Steering Systems and Accessories
for Commercial Vehicles
Steering the right way.
Many tonnes safely controlled
– and what's behind it.
Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Page
ZF Servocom® . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
ZF Servocomtronic® . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Dual-Circuit Steering Systems . . . . . . . . . . . . . . . . . . . . 12
ZF Servocom® RAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
ZF Servocom® RAS-EC . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Semi-Integral Power Steering Gears . . . . . . . . . . . . . . . 25
Steering Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Oil Reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Power Cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Steering columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Bevel Boxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
The steering gear is something like the
soul of the commercial vehicle. The
driver is in permanent contact with it,
via the steering gear he notices the
condition of the road and the response
of the vehicle. Here the quality of the
steering gear is decided – with light
operation and comfort on the one
hand and as much feedback from the
road as possible on the other. Essential
for this is a carefully balanced interaction of high-precision mechanics and
a finely controllable hydraulic system,
which we have perfected with our
steering systems. These are preferred,
by the vehicle manufacturers, because
they are compact and reliable, and, by
the drivers, because they are comfortable and precise.
Such a high technical level is achieved
only by someone who is master of the
complete system. That’s why we dedicate ourselves to all components. Steering columns, steering pumps, valves,
oil reservoirs and other peripheral parts
contribute their share to reliability.
Ball-Track Telescopic Shafts/Ball-Track Relay Shafts . . . 35
Universal Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Pressure Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
ZF Servocom® and ZF Servocomtronic® are
registered trademarks of ZF.
2
Worth knowing
ZF Steering Systems.
Steering toward the future.
As a joint venture of Robert Bosch
GmbH and ZF Friedrichshafen AG,
ZF Lenksysteme GmbH has produced
power steering systems for passenger
cars and commercial vehicles for several decades. The products fulfill every
specified requirement. With the ZF
Servocom, the tried-and-tested recirculating-ball power steering principle
has been continually further developed, protected by numerous
patents, and adapted to suit specific
vehicle requirements. And for a great
variety of special applications and for
compliance with legal safety criteria,
it is logic to use our semi-integral and
dual-circuit power steering systems.
New courses were also taken in the
field of rear axle steering systems.
The ZF Servocom RAS (Rear Axle
Steering), which is also suitable for
retrofit, and the electronically controlled ZF Servocom RAS-EC (Rear
Axle Steering – Electronically Controlled) can completely do without
the steering linkage known to date
which is heavy and imprecise.
with the ZFLS Steer-by-Wire Steering
System. And as a member of a European consortium of vehicle manufacturers, suppliers, users, and institutions, we are preparing the implementation of automatic driving in
goods traffic.
Future-oriented innovative
approaches regarding
functions, design
and safety are
opened up
An outstanding result of innovative
further development is the ZF Servocomtronic. This steering gear designed on the basis of the Servocom is
electronically controlled and speedsensitive. It is characterized by easy,
comfortable steering in maneuvering
as well as a safe road feel at increasing speed.
Installation schematic of a ZF Servocom
with height and tilt adjustable steering
column, ball-track relay shaft, steering
pump and oil reservoir.
Worth knowing
3
ZF Servocom®
Mechanical construction
The ZF Servocom, a compact-design
recirculating-ball power steering
gear, basically comprises a sturdy
cast-iron housing (1) with integrated
mechanical steering gear, control
valve and power cylinder.
A turning movement at the steering
wheel is transmitted via the output
shaft and the valve slide (3) in the
control valve to the worm (4) and,
via an endless ball chain (7), transformed into an axial movement of
the piston (2). At the same time, the
sector shaft (6), which is arranged at
right angles to the longitudinal axis
of the piston, is caused to rotate by
the meshing of teeth. The drop arm
mounted on the sector shaft moves
the steering linkage which goes to
the steering arms, thus causing the
wheels to be turned.
The basic components of the
Servocom control valve are the valve
slide, with six control grooves on its
surface area, and a worm in whose
valve bore axial grooves are provided which are matched to the control grooves. Centralizing the valve
slide (neutral position) is done by a
torsion bar (5) which provides at the
same time the connection between
Figure at top:
ZF Servocom, type 8098.
Figure on page 5:
ZF Servocom, type 8098,
control valve in neutral position.
4
ZF Servocom
the valve slide and the worm. The
valve slide and the worm run in antifriction bearings to ensure the precision of operation and the functional
safety of the control valve even at
high pressures.
If a torque is transmitted to the
valve slide or the worm from the
steering wheel or the steered
wheels, a relative rotary motion
influenced by the torsion bar will
occur between the valve slide and
the worm. The valve slide is thereby
caused to change its position in relation to the worm valve bore surrounding it, so that the relative positions of the control grooves are
changed, too. Pressurized oil can
now flow through connecting bores
to one of the two power cylinder
chambers (ZL or ZR) and assists the
axial movement of the piston. When
the steering wheel is released, the
1
2
3
4
5
6
7
8
Housing
Piston
Valve slide
Worm
Torsion bar
Sector shaft
Ball chain
Feed oil radial groove
9
10
11
12
13
14
15
16
action of the twisted torsion bar
makes the control grooves return to
the neutral position, and the same
system pressure will exist in both of
the power cylinder chambers.
Basic hydraulic function of the
Servocom control valve
The hydraulic fluid delivered by an
engine-driven steering pump (21)
flows through a connecting bore in the
housing bottom, via the feed oil radial
groove (8) and transverse bores in the
valve portion of the worm (4), onward
to the three feed oil control grooves
(9) of the valve slide (3). In the valve
neutral position (see fig. on page 5),
the oil flows, over the open feed oil
control edges (10), to all axial grooves
(11) of the worm head and from there,
over the open return oil control edges
(13), also to the return oil control
grooves (12) of the valve slide. From
Feed oil control groove
Feed oil control edge
Axial groove
Return oil control groove
Return oil control edge
Return oil chamber
Radial groove
Radial groove
17
18
19
20
21
22
ZL
ZR
Hydraulic steering limiter
Pressure relief valve
Replenishing valve
Flow limiting valve
Steering pump
Oil reservoir
Power cylinder, left
Power cylinder, right
14
13
11
22
8
21
10
20
1
15
12
9
16
3
18
14
ZL
2
17
7
4
5
6
ZR
16
15
8
19
ZF Servocom
5
14
13
11
22
8
21
10
20
1
15
12
9
16
3
18
14
ZL
2
17
7
4
5
6
ZR
16
15
8
19
ZF Servocom, type 8098,
control valve in working position.
Steering wheel turned clockwise.
6
ZF Servocom
1
2
3
4
5
6
7
8
Housing
Piston
Valve slide
Worm
Torsion bar
Sector shaft
Ball chain
Feed oil radial groove
9
10
11
12
13
14
15
16
Feed oil control groove
Feed oil control edge
Axial groove
Return oil control groove
Return oil control edge
Return oil chamber
Radial groove
Radial groove
17
18
19
20
21
22
ZL
ZR
Hydraulic steering limiter
Pressure relief valve
Replenishing valve
Flow limiting valve
Steering pump
Oil reservoir
Power cylinder, left
Power cylinder, right
these grooves the oil flows back, via
bores, to the return oil chamber (14)
inside the worm and from there to the
oil reservoir (22). At the same time,
the radial grooves (15 and 16) of
the control valve and their associated
connections provide for a connection
between the right-hand (ZR) and lefthand power cylinder chambers (ZL).
When turning the steering wheel
clockwise (fig. on page 6), the piston
(2) will move to the right in the piston
bore if it is a right-hand ball thread.
