Service Manual Travel hydraulic

Function

The chapter is divided into two function blocks:

A. Function Travel hydraulic (replenishing circuit, closed circuit, brake) Functional schematic, see page 8/10
B. Function Cooling circuit and hydraulic fan control Functional schematic, see page 9/10

A. Travel hydraulic- Function

1. Neutral function

1.1. Tandem gear pump 15 - Replenishing

When the Diesel engine is running, the gear pumps 15 supply oil from the hydraulic tank 80 via the shut off
valve 81 to valve block 16/P1+P2.
Oil not required in the pressure section of the system flows via the pressure relief valves 17/ valve block 16-T
return filter 82 into the hydraulic tank 80.
From valve block 16 -F F-filter block 18
-P2 check valve 52 accumulator 55 (blocked)
solenoid valve Y4 (safety control WH) blocked.
-SP console 61 solenoid valve Y13 (parking brake) blocked.
The temperature sensor B24 on the distributor section 19 senses the oil temperature and reports it to the
electronic fan control.
The oil pressure switch B6 and the test connection SP to monitor and control the replenishing pressure are
installed on the distributor section 19.
After lowering the safety lever, the solenoid valve Y4 switches to outlet B pilot control - working hydraulic.
Distribution of the oil flow after distributor section 19 F-var. displ. pump 2 right.
F-var. displ. pump 2 left.
E-var. displ. motors 20 right.
E-var. displ. motors 20 left.
Oil flow in var. displ. pump 2 right / left from F to
replenishing function check valve 10 replenishing - closed circuit
replenishing function check valve 11 replenishing - closed circuit
orifice 12 pump flushing
pilot piston 8 control cylinder 9
proportional solenoid valves Y9/Y10 or Y11/Y12 blocked.
The passages of the servo control piston 8 to and from the control cylinders 9 are significantly restricted. All
control cylinders are actuated evenly with pressurized oil and keep the swash plates of pumps 4 in hydraulic
zero position = 0° swivel angle.
The oil flowing off via the servo control piston 8 and orifice 12 as well as the existing leak oil lubricate, cool and
flush the drive gears of the var. displ. pumps 2 and flows via housing connections T into the cooling circuit, see
section „B. Cooling circuit and hydraulic fan control“.
LWT / Ausgabe / Edition / Edition / 01.2009

1.2. Variable displacement pumps 2 and variable displacement motors 20 – closed circuits
There is no oil circulation within the closed circuits. Due to the zero position of the swash plates in the variable
displacement pumps 2, there is no piston stroke despite the rotation of the pump housing and the pistons, and
therefore no oil flow. The closed circuits, however, are filled with 20+5 bar pressurized oil via the check valves
10 and 11 (see section 1).
The 3/3 directional valves 25 in the variable displacement motors 20 are in center position, all connections are
blocked, there is no discharge as long as there is no oil flow coming from the variable displacement pumps 2.
Via the replenishing on connection E, pressurized oil is directed through the parallel passage on the pilot piston
23 to the regulator side control piston 22 and the axial piston unit 21 is held in neutral position = maximum
swivel angle.

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Travel hydraulic Service Manual
Function

2. Operating functions – Travel operation

The regulation of the travel operation is made by the electronic control via the proportional valves Y7 to Y12,
which are installed on the var. displ. pump covers and the var. displ. motors. The current feed of the solenoid
valves and therefore the regulating pressure build up is corresponding to the lever deflection on the travel
joystick. For pump design / flow outlet see section 8.1.
The determination of the direction in travel operation is made on the variable pumps 2 for:
Electronic control: Regulating pressure build up: Flow: Pump Motor:
Travel forward solenoid valve Y9 Z P B
solenoid valve Y11 Y S A
Travel reverse solenoid valve Y10 Y S A
solenoid valve Y12 Z P B
When driving in turns, the current of the pump flow is changed according to the selected radius, until counter
rotation.
The proportional solenoid valves of the var. displ. motors are energized separately by the electronic control.
During the complete travel operation, solenoid valve Y13 is also energized. The valve shifts to position
“Diagonal passage”, the pressurized oil available on P flows via outlet B
travel gear 45 disk brakes 46 / Brake vented.
Depending on the travel resistance (= rotational resistance of the variable motors) a hydraulic pressure is
created in the pump flow = high pressure side of closed circuits. In the return flow from the variable motors, a
fill pressure of the replenishing (SP-pressure) is available = low pressure side of closed circuits.
High pressure side - Travel forward = System part - test point M1 (ML1/MR1)
High pressure side - Travel reverse = System part - test point M2 (ML2/MR2)

