ZF Servocom - Paul Nutzfahrzeuge

S e r v i c e
8352 W 5/00 e
Function
Maintenance
Inspection
Troubleshooting
ZF Servocom ® RAS
(Rear Axle Steering)
ZF Lenksysteme GmbH
D-73522 Schwaebisch Gmuend
Telephone (07171) 31-0 Fax (07171) 31-4396

Contents / System description / Notes
Contents:
1 Functional description ................................................................ 2
2 Safety instructions ................................................................... 5
3 Maintenance ........................................................................ 5
4 Inspection .......................................................................... 6
5 Disassembly of the ZF-Servocom RAS ................................................... 7
6 Installation of the ZF-Servocom RAS .................................................... 8
7 Charging with gas ................................................................... 9
8 Filling with oil ...................................................................... 10
9 Bleeding ........................................................................... 10
10 Settings and tightening torques ....................................................... 11
11 Special tools ....................................................................... 12
12 Troubleshooting ..................................................................... 13
System description:
! The ZF-Servocom RAS is a rear axle steering system which hydrostatically transmits the steering
movement of the front axle to the rear axle (as in the case of a hydraulic braking system).
! The hydrostatic power is transmitted from the master cylinder (front axle) via hydraulic connections
to the centering cylinder (rear axle).
Notes:
! This manual is exclusively intended to supplement the existing instructions for operation, maintenance
and inspection of the ZF-Servocom steering systems.
! All the work described here should be carried out within the framework of routine maintenance and
inspection of the ZF-Servocom.
! All the settings and test values or dimensions listed are standard values which apply for all versions.
Exception: The individual vehicle manufacturers may prescribe different values. Please take note
of the service information circulars and the spare parts list!
ZF-Servocom RAS
Page
1

Function
1 Functional description
1.1 Mechanical design
1.2 Function
2
1 ZF-Servocom 3 Centering cylinder
2 Master cylinder with 4 Hydraulic accumulator
pressure limiting valve 5 Hydraulic lines
and replenishing vavle 6 Pressure switch
7 Connecting rod
! The steering movement is transmitted from the ZF-Servocom or front axle steering (1) mechanically
via a connecting rod (7) and hydraulically via hydraulic lines (5) to the master cylinder (2) which is
mounted on the front axle or vehicle chassis.
⎨ The master cylinder (2) hydrostatically transmits the steering movement via hydraulic lines (5) to the
centering cylinder (3) on the trailing axle. This axle is steered by the centering cylinder via the piston
rod.
⎨ The hydraulic accumulator (4) improves the rigidity of the hydrostatic transmission system and acts
permanently on the two centering pistons which are integrated into the centering cylinder (3).
⎨ Pressure limiting valves are installed in the master cylinder (2) in order to protect the hydrostatic
system against impermissibly high pressures which could damage the system.
⎨ The master cylinder (2) also includes replenishing valves to prevent the formation of a vacuum in
the unpressurized cylinder area and thus facilitate the steering movement.
The pressure switch (6) monitors the system pressure and is connected to a pilot lamp.
Additional information for versions with cut-off valve (8):
The cut-off valve (8) (see figure on page 18) connects the lines L1 and L2 in case of breakdown of the
hydraulic assistance of the front axle steering gear.
This produces the fact that the ZF-Servocom RAS does not co-steer and that no additional actuating
forces are required at the front axle steering gear.
ZF-Servocom RAS

Function
1.3 Function when travelling straight ahead
Operating pressure - front axle steering
System pressure
Accumulator pressure (gas-charged)
! The master cylinder (2) is designed so that the cylinder areas Z1 and Z2 are connected when travelling
straight ahead and when the steering angle is less than 5 o . This ensures that the four cylinder
areas Z3, Z4, Z5 and Z6 in the centering cylinder (3) are pressurized by the hydraulic accumulator
(4), thus holding the trailing axle in position for travel straight ahead.
⎨ Short-circuiting cylinder areas Z1 and Z2 and centering the trailing axle additionally ensures that the
master cylinder and centering cylinder (3) are synchronized. This compensates any hydrostatic offsets
which may occur, for example as a result of minute leaks.
ZF-Servocom RAS
3