Due to the simultaneous rotation of
the valve slide (3) to the right, the
pressurized oil is directed, over the
further opened feed oil control edges
(10), to the three associated axial
grooves (11), via bores to the radial
groove (16), and, via a connection, to
the left-hand cylinder chamber (ZL),
whereby the piston movement is
hydraulically assisted. An individually
adaptable pressure build-up is achieved
by the fact that the partially or fully
closed feed oil control edges (10)
restrict or prevent a connection
between the pressure oil inlet and the
other three axial grooves (11) connected to the radial groove (15). At the
same time, the pressure oil outlet
toward the pressurized axial grooves is
restricted or prevented, too, by the
closing return oil control edges (13).
The oil displaced by the piston (2) from
the right-hand power cylinder chamber (ZR) first flows through a connection to the radial groove (15) and,
through transverse bores, to the associated axial grooves and onward to the
return oil control grooves (12) over the
further opened return oil control
edges (13). From here, the further
return flow of the oil to the oil reservoir (21) takes places via the connecting bores leading to the return oil
chamber (14). When the steering
wheel is turned counterclockwise, the
operating sequence will be analogous
to the above.
Hydraulic steering limiter
To protect the steering linkage, the
wheel lock stops and the steering
pump from excessive loads at the maximum wheel lock angles, the ZF
Servocom has a mechanically adjustable or automatically on the vehicle
adjusting hydraulic steering limiter
(17). This device integrated in the
piston (2) is always closed due to the
oil pressure in the right-hand or lefthand power cylinder chamber. It is only
just before each piston end position
that the steering limiter valve is opened by the contact of the valve pin with
the adjusting screw or sleeve. Thus,
that cylinder chamber which is under
high pressure is connected with the
opposite return oil chamber. As a result
of the pressure drop, hydraulic assistance is reduced heavily. The steering
wheel can only be turned up to the
road wheel lock stop by using increased effort.
Further features
The ZF Servocom is fitted with a pressure relief valve (18) which limits the
delivery pressure of the steering pump
at the maximum specified pressure.
Also, a replenishing valve (19) can be
fitted to the housing or the valve slide,
if required. This valve allows to suck oil
from the return flow if the vehicle
must be steered without hydraulic assistance.
Mechanically adjustable (top)
and automatically adjusting steering
limiter (bottom).
Steering limiter valve opened,
oil pressure greatly reduced.
ZF Servocom
7
ZF Servocomtronic ®
Pressure p [bar]
140
reaction device makes it possible to
vary steering effort with vehicle speed.
120
0 km/h
100
20 k
m/h
80
50
km
/h
60
40
20
20
15
10
5
0
h
m/
0k
0
1
5
10
15
20
Torque required at the steering wheel [Nm]
Design and function
The ZF Servocomtronic is a speed-sensitive recirculating-ball power steering
gear for trucks and buses. The use of
advanced electronics, of an electro-hydraulic transducer, and of a hydraulic
Figure at top:
ZF Servocomtronic – diagram of characteristic
curves. It shows the change in pressure and
steering wheel torque as a function of vehicle
speed. The course of the characteristics can
be matched to the specific character of the
vehicle.
8
ZF Servocomtronic
Application of the ZF Servocomtronic
requires either an electronic speedometer (1) or a suitable ABS control
unit. The speed signals coming from
one of these units are transmitted to
the electronic control unit (2) which
can either be a separate component or
integrated in the existing vehicle electronics. The signals are analyzed by the
microprocessor of the Servocomtronic
control unit and converted into a controlled electric current which actuates
the electro-hydraulic transducer. On
the basis of this influence, the transducer, which is directly attached to the
housing cover, determines the hydraulic
reaction of the control valve and, thus,
the amount of input torque at the
steering wheel.
This speed-dependent influencing of
steering ensures that static steering as
well as steering at low vehicle speeds,
e.g. in parking maneuvers, require
minimal effort. As the hydraulic reaction changes in proportion to the vehicle speed, the steering effort increases
as the vehicle goes faster (see fig. at
top). At higher speeds the driver thus
has particularly good road contact and
is able to steer the vehicle precisely
and with directional stability.
Figure on page 9:
1 Electronic speedometer
Schematic representation of the
on the vehicle
ZF Servocomtronic, type 8098.
2 Electronic control unit
(microprocessor, 12 V)
3 Electro-hydraulic
transducer
4 ZF Servocomtronic
5 Steering pump
6 Oil reservoir with fine
filter
7 Ball-track relay shaft
8 Height and tilt adjustable
steering column
Mechanical construction
A further advantage of the ZF Servocomtronic is that oil pressure and flow
rate are never reduced and can therefore be utilized immediately in emergencies where sudden and unexpected
steering corrections may become
necessary. These features bring about
extraordinary precision of steering,
together with a high safety standard
and optimum steering comfort.
The basic steering gear for the ZF Servocomtronic is the well-proven ZF Servocom – millions of which have already
been fitted – based on the design and
operating principle described on pages
4 to 7. The difference from the description there is that in the axial bore of
the worm a torsionally resistant bellows (5, see fig. on page 10) is arranged
whose lower end is connected with the
worm such that rotation is prevented.
1
2
The upper part of the bellows forms a
centering bush (26) and presses, with
an axial spring force which is matched
to the specific vehicle, against a prism
centering device with two rollers (27).
When driving straight ahead, this has
especially positive effects on the exact
centralizing of the control valve. When
subjected to hydraulic pressure, a reaction piston (28) located in the cylinder
cover concentrically with the worm
also applies, via a spacer tube (29),
load on the prism centering device and
makes necessary, in addition to the
spring force of the bellows, further
force for the displacement of the control valve from its neutral position. The
amount of that hydraulic reaction is
determined by the indicated instantaneous vehicle speed and the opening
position of the electro-hydraulic
transducer resulting from this.
+
–
8
7
6
4
5
3
ZF Servocomtronic
9
14
13
11
22
8
21
10
20
24
25
23
1
15
12
9
16
3
18
30
14
ZL
2
17
7
4
29
5
ZR
16
8
15
19
27
26
31
32
6
28
ZF Servocomtronic, type 8098.
Control valve in working
position. Steering wheel turned
clockwise, driving at high
speed, transducer valve fully
opened, maximum reaction
limited by cut-off valve.
1
2
3
4
5
6
7
8
10
Housing
Piston
Valve slide
Worm
Bellows
Sector shaft
Ball chain
Feed oil radial
groove
9
10
11
12
13
14
15
16
17
ZF Servocomtronic
Feed oil control groove
Feed oil control edge
Axial groove
Return oil control groove
Return oil control edge
Return oil chamber
Radial groove
Radial groove
Hydraulic steering limiter
18
19
20
21
22
23
24
25
26
Pressure relief valve
Replenishing valve
Flow limiting valve
Steering pump
Oil reservoir
Electronic speedometer
Electronic control unit (12 V)
Electro-hydraulic transducer
Centering bush
27
28
29
30
31
32
ZL
ZR
Roller
Reaction piston
Tube
Reaction chamber
Orifice
Cut-off valve
Power cylinder, left
Power cylinder, right
Function of the
ZF Servocomtronic
At low speeds, e.g. in maneuvering,
the electronic speedometer (23) or the
ABS control unit transmit very few signals to the microprocessor integrated
into the electronic control unit (24).
The microprocessor analyzes the signals and passes them to the electrohydraulic transducer (25) in the form
of a correspondingly adapted control
current. Due to the maximum current
existing in that driving mode, the
transducer valve closes and prevents
pressure build-up in the reaction
chamber (30). An orifice (31) ensures
that there is also return pressure level
in the reaction chamber. Thus, owing
to the elimination of reaction, the
steering is light in operation and can
be handled with little effort.
