lenksysteme_nkw_e
TRANSCRIPT
Steering Systems and Accessoriesfor Commercial Vehicles
Many tonnes safely controlled – and what's behind it.
The steering gear is something like thesoul of the commercial vehicle. The driver is in permanent contact with it,via the steering gear he notices thecondition of the road and the responseof the vehicle. Here the quality of thesteering gear is decided – with lightoperation and comfort on the onehand and as much feedback from theroad as possible on the other. Essentialfor this is a carefully balanced inter-action of high-precision mechanics anda finely controllable hydraulic system,which we have perfected with oursteering systems. These are preferred,by the vehicle manufacturers, becausethey are compact and reliable, and, bythe drivers, because they are comfort-able and precise.
Such a high technical level is achievedonly by someone who is master of thecomplete system. That’s why we dedi-cate ourselves to all components. Steer-ing columns, steering pumps, valves,oil reservoirs and other peripheral partscontribute their share to reliability.
Steering the right way.
ZF Servocom® and ZF Servocomtronic® are registered trademarks of ZF.
2 Worth knowing
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
Ball-Track Telescopic Shafts/Ball-Track Relay Shafts . . . 35
Universal Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Pressure Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
ZF Steering Systems.Steering toward the future.
As a joint venture of Robert BoschGmbH and ZF Friedrichshafen AG, ZF Lenksysteme GmbH has producedpower steering systems for passengercars and commercial vehicles for sev-eral decades. The products fulfill everyspecified requirement. With the ZFServocom, the tried-and-tested recir-culating-ball power steering principlehas been continually further de-veloped, protected by numerouspatents, and adapted to suit specificvehicle requirements. And for a greatvariety of special applications and forcompliance with legal safety criteria,it is logic to use our semi-integral anddual-circuit power steering systems.
An outstanding result of innovativefurther development is the ZF Servo-comtronic. This steering gear desig-ned on the basis of the Servocom iselectronically controlled and speed-sensitive. It is characterized by easy,comfortable steering in maneuveringas well as a safe road feel at increas-ing speed.
New courses were also taken in thefield of rear axle steering systems.The ZF Servocom RAS (Rear AxleSteering), which is also suitable forretrofit, and the electronically con-trolled ZF Servocom RAS-EC (RearAxle Steering – Electronically Con-trolled) can completely do withoutthe steering linkage known to datewhich is heavy and imprecise.
Future-oriented innovativeapproaches regardingfunctions, design and safety areopened up
with the ZFLS Steer-by-Wire SteeringSystem. And as a member of a Euro-pean consortium of vehicle manufac-turers, suppliers, users, and institu-tions, we are preparing the imple-mentation of automatic driving ingoods traffic.
Installation schematic of a ZF Servocomwith height and tilt adjustable steeringcolumn, ball-track relay shaft, steeringpump and oil reservoir.
Worth knowing 3
action of the twisted torsion barmakes the control grooves return tothe neutral position, and the samesystem pressure will exist in both ofthe power cylinder chambers.
Basic hydraulic function of theServocom control valve
The hydraulic fluid delivered by anengine-driven steering pump (21)flows through a connecting bore in thehousing bottom, via the feed oil radialgroove (8) and transverse bores in thevalve portion of the worm (4), onwardto the three feed oil control grooves(9) of the valve slide (3). In the valveneutral position (see fig. on page 5),the oil flows, over the open feed oilcontrol 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
the valve slide and the worm. Thevalve slide and the worm run in anti-friction bearings to ensure the preci-sion of operation and the functionalsafety of the control valve even athigh pressures.
If a torque is transmitted to thevalve slide or the worm from thesteering wheel or the steeredwheels, a relative rotary motioninfluenced by the torsion bar willoccur between the valve slide andthe worm. The valve slide is therebycaused to change its position in rela-tion to the worm valve bore sur-rounding it, so that the relative posi-tions of the control grooves arechanged, too. Pressurized oil cannow flow through connecting boresto one of the two power cylinderchambers (ZL or ZR) and assists theaxial movement of the piston. Whenthe steering wheel is released, the
Mechanical construction
The ZF Servocom, a compact-designrecirculating-ball power steeringgear, basically comprises a sturdycast-iron housing (1) with integratedmechanical steering gear, controlvalve and power cylinder.
A turning movement at the steeringwheel is transmitted via the outputshaft and the valve slide (3) in thecontrol valve to the worm (4) and,via an endless ball chain (7), trans-formed into an axial movement ofthe piston (2). At the same time, thesector shaft (6), which is arranged atright angles to the longitudinal axisof the piston, is caused to rotate bythe meshing of teeth. The drop armmounted on the sector shaft movesthe steering linkage which goes tothe steering arms, thus causing thewheels to be turned.
The basic components of theServocom control valve are the valveslide, with six control grooves on itssurface area, and a worm in whosevalve bore axial grooves are pro-vided which are matched to the con-trol grooves. Centralizing the valveslide (neutral position) is done by atorsion bar (5) which provides at thesame time the connection between
4 ZF Servocom
ZF Servocom®
Figure at top:ZF Servocom, type 8098.
Figure on page 5:ZF Servocom, type 8098, control valve in neutral position.
1 Housing2 Piston3 Valve slide4 Worm5 Torsion bar6 Sector shaft7 Ball chain8 Feed oil radial groove
9 Feed oil control groove10 Feed oil control edge11 Axial groove12 Return oil control groove13 Return oil control edge14 Return oil chamber15 Radial groove16 Radial groove
17 Hydraulic steering limiter18 Pressure relief valve19 Replenishing valve 20 Flow limiting valve21 Steering pump22 Oil reservoirZL Power cylinder, leftZR Power cylinder, right
ZF Servocom 5
14
13
11
8
10
1691215
14 ZL 2 17 7 4 5 6 ZR 16 15
22
21
20
1
3
18
8
19
6 ZF Servocom
ZF Servocom, type 8098, control valve in working position.Steering wheel turned clockwise.
1 Housing2 Piston3 Valve slide4 Worm5 Torsion bar6 Sector shaft7 Ball chain8 Feed oil radial groove
9 Feed oil control groove10 Feed oil control edge11 Axial groove12 Return oil control groove13 Return oil control edge14 Return oil chamber15 Radial groove16 Radial groove
17 Hydraulic steering limiter18 Pressure relief valve19 Replenishing valve20 Flow limiting valve21 Steering pump22 Oil reservoirZL Power cylinder, leftZR Power cylinder, right
14
13
11
8
10
1691215
14 ZL 2 17 7 4 5 6 ZR 16 15
22
21
20
1
3
18
8
19
Mechanically adjustable (top) and automatically adjusting steeringlimiter (bottom).Steering limiter valve opened,oil pressure greatly reduced.