Function
1.4 Function when steering to the right
⎩
⎨
Operating pressure - front axle steering / return flow
Operating pressure - front axle steering
System pressure / return flow
System pressure
Accumulator pressure (gas-charged)
! Cylinder space Z7 in the master cylinder (2) is pressurized by the ZF-Servocom or front axle steering
(1) and displaces the piston rod to the right.
⎨ This pressurizes the hydraulic fluid in cylinder space Z1, as well as in cylinder space Z6 via hydraulic
lines (5) if the steering angle is greater than 5 o .
⎨ The piston rod of the centering cylinder (3) is consequently displaced to the left and the trailing axle
is steered to the left.
4
ZF-Servocom RAS
⎩

Safety instructions / Maintenance
2 Safety instructions
Attention:
! Lines and connections, etc. must not be opened until the system pressure of the ZF-Servocom RAS
has been relieved.
⎨ The bleeder valves must be opened slowly to bleed the system.
⎨ Note that at least the system pressure is still present in the hydrostatic system even when the engine
has been switched off.
! The front axle steering must not be actuated when connections or lines, etc. on the rear axle
steering system have been opened, since the high resultant pressures may cause injuries.
3 Maintenance
3.1 Maintenance intervals
⎨ The maintenance intervals which apply for the ZF-Servocom steering system or which are specified
by the vehicle manufacturer also apply for the ZF-Servocom RAS.
Note any other or additional intervals specified by the vehicle manufacturer.
3.2 Maintenance work
⎨ Check oil level
ZF-Servocom RAS:
The oil level cannot be checked. This is also unnecessary, since oil cannot be lost during normal
operation and when the ZF-Servocom RAS is in good condition.
ZF-Servocom (front axle steering):
Check oil level in oil tank.
If the oil level exceeds the maximum permitted, this may be due to a leak in the master cylinder.
⎨ Check for external leaks
Hydraulic lines, connections, master cylinder, centering cylinder and hydraulic accumulator must
be visually inspected for leaks.
There must not be any visible leakage. Only the piston rods of the master and centering cylinders
may be coated with a film of oil. The formation of drops is not permissible.
⎨ Check bellows
Check that the bellows is seated correctly and undamaged.
The bellows must not be twisted.
ZF-Servocom RAS
5

Inspection
4 Inspection
4.1 Inspection intervals
! The same inspection intervals apply for the ZF-Servocom RAS as for the ZF-Servocom steering.
⎨ Note any other or additional intervals specified by the vehicle manufacturer.
4.2 Inspection work
4.2.1 Carry out the maintenance work described in section 3.2.
6
Bleeder valve
Connection
4.2.2 Check system pressure on hydraulic accumulator
⎨ Remove cap on hydraulic accumulator.
ZF-Servocom RAS
Tool [1]
Valve B
Spindle A
⎨ Slightly loosen the hexagon socket head screw of connection A2 (back off approx. one-quarter
turn anticlockwise).
⎨ Screw tool [1] onto connection A2 by hand with the union nut.
⎨ Turn pressure gauge so that it can be read off conveniently.
⎨ Close valve B.
⎨ Turn spindle A of tool [1] to the left until the pointer on the pressure gauge begins to move. Then
give it one more full turn.

Inspection
! Read off the system pressure. Required: 15 +1/-3 bar (see note on page 1).
Note:
If the system pressure corresponds with the required value, the accumulator pressure need not be
checked as described in section 4.2.3 since it will invariably be correct in such cases.
⎨ Tighten the hexagon socket head screw with spindle A in order to close the valve.
⎨ Unscrew the union nut and remove tool [1].
⎨ Tighten the hexagon socket head screw with a torque of 25 Nm and leak-test the valve with
sealing spray or soapy water.
⎨ Refit the cap.
4.2.3 Check hydraulic accumulator pressure
⎨ Connect tool [2] to connection A1.
Tool [2]
⎨ Turn the valve anticlockwise with tool [2] to relieve the system pressure. At the same time, check
whether the pilot indicator installed in the cab switches at a remaining pressure of 2...5 bar.
⎨ Connect tool [1] to connection A2 as described in section 4.2.2.
⎨ Read off the accumulator pressure. Required: 10 +1/-3 bar (see note on page 1).
ZF-Servocom RAS
Valve
Note:
If the accumulator pressure does not correspond to the required value, the accumulator pressure
must be increased as described in section 7 or the hydraulic accumulator replaced.
⎨ Close the valve of tool [2] by turning clockwise.
⎨ Check whether the oil tank of tool [2] has been filled (oil grade see section 6.1).
⎨ Build up the system pressure with tool [2] by actuating the hand pump. Required: 15 +1/-3 bar
(see note on page 1).
⎨ Remove tool [1] as described in section 4.2.2.
⎨ Close the valve of the hydraulic accumulator as described in section 4.2.2.
⎨ Remove tool [2].
4.2.4 Check cut-off valve (8) and switch relay (9)
⎨ Connect cable plug of the signal sensor (10) to earth.
⎨ Steer the steering wheel so that the steering output shaft angle is bigger than 5 o .
⎨ The ZF-Servocom RAS must not co-steer.
⎨ If the ZF-Servocom RAS co-steers, the failure must be removed - see troubleshooting.
7