2.1. Regulation - var. displ. pumps 2 starting from neutral function as described in section A
Due to the current feed of a proportional solenoid valve Y9/Y11or Y10/Y12, the valve is opened, the
pressurized oil from the system part “Replenishing” flows to a side of the actuator cylinder 7 and builds up a
hydraulic regulating pressure in proportion to the magnetic force.
The piston of the actuator cylinder 7 is moved by the regulating pressure against the regulating spring
assembly until a balance of forces is created.
The servo control piston 8 is moved into the opposite direction via a mechanical connection. It opens the oil
intake port to the side of the swash plate control – control cylinder 9, where there was also regulating pressure
build up and allows the oil to flow off into the pump from the opposite side of the swash plate control – control
cylinder 9. The swash plate pump 4 is moved in flow direction against Q max.
Low current small flow volume
High current large flow volume
As the swash plate is moved, the servo control piston 8 is moved back in the servo control piston housing by
the mechanical connection until the housing bores and the servo control piston are back to a negative overlap =
neutral position. Both sides of the swash plate control – control cylinder 9 are actuated right away with
pressurized oil and the swash plate is held hydraulically on the selected swivel angle.
LWT / Ausgabe / Edition / Edition / 01.2009

If current increases, the pumps are shifted in direction of Qmax until the mechanical regulating stop of the
variable pump is reached.
If current decreases, the swash plate pump 4 is moved in direction Qmin / neutral position, regulation / control
sequence as described above.

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 Service Manual Travel hydraulic
Function

2.2. Regulation - var. displ. motors 20 starting from neutral function as described in section A
When starting to travel, the 3/3 -directional valve 25 is pressurized on one side with low pressure, on the other
side with high pressure from the closed loop circuit and is moved accordingly towards the low pressure side.
This results in the following oil flow, in addition to the closed loop circuit: (applies to both motors per side)
Low pressure side directional valve 25 orifice 26 pressure relief valve 24 motor housing
(lubrication / flushing) cooling circuit; see section „B“.
High pressure side directional valve 25 blocked.
Due to the energization of the proportional solenoid valves Y7/Y7.1 or Y8/Y8.1, the servo control piston 23 is
moved against the regulating spring to shift position” diagonal passages”. The regulator side (spring side)
control piston is relieved to the tank, the opposite control piston is actuated with pressurized oil and the axial
piston unit is moved in direction Qmin.
With the movement in direction Qmin , the return spring pushes the servo control piston 23 increasingly against
the force of the proportional solenoid valve. If the spring force equals the magnetic force, the servo control
piston 23 is held in center position, the channels to the control piston 22 are closed and the axial piston unit is
held on the selected angle.
As current increases, it continues to move in direction Qmin. The maximum possible current / adjustment is
limited by the electronic control.
As current is decreased, the axial piston unit 21 is moved again in direction Qmax = neutral position, whereby
the control pressure is moved the same way as in neutral function via the pilot piston 23 - ”parallel passage” to
the piston bottom side of the control piston 22 and the piston rod side is relieved. Intermediate positions for
equal force between solenoid and spring are as described above.

2.3. Closed circuits, starting from neutral function as described in section A

The flow of swash plate pump 4 is directed via two line connections to the connection sides of variable
motors 20 / axial piston unit 21, actuates the pistons and rotates the unit.
Due to the output rotation, the oil flows to the opposite side and flows via a second line connection back to the
suction side of the variable pump 2 – swash plate pump 4.
The oil discharged from the variable motor 20 is also replenished again in the variable pump 2 on the low
pressure side = suction side via the replenishing function of check valves 10 or 11 – see neutral function,
paragraph 1.
On the high pressure side = flow side, the replenishing function is held by high pressure and the maximum
pressure is limited by the PR valve 5 or 6 to 420+30/-10 bar.
If the pressure increases above this value, the PR valve opens and allows the oil to flow off via the suction
function of the opposite PR valve to the low pressure.
The second closed circuit works the same way.
The pressures in the closed circuits can be measured on test points ML1/MR1 and ML2/MR2 on test
console 60 with suitable pressure gauges. See Special tools, section 2.1.
Observe the safety guidelines – see section 1.2.
LWT / Ausgabe / Edition / Edition / 01.2009

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Travel hydraulic Service Manual
Function

3. Special function – Brake control in towing operation

In case of failure of the machine drive, the brake function (service brake) of the hydrostatic drives and the
mechanical parking brake can be released manually / hydraulically.
Observe the guidelines in the operating instructions and corresponding safety guidelines.

3.1. To release the brakes

On console 61, remove and close off the hose from the fitting.
- Connect ML1 ML2 and MR1 MR2 on the test console 60 with hose lines (supplied in the tool box of
the machine) = release of service brake.

- Install special tool to take on the CO2 cartridge on fitting on console 61 – see Operating instructions.

- Screw the CO2 cartridge into the receptacle.

