lenkungskomponenten gb
TRANSCRIPT
Technical Information about Steering Gears and Steering Pumps
Construction, Function, Versions
3
ZF Parts Steering Gears and Steering Pumps -Know-How Straight from the Source
As indispensable as an engine and wheels may be for a vehicle, as important as brakes are for its
safety, the overall quality of a vehicle also depends on its steering system.
The experience of ZF Lenksysteme GmbH, a joint venture between ZF and Bosch, backs the ZF Parts
product line. The company is the technical leader in original equipment, developing and manufacturing
ultra-modern steering systems for nearly every automotive manufacturer.
ZF Trading delivers its expertise to the spare-parts market through its ZF Parts brand, constantly
modifying its lineup to match the needs of the market. The brand is your guarantee for reliability
and durability, covering most automobiles in Western Europe.
As Good As New
• Components that cannot be refurbished to tolerances prescribed by automotive manufactu-rers are replaced by new, original equipment.
• We replace all seals and plastic parts.
• Every component is marked and, if the part is returned, withdrawn from circulation.That means that every component is refurbishedonly once.
It’s the only way to assure constant quality, safety
and the same performance as a new part.
New or refurbished, ZF Parts components always
conform to the same specifications as for original
equipment, and are subjected to the same testing
methods.
There is a direct link between the refurbishing
process used for ZF Parts steering components
and the manufacturers’ quality norms that govern
production in ZF Lenksysteme GmbH’s plants.
The same quality standards apply to both new
and refurbished parts, drawing upon the technology
and the know-how that we have accumulated as
an original-equipment manufacturer.
ZF Parts refurbishes every component according
to the strictest quality standards:
• certification under VDA 6.1 and QS 9000
• DIN EN ISO 14001 environmental certificate
• audited in accordance with automotivemanufacturers’ QS systems
Professional Remanufacturing
4
5
Steering gears 7
Rack-and-pinion power steering 8
Compact power steering 14
Electro-hydraulic steering system 16
ZF-Servotronic® 2 18
Steering pumps 27
Vane pump 28
Energy savings by vane pumps 32
Power pack 33
VARIOSERV® variable-displacement pump 34
Tandem pump 36
Radial-flow pump 37
Contents
6
7
Functional, Convenient and Enjoyable
The steering system is the most critical interface between a car and its driver. Today’s steering systems
have to meet exacting standards. First, they need to provide a greater variety of functions in order
to make driving more convenient and enjoyable. Second, they have to provide for driver-assistance
systems. Third, they need to be energy efficient to increase mileage.
Steering should be precise, without any play, easy and direct, compact and lightweight. What’s more,
it has to convey a perfect road feel to the driver and make it easy to return to the straight-ahead
position. It should be shock free, and require a minimum of maintenance while providing maximum
safety and comfort. And it needs to do all of this at low speeds as well as at high speeds.
In short, safety, ease and comfort – steering makes a major difference in the quality of a car.
Rack-and-pinion power steeringCompact power steering Electro-hydraulic steering systemZF-Servotronic® 2
ZF Parts Steering Gears
8
Rack-and-pinion power steering
Design
Rack-and-pinion power steering essentially
consists of a mechanical steering gear, a steering
valve and a tube-shaped working cylinder.
Wear-free plain bearings within the power
cylinder (1) guide the rack (2) with integrated
piston (3).
The pinion (4) located near the valve meshes with
the rack. A spring-loaded yoke presses the rack
against the pinion so that there is no play.
A torsion bar (6) connects the pinion to the valve
rotor (5).
The pinion translates the rotary motion of the
steering wheel into an axial motion of the rack
(sideward movement).
A rotary valve provides hydraulic assistance.
It consists of a valve rotor (5) with six grooves
on the housing surface and a valve sleeve (7),
fixed to the pinion (4).
The bore of the valve sleeve contains matching
axial grooves to the grooves on the valve rotor.
The valve rotor is centered (neutral position)
by means of a conventional or a positive center-
feel torsion bar (6) which is also the connecting
element between the valve rotor, the pinion
and the valve sleeve.
The valve rotor and pinion run on anti-friction
bearings to ensure reliable operation of the
steering valve even at high pressures.
If torque is applied to the valve rotor or to the
pinion-valve sleeve complex from the steering
wheel or from the wheels, the torsion bar will
exert an influence on the relative torsion between
the valve rotor and the sleeve. The valve rotor
turns against the surrounding valve-sleeve bore,
thus changing the relative position of the grooves.
This allows power-steering fluid under pressure
to flow via hoses to either of the two working-
cylinder chambers (left or right cylinder) and
assists the axial movement of the rack initiated
by turning the steering wheel. If, however, axial
movement of the rack is caused by the wheels,
the steering valve will direct fluid to whatever
cylinder chamber counteracts such axial motion,
even though the steering wheel is held in posi-
tion. This braking effect dampens road shocks.
Once the steering wheel has been released, the
torsion bar brings the grooves back into their
neutral position and both working-cylinder
chambers return to equilibrium.
9
Basic hydraulic function of the rotary valve
The torsion bar (6), the valve rotor (5) and the
valve sleeve (7) are particularly important to
the function of the valve.
In the neutral position, all fluid is under a con-
tinuous pressure of approx. 2 bar. Twisting the
torsion bar against resistance causes the valve
to deliver fluid under pressure to the appropriate
cylinder chamber.
Due to the overlap of the guide edges (11), the
intake ports close and block the flow of fluid to
the axial grooves (12) on the valve sleeve, which
is connected to the opposite cylinder chamber
via the upper radial groove.