As the driving speed increases, the
speed signals become more frequent
and, after having been converted by
the microprocessor, cause a reduction
in the amount of control current transmitted to the electro-hydraulic transducer. As a result, the transducer valve
takes up an opening position adapted
to the instantaneous vehicle speed and
allows a limited oil supply from the
feed oil radial groove (8), via a tube,
to the reaction chamber (30). Via the
spacer tube, the reaction piston now
presses against the prism centering
device and makes necessary more force
for the displacement of the control
valve. Thus, this mode of operation of
the hydraulic reaction requires an individually established higher steering
wheel torque until a determined
hydraulic assistance is raised in the
right-hand (ZR) or left-hand cylinder
chamber (ZL).
At high driving speeds (fig. on page
10), for instance on the motorway, the
transducer valve is fully open owing to
a very low or non-existing control current for the actuation of the transducer. This enables maximum pressure
supply from the feed oil radial groove
(8) to the reaction device. When the
steering wheel is turned clockwise, the
reaction pressure increases in accordance with the existing operating
pressure and pressurizes the reaction
piston from the reaction chamber (30).
As soon as the reaction pressure determined for a specific vehicle reaches its
upper limit, the oil is discharged to the
return oil chamber (14) through the
cut-off valve (32) to avoid a further
increase in reaction pressure. The input
torque at the steering wheel thus
achieved will not now rise any more
and gives a safe driving feel owing to
optimum road contact.
Safety of the ZF Servocomtronic
Even in the event of a failure of the
vehicle electrical system or any other
electrical fault, the steering gear
remains fully operational. In such
exceptional cases the ZF Servocomtronic will work at maximum hydraulic
reaction (high-speed characteristic)
due to the mechanically forced opening of the transducer valve. When
speed signals all of a sudden are not
transmitted any more during driving,
for instance due to lack of cable contact or a defective speedometer, the
highly advanced microprocessor in the
electronic control unit is in a position
to derive a constant control current
from the last speed signals evaluated.
This ensures a constant steering performance until the vehicle engine is
turned off. When the engine is started
again, maximum hydraulic reaction
conforming to the high-speed characteristic will develop again.
ZF Servocomtronic, type 8098.
ZF Servocomtronic
11
ZF Dual-Circuit Steering System
with 2 Pumps
Design and function
The installation of dual-circuit steering
systems is required for vehicles with
high steered axle loads or with more
than one steered axle in order to meet
statutory safety criteria. These requirements are met by the modified dualcircuit steering system with 2 pumps
which builds on the basic concept of
the ZF Servocom (see pages 4 to 7).
A crucial component in this system is
the change-over valve (6) adapted at
the input stub shaft end. In normal
operation it monitors the readiness for
operation of the engine-driven steering pump 1 (1) and ensures the connection to a power cylinder (5) which is
necessary for additional comfortable
steering assistance. The change-over
valve delivers the oil flow generated
by the wheel-driven steering pump 2
(2) directly, i.e. without being utilized,
back to the second oil reservoir 2 (4).
In an emergency, which is very rare, for
instance when the engine has stopped
or the performance of the enginedriven pump is reduced, the two
switching pistons (9 and 10) are pressed
with spring force against the associated contact switches (7), owing to the
insufficient pump pressure. At the
same time, this makes one switching
piston (9) direct the pressure oil from
the wheel-driven steering pump 2 to
the control valve of the steering
system. The second switching piston
(10) shuts off the connection to the
power cylinder, which means that the
lower output flow from the wheeldriven pump is available for the safe
operation of the steering system. This
ZF Servocom, type 8099.
Modified dual-circuit steering system.
Fig. on page 13:
Schematic representation of the
modified dual-circuit steering
system, ZF Servocom, type 8099
(right-hand).
Normal function for right-hand
turning. Both pumps deliver oil.
Power cylinder with pressure
assistance.
12
ZF Dual-circuit steering system
1
2
3
4
5
6
7
condition is indicated to the driver by
means of a pilot lamp. Also, the steering wheel effort is increased in extreme
steering situations, due to the inactive
power cylinder.
Steering pump 1
Steering pump 2
Oil reservoir 1
Oil reservoir 2
Power cylinder
Change-over valve
Contact switches
8
9
10
11
12
ZL
ZR
Feed oil radial groove
Switching piston (inside)
Switching piston (outside)
Check valve
Check valve
Power cylinder, left
Power cylinder, right
3
4
2
7
1
11
1
12
2
ZR
4
3
9
6
ZL
10
5
ZL
ZR
8
ZF Dual-circuit steering system
13
3
4
2
1
7
11
1
2
12
4
ZR
3
9
5
ZL
Schematic representation of the modified
dual-circuit steering system,
ZF Servocom, type 8099 (right-hand).
Emergency function for right-hand turning.
Only the wheel-driven steering pump delivers oil. Power cylinder without pressure
assistance.
14
ZF Dual-circuit steering system
ZR
1
2
3
4
5
6
7
Steering pump 1
Steering pump 2
Oil reservoir 1
Oil reservoir 2
Power cylinder
Change-over valve
Contact switches
8
8
9
10
11
12
ZL
ZR
Feed oil radial groove
Switching piston (inside)
Switching piston (outside)
Check valve
Check valve
Power cylinder, left
Power cylinder, right
6
ZL
10
Basic hydraulic function of the
change-over valve
The oil flow generated by the enginedriven steering pump 1 (1) after engine start will pressurize the inner (9)
and outer switching piston (10) in the
change-over valve (6) and displace
both in a direction contrary to the contact switches (7) and against a specifically determined spring force.
At that position of the inner switching
piston, the pressure oil can get to the
feed oil radial groove (8) on the control valve via an internal connection as
is the case on a standard ZF Servocom.
Via the opened radial groove, the
return oil from the cylinder chambers
(ZL/ZR) flows back to the oil reservoir 1
(3). The oil flow from the wheel-driven
steering pump (2) is directed back to
the oil reservoir 2 (4), without being
utilized, via the opened radial groove.
The position of the outer switching
piston allows the free exchange of
pressure oil between the associated
cylinder chambers (ZL/ZR) and the
separate power cylinder (5).
This position of the inner switching
piston allows the supply flow of the
pressure oil generated by the wheeldriven steering pump, via a connection,
to the feed oil radial groove (8). The
direct return flow to the oil reservoir 2
(4) which takes place in normal operation is prevented. Additionally, a check
valve (11) prevents the outflow of oil to
the oil reservoir 1 (1) via the enginedriven steering pump. Now the return
oil flowing from the cylinder chambers
(ZL/ZR) passes back to the oil reservoir 2
(4) via the opened radial groove. In this
switching mode, an outflow of the
return oil to the oil reservoir 1 (3) is not
possible.
At that position, the outer switching
piston (10) completely shuts off the
connection between the cylinder
chambers (ZL/ZR) and the power cylinder. This means that the steering gear
pressure or return oil respectively,
which is radially available at the
switching piston, cannot pass through
to the two radial grooves on the
switching piston. But, via a channel
system, these radial grooves permit the
mutual exchange of oil in the now
inactive hydraulic circuit of the power
cylinder. Check valves (12) between the
shut-off hydraulic circuit and the
cylinder chambers provide a balanced
pressure level.
When the engine has stopped or a specifically determined amount of oil (fig.
on page 14) is not reached, both
switching pistons are pressed against
the contact switches by spring force.
ZF Servocom, type 8099.
Modified dual-circuit steering system.
ZF Dual-circuit steering system
15
ZF Dual-Circuit Steering System
with 3 Pumps
Design and function
The installation of dual-circuit steering
systems with 3 pumps is required for
commercial vehicles with especially
high steering axle loads or with several
steered axles in order to meet the statutory safety criteria and to ensure the
necessary steering comfort. The steering gear used is the basic concept of
the ZF Servocom (see pages 4 to 7).