Disassembly / Installation
5 Disassembly of the ZF-Servocom RAS
8
Attention:
Note the safety instructions in section 2.
5.1 Master and centering cylinders
⎨ Mark connections and installed position of cylinders.
⎨ Disconnect master and centering cylinders and drain oil.
5.2 Hydraulic accumulator
⎨ Unscrew union piece from hydraulic accumulator.
⎨ Release catches and remove hydraulic accumulator.
6 Installation of the ZF-Servocom RAS
6.1 Master and centering cylinders
6.1.1 Preparing cylinders for installation
⎨ Move piston rod to middle position.
Tool [3]
Master cylinder
ZF-Servocom RAS
Tool [4]
Centering cylinder
⎨ Mount tool on piston rod so that bevel on left-hand side (see arrow on tool) positively engages
the bevel at the end of the piston rod. Move piston rod until the right-hand side of the tool (tool
[3]) or the shoulder (tool [4] - see marking on tool) contacts the face end of the cylinder.
⎨ Fill master and centering cylinders with oil.
Oil grade:
Only oil grades approved by the vehicle manufacturer (e.g. Pentosin CHF 11S) may be used in the
ZF-Servocom RAS (without ZF-Servocom steering). The oil must not be mixed with other grades.
Attention:
In the case of the master cylinder, only ports L1, L2 and L3 in the centre of the cylinder (see diagram
on page 6) may be filled with the above oil grade.
The external ports L4 and L5 must be filled with the oil grade used in the ZF-Servocom or front
axle steering.
6.1.2 Installation instructions
⎨ Screw ball joint onto piston rod.
Attention:
Note values X and Y for the minimum screw depth.

Installation / Charging with gas
Value X
Piston rod
ZF-Servocom RAS
Value Y
Cylinder pipe
⎨ Value X (piston rod): Master cylinder: max. 13 mm (for 8346 974 163:max. 31.5 mm)
Centering cylinder: max. 31.5 mm
Value Y (cylinder pipe): Master cylinder: max. 14 mm
Centering cylinder: max. 19 mm
⎨ Oil the mating faces and screw thread of the ball joint.
⎨ Turn the steered wheels into the exact straight-ahead position.
⎨ Install the master and centering cylinders so that the line connections point upwards.
⎨ Screw the ball joint mounted on the cylinder pipe onto the frame (master cylinder) and axle (centering
cylinder) and lock it in place.
Tightening torque: Clamp: 45...50 Nm
Castellated nut: 250...280 Nm (for 8346 974 163: 210...230 Nm)
⎨ Move piston rod to middle position as described in section 6.1.
⎨ Oil the screw thread and mating face.
⎨ After connection to the piston rod, fit the ball joint into the wheel steering lever by twisting and
turning.
Attention:
Note values X and Y.
Tighten (torque see above) and secure the clamp and castellated nut.
Check sealing edges, sealing faces and sealing rings of the connections for signs of damage.
Connect the piping so that it is oil-tight.
⎨ Following the installation of the centering and the master cylinder, the chassis geometry must be
measured and readjusted, if necessary.
6.2 Hydraulic accumulator
⎨ Insert hydraulic accumulator in union piece and secure it.
⎨ Tighten union piece (Ermeto GE) with a torque of 80+10 Nm.
7 Charging with gas
⎨ Connect tool [1] to connection A2 as described in section 4.2.2.
Attention:
Only nitrogen may be used as the charging gas. Explosion hazard!
Only nitrogen cylinders with pressure reducing valve and relief valve may be used.
⎨ Connect charging hose to tool [1] and nitrogen cylinder.
9