The closed guide ports prevent pressurized fluid
from flowing back into the reservoir. The piston
(3) displaces the fluid from the cylinder cham-
ber onto the rack (2) causing it to flow over the
radial groove of the valve sleeve to the return
grooves (13) of the valve rotor and, from there,
back to the reservoir (20).
11.. HHoouussiinngg 22.. RRaacckk 33.. PPiissttoonn 44.. PPiinniioonn 55.. VVaallvvee rroottoorr 66.. TToorrssiioonn bbaarr 77.. VVaallvvee sslleeeevvee 88.. TTiiee rroodd 99.. FFeeeedd ooiill rraaddiiaall ggrroooovvee 1100.. FFeeeedd ooiill ccoonnttrrooll ggrroooovvee 1111.. FFeeeedd ooiill ccoonnttrrooll eeddggee 1122.. AAxxiiaall ggrroooovvee 1133.. RReettuurrnn ooiill ccoonnttrrooll ggrroooovvee 1144.. RReettuurrnn ooiill ccoonnttrrooll eeddggee 1155.. RReettuurrnn ooiill cchhaammbbeerr 1166.. RRaaddiiaall ggrroooovvee 1177.. RRaaddiiaall ggrroooovvee 1188.. PPrreessssuurree rreelliieeff aanndd ffllooww lliimmiittiinngg vvaallvvee 1199.. SStteeeerriinngg ppuummpp 2200 ..OOiill rreesseerrvvooiirr ZZLL PPoowweerr ccyylliinnddeerr,, lleefftt ZZRR PPoowweerr ccyylliinnddeerr,, rriigghhtt
10
Special equipment
The rack-and-pinion power steering gear can
be fitted with number of equipment options to
optimize steering under a variety of different
motoring conditions.
Variable ratio
Rack-and-pinion power steering can also come
with a variable-ratio option. With variable ratio,
the rack teeth come with different modules and
mesh angles. Steering reacts normally in the mid
position (straight ahead). As the steering wheel
turns more (right and left), the ratio diminishes,
making steering more direct. The maximum
variance between the greatest and the smallest
ratio is limited to 35%. The result is an unusually
low two turns from lock to lock.
The extraordinary ease of handling afforded by
variable ratio rack-and-pinion power steering is
equally suitable for mid-size passenger cars and
light trucks as it is for sports cars. It allows for
precise and rapid steering reactions at higher
speeds without running the risk of over-steering
and it optimizes handling when maneuvering
into or out of a parking space, when turning in
tight places, and during extreme cornering.
SScchheemmaattiicc rraattiioo ddiiaaggrraamm ffoorr ZZFFrraacckk aannddppiinniioonn ppoowweerr sstteeeerriinnggggeeaarr wwiitthh vvaarriiaabblleerraattiioo.. OOnneeffuullll ttuurrnn ooff tthhee sstteeeerriinngg wwhheeeelliisssshhoowwnn aass 336600°°oonn tthhiiss ggrraapphh..
11
Positive center-feel torsion bar
The torsion bar is an extremely important com-
ponent of the steering valve. Its design is critical
to the feel and the precision of steering at diffe-
rent speeds. When the steering valve is activated
(when the wheel begins to turn), it is necessary
to overcome the resistance of the torsion bar.
The steering valve would be too easy to activate
without this resistance, rendering it imprecise
and hard to control. The positive center-feel (PCF)
torsion bar was developed on the basis of the
common torsion bar (see illustration above) to
optimize the steering valve when initiating
steering maneuvers (and thus handling at higher
speeds). The positive center-feel torsion bar is
used with Servotronic steering valves as well as
in conventional rotary radial-piston valves.
The compact component consists essentially of
a torsion bar, a preloaded, torsion-proof metal
bellows spring, two sleeve-shaped end pieces
and prismatic centering with spheres. The torsion
bar is joined to both end pieces. The metal bel-
lows spring is located coaxially to the torsion
bar. It is attached at one end to one of the end
pieces and to a centering piece at the other end.
The prismatic centering consists of two cente-
ring pieces, each possessing prism-shaped
recesses on one side. Between these recesses
are cages containing spheres. The prismatic
centering determines the zero torsion setting of
the axially preloaded metal bellows spring.
Right at the beginning of torsional motion, the
metal bellows spring generates substantial
additional torsional resistance. As soon as the
prisms of the two centering pieces are displaced
against each other (starting at a torsional angle
of approx. 0.2°), the high rigidity of the metal
bellows spring is uncoupled and the torsional
rigidity behaves in a similar manner to a standard
torsion bar.
Fitment of the PCF torsionbar to the ZF rotary valve.
12
Nominal characteristic
This term refers to a curve characterized by an
extremely narrow tolerance. Conically formed
guide grooves and leading edges on the rotary
valve and on the valve sleeve permit optimum
configuration of the valve’s performance charac-
teristics to manufacturers’ specifications.
Hydraulic limiting
There are a number of reasons – functional,
financial and environmental – in favor of inte-
grating hydraulic limiters into rack-and-pinion
power steering. A chassis with sufficient self-
aligning properties is a prerequisite in any case.
Specially developed connecting profiles in the
cylinder bore permit fluid to flow under high
pressure from the cylinder chamber into the
return chamber (separated by the piston) just
before the end of the stroke. The resultant drop
in pressure at the lock protects the steering pump
and the steering gear from excessive loads. The
greatly reduced hydraulic boost caused by the
drop in pressure causes increased operating
torque at the steering wheel, signaling the driver
that maximum wheel lock is imminent. As the
hydraulic limiter reduces the power requirement
of the steering pump, the engine idling speed
can be set lower. This saves fuel and helps protect
the environment.