ZF Servocom 7
these grooves the oil flows back, viabores, to the return oil chamber (14)inside the worm and from there to theoil reservoir (22). At the same time,the radial grooves (15 and 16) of the control valve and their associatedconnections provide for a connectionbetween the right-hand (ZR) and left-hand power cylinder chambers (ZL).
When turning the steering wheelclockwise (fig. on page 6), the piston(2) will move to the right in the pistonbore if it is a right-hand ball thread.Due to the simultaneous rotation ofthe valve slide (3) to the right, thepressurized oil is directed, over thefurther opened feed oil control edges(10), to the three associated axial grooves (11), via bores to the radialgroove (16), and, via a connection, tothe left-hand cylinder chamber (ZL),whereby the piston movement ishydraulically assisted. An individuallyadaptable pressure build-up is achievedby the fact that the partially or fullyclosed feed oil control edges (10)restrict or prevent a connection between the pressure oil inlet and theother three axial grooves (11) con-nected to the radial groove (15). At thesame time, the pressure oil outlettoward the pressurized axial grooves isrestricted or prevented, too, by the closing return oil control edges (13).
The oil displaced by the piston (2) fromthe right-hand power cylinder cham-ber (ZR) first flows through a con-nection to the radial groove (15) and,through transverse bores, to the asso-ciated axial grooves and onward to thereturn oil control grooves (12) over thefurther opened return oil controledges (13). From here, the furtherreturn flow of the oil to the oil reser-voir (21) takes places via the con-necting bores leading to the return oilchamber (14). When the steeringwheel is turned counterclockwise, theoperating sequence will be analogousto the above.
Hydraulic steering limiter
To protect the steering linkage, thewheel lock stops and the steeringpump from excessive loads at the max-imum wheel lock angles, the ZFServocom has a mechanically adjust-able or automatically on the vehicleadjusting hydraulic steering limiter(17). This device integrated in thepiston (2) is always closed due to the
oil pressure in the right-hand or left-hand power cylinder chamber. It is onlyjust before each piston end positionthat the steering limiter valve is open-ed by the contact of the valve pin withthe adjusting screw or sleeve. Thus,that cylinder chamber which is underhigh pressure is connected with theopposite return oil chamber. As a resultof the pressure drop, hydraulic assist-ance is reduced heavily. The steeringwheel can only be turned up to theroad wheel lock stop by using in-creased effort.
Further features
The ZF Servocom is fitted with a pres-sure relief valve (18) which limits thedelivery pressure of the steering pumpat the maximum specified pressure.Also, a replenishing valve (19) can befitted to the housing or the valve slide,if required. This valve allows to suck oilfrom the return flow if the vehiclemust be steered without hydraulic as-sistance.
8 ZF Servocomtronic
ZF Servocomtronic®
Figure at top:ZF Servocomtronic – diagram of characteristiccurves. It shows the change in pressure andsteering wheel torque as a function of vehiclespeed. The course of the characteristics canbe matched to the specific character of thevehicle.
Design and function
The ZF Servocomtronic is a speed-sensi-tive recirculating-ball power steeringgear for trucks and buses. The use ofadvanced electronics, of an electro-hy-draulic transducer, and of a hydraulic
Figure on page 9:Schematic representation of theZF Servocomtronic, type 8098.
5 Steering pump6 Oil reservoir with fine
filter7 Ball-track relay shaft8 Height and tilt adjustable
steering column
1 Electronic speedometer on the vehicle
2 Electronic control unit (microprocessor, 12 V)
3 Electro-hydraulic transducer
4 ZF Servocomtronic
Pressure p [bar]
Torque required at the steering wheel [Nm]
0 km
/h
20 k
m/h
50 k
m/h
100 km/h
20 15 10 5 0 5 10 15 20
140
120
100
80
60
40
20
reaction device makes it possible tovary steering effort with vehicle speed.
Application of the ZF Servocomtronicrequires either an electronic speed-ometer (1) or a suitable ABS controlunit. The speed signals coming fromone of these units are transmitted tothe electronic control unit (2) whichcan either be a separate component orintegrated in the existing vehicle elec-tronics. The signals are analyzed by themicroprocessor of the Servocomtroniccontrol unit and converted into a con-trolled electric current which actuatesthe electro-hydraulic transducer. Onthe basis of this influence, the trans-ducer, which is directly attached to thehousing cover, determines the hydraulicreaction of the control valve and, thus,the amount of input torque at thesteering wheel.
This speed-dependent influencing ofsteering ensures that static steering aswell as steering at low vehicle speeds,e.g. in parking maneuvers, requireminimal effort. As the hydraulic reac-tion changes in proportion to the vehi-cle speed, the steering effort increasesas the vehicle goes faster (see fig. attop). At higher speeds the driver thushas particularly good road contact andis able to steer the vehicle preciselyand with directional stability.
A further advantage of the ZF Servo-comtronic is that oil pressure and flowrate are never reduced and can there-fore be utilized immediately in emer-gencies where sudden and unexpectedsteering corrections may become necessary. These features bring aboutextraordinary precision of steering, together with a high safety standardand optimum steering comfort.
Mechanical construction
The basic steering gear for the ZF Ser-vocomtronic is the well-proven ZF Ser-vocom – millions of which have alreadybeen fitted – based on the design andoperating principle described on pages4 to 7. The difference from the descrip-tion there is that in the axial bore ofthe worm a torsionally resistant bel-lows (5, see fig. on page 10) is arrangedwhose lower end is connected with theworm such that rotation is prevented.
The upper part of the bellows forms acentering bush (26) and presses, withan axial spring force which is matchedto the specific vehicle, against a prismcentering device with two rollers (27).When driving straight ahead, this hasespecially positive effects on the exactcentralizing of the control valve. Whensubjected to hydraulic pressure, a reac-tion piston (28) located in the cylindercover concentrically with the wormalso applies, via a spacer tube (29),load on the prism centering device andmakes necessary, in addition to thespring force of the bellows, furtherforce for the displacement of the con-trol valve from its neutral position. Theamount of that hydraulic reaction isdetermined by the indicated instan-taneous vehicle speed and the opening
position of the electro-hydraulictransducer resulting from this.