Charging with gas / Filling with oil / Bleeding
! Set pressure reducing valve to 10+1 bar (see note on page 1).
! Slowly open the shutoff valve on the nitrogen cylinder.
! Open valve B of tool [1].
10
ZF-Servocom RAS
Tool [1]
Valve B
! When the accumulator pressure reaches 10+1 bar (see note on page 1),
close the shutoff valve on the cylinder and valve B on tool [1].
Spindle A
! Check the accumulator pressure again after waiting 5 minutes, during which time the temperature
is equalized.
! Remove tool [1] as described in section 4.2.2.
! Close the valve of the hydraulic accumulator as described in section 4.2.2.
8 Filling with oil
! Oil grade:
Only oil grades approved by the vehicle manufacturer (e.g. Pentosin CHF 11S) may be used in the
ZF-Servocom RAS (without ZF-Servocom steering). The oil must not be mixed with other grades.
! Amount of oil:
Approx. 6 litres are required to fill the complete ZF-Servocom RAS (without ZF-Servocom steering
or front axle steering).
! Move master cylinder to middle position (see section 6.1).
! Connect tool [2] to connection A1. Fill the oil tank of tool [2] with oil (oil grade as above) in order
to avoid pumping air.
! In order to facilitate the filling and subsequent bleeding process, it is advisable to open some of the
bleeder valves of lines L3, L1 and L2 (see diagram on page 6) and to close them again when oil
emerges.
! Increase the system pressure to 15+1 bar (see note on page 1) by actuating the hand pump.
9 Bleeding
9.1 ZF - Servocom RAS
! Connect tool [2] to connection A1. Fill the oil tank of tool [2] with oil (oil grade as above) in order
to avoid pumping air.
! Actuate the hand pump of tool [2] to build up pressure and ensure that the system is pressurized
during the entire bleeding process.

Bleeding / Settings and tightening torques
! Open the highest bleeder valve in line L3 until air bubbles no longer escape; then reclose it so that
it is oil-tight.
! Open the other bleeder valves in line L3 and then reclose them so that they are oil-tight.
! Bleed lines L1 and L2 as described above.
! Bleed the cylinder ports and then reclose them so that they are oil-tight.
! Check the system pressure. Required: 15+1 bar (see note on page 1)
9.2 ZF-Servocom or front axle steering and corresponding circuit of master cylinder
! Start the engine.
! Open the bleeder valves or ports of lines L4 and L5 until air bubbles no longer escape; then reclose
them so that they are oil-tight.
! Turn the steering from one limit position to the other several times.
! This allows any remaining air in the oil to escape via the oil tank.
! The oil level in the oil tank may rise by 1...2 cm when the engine is switched off and the system
has been fully bled.
9.3 Bleed the ZF-Servocom RAS again as described in section 9.1
! Check the system pressure. Required: 15�1 bar (see note on page 1)
! Remove tool [2].
10 Settings and tightening torques
- Settings:
Accumulator pressure: Check: 10+1/-3 bar (see note on page 1)
Charging: 10�1 bar (see note on page 1)
System pressure: Check: 15+1/-3 bar (see note on page 1)
Charging: 15�1 bar (see note on page 1)
Value X: Master cylinder: max. 13 mm (for 8346 974 163max. 31.5 mm)
Centering cylinder: max. 31.5 mm
Value Y: Master cylinder: max. 14 mm
Centering cylinder: max. 19 mm
- Tightening torques:
Hydraulic accumulator: Hexagon socket head screw 25 Nm
Union piece (Ermeto GE) 80+10 Nm
Master and centering cylinders: Clamp 45...50 Nm
Castellated nut 250...280 Nm
(for 8346 974 163: 210...230 Nm)
ZF-Servocom RAS
11

Special tools
11 Special tools
Note:
The special tools listed below refer to the standard version and the design version on the basis of which
the entire manual has been compiled. Other tools may consequently be required for the particular unit
in question.
Tool [1]
12
ZF-Servocom RAS
Part. No.
Test device 7016 798 516
Tool [2]
Hydraulic unit for 7418 798 563
filling with oil
Tool [3]
Gauge for 8346 798 301
master cylinder
(For 8346 974 163:
8346 798 303)
Tool [4]
Gauge for 8346 798 302
centering cylinder