13
Full-lock damping
If required by the automotive manufacturer
rack-and-pinion power steering systems can be
fitted with flexible plastic components on the
stroke limiters to counteract annoying noises
at maximum wheel lock.
Steering damping
The absence of shocks from the road and the
chassis through the steering wheel is a sign of
superior steering. Additional technical solutions
are possible in particular cases, such as fixing
the steering gear using rubber mounts, attaching
a damper parallel to the steering system, or tie
rods with flexible elements. It is also possible
to use valve and baffle systems (variable baffles)
to work as damping.
14
Compact power steering
The compact power steering gear is based on a
modular design consisting mainly of three sub-
assemblies: a mechanical steering gear, a valve
and a working cylinder.
Its design essentially corresponds to that of the
rack-and-pinion power steering already described.
A bleeder hole in the rack provides for equilibrium
between both bellows chambers.
The compact rotary valve controls the pressurized
fluid needed for power assistance. The design
consists of a valve body (7) which exhibits eight
guide grooves in its valve bore and a control
11.. HHoouussiinngg 22.. CCyylliinnddeerr ttuubbee 33.. RRaacckk 44.. PPiissttoonn 55.. PPiinniioonn 66.. TToorrssiioonn bbaarr 77.. VVaallvvee bbooddyy 88.. CCoonnttrrooll bbuusshh 99.. TTiiee rroodd 1100.. FFeeeedd ooiill rraaddiiaall ggrroooovvee 1111.. FFeeeedd ooiill ccoonnttrrooll ggrroooovvee 1122.. FFeeeedd ooiill ccoonnttrrooll eeddggee 1133.. AAxxiiaall ggrroooovvee1144.. RReettuurrnn ooiill ccoonnttrrooll ggrroooovvee 1155.. RReettuurrnn ooiill ccoonnttrrooll eeddggee 1166.. RReettuurrnn ooiill cchhaammbbeerr 1177.. RRaaddiiaall ggrroooovvee 1188.. RRaaddiiaall ggrroooovvee 1199.. PPrreessssuurree rreelliieeff aanndd ffllooww lliimmiittiinngg vvaallvvee 2200.. SStteeeerriinngg ppuummpp 2211.. OOiill rreesseerrvvooiirr ZZLL PPoowweerrccyylliinnddeerr,, lleefftt ZZRR PPoowweerr ccyylliinnddeerr,, rriigghhtt
bushing (8) securely attached to the pinion (5).
The surface of the control bushing likewise
contains eight matching axial grooves.
Centering (neutral position) of the valve body
occurs by means of a conventional or a positive
center-feel torsion bar (6) which also provides
the link between the valve body, the pinion and
the control bushing.
The rest of the design and the function corres-
pond to those of rack-and-pinion power steering.
15
Basic hydraulic function of the compact rotary
valve
The basic function of the compact rotary valve
corresponds to that of the normal rotary valve,
except that it uses three feed grooves instead of
four.
SScchheemmaattiicc rreepprreesseennttaattiioonn oofftthhee ZZFF ccoommppaacctt ppoowweerr sstteeeerriinnggggeeaarr wwiitthhvvaarriioouuss vvaallvvee ccoonnffiigguurraattiioonnss..1. Compact rotary valve2. Standard rotary valve3. Servotronic rotary valve
Special equipment
Like the other power-steering systems, the compact
power steering can be fitted with a number of
equipment options to optimize steering under a
variety of different motoring conditions.
Beyond the design features already mentioned
on pages 8 to 13, the steering gear can be fitted
with three different types of valve configuration.
Modular design is the key here. The compact
rotary valve, for example, comes into play when
space is limited, while the standard rotary valve
accommodates larger fluid flows.
The Servotronic rotary valve with speed-sensitive
control is the system of choice to assure optimum
ease of steering.
16
Electro-Hydraulic Power Steering (EHPS)
Electro-hydraulic system benefits
Usually, the engine permanently drives the steering pump. However, this arrangement is not energy
efficient. That is why ZF developed the EHPS system (EHPS = Electro Hydraulic Power Steering)
and the power pack. The vehicle’s electrical system drives the latter, resulting in energy savings
of up to 75%, depending on the load and the control strategy. Of course, all the familiar advantages
of rack-and-pinion power steering or compact power steering such as feel, precision and safety,
remain.
Aside from saving energy, there are other advantages if the steering system works independently
of an internal-combustion engine. Power steering remains available even if the engine stalls. The
electro-hydraulic steering system can be delivered to a manufacturer’s assembly line just in time
as a completely functional, fully tested system module.
17
Power pack
The electrically driven power pack was designed as a modular system. It is also suitable for other
types of applications as a stand-alone unit.
Thanks to several variable components, the power pack can adapt to a wide variety of demands.
Whereas the EP 1 series operates with a direct-current brush motor, the EP 2 series employs a
brushless direct-current motor. The pump element operates on the rolling vane principle (2).
There are several ways to fine tune the displacement in conjunction with control strategies.
Integrated into the control device (3), they translate the data entered into just the amount of pressure
needed. Data could include steering and vehicle speed, but could also refer to power flow to the
electric motor (5) or specific vehicle information desired by a customer. The fluid reservoir (1)
and its bracket (4) are suitable for vertical or horizontal mounting, thus allowing for flexible use
of space.