- Bring the starter switch, emergency off button and safety lever in operating position. Press the scroll key
and the towing button in the cab at the same time, on the display blinks the word "TOWING". For final
release of the brakes, deflect the travel joystick to the front = solenoid valve Y13 is energized / shifted
and via passage P B, the parking brakes 46 are released by the available pressurized oil.

The release of the brake is shown on the instrument panel via switch B5.
During towing, the displaced oil from the variable motors circulated via connections ML1 ML2 or MR1 MR2
in the closed loop circuit.

3.2. Emergency braking in towing operation

Interrupt the current circuit to solenoid valve Y13 by
- bringing the travel joystick to neutral or
- pressing the emergency off button or
- raising the safety lever or
- bringing the starter switch in position "0".
Solenoid valve Y13 moves to neutral position, the pressure to the parking brakes is relieved via passage B T
and the brakes are applied.
The existing accumulator volume of the CO2 cartridge usually allows the parking brakes to be released again.

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 Service Manual Travel hydraulic
Function

B. Cooling circuit and hydraulic fan control

For functional schematic, see page 9/10

1. Cooling circuit
The leak oil or oil from the discharge (cooling and flushing of components) is directed via the housing
connections T on the var. displ. pumps 2 or U on the var. displ. motors 20 via the strainers 71 and collector
block 70 to the oil cooler 67 and cooled into the tank 80.
When the oil is cold, a back pressure is created in front of the oil cooler 67. It opens the check valve 72 and a
large amount of the oil flow from the collector block 70 is directed uncooled to the pump 15/S1 = oil cooler
bypass for quicker warm up.
At operating temperature, the back pressure in front of the oil cooler drops and the check valve 72 remains
closed. The complete oil return flow flows via oil cooler 67, the pump 15/1 receives only cooled oil from the tank.

2. Hydraulic fan drive – Regulating pump 30

When the Diesel engine is at a standstill, the axial piston unit 31 of the regulating pump 30 is on maximum angle
= Qmax.
With the starting procedure of the Diesel engine, the pump 30 starts to supply oil, the system pressure
increases and the hydro motors 40 and 42 for the fan drive start to turn.

2.1. Electronic regulation:

The electronic fan control is handled by the electronic box A25 of the Diesel engine.
A25 regulates, depending on the signal values of the temperature sensors B24/B707 as well as B708 the
energization of the proportional pressure cut off valve Y36 and limits the pressure build up in the hydraulic fan
drive = the fan speed.

A25 B24 – Hydraulic oil temperature

B707 –Charge air temperature
B708 – Coolant temperature
Y36 – Proportional solenoid valve – fan control
When the machine is cold, Y36 is energized with the maximum value by A25 and it regulates the possible
pressure build up in the hydraulic fan drive - pump 30 hydro motors 40/42 - to the adjusted stand by
pressure (30±5 bar) (fans turn at minimum speed).
With increasing media temperature (hydraulic oil, Diesel engine coolant, charge air), the energization of Y36 is
proportionally reduced and as a result, the pressure in the fan drive / the fan speed is increased accordingly.
If one of the media reaches the operating temperature, Y36 is energized by A25 with minimum value and it
regulates the pressure level in the fan drive to the maximum normal operating pressure (approx. 205 bar).
If there is no current, for example if there is a broken wire, Y36 mechanically / hydraulically regulates the
pressure level in the fan drive to the adjusted spring value of the pressure cut off = maximum fan speed (fail-
save system) - p = 205±5 bar, see section 6.4, Test and adjustment.
For wiring, see section 9 – Electrical system, fan speeds min/max in normal operation, see page 3.1.
LWT / Ausgabe / Edition / Edition / 01.2009

2.2. Pump regulation:

The pressure in the system created by the flow of pump 30 acts on the load sensing regulator 36 against its
spring.
On the proportional pressure cut off valve, Y36 acts due to the magnetic force created by the energization of
the solenoid valve (regulated by the electronic box A25) and the pressure against the spring of the valve via the
restrictor bore 37. If the sum of these forces exceeds the spring force, the valve Y36 opens and allows the
pressurized oil to flow off into the housing / to the tank. The remaining pressure (= regulating pressure)
between the restrictor bore 37 and the pressure cut off valve Y36 supports the spring force on the LS regulator
36.
If the force of the system pressure on the LS regulator 36 exceeds the sum from spring value and regulating
pressure, the regulator is moved in direction of the spring and pressurized oil is directed to the control cylinder

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Travel hydraulic Service Manual
Function