The high safety potential of this steering system is based on the existence of
two steering circuits which are completely separate from each other and
complement each other favorably in
normal operation. Steering circuit I is
supplied, on a priority basis, with pressure oil from the an engine-driven
steering pump 1 (1). From a determined oil flow, a standby valve (2) in
the valve block (3) controls the supply
of the oil from that pump, via the control valve 1 (4), to the steering gear
and to the parallel-connected power
cylinder (5) that may be fitted in the
particular case. At the described
switching position of the standby
valve, the oil flow generated by the
wheel-driven steering pump 2 (6) is, at
the same time, delivered back to the
common oil reservoir 1 (8), without
being utilized, but monitored by a
flow indicator (7).
In steering circuit II, the oil pressurizing, via the control valve 2 (12), the
power cylinder (13) fitted downstream
is also supplied by an engine-driven
pump 3 (9). A flow indicator (11) between the control valve 2 and the
separate oil reservoir 2 (10) monitors
the flowing oil.
Figure on page 17:
Schematic representation of the dualcircuit steering system ZF Servocom,
type 8099 (right-hand).
Normal function for right-hand turning.
Both control valves in working position.
Presssure buildup in steering circuit II
delayed. All pumps deliver oil.
16
ZF Dual-circuit steering system
1
2
3
4
5
6
7
8
9
Steering pump 1
Standby valve
Valve block
Control valve 1
Power cylinder
Steering pump 2
Flow indicator
Oil reservoir 1
Steering pump 3
10
11
12
13
14
15
16
17
Oil reservoir 2
Flow indicator
Control valve 2
Power cylinder
Steering limiter valve
Radial groove
Radial groove
Piston position indicator
Steering circuit I
Steering circuit II
8
6
1
10
9
7
11
3
4
2
12
17
15
16
5
14
13
ZF Dual-circuit steering system
17
Steering circuit I
Steering circuit II
8
6
1
10
9
7
11
3
4
2
17
12
15
16
5
14
Schematic representation of the dual-circuit
steering system ZF Servocom, type 8099
(right-hand).
Emergency function for left-hand turning.
Both control valves in working position. Only
the wheel-driven steering pump delivers oil.
18
ZF Dual-circuit steering system
13
1
2
3
4
5
6
7
8
9
Steering pump 1
Standby valve
Valve block
Control valve 1
Power cylinder
Steering pump 2
Flow indicator
Oil reservoir 1
Steering pump 3
10
11
12
13
14
15
16
17
Oil reservoir 2
Flow indicator
Control valve 2
Power cylinder
Steering limiter valve
Radial groove
Radial groove
Piston position indicator
Only in an emergency, which is very
rare, for instance when the engine has
stopped, the system is limited to the
safe operation of steering circuit I. Due
to the lack of oil flow from the stopped
steering pump 1, the standby valve is
not moved, which means that the pressure oil flow generated by the wheeldriven steering pump 2 is now directed,
through the standby valve, to the control valve 1 and can thus be effective in
the steering gear and power cylinder.
At the same time, the standby valve
prevents the outflow of oil through the
flow indicator and indicates that condition to the driver by means of a pilot
lamp. Additionally, a piston position
indicator (17) monitors the function of
the standby valve. A flow indicator between the control valve 2 and the oil
reservoir 2 notifies the driver, by means
of a further pilot lamp, of the lack of
oil flow. When the pilot lamps go on,
an early check and maintenance of the
steeering system will be necessary for
safety reasons.
in the same manner as on the standard
single-circuit steering gear (see pages
4 to 7).
In steering circuit II, however, the individually determinable reduction of
pressure in the power cylinder is
controlled by two adjustable steering
limiter valves (14). These are arranged
in the housing cover of the steering
gear and are opened by a cam of the
sector shaft. Via pipes, both valves are
connected to the respective radial
grooves (15 and 16) of control valve 2
and thus, indirectly, also to the lines to
the power cylinder.
Hydraulic steering limiter
To protect the steering linkage, wheel
lock stops and steering pump from
excessive loads, both steering circuits
are usually equipped with a hydraulic
steering limiter. The hydraulic steering
assistance in steering circuit I is limited
Installation schematic of a ZF Servocom
dual-circuit steering system with 3 pumps,
2 oil reservoirs, valve block, flow indicator,
power cylinder, steering column and
ball-track relay shaft.
ZF Dual-circuit steering system
19
1
ZF Servocom® RAS
Rear Axle Steering
System
5
2
System description
3
The rear axle steering system ZF Servocom RAS (Rear Axle Steering) is optimized both with regard to cost and
weight and is particularly suitable for
the forced steering of nondriven rear
axles. The steering work is done by the
front axle steering system (1) and a
power cylinder portion in the master
cylinder (2).
4
Up to now, comparable systems have
predominantly been implemented
according to the mechanical principle,
consisting of drag links, intermediate
arms, etc. Such mechanical transmission systems need a lot of installation
space because of the swivelling ranges
of the levers and drag links. In addition, a fairly heavy weight is typical of
such systems. Another disadvantage is
the inadequate steering stiffness, i.e.
because of the elasticity of the transmission parts the steered rear axle is
not stabilized enough and is prone to
oscillations during straight line driving.
Compared to mechanical steering systems, the rear axle steering system ZF
Servocom RAS achieves better results
in many respects.
Schematic representation of the rear axle
steering system ZF Servocom RAS, type 8098.
1
2
3
4
5
20
ZF Servocom RAS
The content of the RAS system is basically made up of 2 special power cylinders and a hydraulic accumulator (3).
In conformity with the steering angle
movement, the master cylinder (2) fitted at the front axle pumps oil into the
associated cylinder chamber of the
centering cylinder (4) arranged at the
the steered rear axle. In this way, the
steering movement of the front axle is
ZF Servocom
Master cylinder
Hydraulic accumulator
Centering cylinder
Additional lines
hydrostatically transmitted to the rear
axle. The steering angle ratio between
front axle and rear axle is determined
by the vehicle manufacturer by rating
the length of the steering arms appropriately.
The master cylinder is fitted with an
automatic synchronizer system and a
pressure relief valve. In a definable
steering angle range of the front axle,
e.g. up to about ± 5°, both cylinder
chambers of the master cylinder are
shortcircuited. Within this small steer-
ing angle range of the front axle, the
rear axle is put and kept in straight
ahead position with the help of a
hydraulic centering device in the centering cylinder. Owing to this automatic synchronization, hydrostatic mismatches in the cylinders, which seldom
occur, can be compensated for.
At steering angles of more than about
± 5° at the front axle, the cylinder
chambers of the master cylinder are
separated from each other and sealed
hermetically. This means that at steering angles at the front axle exceeding
± 5° the hydrostatic steering system is
in operation. A control system in the
master cylinder prevents that in
straight ahead position of the front
axle and at high actuating forces at the
steered rear axle, caused for example
by driving over obstacles in forward
or reverse, the kinematic conditions
change considerably.
An accumulator is assigned to the
hydrostatic system. The function of this
accumulator is to optimize the stiffness
of the hydostatic transmission system
by preloading the transmission lines
with the centering pressure. Besides,
the accumulator pressure acts permanently on the two hydraulic centering
pistons which are integrated into the
centering cylinder. Experience has
shown that the centering and preload
pressure is approximately 15 bar and
thus meets the safety criteria required
for driving without hydraulic assistance. If due to the extra steering work
performed by the rear axle the steering comfort becomes too low, i.e. the
hydraulic pressure reserve becomes
too small, the master cylinder can be
subjected to steering pressure via additional lines (5) in order to provide sufficient steering comfort, e.g. during
static steering.