ZF Servocomtronic 9
1
2
8
7
4
6
5
3
+–
10 ZF Servocomtronic
ZF Servocomtronic, type 8098.Control valve in working position. Steering wheel turnedclockwise, driving at high speed, transducer valve fully opened, maximum reaction limited by cut-off valve.
14
13
11
8
10
1691215
1430 ZL 2 17 7 4 29
24 25 23
31 32 28
5 ZR 16 15
22
21
20
27
26
6
1
3
18
8
19
18 Pressure relief valve19 Replenishing valve20 Flow limiting valve21 Steering pump22 Oil reservoir23 Electronic speedometer24 Electronic control unit (12 V)25 Electro-hydraulic transducer26 Centering bush
27 Roller28 Reaction piston29 Tube30 Reaction chamber31 Orifice32 Cut-off valveZL Power cylinder, leftZR Power cylinder, right
9 Feed oil control groove10 Feed oil control edge11 Axial groove12 Return oil control groove13 Return oil control edge14 Return oil chamber15 Radial groove16 Radial groove17 Hydraulic steering limiter
1 Housing2 Piston3 Valve slide4 Worm5 Bellows6 Sector shaft7 Ball chain8 Feed oil radial
groove
Function of the ZF Servocomtronic
At low speeds, e.g. in maneuvering,the electronic speedometer (23) or theABS control unit transmit very few sig-nals to the microprocessor integratedinto the electronic control unit (24).The microprocessor analyzes the sig-nals and passes them to the electro-hydraulic transducer (25) in the formof a correspondingly adapted controlcurrent. Due to the maximum currentexisting in that driving mode, thetransducer valve closes and preventspressure build-up in the reactionchamber (30). An orifice (31) ensuresthat there is also return pressure levelin the reaction chamber. Thus, owingto the elimination of reaction, thesteering is light in operation and canbe handled with little effort.
As the driving speed increases, thespeed signals become more frequentand, after having been converted bythe microprocessor, cause a reductionin the amount of control current trans-mitted to the electro-hydraulic trans-ducer. As a result, the transducer valvetakes up an opening position adaptedto the instantaneous vehicle speed and
allows a limited oil supply from thefeed oil radial groove (8), via a tube,to the reaction chamber (30). Via thespacer tube, the reaction piston nowpresses against the prism centeringdevice and makes necessary more forcefor the displacement of the controlvalve. Thus, this mode of operation ofthe hydraulic reaction requires an indi-vidually established higher steeringwheel torque until a determinedhydraulic assistance is raised in theright-hand (ZR) or left-hand cylinderchamber (ZL).
At high driving speeds (fig. on page10), for instance on the motorway, thetransducer valve is fully open owing toa very low or non-existing control cur-rent for the actuation of the trans-ducer. This enables maximum pressuresupply from the feed oil radial groove(8) to the reaction device. When thesteering wheel is turned clockwise, thereaction pressure increases in accord-ance with the existing operating pressure and pressurizes the reactionpiston from the reaction chamber (30).As soon as the reaction pressure deter-mined for a specific vehicle reaches itsupper limit, the oil is discharged to thereturn oil chamber (14) through the
cut-off valve (32) to avoid a furtherincrease in reaction pressure. The inputtorque at the steering wheel thusachieved will not now rise any moreand gives a safe driving feel owing tooptimum road contact.
Safety of the ZF Servocomtronic
Even in the event of a failure of thevehicle electrical system or any otherelectrical fault, the steering gearremains fully operational. In suchexceptional cases the ZF Servocom-tronic will work at maximum hydraulicreaction (high-speed characteristic)due to the mechanically forced open-ing of the transducer valve. Whenspeed signals all of a sudden are nottransmitted any more during driving,for instance due to lack of cable con-tact or a defective speedometer, thehighly advanced microprocessor in theelectronic control unit is in a positionto derive a constant control currentfrom the last speed signals evaluated.This ensures a constant steering per-formance until the vehicle engine isturned off. When the engine is startedagain, maximum hydraulic reactionconforming to the high-speed charac-teristic will develop again.
ZF Servocomtronic 11
ZF Servocomtronic, type 8098.
12 ZF Dual-circuit steering system
ZF Dual-Circuit Steering System with 2 Pumps
ZF Servocom, type 8099.Modified dual-circuit steering system.
piston (9) direct the pressure oil fromthe wheel-driven steering pump 2 tothe control valve of the steeringsystem. The second switching piston(10) shuts off the connection to thepower cylinder, which means that thelower output flow from the wheel-driven pump is available for the safeoperation of the steering system. This
condition is indicated to the driver bymeans of a pilot lamp. Also, the steer-ing wheel effort is increased in extremesteering situations, due to the inactivepower cylinder.
Fig. on page 13:Schematic representation of themodified dual-circuit steeringsystem, ZF Servocom, type 8099(right-hand).
Normal function for right-hand turning. Both pumps deliver oil.Power cylinder with pressure assistance.
1 Steering pump 12 Steering pump 23 Oil reservoir 14 Oil reservoir 25 Power cylinder6 Change-over valve7 Contact switches
8 Feed oil radial groove9 Switching piston (inside)
10 Switching piston (outside)11 Check valve12 Check valveZL Power cylinder, leftZR Power cylinder, right
Design and function
The installation of dual-circuit steeringsystems is required for vehicles withhigh steered axle loads or with morethan one steered axle in order to meetstatutory safety criteria. These require-ments are met by the modified dual-circuit steering system with 2 pumpswhich builds on the basic concept ofthe ZF Servocom (see pages 4 to 7).
A crucial component in this system isthe change-over valve (6) adapted atthe input stub shaft end. In normaloperation it monitors the readiness foroperation of the engine-driven steer-ing pump 1 (1) and ensures the con-nection to a power cylinder (5) which isnecessary for additional comfortablesteering assistance. The change-overvalve delivers the oil flow generatedby 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, forinstance when the engine has stoppedor the performance of the engine-driven pump is reduced, the two switching pistons (9 and 10) are pressedwith spring force against the associ-ated contact switches (7), owing to theinsufficient pump pressure. At thesame time, this makes one switching
ZR
ZR
12
8ZL
ZL 106
5
9
3
4
2
1
11
7 1
3 4
2
ZF Dual-circuit steering system 13
14 ZF Dual-circuit steering system
Schematic representation of the modifieddual-circuit steering system, ZF Servocom, type 8099 (right-hand).