13
12 Troubleshooting
This troubleshooting procedure presupposes that there The applicated letters and numbers (e.g. 2, L5) refer to the illustration are
no defects in the front axle steering! on page 17.
Attention: Note the safety instructions in section 2.
Fault Cause Check Remedy
Pressure switch ! Loss of oil; ! Check steering system ! Eliminate cause of leak.
indicator lights up when external leak in system for external leaks (visual examination) Fill system with oil 15�1 bar (see note
vehicle power supply is and system pressure - required: on page 1) and bleed it.
switched on 15+1-3 bar (see note on page 1) See operating instructions.
! Internal leak in ! Check oil level in oil ! Seal or replace cylinder
master cylinder (2) tank (is too high)
! Hydraulic accumulator (4) ! Check maximum pressure ! Charge hydraulic accumulator
defective in hydraulic accumulator - required: with nitrogen or replace it.
10 +1/-3 bar (see note on page 1) 10�1 bar (see note on page 1)
- see operating instructions
! Internal leak in ! Open screw plug A3 ! Seal or replace centering cylinder
centering cylinder (3) - oil must not emerge
oil leaking from L3 to A3
Pressure switch indicator lights ! Press switch (6) defective ! Check correct functioning of ! Replace pressure switch
up when vehicle power supply pressure switch as directed by vehicle
is switched on although manufacturer
RAS is functioning correctly
Indicator does not light ! Contactofpressure ! Check correct functioning of pressure ! Replace pressure switch
up although RAS is not switch (6) is sticking switch as directed by vehicle
functioning correctly manufacturer
! Indicator defective ! Check correct functioning of indicator ! Replace indicator
! Electric lead defective ! Check wiring for continuity with the aid ! Replace wiring or repair break
of a multi-purpose measuring instrument in connection
Oil tank indicator ! Internal leak in master cylinder ! Check system pressure ! Seal or replace cylinder
lights up while steering (2) - oil leaking leaking (is too high) - required:
from L5 to L2 or from L4 to L1 15+1-3 bar (see note on page 1)

14
Fault Cause Check Remedy
Synchronization error. ! Mechanical connecting elements ! Visual examination of ! Replace defective parts
Rear axle does not centre between axle (ball joints, mechanical connecting elements
back to the precise piston rod, steering knuckle,
straight-ahead position etc.) and cylinder are stiff,
- only when stationary bent, with backlash
-alsowhendriving
- in one direction
- in both directions
! Air in RAS steering system ! Bleed steering system and ! Fill system with oil 15�1 bar
check system pressure - required: (see note on page 1) and bleed it
15 +1 -3 bar (see note on page 1) (see operating instructions)
! Geometry of rear ! Check rear axle for ! Set axle values as directed
axle not OK specified axle values by vehicle manufacturer
! System pressure ! Check steering system for external ! Repair leak
too low, external leak leaks (visual examination) and check Fill with oil 15�1 bar (see note on
existing system pressure - required: page1) and bleed system
15 +1 -3 bar (see note on page 1) (see operating instructions)
! Gas pressure of ! Check accumulator pressure ! Charge hydraulic accumulator with
accumulator (4) too low - required: 10+1-3 bar (see note on nitrogen 10+1 bar (see note on page 1)
page 1) or replace it (see operating instructions)
! Internal leak in centering ! Remove screw plug (A3) - oil must not ! Seal or replace cylinder
cylinder (3) - oil leaking emerge from the open cylinder port.
from L3 to A3
Middle position of master ! Set wheels to straight-ahead position. ! Set cylinders to middle position with
and/or centering cylinder Check middle position of piston rod wheels in straight-ahead position.
offset in relation to in master and centering cylinders See operating instructions
straight-ahead position for setting
of wheels