The ZF Servocom RAS concept does not
need additional electric and electronic
components and is also suitable for
later installation because of a small
variety of parts. What has to be stressed particularly is the improvement in
vehicle dynamics during straight line
driving due to the hydraulically centered rear axle. Better maneuverability, less tire wear, and fuel saving are
further assets of the system.
2
4
ZF Servocom RAS
21
ZF Servocom® RAS-EC
Rear Axle Steering System
Application
Modern commercial vehicles have to
correspond more and more to the
requirements of the clients for economy, environmental protection, legislation and special applications. To this,
the electronically controlled rear axle
steering system ZF Servocom RAS-EC
(Rear Axle Steering – Electronically
Controlled) makes an important contribution and at the same time it offers
a basis for entirely new solutions for
1
2
3
5
the development of commercial vehicles. That's because the ZF RAS-EC is
particularly suitable for commercial
vehicles with a very big wheelbase and
several rear axles.
The increasingly heavy traffic as well as
modern city planning concepts require
commercial vehicles with a big loading
capacity and good maneuverability in
order to supply and dispose of the
goods for the city centers. The active
rear axle steering system increases the
maneuverability of the vehicle, particularly during parking maneuvers,
because here, in contrast to adhesionsteered axles, steering operations
are also carried out when the
vehicle is reversed. Owing to
the optimum kinematic
conditions, tire wear is
reduced and the traction at this axle is
4
increased.
The
forced steering of
rear axles offers
6
7
9
8
the additional advantage of an immediate build-up of side force and therefore leads to an increase in vehicle stability and driving safety.
The RAS-EC does not require a mechanical connection between the front and
rear axles, which makes the adaptation
in series production as well as retrofitting a lot easier. The possibility of endof-assembly-line programming of the
electronic control opens up an adaptation to changed geometric quantities
(wheelbase, steering angle) or to a
particular application pattern of the
vehicle. Because of that, no modification of mechanical power transmission
components is necessary.
Due to the electronic control, the steering angle at the rear axle can be altered as a function of whatever driving
parameters are chosen. This means for
example that by opposite steering
motions of rear and front wheels in
the low speed range maximum maneuverability can be achieved. Same-direction rear/front wheel steering motions
at medium and high speeds lead to
exact straight line driving and vehicle
stabilization during lane changes.
Special steering programs, such as rear
swing-out minimization or constant
ratio with same-direction rear/front
wheel steering motions in the low
speed range, additionally increase the
10
11
22
ZF Servocom RAS-EC. Basic diagram
for vehicle 6x2x4 with steered trailing
axle (nondriven).
12
ZF Servocom RAS-EC
1
2
4
3
advantages of the RAS-EC. The electronic control of the rear axle steering
system is provided with a diagnostic
interface (KWP 2000). Dectected errors
are stored in the memory and can be
read out for service and/or for troubleshooting purposes.
5
6
7
System description
9
8
The requirements for rear axle steering
systems are described in the directive
EC 70/311. When an error in the steering system occurs, no safety critical driving situation is permissible.
Derived from this, the safety concept
for the RAS-EC is as follows:
13
10
• For nondriven axles, with self-centering characteristics the ”axle free“
safe mode is recommended, i.e. the
axle will then work like a trailing
axle.
Precondition for this safety philosophy
is a redundant electronic system so
that system errors are detected and
the appropriate safety reactions are
initiated.
Mechanical construction
Hydraulic components
On a ZF RAS-EC, the operating circuit
is supplied with pressure oil from an
engine-driven pump. The required
1
2
3
4
5
6
7
ZF Servocom
Steering angle sensor
CAN-Bus
Diagnosis
Oil reservoir
Steering pump
Pressure filter
steering movement of the rear axle is
introduced by a proportional valve
connected to the electric control unit.
The necessary steering forces are generated by a hydraulic power cylinder at
the rear axle. By means of a pressure
relief valve the components are protected from overload. If an error occurs, the pressurization of the operating circuit can be removed with a
cut-off valve which is switched through
in the zero-current condition.
Sensors
8
9
10
11
12
13
ZF Servocom RAS-EC. Basic diagram
for vehicle 6x2x4 with steered leading
axle (nondriven).
Control valve
Electronic control unit
24V supply
Power cylinder
Turning angle sensor
Power cylinder
with linear sensor
For the measurement of the instantaneous values at the front and rear axles
non-contact sensors are used. These
sensors have a long service life, operate over a large temperature range
and are accommodated in a watertight
housing. To make it possible to check
the proper function of these sensors,
they are of redundant design and
intrinsically safe.
ZF Servocom RAS-EC
23
I Steering angle I rear axle [°]
VA_LW_max[°]*
12
max. steering angle rear axle*
0-15 km/h
V [km/h]
10
8
20 km/h
6
25 km/h
4
2
Vneutral [km/h]*
0
0
10
20
30
40
* These parameters can be programmed at the end of the assembly line.
Electrical structure
The control unit is a system with 2
channels with mutual monitoring of
the readiness for operation of the
channels and includes all the components necessary for sensor evaluation,
nominal-value calculation and valve
actuation.
The control circuit basically consists of
a 16-bit high-capacity microcontroller.
This computer acquires all incoming
data and carries out a plausibility
check. Using a graph of steering angle
characteristics, the nominal value of
the rear axle is generated from the
vehicle speed and the front axle steering angle data. Then, from the nominal value and the steering angle of the
rear axle, the quantity for the actuation of the control valve is computed.
The valves are actuated via short-circuit-proof outputs and their function is
then reread and checked by the microcontroller.
In order to adapt the function of the
rear axle steering system to the vehicle
conditions additional sensors can be
ZF Servocom RAS-EC. Basic diagram of a
steering angle characteristic curve.
24
ZF Servocom RAS-EC
50
Steering angle
front axle [°]
read in. Thus, due to the axle geometry, actuation of the rear axle is not
possible when the vehicle is at a standstill and the brake is applied at the
same time. This for example can be
noticed when the control unit also
reads in the brake pressure. In this
case, the rear axle is not actuated until
either the brake has been released or a
speed is measured.
The safety circuit includes an 8-bit
microcontroller. This device also acquires all incoming data and carries out
a plausibility check. The nominal value
of the rear axle calculated by it is compared with the actual value. The control computer of the operating circuit
and the monitoring computer of the
safety circuit are connected with each
other via an interface and can thus
exchange and compare input, intermediate and output quantities. Additionally, status information is exchanged via this interface in order to guarantee and check the correct function of
the rear axle steering gear. If an error
is detected, both the control computer
and the monitoring computer are in a
position to automatically switch off
the outputs and thus initiate the safe
mode. For the localization of errors
and/or for the maintenance of the rear
axle steering system, the control unit
has full diagnostic capability. Thus, the
values from the sensors for instance
can be read out and checked via the
diagnostic interface (KWP 2000). An
integrated CAN interface allows data
exchange with other control units
during operation. By means of the
integrated diagnostic interface the
control unit can be programmed at the
end of the assembly line, and during
vehicle production it can be adapted
to the conditions on the target vehicle.
If necessary, this can for instance be
used to carry out an electronic alignment of the sensors.
Special functions
The system allows the processing of
numerous signals such as those from
brake, door switch and other switches.
Together with the speed signal, the
front axle steering angle etc. it’s possible to implement a variety of special
functions.
ZF Semi-Integral Power Steering Gears
Application
Steering gears of this type are used on
vehicles which require high steering
effort due to their high steering axle
load and on which the required piston
displacement exceeds the volume
which can economically be accommodated in the power cylinder of a steering gear of integral design. Application begins at steering axle loads of
approx. 8 tonnes on vehicles with king
pin steering.