Emergency function for right-hand turning.Only the wheel-driven steering pump deli-vers oil. Power cylinder without pressureassistance.
1 Steering pump 12 Steering pump 23 Oil reservoir 14 Oil reservoir 25 Power cylinder6 Change-over valve7 Contact switches
8 Feed oil radial groove9 Switching piston (inside)
10 Switching piston (outside)11 Check valve12 Check valveZL Power cylinder, leftZR Power cylinder, right
ZR
ZR
12
8ZL
ZL 109
5
6
3
4
2
1
11
71
3 4
2
ZF Servocom, type 8099.Modified dual-circuit steering system.
Basic hydraulic function of thechange-over valve
The oil flow generated by the engine-driven steering pump 1 (1) after en-gine start will pressurize the inner (9)and outer switching piston (10) in thechange-over valve (6) and displaceboth in a direction contrary to the con-tact switches (7) and against a specif-ically determined spring force.
At that position of the inner switchingpiston, the pressure oil can get to thefeed oil radial groove (8) on the con-trol valve via an internal connection asis the case on a standard ZF Servocom.Via the opened radial groove, thereturn oil from the cylinder chambers(ZL/ZR) flows back to the oil reservoir 1(3). The oil flow from the wheel-drivensteering pump (2) is directed back tothe oil reservoir 2 (4), without beingutilized, via the opened radial groove.The position of the outer switchingpiston allows the free exchange ofpressure oil between the associatedcylinder chambers (ZL/ZR) and theseparate power cylinder (5).
When the engine has stopped or a spe-cifically determined amount of oil (fig.on page 14) is not reached, both switching pistons are pressed againstthe contact switches by spring force.
This position of the inner switchingpiston allows the supply flow of thepressure oil generated by the wheel-driven steering pump, via a connection,to the feed oil radial groove (8). Thedirect return flow to the oil reservoir 2(4) which takes place in normal opera-tion is prevented. Additionally, a checkvalve (11) prevents the outflow of oil tothe oil reservoir 1 (1) via the engine-driven steering pump. Now the returnoil flowing from the cylinder chambers(ZL/ZR) passes back to the oil reservoir 2(4) via the opened radial groove. In thisswitching mode, an outflow of thereturn oil to the oil reservoir 1 (3) is notpossible.
At that position, the outer switchingpiston (10) completely shuts off theconnection between the cylinderchambers (ZL/ZR) and the power cylin-der. This means that the steering gearpressure or return oil respectively,which is radially available at the switching piston, cannot pass throughto the two radial grooves on the switching piston. But, via a channelsystem, these radial grooves permit themutual exchange of oil in the nowinactive hydraulic circuit of the powercylinder. Check valves (12) between theshut-off hydraulic circuit and the cylinder chambers provide a balancedpressure level.
ZF Dual-circuit steering system 15
16 ZF Dual-circuit steering system
ZF Dual-Circuit Steering Systemwith 3 Pumps
Figure on page 17:Schematic representation of the dual-circuit 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.
1 Steering pump 12 Standby valve3 Valve block4 Control valve 15 Power cylinder6 Steering pump 27 Flow indicator8 Oil reservoir 19 Steering pump 3
10 Oil reservoir 211 Flow indicator12 Control valve 213 Power cylinder14 Steering limiter valve15 Radial groove16 Radial groove17 Piston position indicator
Design and function
The installation of dual-circuit steeringsystems with 3 pumps is required forcommercial vehicles with especiallyhigh steering axle loads or with several
steered axles in order to meet the stat-utory safety criteria and to ensure thenecessary steering comfort. The steer-ing gear used is the basic concept ofthe ZF Servocom (see pages 4 to 7).
The high safety potential of this steer-ing system is based on the existence oftwo steering circuits which are comple-tely separate from each other andcomplement each other favorably innormal operation. Steering circuit I issupplied, on a priority basis, with pres-sure oil from the an engine-drivensteering pump 1 (1). From a deter-mined oil flow, a standby valve (2) inthe valve block (3) controls the supplyof the oil from that pump, via the con-trol valve 1 (4), to the steering gearand to the parallel-connected powercylinder (5) that may be fitted in theparticular case. At the describedswitching position of the standbyvalve, the oil flow generated by thewheel-driven steering pump 2 (6) is, atthe same time, delivered back to thecommon oil reservoir 1 (8), withoutbeing utilized, but monitored by aflow indicator (7).
In steering circuit II, the oil pressur-izing, via the control valve 2 (12), thepower cylinder (13) fitted downstreamis also supplied by an engine-drivenpump 3 (9). A flow indicator (11) be-tween the control valve 2 and theseparate oil reservoir 2 (10) monitorsthe flowing oil.
ZF Dual-circuit steering system 17
8
10
Steering circuit I Steering circuit II
9
11
4
12
15
16
13145
6
1
7
3
2
17
18 ZF Dual-circuit steering system
Schematic representation of the dual-circuitsteering system ZF Servocom, type 8099 (right-hand).
Emergency function for left-hand turning.Both control valves in working position. Onlythe wheel-driven steering pump delivers oil.
1 Steering pump 12 Standby valve3 Valve block4 Control valve 15 Power cylinder6 Steering pump 27 Flow indicator8 Oil reservoir 19 Steering pump 3
10 Oil reservoir 211 Flow indicator12 Control valve 213 Power cylinder 14 Steering limiter valve15 Radial groove16 Radial groove17 Piston position indicator
8
10
Steering circuit I Steering circuit II
9
11
4
12
15
16
13145
6
1
7
3
217
ZF Dual-circuit steering system 19
Installation schematic of a ZF Servocomdual-circuit steering system with 3 pumps, 2 oil reservoirs, valve block, flow indicator,power cylinder, steering column andball-track relay shaft.
Only in an emergency, which is veryrare, for instance when the engine hasstopped, the system is limited to thesafe operation of steering circuit I. Dueto the lack of oil flow from the stoppedsteering pump 1, the standby valve isnot moved, which means that the pres-sure oil flow generated by the wheel-driven steering pump 2 is now directed,through the standby valve, to the con-trol valve 1 and can thus be effective inthe steering gear and power cylinder.At the same time, the standby valveprevents the outflow of oil through theflow indicator and indicates that condi-tion to the driver by means of a pilotlamp. Additionally, a piston positionindicator (17) monitors the function ofthe standby valve. A flow indicator be-tween the control valve 2 and the oilreservoir 2 notifies the driver, by meansof a further pilot lamp, of the lack ofoil flow. When the pilot lamps go on,an early check and maintenance of thesteeering system will be necessary forsafety reasons.
