ZF Rack and Pinion Power Steering Gears

Transcription

for Passenger Cars and Light Commercial Vehicles
Steering the right way.
Millions of vehicles.
We can do the steering.
little maintenance as possible and, at
As vital for the vehicle as the engine
low speeds.
the same time, offer a maximum of
safety and comfort at both high and
and the wheels, as important for
safety as the brake – the steering
When it comes to safety potential,
system is at the heart of every vehicle.
the power steering gear really comes
Whether it be a question of ope-
into its own at high speeds. Abrupt
ration, safety or ride comfort, one
steering corrections – when maneu-
thing is clear – the overall quality of
vering to avoid an obstacle, for
the vehicle also depends on the
example – are easy to master thanks
quality of the steering. The require-
to the power assistance provided by
ments are clearly defined – the
the servo mechanism. Also, the
steering must be precise and free
hydraulic system will compensate for
from play, easy to operate and direct,
the most part for sudden, one-sided
compact
weight.
steering forces. The driver is able to
Moreover, it should give the driver a
and
light
in
react to the surprise, keep the vehicle
good "feel" of the road and favor the
on track and bring it to a halt. And all
return of the steered wheels into the
this only requires a little extra effort
straight-ahead position. It has to
on the steering wheel.
work shock-free, should require as
2
Worth knowing
ZF Lenksysteme. Steering
toward the future.
As a joint venture of Robert Bosch
steering in parking operations and a
independent steering interventions
feeling of safe driving at increasing
without the need to separate the
speed as a result of more stiffness at
mechanical
the steering wheel.
steering wheel and front axle. The
GmbH and ZF Friedrichshafen AG, ZF
connection
between
variable steering ratio coupled with
Lenksysteme GmbH has produced
The ZF Active Steering sets new stan-
this clearly increases driving comfort
power steering systems for passenger
dards for implementing both kinematic
at low speeds.
cars and commercial vehicles for
steering assistance functions and
several decades. The products fulfill
driving stabilizing functions.
every specified requirement. Take rack
It allows driver-
and pinion and recirculating ball
power steering gears, for example.
These
tried-and-tested
steering
designs have been continually further
developed, protected by numerous
patents, and adapted to suit specific
vehicle requirements. The development of the ZF compact power
steering gear involved the forging
of new paths both in valve
design
and
in
production
processes.
One product of this approach
that really stands out is the ZF
Servotronic, an electronically
controlled and speed-sensitive
rack and pinion power steering
gear for passenger cars and light
commercial vehicles. It is characterized by easy, comfortable
Servotronic® is a registered trademark
of ZF Lenksysteme GmbH.
Installation schematic for a ZF compact
power steering gear with steering column,
engine-driven steering pump and oil
reservoir as well as pressure, suction and
return lines.
Worth knowing
3
ZF rack and pinion
power steering gear
with constant or
variable ratio, rotary
valve, integral housing
and end take-off.
Mechanical construction
The ZF rack and pinion power
(6). The rotary motion of the steering
steering gear basically comprises a
wheel is transformed by the pinion
mechanical steering gear, the steering
into an axial motion of the rack and
valve and an integrated power
transferred to the steering arms by tie
cylinder.
rods (8).
The rack (2) with the integrated
The ZF rotary valve is a valve design
piston (3) is guided in wear-free plain
that has proved itself time and time
bearings within the power cylinder of
againin millions of applications. It is
14
the housing (1). The pinion (4), which
used
9
runs in bearings in the valve area,
pressurized oil needed for power
meshes with the rack teeth. The rack
assistance. This valve design mainly
is pressed against the pinion by
comprises the valve rotor (5), which
means of a spring-loaded yoke so
has six control grooves on its surface
that the teeth always mesh without
area, and the valve sleeve (7) securely
play. The pinion is connected with the
connected with the pinion (4).
in
order
to
control
15
the
11
12
13
17
10
16
valve rotor (5) through a torsion bar
1 Housing
2 Rack
3 Piston
4 Pinion
5 Valve rotor
6 Torsion bar
7 Valve sleeve
8 Tie rod
4
9 Feed oil radial groove
10 Feed oil control groove
11 Feed oil control edge
12 Axial groove
13 Return oil control groove
14 Return oil control edge
15 Return oil chamber
16 Radial groove
Rack and pinion power steering gears
17 Radial groove
18 Pressure relief and flow
limiting valve
19 Steering pump
20 Oil reservoir
ZL Power cylinder, left
ZR Power cylinder, right
ZL
3
ZR
1
The valve bore of the valve sleeve has
rotor and pinion run in anti-friction
axial grooves that are matched to the
bearings to ensure the precision of
change its position in relation to the
control grooves of the valve rotor. The
operation and the functional safety of
valve sleeve bore surrounding it, so
centering of the valve rotor (neutral
the steering valve even at high
that the relative positions of the
position) is by means of a conven-
pressures.