Another possibility for the use of semiintegral power steering gears exists
when, due to its length or offset, the
drag link is unable to transmit the
required steering forces. The number
and size of the power cylinders used
can be chosen such that at the maximum steering forces which occur and
at the required steering wheel turning
rate the vehicle can be steered with
full hydraulic assistance.
turning movements, thus causing the
sector shaft to rotate.
The control valve is located in the valve
housing concentrically with the input
shaft. When the worm shaft is rotated,
the control valve is moved axially back
and forth. This will displace the control
edges such that the pressure oil gets
from the steering pump to one power
cylinder chamber. When the steering
wheel is released, the valve is returned
to its neutral position by spring force;
the return flow is thus maintained. The
valve housing is also provided with the
connections for the pressure and return lines as well as for the lines to the
power cylinder.
Mechanical steering limiter
Depending on customers’ requests, the
semi-integral power steering gears can
be fitted with mechanical steering
limiter. It will avoid that the wheels are
turned up to the wheel lock stops with
full hydraulic pressure. This protects
the steering linkage components from
excessive loads.
Design and function
The semi-integral power steering gear
(single-circuit design) comprises a complete mechanical steering gear. In it,
the steering effort is transmitted from
the input shaft, via a ball screw thread,
to the steering nut and from there, via
a tooth system, to the sector shaft. The
steering nut is moved up and down by
Installation schematic of a semi-integral
power steering gear type 7421, dualcircuit design on a mobile crane. With
engine-driven vane pump, wheel-driven
radial piston pump and with one power
cylinder each at the steering axles.
ZF Semi-integral power steering gears
25
Semi-integral power steering
gear, dual-circuit design
Extra-heavy and special-purpose vehicles with very high steering axle loads
and speeds in excess of 62 km/h cannot
usually be steered without exceeding
the steering effort limits prescribed by
law if hydraulic assistance fails. For
such applications our dual-circuit steering gears of semi-integral design with
two independent steering valves for
the control of the pressure oil in two
entirely separate circuits are available.
Vehicles thus equipped can still be
steered fully even if pressure oil is lost,
for instance due to pipe breakage in
one steering circuit. Normally, one circuit of the steering system is fed by an
engine-driven pump and the other by
a wheel-driven steering pump.
the steering circuits is provided with a
wheel-driven emergency steering
pump whose output flow is directed,
under normal operating conditions of
the two engine-driven pumps, into the
return flow to the oil reservoir. If the
engine-driven pump fails, the pressurized oil from the emergency steering
To allow the wheels to be turned even
if the engine is stopped, pressure oil
supply to the two circuits by one engine-driven steering pump each has
been provided for. Additionally, one of
Figure at top:
Semi-integral power steering gear, type 7421,
single-circuit design with mechanical steering
limiter and flange-mounted bevel box.
26
Semi-integral power steering gear, type
7421, dual-circuit design with mechanical
steering limiter.
ZF Semi-integral power steering gears
pump is automatically supplied to the
steering gear via a standby valve, with
the result that vehicle steerability is
maintained in any event. Thus, dualcircuit semi-integral power steering
gears are an important contribution to
safety in road traffic.
ZF Steering Pumps
Application
The main function of a steering pump
is to generate a sufficient amount of
the oil flow required for operating a
hydraulic steering system. Convincing
advantages which suggest the consistent installation of steering pumps
from ZF Lenksysteme GmbH on commercial vehicles are in particular the
compact design, the high efficiency at
low weight as well as the possibilities
of individual adaptation due to modular system configuration. Depending
on the type of pump, such pumps can
be attached to the vehicle engine or
the compressor. For driving them, different elements of transmission such as
V-belt pulleys, cross-slotted discs or
gears are utilized. Radial piston pumps
which act as emergency steering
pumps are wheel-driven, i.e. by an axle
or the gearbox output.
pressure plate. These sealing faces
have two suction and pressure zones
opposite to each other. Thus, during
each revolution the ten chambers formed by the vanes located in the rotor
deliver an amount of oil which is twice
their chamber volume. Also, this double
1
2
arrangement of the suction and pressure zones neutralizes the radial
hydraulic loads acting on the rotor. The
pressure between the vanes and the
cam ring, which is decisive for function, is produced by the radial centrifugal force of the vanes when the
6
7
ZF Vane pump FN 4
On this type of pump, the pumping
element which basically consists of a
rotor (3), ten vanes (4), a cam ring (5)
and the pressure plate (6) is accommodated in the light-alloy housing (1).
The axial end is formed by a cover (7),
which is also made of light alloy, with
a sealing face made to conform to the
8
5
4
3
Representation of the function of the
ZF vane pump FN 4, type 7685.
1
2
3
4
5
6
7
8
Housing
Shaft
Rotor
Vane
Cam ring
Pressure plate
Cover
Pressure relief and flow limiting valve
ZF Steering Pumps
27
load. With a view to technical necessity and economic volume of production, it is also possible to integrate into
this pump type design features which
allow radial drive loads, for example
via a gear, and a pressure level of 200
bar maximum.
drive shaft is rotated. Additional pressure is achieved by pressurizing the
inside faces of the vanes with pressure
oil.
It is also possible to mount the oil
reservoir directly on the pump. This
saves a hose and assembly costs at the
vehicle manufacturer.
The oil flow generated in the crescentshaped pressure chambers is supplied
to the flow limiting valve (8) positioned longitudinally to the shaft (2) and
limited at a set value. If relief of the
type-specific maximum pressure of the
pump (165 or 180 bar respectively) is
not carried out within the steering
gear as is recommended, this can in a
specific instance be done by means of
a pressure relief valve integrated in the
flow limiting valve.
28
ZF Steering pumps
The basis for the modular-design pump
is the short-length light-alloy cover. It
includes a pumping element which
features two pressure plates and is
equal in its principle of hydraulic operation to the vane pump FN 4 described
above. The cover also incorporates the
flow limiting valve (if required, with
integrated pressure relief), arranged at
right angles to the drive shaft, and the
suction and pressure ports. The pressure port is optionally feasible on the
right or left. The symmetrical flange
bolt pattern of housing and cover
allows fitting in any one of 4x90° positions.
The variable use of this pump is also
based on the rugged shaft bearing
system in the light-alloy housing. The
pump is perfectly suitable to safely
absorb the axial and radial drive loads
if a clutch disc, V-belt pulley or gear is
used. In a gear drive, the drive-side
antifriction bearing can be lubricated
by engine oil.
The design of the vane pump FN 4 is
mainly intended for attachment to the
air compressor or to the generator by
means of a cross-slotted disc for the
transmission of torque free from radial
Figure at top:
ZF Vane pump FN 4, type 7685.
Longitudinally fitted pressure relief and
flow limiting valve.
ZF Vane pump FN 31
Right-hand figure:
ZF Vane pump FN 31, type 7685.
With short-length housing and transversely fitted flow limiting valve.
ZF Vane pump FN 32
ZF Radial piston pump
The use of this pump, which is largely
identical to the FN 31 range, is appropriate in applications where a comparatively high displacement is needed.
The theoretical displacement is 32
cm3/revolution.
The same direction of delivery of the
radial piston pump for both clockwise
and counterclockwise direction of drive
is the reason for its principal application as a wheel-driven emergency
steering pump on commercial vehicles.
Besides the high pressure level of 200
bar maximum, the suction control is of
particular importance. It ensures that
the output flow remains the same in
spite of varying pump speed and only
that amount of oil is sucked in which is
necessary for steering. Thus, no flow
limiting valve is required.