ZZFF PPoowweerr ppaacckk EEPP 22..FFoorr vveerrttiiccaall iinnssttaallllaattiioonn..1. Oil reservoir2. Roller cell pump3. Control unit
(integrated into housing) 4. Bracket (variable)5. Electric motor
See chapter “Steering Pumps”for more detailed description.
18
ZF-Servotronic® 2
Design and function
Either the proven rack-and-pinion power steering system or, if necessary, the compact power steering
system can serve as the basis for the Servotronic 2. A modified rotary valve from the rack-and-pinion
power steering system is used. By employing modern electronics and an electro-hydraulic transducer,
and by modifying the steering valve, the Servotronic 2 can operate as a function of the momentary
vehicle speed in contrast to common power steering systems.
A prerequisite for using the Servotronic 2 is either an electronic speedometer or a suitable ABS
control unit. The speed signals from either of these two devices go to an electronic control device
which can be either a separate component, or can be integrated into the vehicle’s electrical system.
The microprocessor in the Servotronic control unit analyzes the speed signals and converts them
into a regular electrical current that activates the electro-hydraulic transducer. The transducer, which
is directly attached to the valve, regulates the hydraulic reaction at the rotary valve and thus, the
torque necessary to operate the steering wheel.
19
The steering effort is related to the vehicle speed, which means that it is minimal at low speeds or
when the vehicle is stationary (such as in parking). Since the hydraulic reaction changes in proportion
to vehicle speed, more effort is needed as speed increases (see illustration above). The driver thus
enjoys particularly good road feel at higher speeds, allowing him to steer precisely. Another advantage
of Servotronic 2 is that fluid pressure and flow never diminish. They can be summoned up imme-
diately in emergencies where sudden and unexpected steering corrections may become necessary.
These features combine to assure extraordinarily high precision, safety and ease of steering.
SScchheemmaattiicc rreepprreesseennttaattiioonn ooff tthhee ZZFF SSeerrvvoottrroonniicc 22 bbaasseedd oonn tthhee rraacckk aanndd ppiinniioonn ppoowweerr sstteeeerriinngg ggeeaarr,, ttyyppee 77885522..1. Electronic speedometer in the vehicle 2. Electronic control unit (ECU) 3. Electro-hydraulic transducer 4. Rack and pinion power steering gear 5. Steering pump 6. Oil reservoir with fine filter 7. Anti-vibration expansible hose 8. Manually adjustable steering column
20
Design
The design corresponds to that of the rack-and-
pinion hydraulic gear already described.
The rotary valve, which was adapted to meet
Servotronic 2 requirements, controls the pressuri-
zed fluid needed for power boost. The essential
components in this valve design are the valve
rotor (16) with at least six control grooves on its
surface, and a valve sleeve (17) which is con-
nected to the pinion (20). The bore of the valve
sleeve contains matching axial grooves.
Centering (neutral position) of the valve rotor
occurs primarily by means of a torsion bar (15)
that also links the valve rotor, the pinion and
the valve sleeve. The prism-guided spheres (13)
between the centering piece (14) fixed to the
valve sleeve and the spring-loaded (10) reaction
piston (9), reinforce the centering effect (similar
to the positive center-feel torsion bar) and help
determine the hydraulic reaction. The coaxially
guided reaction piston is connected via two
axially located sphere races to the valve rotor.
11.. EElleeccttrroonniicc ssppeeeeddoommeetteerr 22.. EElleeccttrroonniicc ccoonnttrrooll uunniitt ((EECCUU)) 33.. EElleeccttrroo--hhyyddrraauulliicc ttrraannssdduucceerr 44.. FFeeeedd ooiill rraaddiiaallggrroooovvee 55 RRaaddiiaall ggrroooovvee 66.. RRaaddiiaall ggrroooovvee 77.. RReettuurrnn ooiill cchhaammbbeerr 88.. RReeaaccttiioonn cchhaammbbeerr 99.. RReeaaccttiioonn ppiissttoonn1100.. CCoommpprreessssiioonn sspprriinngg 1111.. CCuutt--ooffff vvaallvvee 1122.. OOrriiffiiccee 1133.. BBaallll 1144.. CCeenntteerriinngg ppiieeccee 1155.. TToorrssiioonn bbaarr 1166.. VVaallvvee rroottoorr 1177.. VVaallvvee sslleeeevvee 1188.. PPiissttoonn 1199.. HHoouussiinngg 2200.. PPiinniioonn 2211.. RRaacckk 2222.. TTiiee rroodd 2233.. FFeeeedd ooiill ccoonnttrroollggrroooovvee 2244.. FFeeeedd ooiill ccoonnttrrooll eeddggee 2255.. AAxxiiaall ggrroooovvee 2266.. RReettuurrnn ooiill ccoonnttrrooll ggrroooovvee 2277.. RReettuurrnn ooiill ccoonnttrrooll eeddggee2288.. PPrreessss.. rreelliieeff aanndd ffllooww lliimmiittiinngg vvaallvvee 2299.. SStteeeerriinngg ppuummpp 3300.. OOiill rreesseerrvvooiirr ZZLL PPoowweerr ccyylliinnddeerr,, lleefftt ZZRR PPoowweerr ccyylliinnddeerr,, rriigghhtt
21
Basic hydraulic function of the rotary valve
See page 9 for a description of how a rotary
valve works.