Hydraulic steering limiter
To protect the steering linkage, wheellock stops and steering pump fromexcessive loads, both steering circuitsare usually equipped with a hydraulicsteering limiter. The hydraulic steeringassistance in steering circuit I is limited
in the same manner as on the standardsingle-circuit steering gear (see pages 4 to 7).
In steering circuit II, however, the indi-vidually determinable reduction ofpressure in the power cylinder is controlled by two adjustable steeringlimiter valves (14). These are arranged
in the housing cover of the steeringgear and are opened by a cam of thesector shaft. Via pipes, both valves areconnected to the respective radialgrooves (15 and 16) of control valve 2and thus, indirectly, also to the lines tothe power cylinder.
20 ZF Servocom RAS
ZF Servocom® RASRear Axle Steering System
Schematic representation of the rear axlesteering system ZF Servocom RAS, type 8098.
1 ZF Servocom2 Master cylinder3 Hydraulic accumulator4 Centering cylinder5 Additional lines
System description
The rear axle steering system ZF Servo-com RAS (Rear Axle Steering) is opti-mized both with regard to cost andweight and is particularly suitable forthe forced steering of nondriven rearaxles. The steering work is done by thefront axle steering system (1) and apower cylinder portion in the mastercylinder (2).
Up to now, comparable systems havepredominantly been implementedaccording to the mechanical principle,consisting of drag links, intermediatearms, etc. Such mechanical transmis-sion systems need a lot of installationspace because of the swivelling rangesof the levers and drag links. In add-ition, a fairly heavy weight is typical ofsuch systems. Another disadvantage isthe inadequate steering stiffness, i.e.because of the elasticity of the trans-mission parts the steered rear axle isnot stabilized enough and is prone tooscillations during straight line driving.
Compared to mechanical steering sys-tems, the rear axle steering system ZFServocom RAS achieves better resultsin many respects.
The content of the RAS system is basic-ally made up of 2 special power cylin-ders and a hydraulic accumulator (3).In conformity with the steering anglemovement, the master cylinder (2) fit-ted at the front axle pumps oil into theassociated cylinder chamber of thecentering cylinder (4) arranged at thethe steered rear axle. In this way, thesteering movement of the front axle is
hydrostatically transmitted to the rearaxle. The steering angle ratio betweenfront axle and rear axle is determinedby the vehicle manufacturer by ratingthe length of the steering arms appro-priately.
The master cylinder is fitted with anautomatic synchronizer system and apressure relief valve. In a definablesteering angle range of the front axle,e.g. up to about ± 5°, both cylinderchambers of the master cylinder areshortcircuited. Within this small steer-
2
4
5
1
3
ZF Servocom RAS 21
ing angle range of the front axle, therear axle is put and kept in straightahead position with the help of ahydraulic centering device in the cen-tering cylinder. Owing to this automa-tic synchronization, hydrostatic mis-matches in the cylinders, which seldomoccur, can be compensated for.
At steering angles of more than about± 5° at the front axle, the cylinderchambers of the master cylinder areseparated from each other and sealedhermetically. This means that at steer-ing angles at the front axle exceeding± 5° the hydrostatic steering system isin operation. A control system in themaster cylinder prevents that instraight ahead position of the frontaxle and at high actuating forces at thesteered rear axle, caused for example
by driving over obstacles in forward or reverse, the kinematic conditionschange considerably.
An accumulator is assigned to thehydrostatic system. The function of thisaccumulator is to optimize the stiffnessof the hydostatic transmission systemby preloading the transmission lineswith the centering pressure. Besides,the accumulator pressure acts perman-ently on the two hydraulic centeringpistons which are integrated into thecentering cylinder. Experience hasshown that the centering and preloadpressure is approximately 15 bar andthus meets the safety criteria requiredfor driving without hydraulic assist-ance. If due to the extra steering workperformed by the rear axle the steer-ing comfort becomes too low, i.e. the
hydraulic pressure reserve becomestoo small, the master cylinder can besubjected to steering pressure via add-itional lines (5) in order to provide suf-ficient steering comfort, e.g. duringstatic steering.
The ZF Servocom RAS concept does notneed additional electric and electroniccomponents and is also suitable forlater installation because of a smallvariety of parts. What has to be stres-sed particularly is the improvement invehicle dynamics during straight linedriving due to the hydraulically cen-tered rear axle. Better maneuverabil-ity, less tire wear, and fuel saving arefurther assets of the system.
4
2
Application
Modern commercial vehicles have tocorrespond more and more to therequirements of the clients for econ-omy, environmental protection, legisla-tion and special applications. To this,the electronically controlled rear axlesteering system ZF Servocom RAS-EC(Rear Axle Steering – ElectronicallyControlled) makes an important con-tribution and at the same time it offersa basis for entirely new solutions for
ZF Servocom® RAS-ECRear Axle Steering System
ZF Servocom RAS-EC. Basic diagramfor vehicle 6x2x4 with steered trailing axle (nondriven).
the development of commercial ve-hicles. That's because the ZF RAS-EC isparticularly suitable for commercialvehicles with a very big wheelbase andseveral rear axles.
The increasingly heavy traffic as well asmodern city planning concepts requirecommercial vehicles with a big loadingcapacity and good maneuverability inorder to supply and dispose of thegoods for the city centers. The activerear axle steering system increases themaneuverability of the vehicle, par-ticularly during parking maneuvers,because here, in contrast to adhesion-
steered axles, steering operationsare also carried out when the
vehicle is reversed. Owing tothe optimum kinematic
conditions, tire wear isreduced and the trac-
tion at this axle isincreased. Theforced steering ofrear axles offers
the additional advantage of an imme-diate build-up of side force and there-fore leads to an increase in vehicle sta-bility and driving safety.
The RAS-EC does not require a mechan-ical connection between the front andrear axles, which makes the adaptationin series production as well as retrofit-ting a lot easier. The possibility of end-of-assembly-line programming of theelectronic control opens up an adapta-tion to changed geometric quantities(wheelbase, steering angle) or to aparticular application pattern of thevehicle. Because of that, no modifica-tion of mechanical power transmissioncomponents is necessary.