ZF Tandem pump TN 4
This pump combination consists of a
vane pump FN 4 and a gear pump
whose housings are bolted together.
Both systems are driven by the same
pump shaft, but generate two independent output flows. While the vane
pump is used for the pressure supply to
the hydraulic steering system, it is the
function of the gear pump to ensure
the presupply of fuel. The drive, free
from radial loading, is by the air compressor.
Depending on the type, several pistons
are driven by an eccentric shaft supported on antifriction bearings and perform a stroke in radially arranged cylinder bores. Preloaded compression
springs provide for the return of the
pistons. In the process, the pistons are
immersed in the oil-filled suction chamber and their interior can be filled with
oil through transverse bores. The subsequent discharge stroke forces the oil
into the pressure line. Automatic outlet
valves prevent the pressure oil from
flowing back into the piston chamber.
Depending on type and application,
pressure relief is by a valve either in the
pump or at some other suitable location of the hydraulic system.
ZF Tandem pump TN 31
The basis of this pump type is the vane
pump FN 31 and a gear pump. Again,
both housings are bolted together,
and both systems are driven by the
same pump shaft. Two independent
output flows are generated. The pressure oil from the vane pump is again
intended for the supply to hydraulic
steering systems, whereas the gear
pump is used for the presupply of fuel.
ZF Radial piston pump, type 8605,
with 8 cylinders and suction-controlled
output flow.
ZF Steering pumps
29
ZF Oil Reservoirs
1
The oil reservoirs designed by ZF
Lenksysteme GmbH for commercial
vehicles are matched to the specific
requirements of such vehicles, which
offers the manufacturers the complete
peripheral equipment for hydraulic
steering systems from a single source.
2
Depending on type and application,
the oil reservoirs made of heat-resistant plastic or sheet steel have a filling
capacity of 0.75 to 2.3 dm3. Plastic oil
reservoirs are fitted with a filter with
integrated filter safety valve. The
sheet-steel oil reservoir also has, in
addition to a micro-filter cartridge, a
safety valve, and for special applications it can be provided with a replenishing valve and standby capacity.
For oil level checks, the dipstick is marked to show the minimum and maximum levels. All oil reservoirs can alternatively be fitted with an electric
oil level indicator. In this case, a pilot
lamp on the dashboard indicates when
the oil level drops to the specified minimum. And by using a transparent
reservoir top a rapid visual check can be
made from outside.
3
1 ZF Oil reservoir, type 7632. Of plastic,
with electric oil level indicator, capacity
approx. 0.75 dm3.
2 ZF Oil reservoir, type 7636. Of sheet
steel, with micro-filter cartridge and
safety valve, capacity approx. 1 dm3.
3 ZF Oil reservoir, type 7636. Of plastic,
capacity approx. 1.5 dm3.
30
ZF Oil reservoirs
ZF Power Cylinders
As well as being used in industry and in
toolmaking and machine building,
double-acting power cylinders are
indispensable for operating semi-integral power steering gears, dual-circuit
steering systems or ZF Servocom RASEC rear axle steering systems. They are
also used to assist hydraulic power
steering gears if the torque of the steering gear alone is not sufficient. Various
sizes and different types of mounting
with thread, ball joints, articulated
support or end fitting are decisive
for the universal possibilities of application of this component.
The inner diameter of the cylinder
tube of the power cylinders is designed
as a slide face for the piston, and the
cylinder tube is closed on one side by a
weld-on end piece. On the opposite
side, a piston rod guide is used whose
function is to seal and guide the piston
rod. The piston attached to the piston
rod separates the two cylinder chambers and, depending on pressurization,
displaces the piston rod or maintains
its position. Pressure oil is supplied to
the two cylinder chambers through
the lines fitted on the threaded connectors. A high degree of corrosion
protection is achieved by using a corrosion-resistant piston rod and an
electro-zinc-plated cylinder surface.
The dual-circuit cylinder is preferentially used in confined installation spaces. Integrated in it are, in effect, two
power cylinders which can work either
synchronously or individually.
ZF Power cylinder, type 8346,
standard range.
ZF Power cylinders
31
ZF Steering Columns
Application
There is no doubt that an ergonomically designed driver's place on trucks,
buses and special vehicles has favorable effects on the driver's well-being.
The use of a ZF steering column which
allows continuous adjustment of
height and tilt of steering wheel, coupled with further comfort functions, is
especially advantageous.
32
Requirements and design
Features
Advantages
Continuously adjustable
height and tilt of steering
wheel
Ergonomic driver’s
place
Pneumatic assistance of the
positioning device
Low adjusting forces
Clamping is achieved by spring force
Safe locking
Integration of steering wheel lock,
steering column switches, clock spring
and steering angle sensor
Reduced costs for logistics and
assembly at the customer
Dashboard can be attached
around the steering column
Optimum ergonomic position
of the display
Free of play
Improved comfort and
safety
Maintenance-free
No servicing required
Pedal box can be
integrated
Reduced costs for logistics and
assembly at the customer
ZF Steering columns
Decades of experience in the manufacture of steering columns have enabled
us to meet the requirements detailed
below to a very high degree and to
improve on these by innovative design
work:
• Low friction so as not to adversely
affect the self-centering characteristics of the steering gear and to allow
hysteresis-free transmission of the
input torques.
• Optimum arrangement of the universal joints in order to minimize input torque variations while steering.
• Minimal sliding forces in the balltrack telescopic shafts or ball-track
relay shafts to avoid unwanted
vibrations at the steering wheel.
• Length compensation to accommo-
ZF Steering column, type 7360.
With light-alloy mounting bracket and
externally mounted gas-filled spring.
7
1
8
2
9
10
date the relative movement between the driver's cab and the chassis because of tilting truck cabs, possible conditions of elasticity on
buses and height and tilt adjustability of the steering column.
• For reasons of comfort, the forces
for tilt and height adjustment of the
steering wheel must be low.
• No maintenance during the service
life of the vehicle.
Depending on the specific application,
the mounting bracket is made of light
alloy or sheet steel. During the adjustment process, the necessary weight
compensation of the steering unit
with steering wheel, steering column,
switch and, if applicable, dashboard, is
made possible by a gas-filled spring
integrated into the ball-track relay
shaft or by a cylindrical compression
spring. It is also possible to fit the gasfilled spring between the mounting
bracket and the inner panel.
ZF Steering column, type 7360.
Installation schematic of the height
and tilt adjustable steering column
with steering gear.
11
12
3
4
13
5
14
6
Further design features and the advantages resulting from these can be seen
from the table on page 32.
1
2
3
4
5
6
7
Dashboard
Adjusting unit
Mounting bracket, fixing
Pedal
Ball-track relay shaft
ZF Servocom steering gear
Tilt angle
8
9
10
11
12
13
14
Height adjustment
Clock spring ”integrated“
Steering angle sensor ”integrated“
Ignition and starting switch
Universal joint = tilt axis
Gas-filled spring
Drag link
ZF Steering columns
33
ZF Bevel Boxes
ZF Bevel boxes are used on vehicles
where a direct connection to the steering column is not possible due to the
arrangement of the steering gear.
They can be flange-mounted directly
to the steering gear or be delivered for
separate installation. The standard
bevel box has a shaft angle of 90° and
a weight of only 2.1 kg. Special versions
with a 77° angle or a double output
drive are also available.
1
2
3
ZF Bevel box, type 7860.
Installation schematic with horizontally
fitted steering gear.
34
ZF Bevel boxes
The input and output shafts of the
bevel boxes are carried in antifriction
bearings. Both shafts carry a straight
bevel gear with the same number of
teeth, which means that the ratio is
1:1. Especially beneficial effects on
noise and wear performance has the
lifetime lubrication of the mechanical
ratio system integrated in a sturdy
light-alloy housing.