control grooves are changed, too.
tional or a positive center feel torsion
bar (6) which is also the connecting
The valve rotor is thereby caused to
Pressurized oil can now flow through
If a torque coming from the steering
pipes to one of the two power
element between the valve rotor, the
wheel or from the steered road
cylinder chambers (ZL or ZR) and
pinion and the valve sleeve. The valve
wheels is transmitted to the valve
assists the axial movement of the rack
rotor or to the securely connected
if the turning motion is initiated from
pinion and valve sleeve, respectively, a
the steering wheel. If, however, the
relative rotary motion influenced by
axial movement of the rack is caused
the torsion bar will occur between
by the road wheels, the steering valve
the valve rotor and the valve
will, in spite of the steering wheel
sleeve.
being held, direct the pressurized oil
to that power cylinder chamber
which counteracts rack axial movement. This braking effect dampens
6
road shocks. When the steering
20
5
wheel is released, the action of the
19
7
twisted torsion bar makes the control
18
16
grooves
return
to
the
neutral
position, and the same system
pressure will exist in both of the
9
power cylinder chambers.
17
15
ZF rack and pinion power steering gear,
type 7852, with end take-off.
Rotary valve in neutral position.
2
4
8
5
6
20
5
13 17
10
ZL
15
7
14
16
9
9
11
17
12
15
19
18
16
3
ZR
1
2
8
4
Basic hydraulic function of
the ZF rotary valve
The hydraulic fluid delivered by the
oil control edges (14), also to the
wheel clockwise (top fig.), the rack
steering pump (19) flows through a
return oil control grooves (13) of the
with the integrated piston will move
connnecting bore in the valve area,
valve rotor. From these grooves the oil
to the right in the piston bore (to the
via the feed oil radial groove (9) and
flows back, via connecting bores, to
left in the installed position on the
transverse bores in the valve sleeve
the return oil chamber (15) and from
vehicle) if the pinion is situated on
(7), onward to the three feed oil
there to the oil reservoir (20). At the
top. Due to the simultaneous rotation
control grooves (10) of the valve
same time, the radial grooves (16 and
of the valve rotor to the right, the
rotor. In the valve neutral position
17) of the valve body and their
pressurized oil is directed, over the
(fig. see page 4/5) the oil flows, over
associated
a
further opened feed oil control edges
the open feed oil control edges (11),
connection between the right-hand
(11), to the three associated axial
to all valve sleeve axial grooves (12)
(ZR) and left-hand (ZL) power cylinder
grooves (12), via bores to the radial
and from there, over the open return
chambers. When turning the steering
groove (17), and, via a pipe, to the
Top figure:
1 Housing
2 Rack
3 Piston
4 Pinion
5 Valve rotor
6 Torsion bar
7 Valve sleeve
8 Tie rod
ZF rack and pinion power steering gear,
type 7852, with end take-off.
Rotary valve in working position, steering
wheel turned clockwise.
6
pipes
provide
for
12 Axial groove
16 Radial groove
17 Radial groove
limiting valve
19 Steering pump
20 Oil reservoir
left-hand power cylinder chamber
Special equipment
The ZF variable ratio rack and pinion
(ZL), whereby the rack movement is
power steering gear with its extra-
hydraulically assisted. An individually
The ZF rack and pinion power
ordinary ease of handling is just as
adaptable
is
steering gear can be fitted with a
suitable for mid-size saloons and light
achieved by the fact that the partially
number of equipment options in
commercial vehicles as for sporty
or fully closed feed oil control edges
order to adapt it in an optimum way
vehicles. It enables precise and rapid
(11) restrict or prevent a connection
to the kinematic and motorization
steeering reactions at higher speeds
between the pressure oil inlet and the
conditions existing on the vehicle.
without running the risk of abruptly
pressure
build-up
other three axial grooves connected
pulling the steering wheel to the right
to the radial groove (16). At the same
Variable ratio
or left, and optimizes the handling of
time, the pressure oil outlet toward
Besides the constant ratio version, the
the vehicle when maneuvering it into
the pressurized axial grooves is
rack and pinion power steering gear
or out of a parking space, when
restricted or prevented, too, by the
can also be made available with a
turning it round in tight areas and
closing return oil control edges (14).
variable ratio. The rack teeth have
during extreme cornering.