Due to the electronic control, the steer-ing angle at the rear axle can be alter-ed as a function of whatever drivingparameters are chosen. This means forexample that by opposite steeringmotions of rear and front wheels inthe low speed range maximum maneu-verability can be achieved. Same-direc-tion rear/front wheel steering motionsat medium and high speeds lead toexact straight line driving and vehiclestabilization during lane changes.Special steering programs, such as rearswing-out minimization or constantratio with same-direction rear/frontwheel steering motions in the lowspeed range, additionally increase the
11
8
10
9
12
7
4
5
6
1
3
2
22 ZF Servocom RAS-EC
ZF Servocom RAS-EC 23
1 ZF Servocom2 Steering angle sensor3 CAN-Bus4 Diagnosis5 Oil reservoir6 Steering pump7 Pressure filter
8 Control valve9 Electronic control unit
10 24V supply 11 Power cylinder12 Turning angle sensor13 Power cylinder
with linear sensor
ZF Servocom RAS-EC. Basic diagramfor vehicle 6x2x4 with steered leading axle (nondriven).
advantages of the RAS-EC. The elec-tronic control of the rear axle steeringsystem is provided with a diagnosticinterface (KWP 2000). Dectected errorsare stored in the memory and can beread out for service and/or for trouble-shooting purposes.
System description
The requirements for rear axle steeringsystems are described in the directiveEC 70/311. When an error in the steer-ing system occurs, no safety crit-ical driving situation is permissible.Derived from this, the safety conceptfor the RAS-EC is as follows:
• For nondriven axles, with self-cen-tering characteristics the ”axle free“safe mode is recommended, i.e. theaxle will then work like a trailingaxle.
Precondition for this safety philosophyis a redundant electronic system sothat system errors are detected andthe appropriate safety reactions areinitiated.
Mechanical constructionHydraulic components
On a ZF RAS-EC, the operating circuit is supplied with pressure oil from anengine-driven pump. The required
steering movement of the rear axle isintroduced by a proportional valveconnected to the electric control unit.The necessary steering forces are gener-ated by a hydraulic power cylinder atthe rear axle. By means of a pressurerelief valve the components are pro-tected from overload. If an error oc-curs, the pressurization of the oper-ating circuit can be removed with acut-off valve which is switched throughin the zero-current condition.
Sensors
For the measurement of the instantan-eous values at the front and rear axlesnon-contact sensors are used. Thesesensors have a long service life, oper-ate over a large temperature rangeand are accommodated in a watertighthousing. To make it possible to checkthe proper function of these sensors,they are of redundant design andintrinsically safe.
8
10
9
13
2
7
4
5
6
1
3
24 ZF Servocom RAS-EC
ZF Servocom RAS-EC. Basic diagram of asteering angle characteristic curve.
Electrical structure
The control unit is a system with 2channels with mutual monitoring ofthe readiness for operation of thechannels and includes all the compon-ents necessary for sensor evaluation,nominal-value calculation and valveactuation.
The control circuit basically consists ofa 16-bit high-capacity microcontroller.This computer acquires all incomingdata and carries out a plausibilitycheck. Using a graph of steering anglecharacteristics, the nominal value ofthe rear axle is generated from thevehicle speed and the front axle steer-ing angle data. Then, from the nomin-al value and the steering angle of therear axle, the quantity for the actu-ation of the control valve is computed.The valves are actuated via short-cir-cuit-proof outputs and their function isthen reread and checked by the micro-controller.
In order to adapt the function of therear axle steering system to the vehicleconditions additional sensors can be
read in. Thus, due to the axle geom-etry, actuation of the rear axle is notpossible when the vehicle is at a stand-still and the brake is applied at thesame time. This for example can benoticed when the control unit alsoreads in the brake pressure. In thiscase, the rear axle is not actuated untileither the brake has been released or aspeed is measured.
The safety circuit includes an 8-bitmicrocontroller. This device also ac-quires all incoming data and carries outa plausibility check. The nominal valueof the rear axle calculated by it is com-pared with the actual value. The con-trol computer of the operating circuitand the monitoring computer of thesafety circuit are connected with eachother via an interface and can thusexchange and compare input, inter-mediate and output quantities. Add-itionally, status information is exchan-ged via this interface in order to guaran-tee and check the correct function ofthe rear axle steering gear. If an erroris detected, both the control computerand the monitoring computer are in aposition to automatically switch off
the outputs and thus initiate the safemode. For the localization of errorsand/or for the maintenance of the rearaxle steering system, the control unithas full diagnostic capability. Thus, thevalues from the sensors for instancecan be read out and checked via thediagnostic interface (KWP 2000). Anintegrated CAN interface allows dataexchange with other control unitsduring operation. By means of theintegrated diagnostic interface thecontrol unit can be programmed at theend of the assembly line, and duringvehicle production it can be adaptedto the conditions on the target vehicle.If necessary, this can for instance beused to carry out an electronic align-ment of the sensors.
Special functions
The system allows the processing ofnumerous signals such as those frombrake, door switch and other switches.Together with the speed signal, thefront axle steering angle etc. it’s pos-sible to implement a variety of specialfunctions.
0 10 20 30 40 50
VA_LW_max[°]*
12
10
8
6
4
2
0
0-15 km/h
20 km/h
25 km/h
Steering anglefront axle [°]
V [km/h]
Vneutral [km/h]*
max. steering angle rear axle*
* These parameters can be programmed at the end of the assembly line.
I Steering angle I rear axle [°]
ZF Semi-Integral Power Steering Gears
Installation schematic of a semi-integralpower steering gear type 7421, dual-circuit design on a mobile crane. Withengine-driven vane pump, wheel-drivenradial piston pump and with one powercylinder each at the steering axles.
Application
Steering gears of this type are used onvehicles which require high steeringeffort due to their high steering axleload and on which the required pistondisplacement exceeds the volumewhich can economically be accommo-dated in the power cylinder of a steer-ing gear of integral design. Applica-tion begins at steering axle loads ofapprox. 8 tonnes on vehicles with kingpin steering.
Another possibility for the use of semi-integral power steering gears existswhen, due to its length or offset, thedrag link is unable to transmit therequired steering forces. The numberand size of the power cylinders usedcan be chosen such that at the max-imum steering forces which occur andat the required steering wheel turningrate the vehicle can be steered withfull hydraulic assistance.
Design and function
The semi-integral power steering gear(single-circuit design) comprises a com-plete mechanical steering gear. In it,the steering effort is transmitted fromthe input shaft, via a ball screw thread,to the steering nut and from there, viaa tooth system, to the sector shaft. Thesteering nut is moved up and down by
turning movements, thus causing thesector shaft to rotate.