4
1
2
3
4
Adjustable steering column
Ball-track relay shaft
Bevel box
ZF Servocom steering gear
Figure at top:
ZF Bevel box, type 7860.
Standard design of light alloy and filler
neck for lifetime lubrication.
ZF Ball-Track Telescopic Shafts
and Ball-Track Relay Shafts
An important link between the steering wheel and steering gear is the balltrack telescopic shaft or the ball-track
relay shaft which is complemented by
universal joints. Their installation has
the advantage that the relative movements existing in driving between the
driver's cab and the chassis do not
adversely affect steering performance.
The total length of the low-wear and
maintenance-free component can be
adapted to the specific vehicle or complemented by an extension tube. To
save weight, the extension tube and
the universal joints can alternatively be
made of light alloy. Also, a comfortable weight compensation for installa-
tion in an adjustable steering column is
achieved by integrating a gas-filled
spring or cylindrical compression
spring into the ball-track telescopic
shaft/ball-track relay shaft.
The major components of the balltrack telescopic shaft / ball-track relay
shaft developed by ZF Lenksysteme
GmbH are a drive shaft with internal
ball tracks and a profiled shaft with
external ball tracks. Two axially arranged rows of balls provide the noisefree connection. This design ensures a
radially play-free, but axially easily
movable connection with an available
stroke of ± 28 mm.
ZF Ball-track relay shaft, type 7025.
ZF Ball-track telescopic shafts, ball-track relay shaft
35
ZF Universal Joints
The use of ZF universal joints ensures a
safe transmission of torques and a high
degree of compensation for misalignment which exists between a separate
steering column and the steering
gear or bevel box respectively. High
strength reserves at an available flexing angle of 68° and a maximum diameter of rotation of only 76 mm are
characteristic of this component.
All universal joints are carried in needle
bearings, are play-free and maintenance-free and, thus, meet the requirements of good steering returnability,
low rotation torques and absolutely
minimal maintenance. Different possible connections such as cylindrical
serrations, internal thread or flange
joint design admit individual adaptation of the installation. The various
joint forks are made of wrought steel
and can be combined optionally.
For protection against corrosion the
surfaces of the universal joints are
electro-zinc-plated.
ZF Universal joint, type 7026.
Connection with cylindrical serrations.
36
ZF Universal joints
ZF Sensors
1
The increasing use of electronically
assisted or controlled steering systems
requires the development and manufacture of special adaptable sensors. In
order to meet the high safety standards for steering systems, our steering
angle and turning angle sensors use a
non-contact inductive measurement
principle. They can be used up to an
effective angle of 100°.
New methods are also available by the
integration of a linear sensor on power
cylinders. The application of a noncontact magnetic principle of measurement means that stroke is measured
directly where the movement is generated. This possibility of comparing
specified and actual values allows calculation and control of the piston rod
stroke with millimeter accuracy.
Due to their high flexibility, these versions offer ideal conditions for the installation in rear axle steering systems.
The best degree of integration is
achieved by the direct adaptation of
the steering angle sensor on the
steering gear. The turning angle sensor
can be positioned in many suitable
places on the vehicle and can also be
used for other applications. The least
loss of installation space is caused by
the linear sensor integrated on a
power cylinder.
2
3
1 ZF Power cylinder, type 8346.
With integrated linear sensor and
ball joints.
2 ZF Turning angle sensor, type 8353.
For universal installation and use.
3 ZF Steering angle sensor, type 8353.
For attachment to the steering gear.
ZF Sensors
37
ZF Pressure Filters
The use of electronically controlled
valves increasingly requires additional
measures to meet with the high expectations regarding the reliability of
steering systems. That's because major
particles in the hydraulic fluid may
result in limited control function. This
can be counteracted effectively by
fitting a pressure filter from ZF Lenksysteme GmbH.
The pressure filter which can be used
up to 210 bar rated pressure is of compact design and offers flexible solutions for attachment. The contamination absorbing capacity can be controlled individually by means of different filter elements, depending on
requirements. Filter elements with
filtration grades of 3, 5, 10 and 20 µm
absolute are available. Depending on
the filter element used, the maximum
permissible flow rate is between 32
and 50 dm3/min.
Differential pressure [bar]
2.0
Figure at top:
ZF Pressure filter, type 8480.
Right-hand figure:
ZF Pressure filter, type 8480.
Performance characteristics on the
complete filter, as a function of the
differential pressure.
38
ZF Pressure filters
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
10
Filtration
Filtration
Filtration
Filtration
20
grade
3 µm
grade
5 µm
grade 10 µm
grade 20 µm
30
40
50
60
Flow rate [dm3/min]
ZF Valves
Due to their different functions and
designs, valves from ZF Lenksysteme
GmbH are perfectly suited for complementing the hydraulic management of
hydraulic steering systems ideally.
That's why they are chiefly fitted on
commercial vehicles which are equipped with a dual-circuit steering system
or a ZF semi-integral power steering
gear. But they are also used in industry,
in toolmaking as well as in machine
building and vehicle manufacture.
Flow indicators
Valve block
In hydraulic steering systems with
emergency steering properties, great
importance is attached to the monitoring of the oil circuit for safety reasons. A flow indicator fitted between
the steering pumps and the steering
gear indicates to the driver the lack of
oil flow by a pilot lamp that goes on.
In this component, standby valve, flow
indicator and – optionally – a pressure
relief valve or piston position indicator
are combined into a single unit. The
advantage of this space-saving block
design compared to individual valves
is easier assembly and installation of
pipes.
Standby valves
They are fitted for maximum pressure
relief in hydraulic systems and to protect the system from excessive
loads. Designs with their own
housings or valve inserts
designed for screwing
into existing housings
are available.
The standby valve coordinates the oil
flow between two steering pumps and
directs it to the steering gear via a
hydraulic line. In normal operation, a
valve spool located in the housing is
displaced by the main pump output
flow up to the stop position after it has
overcome a spring force and opens the
supply flow of this pump to the steering gear. At the same time, this position of the valve spool determines the
return to the oil reservoir of the oil
flow generated by an emergency steering pump. The oil delivered by the
emergency steering pump can get to
the steering gear only if the valve
spool is in its initial position.
Pressure relief valves
1
2
3
4
Figure showing various ZF valves.
1
2
3
4
5
Standby valve, type 7734
Valve block, type 7760
Valve insert, type 8470
Pressure relief valve, type 7753
Flow indicator, type 7780
5
ZF Valves
39
ZFLS 8090P-MBA 8/02e · Printed on paper bleached without use of chlorine
ZF Lenksysteme GmbH:
the systems partner
ZF Lenksysteme GmbH is one of
the largest independent manufacturers of power steering systems for passenger cars and commercial vehicles. Renowned automotive manufacturers from all
over the world value us as a creative and efficient systems partner for the development of new
and innovative solutions.
As a joint venture of Robert Bosch
GmbH and ZF Friedrichshafen AG,
ZF Lenksysteme GmbH offers its
customers a unique source of expertise when it comes to integrating a wide range of top technologies in modules, system modules or entire chassis systems.
The benefits for the manufacturer are clear to see: even shorter
development times and optimized production processes –
with quality standards which just
get better and better.
[Steering the right way]
A Joint Venture of
Robert Bosch GmbH and
ZF Friedrichshafen AG
Technical modifications reserved.
ZF Lenksysteme GmbH
Richard-Bullinger-Strasse 77
D-73527 Schwäbisch Gmünd
Germany
Phone: + 49 (0)71 71 / 31- 0
Telefax: + 49 (0)71 71 / 31- 32 22
www.zf-lenksysteme.com