32. Due to the given surface ratio. the control cylinder 32 moves the axial piston unit 31 against the return
spring 33 to a smaller angle = in direction Qmin. The swivel angle / flow is regulated in such a way via the LS
regulator 36 and the return force of the cylinder 33, that a pressure corresponding to the regulating forces is
created in the system. This pressure determines the rotational speed of the hydro motors / fans.
If the system pressure or the magnetic force determined by the electronic box A25 drops, then the axial piston
unit 31 is moved in reverse order in direction Qmax.
The oil flowing off via restrictor 38 when the pump is regulated is pressure dependents and serves to dampen
the signal as well as to flush the housing.
Test point LA (pressure build up in fan drive) is connected on outlet P of the regulating pump 30.
Oil flow – working circuit:
Hydraulic tank 80 axial piston unit 31 axial piston constant motor 40 gear motor 42
return filter 82 rank 80.
Check valves 41 and 43 are held closed by the pressurized oil.
Oil flow regulation (pump 30 - intern):
Axial piston unit 31 return cylinder 33.
Restrictor 37 load sensing regulator 36
proportional pressure cut off valve Y36
load sensing regulator 36 control cylinder 32.
( Restrictor 38 housing L strainer 85 tank 80.)

LWT / Ausgabe / Edition / Edition / 01.2009

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 Service Manual Travel hydraulic
Function

1 Splitterbox LA System pressure - fan drive

SP Replenishing pressure
2 Var. displ. pump unit - left/right ML1 High pressure Travel forward - left
4 Swash plate pump ML2 High pressure Travel reverse - left
5 Pressure relief valve 420 bar with MR1 High pressure Travel forward - right
suction function - HD forward / M1 MR2 High pressure Travel reverse - right
6 Pressure relief valve 420 bar with
suction function - HD reverse / M2 61 Console (on test console 60)
7 Actuator cylinder
8 Servo control valve 65 Cooler – Water / charge air
9 Control cylinder
10 Check valve – replenishing closed circuit 67 Oil cooler
11 Check valve – replenishing closed circuit
12 Orifice 70 Collector block
15 Tandem gear pump - replenishing 71 Strainer
72 Check valve (1 bar)
16 Valve block - replenishing SP
17 Pressure relief valve 20 bar 80 Hydraulic tank
SD Test point – cold start 81 Shut off valve
82 Return filter
18 Filter complete 83 Bypass valve (3 bar)
19 Distributor section 85 Strainer

20 Var. displ. motor complete - left/right 90 To regulating pump 100 – working hydraulic
21 Axial piston unit 91 To / from control valve block 130 – working
22 Control piston hydraulic
23 Servo piston 92 To / from pilot control 115 – working hydraulic
24 Pressure relief valve 14 bar - discharge 93 Return lines – working hydraulic
25 Directional valve 95 To / from hand pump – tilt operator’s platform
26 Orifice (2.5 mm)
100 Regulating pump – working hydraulic
30 Regulating pump – fan drive
31 Axial piston unit .. Hydraulic oil flow from and to ..
32 Control cylinder (Qmin) .. Hydraulic oil flow from or to ..
33 Return cylinder (Qmax)
35 Regulator cpl. B5 Oil pressure switch – Brake release (8 bar)
36 Load Sensing regulator B6 Oil pressure switch – repl. pressure (15 bar)
37 Restrictor (0.8 mm) B18 Oil pressure switch – filter monitoring (2.5 bar)
38 Restrictor (0.6 mm) B24 Temperature sensor – Hydraulic oil
fan control; 65°C)
40 Axial piston constant motor – fan drive
41 Check valve (0.5 bar) Y4 Solenoid valve – safety control -
working hydraulic
42 Gear motor – fan oil cooler Y7 Prop. solenoid valve – var. displ. motor left rear
43 Check valve (1 bar) Y7.1 Prop. solenoid valve – var. displ. motor left front
Y8 Prop. solenoid valve – var. displ. motor right rear
45 Travel gear, complete left / right Y8.1 Prop. solenoid valve – var. displ. motor right front
46 Ring piston – disk brake Y9 Prop. solenoid valve – var. displ. pump left
LWT / Ausgabe / Edition / Edition / 01.2009

Travel forward
48 Connection flushing line – slipring seal Y10 Prop. solenoid valve – var. displ. pump left
PR 754 F/N xxx-6924 9706 Travel reverse
49 Console flushing connection – slipring seal in oil Y11 Prop. solenoid valve – var. displ. pump right -
cooler reservoir PR 754 F/N xxx-9707 Travel forward
Y12 Pro. solenoid valve – var. displ. pump right -Travel
50 Distributor block reverse
52 Check valve (1 bar) Y13 Solenoid valve – parking brake
55 Accumulator – working hydraulic
Y36 Prop. solenoid – pressure regulating valve fan
control – max.205 bar
60 Test console - test points Y61 Solenoid valve – shut off servo pressure

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Travel hydraulic Service Manual
Function

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 Service Manual Travel hydraulic
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Travel hydraulic Service Manual
Function

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