The control valve is located in the valvehousing concentrically with the inputshaft. When the worm shaft is rotated,the control valve is moved axially backand forth. This will displace the controledges such that the pressure oil getsfrom the steering pump to one powercylinder chamber. When the steeringwheel is released, the valve is returnedto its neutral position by spring force;the return flow is thus maintained. Thevalve housing is also provided with theconnections for the pressure and re-turn lines as well as for the lines to thepower cylinder.
Mechanical steering limiter
Depending on customers’ requests, thesemi-integral power steering gears canbe fitted with mechanical steeringlimiter. It will avoid that the wheels areturned up to the wheel lock stops withfull hydraulic pressure. This protectsthe steering linkage components fromexcessive loads.
ZF Semi-integral power steering gears 25
26 ZF Semi-integral power steering gears
Semi-integral power steering gear, type7421, dual-circuit design with mechanicalsteering limiter.
Semi-integral power steeringgear, dual-circuit design
Extra-heavy and special-purpose ve-hicles with very high steering axle loadsand speeds in excess of 62 km/h cannotusually be steered without exceedingthe steering effort limits prescribed bylaw if hydraulic assistance fails. Forsuch applications our dual-circuit steer-ing gears of semi-integral design withtwo independent steering valves forthe control of the pressure oil in twoentirely separate circuits are available.Vehicles thus equipped can still besteered fully even if pressure oil is lost,for instance due to pipe breakage inone steering circuit. Normally, one cir-cuit of the steering system is fed by anengine-driven pump and the other bya wheel-driven steering pump.
To allow the wheels to be turned evenif the engine is stopped, pressure oilsupply to the two circuits by one en-gine-driven steering pump each hasbeen provided for. Additionally, one of
the steering circuits is provided with awheel-driven emergency steeringpump whose output flow is directed,under normal operating conditions ofthe two engine-driven pumps, into thereturn flow to the oil reservoir. If theengine-driven pump fails, the pressur-ized oil from the emergency steering
pump is automatically supplied to thesteering gear via a standby valve, withthe result that vehicle steerability ismaintained in any event. Thus, dual-circuit semi-integral power steeringgears are an important contribution tosafety in road traffic.
Figure at top:Semi-integral power steering gear, type 7421,single-circuit design with mechanical steeringlimiter and flange-mounted bevel box.
ZF Steering Pumps
ZF Steering Pumps 27
Representation of the function of the ZF vane pump FN 4, type 7685.
1 Housing2 Shaft3 Rotor4 Vane5 Cam ring6 Pressure plate7 Cover8 Pressure relief and flow limiting valve
Application
The main function of a steering pumpis to generate a sufficient amount ofthe oil flow required for operating ahydraulic steering system. Convincingadvantages which suggest the con-sistent installation of steering pumpsfrom ZF Lenksysteme GmbH on com-mercial vehicles are in particular thecompact design, the high efficiency atlow weight as well as the possibilitiesof individual adaptation due to modu-lar system configuration. Dependingon the type of pump, such pumps canbe attached to the vehicle engine orthe compressor. For driving them, dif-ferent elements of transmission such asV-belt pulleys, cross-slotted discs orgears are utilized. Radial piston pumpswhich act as emergency steeringpumps are wheel-driven, i.e. by an axleor the gearbox output.
ZF Vane pump FN 4
On this type of pump, the pumpingelement which basically consists of arotor (3), ten vanes (4), a cam ring (5)and the pressure plate (6) is accommo-dated in the light-alloy housing (1).The axial end is formed by a cover (7),which is also made of light alloy, witha sealing face made to conform to the
pressure plate. These sealing faceshave two suction and pressure zonesopposite to each other. Thus, duringeach revolution the ten chambers for-med by the vanes located in the rotordeliver an amount of oil which is twicetheir chamber volume. Also, this double
arrangement of the suction and pres-sure zones neutralizes the radialhydraulic loads acting on the rotor. Thepressure between the vanes and thecam ring, which is decisive for func-tion, is produced by the radial centri-fugal force of the vanes when the
8
2 7
45
61
3
28 ZF Steering pumps
Figure at top:ZF Vane pump FN 4, type 7685.Longitudinally fitted pressure relief andflow limiting valve.
Right-hand figure:ZF Vane pump FN 31, type 7685. With short-length housing and transverse-ly fitted flow limiting valve.
drive shaft is rotated. Additional pres-sure is achieved by pressurizing theinside faces of the vanes with pressureoil.
The oil flow generated in the crescent-shaped pressure chambers is suppliedto the flow limiting valve (8) position-ed longitudinally to the shaft (2) andlimited at a set value. If relief of thetype-specific maximum pressure of thepump (165 or 180 bar respectively) isnot carried out within the steeringgear as is recommended, this can in aspecific instance be done by means ofa pressure relief valve integrated in theflow limiting valve.
The design of the vane pump FN 4 ismainly intended for attachment to theair compressor or to the generator bymeans of a cross-slotted disc for thetransmission of torque free from radial
ZF Vane pump FN 31
The basis for the modular-design pumpis the short-length light-alloy cover. Itincludes a pumping element whichfeatures two pressure plates and isequal in its principle of hydraulic oper-ation to the vane pump FN 4 describedabove. The cover also incorporates theflow limiting valve (if required, withintegrated pressure relief), arranged atright angles to the drive shaft, and thesuction and pressure ports. The pres-sure port is optionally feasible on theright or left. The symmetrical flangebolt pattern of housing and coverallows fitting in any one of 4x90° pos-itions.
The variable use of this pump is alsobased on the rugged shaft bearingsystem in the light-alloy housing. Thepump is perfectly suitable to safelyabsorb the axial and radial drive loadsif a clutch disc, V-belt pulley or gear isused. In a gear drive, the drive-sideantifriction bearing can be lubricatedby engine oil.
load. With a view to technical neces-sity and economic volume of produc-tion, it is also possible to integrate intothis pump type design features whichallow radial drive loads, for examplevia a gear, and a pressure level of 200bar maximum.
It is also possible to mount the oilreservoir directly on the pump. Thissaves a hose and assembly costs at thevehicle manufacturer.
ZF Radial piston pump, type 8605,with 8 cylinders and suction-controlled output flow.