ZF Servotronic 2, with ZF compactpower steering gear as the basic unit,constant or variable ratio, rotary valveand end take-off
Section of ZF Servotronic 2rotary valve
ZZFF SSeerrvvoottrroonniicc 22rroottaarryy vvaallvvee
Rotary valve in working position. Steering wheel turnedclockwise, highspeed with rapid steering corrections,transducer valve completely open, maximum hydraulic
reaction limited by
22
Function
At low speeds (see illustration on the right) such
as when maneuvering in and out of parking
spaces, the electronic speedometer (1) or the
ABS control unit transmits very few signals to
the microprocessor integrated into the electrical
control unit (2). Once analyzed, these signals
determine the flow of current to the electro-
hydraulic transducer (3). The maximum amperage
that arises during this situation causes the
transducer valve to close, preventing fluid from
flowing from the radial feed groove (4) to the
reaction chamber (8). A baffle (12) ensures that
there is also return pressure in the reaction
chamber. The Servotronic® 2 valve thus behaves
exactly like a normal rotary valve in this situation.
The reaction is eliminated, steering is light and
easy to handle.
11.. EElleeccttrroonniicc ssppeeeeddoommeetteerr 22.. EElleeccttrroonniicc ccoonnttrrooll uunniitt ((EECCUU)) 33.. EElleeccttrroo--hhyyddrraauulliicc ttrraannssdduucceerr 44.. FFeeeedd ooiill rraaddiiaallggrroooovvee 55.. RRaaddiiaall ggrroooovvee 66.. RRaaddiiaall ggrroooovvee 77.. RReettuurrnn ooiill cchhaammbbeerr 88.. RReeaaccttiioonn cchhaammbbeerr 99.. RReeaaccttiioonn ppiissttoonn1100.. CCoommpprreessssiioonn sspprriinngg 1111.. CCuutt--ooffff vvaallvvee 1122.. OOrriiffiiccee 1133.. BBaallll 1144.. CCeenntteerriinngg ppiieeccee 1155.. TToorrssiioonn bbaarr 1166.. VVaallvvee rroottoorr 1177.. VVaallvvee sslleeeevvee 1188.. PPiissttoonn 1199.. HHoouussiinngg 2200.. PPiinniioonn 2211.. RRaacckk 2222.. TTiiee rroodd 2233.. FFeeeedd ooiill ccoonnttrroollggrroooovvee 2244.. FFeeeedd ooiill ccoonnttrrooll eeddggee 2255.. AAxxiiaall ggrroooovvee 2266.. RReettuurrnn ooiill ccoonnttrrooll ggrroooovvee 2277.. RReettuurrnn ooiill ccoonnttrrooll eeddggee2288.. PPrreessss rreelliieeff aanndd ffllooww lliimmiittiinngg vvaallvvee 2299.. SStteeeerriinngg ppuummpp 3300.. OOiill rreesseerrvvooiirr ZZLL PPoowweerr ccyylliinnddeerr,, lleefftt ZZRR PPoowweerr ccyylliinnddeerr,, rriigghhtt
Rotary valve in working position. Steeringwheel turnedcounter-clockwise. Driving at low speed (parking); trans-
ducer valve andcut-off valve shut, no hydraulic reaction.
23
At high speeds (see illustration on the left) the
transducer valve is fully open due to very low
(or no) control current flowing to the transducer.
This permits maximum pressure to the reaction
device from the radial feeder groove (4).
Turning the steering wheel to the right raises the
reaction pressure in accordance with the ambient
pressure and pressurizes the reaction piston from
the reaction chamber (8). As soon as the reaction
pressure for the specific type of car has reached
its upper limit, the fluid is discharged through
the cut-off valve (11) as it opens, into the return
oil chamber (7) so as to prevent a further increase
in reaction pressure. The effort required to turn
the wheel does not increase any more after this
point, and conveys a secure feel thanks to
optimum road contact.
As speed increases, increased signal frequency
(after conversion by the microprocessor) cau-
ses the control current to the electro-hydraulic
transducer to drop. The transducer valve assu-
mes an open position reflecting current vehicle
speed and admits a limited amount of fluid
into the reaction chamber (8) from the radial
feeder groove (4). A baffle (12) prevents larger
amounts of oil from draining into the return
chamber (7) so that pressure increases in the
reaction chamber. This exerts greater pressure
on the prism-guided spheres (13). This has a
particularly positive effect on centering of the
steering valve when driving straight ahead.
Activation of the steering valve causes the
spheres to increase resistance to turning of the
rotary valve. Thus, more effort is required to
turn the wheel until hydraulic assistance
comes into play in the right-hand or the left-
hand cylinder chamber.
24
Variable ratio
Besides a constant ratio, Servotronic 2 also comes
with a variable ratio. With variable ratio, the rack
teeth come with different modules and mesh
angles. Steering reacts normally in the mid
position (straight ahead). As the steering wheel
turns more (right and left), the ratio diminishes,
making steering more direct. The maximum
variance between the greatest and the smallest
ratio is limited to 35%. The result is an unusually
low two turns from lock to lock.
The extraordinary ease of handling afforded by
Servotronic 2 with variable ratio is equally suitab-
le for mid-size passenger cars and light trucks
as it is for sports cars. It allows for precise and
rapid steering reactions at higher speeds without
running the risk of over-steering, and it optimi-
zes handling when maneuvering into or out of
a parking space, when turning in tight places,
and during extreme cornering.