The oil displaced by the piston (3)
varying modules and pressure angles
from the right-hand power cylinder
so that, on the one hand, one can
chamber (ZR) first flows through a
ensure that, around center (i.e. in
pipe to the radial groove (16) and,
straight-ahead driving), the steering
through transverse bores, to the
offers the response the driver is
associated axial grooves and onward
accustomed to. On the other hand, as
to the return oil control grooves (13)
the steering wheel turning angles (to
over the further opened return oil
the right and to the left) become
control edges (14). From here, the
greater, the ratio becomes lower and
further return flow of the oil to the oil
the steering in consequence becomes
reservoir (20) takes place via the
more direct. The difference between
connnecting bores leading to the
the lowest and the highest ratio can
return oil chamber (15). When the
be as much as 35%. As a result,
steering wheel is turned counter-
values as unusually low as approx. 2
clockwise, the operating sequence
steering wheel turns from lock to lock
will be analogous to the above.
can be achieved.
Rack teeth in the end regions
Rack travel (mm)
Turn of steering wheel
Steering wheel turning angle
Rack teeth around center
40°
Schematic ratio diagram for ZF rack and
pinion power steering gear with variable
ratio. One full turn of the steering wheel is
shown as 360° on this graph.
d1
d2
20°
7
Fitment of the PCF torsion bar to the
ZF rotary valve.
Positive center feel torsion bar
controlled manner. In order to
The torsion bar is a most important
optimize the steering valve in the
component in the steering valve.
initial-hydraulic-assistance range and,
Its design is decisive for steering
thereby, the steering characteristics,
accuracy and steering feel at different
in particular at higher speeds, ZF
driving speeds. When the steering
Lenksysteme GmbH has developed
valve is actuated, in other words,
the positive center feel (PCF) torsion
when steering wheel motion begins,
bar on the basis of the conventional
the torsional force of the torsion bar
torsion bar (top figure). Besides the
must be overcome. Without this
conventional rotary valves and rotary
resistance the steering valve would be
spool valves, this PCF torsion bar is
too light and, in consequence, could
used with the ZF Servotronic steering
each of which is provided with a
not be actuated in a precise and
valves.
prism-shaped recess at its front end.
Cage-guided
balls
are
arranged
The main elements of this com-
between these recesses. The zero-
pact component are: a torsion bar,
torsion position of the axially pre-
a preloaded, torsion-proof metal
loaded metal bellows spring is
bellows spring, two sleeve-shaped
determined by the prism centering
end pieces and a prism centering
device.
device provided with balls. The
torsion bar is positively connect-
Right at the beginning of the torsion-
ed with the two end pieces.
al motion, the metal bellows spring
The metal bellows spring,
produces a substantial additional
which is arranged coaxially
torsional resistance. As soon as the
with the torsion bar, is fas-
prisms of the two centering pieces are
tened to one of the end pieces
displaced to one another (starting
at one end and to a centering
from a torsion angle of approx. 0.2°),
piece at the other end.
the high metal bellows stiffness is no
longer present, and torsional rigidity
The
prism
centering
device
consists of two centering pieces
Characteristics of a conventional
and a PCF torsion bar.
8
takes a shape similar to the characteristic of the standard torsion bar.
Nominal characteristic curve
excessive loads. A power assistance
such as fixing the steering gear by
This is a curve characteristic that
heavily reduced by the pressure drop
rubber mounts. But it is also possible
covers a very narrow tolerance band.
causes an increase of the steering
to use valve and orifice systems
Conically formed control grooves
wheel input torque and, thus,
(variable orifices) which are inte-
and control edges on the valve rotor
informs the vehicle driver that
grated into the hydraulic system and
and the valve sleeve, coupled with
maximum wheel lock is imminent.
act as dampers.
optimum configuration of the valve
As the hydraulic steering limiter
Full-lock damping
characteristic curve in line with
reduces the power requirement of
If required, ZF rack and pinion power
vehicle manufacturer specifications.
the steering pump, the engine idling
steering gears can be fitted with
controlled assembly technique, allow
speed can be set at a lower value,
flexible
Hydraulic steering limiter
thus saving a considerable amount
the stroke limiters. The damping
plastic
components
on
There are functional, economical
of fuel which, in turn, protects the
effect in front of the metallic stop
and environmental advantages in
environment.