15
Fault Cause Check Remedy
Synchronization error ! Air in RAS steering system ! Check steering system for ! Eliminate cause of infiltrated air.
Rear axle remains in external leaks Fill system with oil 15�1 bar (see note on
straight-ahead position page 1) - see operating instructions
and does not turn
- only when stationary ! Too little oil in the RAS system. ! Check steering system for Eliminate cause of leak.
- also when driving External leak in piping or in external leaks (visual examination) Fill system with oil 15�1 bar (see note on
- in one direction master and/or centering cylinder and existing system pressure - required: page 1) and bleed it
- in both directions 15+1-3 bar (see note on page 1)
! Leak in control system of ! Check oil leakage at cylinder ports ! Seal or replace cylinder
master cylinder (2) - oil leaking L1 to L3 or L2 to L3, as appropriate.
from L1 or L2 to L3 Turn front axle in the corresponding
directiontobechecked.
Unscrew and seal line
L1 to L3 or L2 and L3.
Set system pressure of RAS in line
L2 or L1 to 20 bar on Minimess port.
Measure leakage oil at open cylinder
ports for approx. 1 minute.
No more than 0.02 dm 3 /min
oil may emerge from both ports taken
together.
! Leak in main cylinder part of ! Set wheels to straight-ahead ! Seal or replace cylinder
centering cylinder (3) - oil position and check oil leakage
leaking from L1 to L2 or vice at cylinder ports
versa L1 or L2, as appropriate.
Unscrew line L1 or L2
from cylinder and seal it.
Set system pressure to 20 bar via
connection A1.
Oil must not emerge from the open
cylinder port.

16
Fault Cause Check Remedy
! Leak from main cylinder part ! Set wheels to straight-ahead position ! Seal or replace cylinder
of centering cylinder (3) to and check oil leakage at cylinder port
centering cylinder part L3.
- oil leaking from L1 to L3 Unscrew and seal line L3.
Set system pressure of RAS to 20 bar
via connection A1.
Oil must not emerge from
the open cylinder port.
! Lines no longer allow oil to pass ! Check piping and hoses for signs ! Replace damaged piping
through of damage.
Measure system pressure at
the corresponding end of the piping.
Additional information for versions with cut-off valve (8)
ZF-Servocom RAS ! Defective cut-off valve (8) ! Start the engine. Interrupt connection ! If ZF-Servocom RAS does not
No function between main power supply and
cut-off valve
function replace cut-off valve (8).
! Defective switch relay (9) ! Switch on ignition (engine off)
Take off main power supply plug from
cut-off valve (8). Measure voltage.
! Replace switch relay (9).
Nominal value: main power supply ! No main power supply voltage
voltage. Start engine. Measure voltage.
Nominal value: 0V
=> replace switch relay (9)
! Defective signal sensor
(e.g.flow rate indicator)
! Check switching function. ! Replace signal sensor
Legend to diagrams on page 17/18:
1 front axle steering 6 pressure switch L1/L2 hydrostatic connecting line to master- and centering cylinder
2 masterzylinder 7 connecting rod L3 pressure line
3 centering cylinder 8 cut-off valve L4/L5 connecting line from front axle steering to mastercylinder
4 hydraulic accumulator 9 switch relay A1 connection tube (oil side)
5 hydraulic lines 10 signal sensor A2 connection tube at hydraulic accumulator (gas side)
(e.g. flow rate indicator) A3 closing screw (centering cylinder)
(E) bleeder valve
A, B, C, D, E, X, Y, T, W, V connection identification sticked up at the aggregates

17
Version without cut-off-valve (8)

Company instruction PAUL NUTZFAHRZEUGE
BLEEDING GUIDE FOR ZF-SERVOCOM
1. Lift vehicle frame on front side.
2. Move steering straight ahead.
3. Stop engine during bleeding.
4. Connect hydraulic pump to non return valve beneath centering cylinder
(see photo).
5. Increase system pressure to 15 bar with hydraulic pump.
6. Attention: Use only Pentosin CH11S (DC-No.: A00 198 03 10), complete
filling appr. 6 ltr.
7. Loose slightly cap nut of pressure switch (close to master cylinder),
hydraulic line L3 and release air. Close cap nut.
8. Bleeding MINIMESS-connectors by turns (hydraulic line L1 and L2).
9. Increase in between system pressure to appr. 15 bar. Minimum pressure
during bleeding > 10 bar.
10. Star engine an steer slowly 5 times left to right to left.
11. Stop engine and steer straight ahead.
12. For oil calming down wait appr. 10 minutes.
13. Repeat bleeding procedure for hydraulic line L1 and L2.
14. Increase hydraulic system pressure to 15 bar.
15. Final performance control.