Safety
Steering remains fully functional even if the elect-
rical system or some other component fails. In
exceptional cases such as these, Servotronic 2
will operate at maximum hydraulic reaction
(high-speed curve) due to the mechanical opening
of the transducer valve. If speed signals are
suddenly no longer transmitted while underway
due to faulty wire contacts or a defective spee-
dometer, the highly developed microprocessor
in the electronic control unit is capable of deriving
a constant control current from the last speed
signals assessed. This ensures consistent steering
until the engine is turned off. When the engine
starts again, the maximum hydraulic reaction
in line with the high-speed curve will be restored.
Special equipment
The Servotronic 2 can be fitted with several
options to optimize steering under a variety of
different motoring conditions.
25
Full-lock damping
If required by the automotive manufacturer, the
Servotronic 2 can be fitted with flexible plastic
components on the stroke limiters to counteract
annoying noise at maximum wheel lock.
Steering damping
The absence of road shocks transmitted through
the steering wheel is a sign of superior steering.
Fixing the steering gear using rubber mounts
and/or tie rods with flexible elements contribute
to steering ease. Valve and baffle systems in the
hydraulic system also help.
Hydraulic limiting
There are a number of reasons – functional,
financial and environmental – in favor of inte-
grating hydraulic limiters into Servotronic 2.
A chassis with sufficient self-aligning properties
is a prerequisite in any case.
Specially developed connecting profiles in the
cylinder bore permit fluid to flow under high
pressure from the cylinder chamber into the
return chamber (separated by the piston) just
before the end of the stroke. The resultant drop
in pressure at the lock protects both the steering
pump and the steering gear from excessive
loads. The greatly reduced hydraulic boost caused
by the drop in pressure causes increased opera-
ting torque at the steering wheel, signaling the
driver that maximum wheel lock is imminent.
As the hydraulic limiter reduces the power requi-
red by the steering pump, the engine idling speed
can be set lower. This saves fuel and helps protect
the environment.
26
27
ZF Parts Steering Pumps
Compact, light and powerful
Compact design characterizes ZF Parts steering pumps. They combine minimum weight and
maximum displacement.
The wide assortment comprises different designs and series including vane, tandem and special
energy-saving pumps. An adjustable pump is still in the development stage which, together with
the motor-driven power pack for electro-hydraulic steering systems, will represent the technology
of the future.
Energy-saving retrofits
The use of special valves on all pumps can lead to considerable energy savings.
Moreover, a hydraulic steering system makes it possible to boost the functionality of the basic pump
substantially, provided the control is sensitive to steering and speed. It is easy to integrate the
necessary electronic controls into the existing electrical system.
Vane pumpEnery savings by vane pumpsAdjustable pumpTandem pump Radial piston pump
28
Vane pump
11.. HHoouussiinngg22.. SShhaafftt33.. RRoottoorr44.. VVaannee55.. CCaamm rriinngg66.. PPrreessssuurree ppllaattee77.. SSuuccttiioonn cchhaammbbeerr88.. PPrreessssuurree cchhaammbbeerr99.. CCoovveerr
1100.. PPrreessssuurree rreelliieeff aannddffllooww lliimmiittiinngg vvaallvvee
Construction
Vane pumps are based on an element that is
usually housed in an alloy case (1). It consists
essentially of a shaft (2), a rotor (3), ten vanes (4),
a cam ring (5), and a pressure plate (6). Depending
on the model, a second pressure plate or control
plate may be employed. A cover (9), also made
of alloy, forms the axial end.
The shaft is guided in the housing by a ball-
bearing or plain bearing system – depending on
the type of drive – and is connected to the rotor
(3). The vanes (4) are located in the ten slots that
radiate around the rotor. The cam ring (5), which
is installed in the housing or cover, surrounds
the rotor together with the lateral pressure/control
plate or cover. The two crescent-shaped pump
chambers are situated between the cylindrical
circumferential surface of the rotor and the
ellipsoidal bore in the cam ring. Their volume
is the product of the greatest possible crescent-
shaped segment between two vanes and the
width of the rotor or the vanes.
Depending on the type of pump, the fluid flow
generated in the crescent-shaped pressure
chambers is directed to the valves for pressure
relief and flow limitation (10). These valves are
positioned alongside or perpendicular to the shaft.
They limit the pressure to preset amounts and
pass the fluid on to the hydraulic power steering
via a hose.
Function
Rotation of the input shaft (2) and the rotor (3)
causes the ensuing centrifugal force to press the
rotor vanes (4) onto the track of the cam ring
(5). This movement is aided by pressurized fluid
which flows from the pressure chamber (8) to
the inner surfaces of the vanes. This results in
the formation of ten separate pump cells between
the ten vanes; they draw fluid in the two crescent-
shaped pump chambers when the volume rises,
and they force fluid into the pressure chamber
when volume drops. Since two suction zones
and two pressure zones always lie opposite each
other, each of the ten pump cells displaces twice
its own volume with every rotation of the input
shaft, thanks to the shape of the cam ring.
Moreover, due to the double-action arrangement
of the intake and pressure zones, the hydraulic
radial forces acting upon the rotor cancel each
other out.
29
Volumetric flow-rate control
Vane pumps can be set to a flow rate which is
constant, falls, or diminishes gradually in order
to match individual systems. In the pump design
offering a constant flow rate (see Variant A in
the illustration at right), an almost constant fluid
flow is fed to the power-steering gear throughout
the entire range of rpms.
The vane pump with falling control curve (Variant
B) is designed to achieve the required flow rate
at a predetermined speed such as 1000 rpm.
It then falls continuously falls as rpms rise. The
reduced flow rate remains sufficient, however, to
allow for maximum hydraulic assistance at all
times. ZFLS developed a special control system
in the pressurized area to reduce hydraulic
assistance at high speeds, thus greatly improving
road feel in the steering wheel.