(full lock) combats unwanted noise
at maximum wheel lock.
integrating a hydraulic steering
limiter into rack and pinion power
Steering damping
steering gears. A chassis with
The main reason for increased levels
sufficient self-aligning properties is,
of steering comfort is the fact that
however, a prerequisite for this.
shocks caused by the road and the
chassis cannot be felt on the steering
Specially developed connecting cross
wheel. In particular cases, additional
sections in the cylinder bore enable
technical solutions can be adopted,
an overflowing of the oil from
the cylinder chamber under high
pressure to the return oil chamber
separated from it by the piston, just
before the end of the stroke. The
pressure drop at the lock stop
resulting from this protects the
steering pump and the mechanical
parts of the steering gear from
9
ZF Compact Power Steering Gears
ZF compact power steering
gear, type 7891, with
compact rotary valve,
pressed-on cylinder tube and
end take-off.
Mechanical construction
The ZF compact power steeering gear
This bore is manufactured using a
surface area of the control bush has
is based on a modular design, mainly
special forming technique. The pinion
eight axial grooves that are matched
consisting of three modules – a
is connected with the valve body (7)
to the control grooves in the
mechanical steering gear, a steering
through a torsion bar (6). The rotary
valve bore.
valve and a power cylinder.
motion of the steering wheel is
transformed by the pinion into an
The light metal housing (1) and the
axial motion of the rack. This motion
form-fitted cylinder tube (2) form the
is transferred to the steering arms by
main
tie rods (9).
body
for
locating
the
mechanical ratio system and the
power cylinder. The rack (3) with the
The ZF compact rotary valve (K-valve)
integrated piston (4) is guided in
–
wear-free plain bearings. The pinion
proportion of extremely precise and
(5), which runs in bearings in the
economical production technology –
valve area, meshes with the rack
is used in order to control the
teeth. The rack is pressed against the
pressurized oil needed for power
pinion by means of a spring-loaded
assistance. This valve design mainly
yoke so that the teeth always mesh
comprises the valve body (7), which
without play. A longitudinal bore in
has eight control grooves in the valve
the rack guarantees air compensation
bore, and a control bush (8) securely
between the two bellows chambers.
connected with the pinion (5). The
1 Housing
2 Cylinder tube
3 Rack
4 Piston
5 Pinion
6 Torsion bar
7 Valve body
8 Control bush
10
9 Tie rod
13 Axial groove
Compact power steering gears
manufactured
using
a
high
17 Radial groove
18 Radial groove
limiting valve
20 Steering pump
21 Oil reservoir
16
14
13
10
11
15
18
12
17
ZL
4
ZR
2
The centering of the valve body
The valve body and pinion run in anti-
(neutral position) is by means of a
friction bearings to ensure the
change its position in relation to the
conventional or a positive center feel
precision of operation and the
control bush inside it, so that the
torsion bar (6) which is also the
functional safety of the steering valve,
relative positions of the control
connecting element between the valve
even at high pressures.
grooves are changed, too. Pressurized
body, the pinion and the control bush.
oil can now flow through pipes to
If a torque coming from the steering
one of the two power cylinder
wheel or from the steered road
chambers (ZL or ZR) and assists the
wheels is transmitted to the valve
axial movement of the rack if the
body or to the securely connected
turning motion is initiated from the
pinion and control bush, a relative
steering wheel. If, on the contrary,
rotary motion influenced by
the axial movement of the rack is
the torsion bar will occur
caused by the road wheels, the
between
valve
steering valve will, in spite of the
body and the con-
the
steering wheel being held, direct the
trol bush.
6
motion of the rack. This braking
20
16
pressure oil to that power cylinder
chamber which counteracts the axial
21
7
The valve body is thereby caused to
effect dampens road impacts.
19
8
When the steering wheel is released,
17
the action of the torsion bar makes
10
neutral position, and the same system
the control grooves return to the
pressure will exist in both of the
18
power cylinder chambers.
ZF compact power steering gear, type 7891,
with end take-off.
