closed centre design ensures

consistent service operation regardless of load.

For a full description of spool and pressure compensator

valve operation see Load Sense - Pressure Compensator

Valve Operation.

A405000

A403680

9803/3280

Issue 1

Section E

Hydraulics

Section E

8 - 19

Circuit Descriptions

8 - 19

Excavator Valve

Precision Control (Servo) (continued)

(Machines from January 2003)

Load Sense - Pressure Compensator Valves

Operation

No service selected (all spools in neutral)

With the spools in the neutral position oil from the pump P

flows to each service spool S and is dead ended. This

generates sufficient pressure (about 15bar) to overcome the

spring E1 and oil flows across the flushing valve spool

assembly E to tank T. Any load acting on the service ram R

is held by the service spool S.

Oil in the load sense gallery LS is vented to tank across the

flow regulator valve F.

The pressure compensator valve PC and load hold check

valves CV are closed.

Operating a service

The service spool S is moved by servo pressure connecting

the oil from the pump P to the gallery P1. The pressure in

this chamber opens the pressure compensator valve PC

allowing oil into the load sense gallery LS. Oil pressure builds

in the gallery LS and the flushing valve spool E begins to

close under the combined action of the oil pressure and

spring force, causing the pump pressure to rise as follows:

Pump pressure = Pressure in load sense gallery LS +

pressure from spring E1.

When the pump pressure has risen sufficiently i.e. to a

pressure equal to the load + spring pressure (15 bar) the load

hold check valve CV moves off its seat and allows oil to flow

into the service gallery A, initiating movement of the service

ram R. Exhaust oil from the other side of the service ram is

directed by the service spool S to tank.

The maximum load sense pressure and hence pump

pressure is set by the load sense relief valve G.

9803/3280

Issue 1

JCB 3CX, 4CX, 214e, 214, 215, 217 & VARIANTS Backhoe Loader. Service Manual - page 14