It is also possible to outfit pumps with a control
curve that diminishes gradually (Variant C).
In this design, the flow rate is already reduced
by a marked transition at medium rpms. This
abbreviated transition phase between maximum
and minimum flow rate can be adjusted indivi-
dually by a special valve system.
BBaassiicc ddiiaaggrraamm ooff ccoonnttrroollcchhaarraacctteerriissttiiccss wwiitthh ccoonnssttaanntt aanndd ffaalllliinngg fflloowwrraattee aasswweellll aass ffllooww rraatteeffaalllliinngg iinn sstteeppss..
30
Vane pump series
CP 14Design
The pressure-relief and flow-limiting valve is
installed transversely to the input shaft on this
kind of pump. Mounting is standardized and
meets the specifications of the Association of
the German Motor Industry (VDA).
Volumetric flow-rate control
Vane pumps can be set to a flow rate which is
constant, falls, or diminishes gradually in order
to match an individual system. (see description
on Page 29)
Energy savings
Use of the ECO (electronically controlled orifice)
valve in this model will result in a remarkable
improvement in energy savings. The valve is
described on Page 32
Design
The CP 1 range with its modular design allows
a variety of mounting possibilities, thus ensuring
that the pump adapts to different vehicles in
optimum fashion. Besides longitudinal mounting
to the input shaft, transverse connection to the
engine by means of a bolt is also possible.
Furthermore, it is possible to redesign the pump
lid for individual mounting needs such as a
multifunctional bracket. This pump is usually
driven by an engine belt.
Volumetric flow-rate control
CP 1 vane pumps can be set to a flow rate which
is constant, falls, or diminishes gradually in
order to match an individual system.
(see description on Page 29)
CP 1
31
FP 4Design
This best-selling pump model permits direct
mounting of the fluid reservoir directly on top of
the pump. This dispenses with one hose line
and cuts assembly costs. Moreover, mounting
is standardized and meets the specifications of
the Association of the German Motor Industry
(VDA). The pump is usually driven by an engine
belt.
Volumetric flow-rate control
FP 4 vane pumps can be set to a flow rate which
is constant, falls, or diminishes gradually in
order to match an individual system. (see des-
cription on Page 29).
Energy savings
Use of the ECO (electronically controlled orifice)
valve in this model will result in a remarkable
improvement in energy savings. The valve is
described on Page 32.
FP 6Design
Development of the FP 6 range took particular
account of new targets set by the automobile
industry for steering pumps with higher hydraulic
capacity. The FP 6 can handle a controlled
flow rate of up to 15 dm3/min. and maximum
pressure of 150 bar.
The function of the pump element and its external
dimensions are largely identical to those of the
FP 4 vane pump. Again, mounting is standardized
and meets the specifications of the Association
of the German Motor Industry (VDA), thus per-
mitting problem-free replacement of competitors’
products. Installation of a plastic fluid reservoir
can dispense with the need for a hose and can
lower assembly costs.
Volumetric flow-rate control
FP 6 vane pumps can be set to a flow rate which
is constant, falls, or diminishes gradually in
order to match an individual system. (See des-
cription on Page 29).
Energy savings
Use of the ECO (electronically controlled orifi-
ce) valve in this model will result in a remarka-
ble improvement in energy savings. The valve
is described on Page 32.
32
Energy savings by vane pumps
Energy savings with ECO
Installing an ECO valve (4) (ECO = Electronically Controlled Orifice) on a vane
pump (1) represents another step towards energy savings. Activated by vehicle
speed and the rate at which the steering wheel is turned, this hydraulic control
unit produces a flow rate as needed to operate the steering gear, thus considerably
expanding the functionality of the basic pump. The pump system shuts off flow
when not needed, chiefly when driving straight ahead. The necessary electronic
controls can be integrated into the existing electrical system.
Advantages:
• Fuel consumption drops by an average of 20 to 40 percent.
• Temperature drops by 15 to 20° C
• Requires approx. 35% less power
• Reduces through-flow pressure by approx. 4 to 6 bar.
SScchheemmaattiicc rreepprreesseennttaattiioonn ooff aa ZZFF sstteeeerriinnggppuummpp wwiitthh EECCOO ttooggeetthheerr wwiitthh aa ZZFF AAccttiivvee SStteeeerriinnggoonn tthhee bbaassiiss ooff aa ZZFF SSeerrvvoottrroonniicc 221. ZF Vane pump FP 6 with ECO 2. ZF Servotronic 2 (basic unit) 3. Servotronic 2 rotary valve 4. Electric motor5. Superposition gear system 6. Oil reservoir with fine filter 7. Electronic control unit 8. Hoses, anti-vibration hose9. Cables to power supply, CAN, ignition, vehicle sensors
Pump comparison foraverage powerinput inthe driving and ECEcycles (test method formeasuring pollutante-missions).
33
Powerpack
SScchheemmaattiicc rreepprreesseennttaattiioonnooff aann eelleeccttrroo--hhyyddrraauulliiccppoowweerr sstteeeerriinngg ssyysstteemm((EEHHPPSS))..With ZF compact powersteering gear, ZF powerpack, sensors and hoses.
Application
A car’s engine normally runs the steering pump
all the time. This is not an ideal solution from an
energy point of view. That is why we developed
the power pack as an important stand-alone unit
for the EHPS (Electro Hydraulic Power Steering).