Compact rotary valve in neutral position.
3
1
5
9
11
6
21
7
20
16 16
14
8
13
17
19
10 10
11 18
15
18
12
17
ZL
4
ZR
2
3
9
5
1
Basic hydraulic
function of the ZF compact
rotary valve
The hydraulic fluid delivered by the
control edges (15), also to the return
When turning the steering wheel
steering pump (20) flows through a
oil control grooves (14) of the control
clockwise (top fig.), the rack (3) with
connecting bore in the valve area, via
bush. From these grooves, the oil
the integrated piston (4) will move to
the feed oil radial groove (10) and
flows back via bores to the return oil
the right in the cylinder tube (2), (to
transverse bores in the valve body (7),
chamber (16) and from there, via the
the left in the installed position on the
onward to the four feed oil control
hose, to the oil reservoir (21). At the
vehicle), if the pinion (5) is situated on
grooves (11) of the control bush (8).
same time, the radial grooves (17 and
top. Due to the simultaneous rotation
In the valve neutral position (fig.
18) of the valve body and their
of the valve body to the right, the
pages 10/11) the oil flows over the
associated
a
pressurized oil is directed, over the
open feed oil control edges (12) to all
connection between the right-hand
further opened feed oil control edges
valve body axial grooves (13) and
(ZR) and left-hand (ZL) power cylinder
(12), to the four associated axial
onward, over the open return oil
chambers.
grooves (13), via bores to the radial
Top figure:
1 Housing
2 Cylinder tube
3 Rack
4 Piston
5 Pinion
6 Torsion bar
7 Valve body
8 Control bush
ZF compact power steering gear, type 7891,
with end take-off.
Compact rotary valve in working position,
steering wheel turned clockwise.
pipes
provide
for
9 Tie rod
13 Axial groove
17 Radial groove
18 Radial groove
limiting valve
20 Steering pump
21 Oil reservoir
ZR power cylinder, right
12
groove (18) and, via a pipe, to the
return oil chamber (16). When the
the steering gear can be optionally
left-hand power cylinder chamber
steering wheel is turned counter-
fitted with three types of valve
(ZL), whereby the rack movement is
clockwise, the operating sequence
configuration. It is the modular
hydraulically assisted.
will be analogous to the above.
design that makes this possible. The
compact rotary valve, for example, is
An individually adaptable pressure
Special equipment
used when space limitations exist,
and the standard rotary valve is used
build-up is achieved by the fact that
the partially or fully closed feed oil
Like the other power steering gears
for relatively large oil flows. The
control edges (12) restrict or prevent
made by ZF Lenksysteme GmbH, the
Servotronic rotary valve with speed-
a connection between the pressure
ZF compact power steering gear can
sensitive control is used where best
oil inlet and the other four axial
also be fitted with a number of
possible steering comfort is to be
grooves (13) connected to the radial
equipment options in order to adapt
achieved.
groove (17). At the same time, the
it in an optimum way to the different
pressure
the
vehicle kinematic and motorization
pressurized axial grooves is restriced
conditions. In addition to the design
or prevented, too, by the closing
features described on pages 7 to 9,
oil
outlet
toward
return oil control edges (15). The oil
displaced by the piston (4) from the
right-hand power cylinder chamber
(ZR) first flows through a pipe to the
radial groove (17) and, through
transverse bores, to the associated
axial grooves and onward, over the
further opened return oil control
edges (15), to the return oil control
grooves (14). From here, the further
return flow of the oil to the oil
1
2
3
reservoir (21) takes place via the
connecting bores leading to the
Schematic representation of the ZF
compact power steering gear with
various valve configurations.
1 Compact rotary valve
2 Standard rotary valve
3 Servotronic rotary valve
13
ZFLS 7830 P - KB 8/08 e
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
ZF Lenksysteme GmbH
systems.
73527 Schwäbisch Gmünd
The benefits for the manufacturer are
Germany
clear to see: even shorter development
Phone: +49 (0)71 71 / 31-0
times and optimized production pro-
Telefax: +49 (0)71 71 / 31-32 22
cesses – with quality standards which
www.zf-lenksysteme.com
just get better and better.
[Steering the right way]
A Joint Venture of
Robert Bosch GmbH and
ZF Friedrichshafen AG
Technical modifications reserved.
Richard-Bullinger-Strasse 77

ZF Rack and Pinion Power Steering Gears

Transcription

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