The power pack runs on the car’s electrical system
and can save up to 75% in energy, depending
on load and control strategy. All the familiar
advantages of a hydraulic steering system remain,
such as safety, precision and road feel.
Aside from saving energy, there are other advan-
tages when the steering system works indepen-
dently of an internal-combustion engine. Power
steering remains available even if the engine
stalls, plus the vehicle performs better in crash
tests. The electro-hydraulic steering system can
be delivered to a manufacturer’s assembly line
just in time as a completely functional, fully
tested system module.
Design
The electrically driven power pack was designed
as a modular system. It is also suitable for other
types of applications as a stand-alone unit.
Thanks to several variable components, the power
pack can adapt to a wide variety of demands.
Whereas the EP 1 series operates with a direct-
current brush motor, the EP 2 series employs a
brushless direct-current motor. The pump element
operates on the rolling vane principle (2).
There are several ways to fine-tune the displa-
cement in conjunction with control strategies.
Integrated into the control device (3), they translate
the data entered into just the amount of pressure
needed. Data could include steering and vehicle
speed, but could also refer to power flow to the
electric motor (5) or specific vehicle information
desired by a customer. The fluid
reservoir (1) and its bracket (4)
are suitable for vertical or
horizontal mounting, thus
allowing for flexible use of
space.
ZF Power pack EP 2. For vertical installation.1. Oil reservoir 2. Roller cell pump 3. Control unit (integrated into housing)
4. Bracket (variable) 5. Electric motor
34
VARIOSERV® variable-displacement pump
FFuunnccttiioonnaall sscchheemmee ooff tthhee vvaarriiaabbllee ddiissppllaacceemmeenntt ppuummpp VVAARRIIOOSSEERRVV..1. Housing 2. Shaft 3. Rotor 4. Vane 5. Cam ring 6. Pressure plate 7. Suction chamber 8. Pressure chamber9. Cover 10. Outer ring 11. Control valve with integrated pressure relief 12. Left-hand chamber13. Right-hand chamber
35
Function
After the pump reaches operating speed at engine
idle, it delivers a constant fluid flow to the
hydraulic power-steering gear. An increase in
engine speed causes the increasing pump pressure
to open the control valve, pressurize the cam
ring and diminish eccentricity with respect to
the rotor (3). The result is that the pump chambers
become smaller and displace a diminished geo-
metric volume. This limited pump operation
prevents excessive generation of pressurized fluid
and, due to the reduced power input, results in
lower energy consumption at high pump speeds.
This makes it possible to simplify cooling of the
steering system.
As engine speed decreases, differential pressure
between the left-hand and right-hand chamber
restores eccentricity between the rotor and the
cam ring, causing the geometric delivery volume
to increase continually.
Pump comparison for averagepowerinput in the driving andECE cycles (test method formeasuring pollutantemissions).
Design
Unlike the double-stroke rotor set of the standard
vane pumps, the cam ring (5) of the single-stroke
rotor set on the VARIOSERV is variable and can
generate, thanks to varying eccentricity, a con-
trollable geometric displacement. The cam ring
varies as a function of engine speed against the
prevailing differential pressure between the
left-hand (12) and the right-hand (13) chamber.
A control valve (11) with integrated pressure
relief, fitted transversely to the shaft (2), influ-
ences performance considerably.
The purpose of this pump is to assist the automobile industry in achieving its targets for CO2
emissions and fuel economy, particularly in upper mid-size and luxury cars. Still further improve-
ment is possible by reducing steering fluid temperature and protecting the pump against possible
overloading.
36
Tandem pumps
The tandem pump range is designed to meet a
wide variety of automotive manufacturer requi-
rements. The two fluid circuits fulfill different
tasks. Whereas Circuit I always supplies the stee-
ring system with pressurized fluid, Circuit II can
be used for the following, thanks to different
pump variants:
• for cooling water for combustion engines
• for supplying hydraulic fan systems
• for supplying roll-stabilization systems
• for power-brake boosting
The same shaft drives both pumps which generate
two separate fluid flows.
Examples of tandem pumps in passenger cars
A frequent combination is a CP 1 vane pump
teamed up with a water pump. The vane pump
supplies the hydraulic steering system with
pressurized fluid while the water pump assures
the transport of cooling water to the engine.
When an FP 4 vane pump and a CP 1 are com-
bined, the FP 4 generates pressurized fluid for
the steering while the CP 1 vane pump drives a
hydraulic fan system. An electromagnetic solenoid
control valve, which is influenced by the electrical
system, determines the CP 1’s flow rate and thus,
the intensity of the hydraulic fan drive. Both
pump elements are connected by an intermediate
housing and a shaft drive.
Still another variant is the combination of an FP 4
or FP 6 vane pump and a radial piston pump.
Here, the vane pump supplies the steering system
while the multi-piston radial pump supplies the
roll-stabilization or the power-brake system.
37
Radial piston pumps
RC 1 radial piston pump
This compact pump type was developed specifically for electro-hydraulic control of continuously
variable automatic transmissions, such as ZF-Ecotronic, on passenger cars. The transmission shaft
drives the pump, which is integrated into the transmission. Suction control generates a constant flow
rate of 27 to 29 dm3/min. throughout the entire range of engine speed at constant power input.
The pressure can vary up to 70 bar.
ZF Trading GmbHObere Weiden 12 . 97424 Schweinfurt . GermanyFon + 49 9721 4756-0 . Fax + 49 9721 4755000
www.zf-trading.com
Driveline and Chassis Technology 3000
4 G
B