ZF Vane pump FN 32
The use of this pump, which is largelyidentical to the FN 31 range, is appro-priate in applications where a compara-tively high displacement is needed.The theoretical displacement is 32cm3/revolution.
ZF Tandem pump TN 4
This pump combination consists of avane pump FN 4 and a gear pumpwhose housings are bolted together.Both systems are driven by the samepump shaft, but generate two inde-pendent output flows. While the vanepump is used for the pressure supply tothe hydraulic steering system, it is thefunction of the gear pump to ensurethe presupply of fuel. The drive, freefrom radial loading, is by the air com-pressor.
ZF Tandem pump TN 31
The basis of this pump type is the vanepump FN 31 and a gear pump. Again,both housings are bolted together,and both systems are driven by thesame pump shaft. Two independentoutput flows are generated. The pres-sure oil from the vane pump is againintended for the supply to hydraulicsteering systems, whereas the gearpump is used for the presupply of fuel.
ZF Radial piston pump
The same direction of delivery of theradial piston pump for both clockwiseand counterclockwise direction of driveis the reason for its principal appli-cation as a wheel-driven emergencysteering pump on commercial vehicles.Besides the high pressure level of 200bar maximum, the suction control is ofparticular importance. It ensures thatthe output flow remains the same inspite of varying pump speed and onlythat amount of oil is sucked in which isnecessary for steering. Thus, no flowlimiting valve is required.
Depending on the type, several pistonsare driven by an eccentric shaft support-ed on antifriction bearings and per-form a stroke in radially arranged cylin-der bores. Preloaded compressionsprings provide for the return of thepistons. In the process, the pistons areimmersed in the oil-filled suction cham-ber and their interior can be filled withoil through transverse bores. The sub-sequent discharge stroke forces the oilinto the pressure line. Automatic outletvalves prevent the pressure oil from flowing back into the piston chamber.Depending on type and application,pressure relief is by a valve either in thepump or at some other suitable loca-tion of the hydraulic system.
ZF Steering pumps 29
30 ZF Oil reservoirs
ZF Oil Reservoirs
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.
The oil reservoirs designed by ZFLenksysteme GmbH for commercialvehicles are matched to the specificrequirements of such vehicles, whichoffers the manufacturers the completeperipheral equipment for hydraulicsteering systems from a single source.
Depending on type and application,the oil reservoirs made of heat-resist-ant plastic or sheet steel have a fillingcapacity of 0.75 to 2.3 dm3. Plastic oilreservoirs are fitted with a filter withintegrated filter safety valve. Thesheet-steel oil reservoir also has, inaddition to a micro-filter cartridge, asafety valve, and for special applica-tions it can be provided with a replen-ishing valve and standby capacity.
For oil level checks, the dipstick is mark-ed to show the minimum and max-imum levels. All oil reservoirs can alter-natively be fitted with an electric oil level indicator. In this case, a pilotlamp on the dashboard indicates whenthe oil level drops to the specified min-imum. And by using a transparentreservoir top a rapid visual check can bemade from outside.
2
1
3
ZF Power Cylinders
ZF Power cylinders 31
As well as being used in industry and intoolmaking and machine building,double-acting power cylinders areindispensable for operating semi-inte-gral power steering gears, dual-circuitsteering systems or ZF Servocom RAS-EC rear axle steering systems. They arealso used to assist hydraulic powersteering gears if the torque of the steer-ing gear alone is not sufficient. Varioussizes and different types of mountingwith thread, ball joints, articulated
support or end fitting are decisive for the universal possibilities of appli-cation of this component.
The inner diameter of the cylindertube of the power cylinders is designedas a slide face for the piston, and thecylinder tube is closed on one side by aweld-on end piece. On the oppositeside, a piston rod guide is used whosefunction is to seal and guide the pistonrod. The piston attached to the piston
rod separates the two cylinder cham-bers and, depending on pressurization,displaces the piston rod or maintainsits position. Pressure oil is supplied tothe two cylinder chambers through the lines fitted on the threaded con-nectors. A high degree of corrosionprotection is achieved by using a cor-rosion-resistant piston rod and anelectro-zinc-plated cylinder surface.
The dual-circuit cylinder is preferen-tially used in confined installation spa-ces. Integrated in it are, in effect, twopower cylinders which can work eithersynchronously or individually.
ZF Power cylinder, type 8346, standard range.
32 ZF Steering columns
ZF Steering Columns
ZF Steering column, type 7360. With light-alloy mounting bracket and externally mounted gas-filled spring.
Application
There is no doubt that an ergonomic-ally designed driver's place on trucks,buses and special vehicles has favor-able effects on the driver's well-being.The use of a ZF steering column whichallows continuous adjustment ofheight and tilt of steering wheel, coup-led with further comfort functions, isespecially advantageous. Requirements and design
Decades of experience in the manufac-ture of steering columns have enabledus to meet the requirements detailedbelow to a very high degree and toimprove on these by innovative designwork:
• Low friction so as not to adverselyaffect the self-centering characteris-tics of the steering gear and to allowhysteresis-free transmission of theinput torques.
• Optimum arrangement of the uni-versal joints in order to minimize in-put torque variations while steering.
• Minimal sliding forces in the ball-track telescopic shafts or ball-trackrelay shafts to avoid unwantedvibrations at the steering wheel.
• Length compensation to accommo-
Features Advantages
Continuously adjustable Ergonomic driver’s
height and tilt of steering place
wheel
Pneumatic assistance of the Low adjusting forces
positioning device
Clamping is achieved by spring force Safe locking
Integration of steering wheel lock, Reduced costs for logistics and
steering column switches, clock spring assembly at the customer
and steering angle sensor
Dashboard can be attached Optimum ergonomic position
around the steering column of the display
Free of play Improved comfort and
safety
Maintenance-free No servicing required
Pedal box can be Reduced costs for logistics and
integrated assembly at the customer
ZF Steering columns 33
1 Dashboard2 Adjusting unit3 Mounting bracket, fixing4 Pedal5 Ball-track relay shaft6 ZF Servocom steering gear7 Tilt angle
8 Height adjustment9 Clock spring ”integrated“
10 Steering angle sensor ”integrated“11 Ignition and starting switch12 Universal joint = tilt axis13 Gas-filled spring14 Drag link
ZF Steering column, type 7360. Installation schematic of the height and tilt adjustable steering column with steering gear.
date the relative movement be-tween the driver's cab and the chas-sis because of tilting truck cabs, pos-sible conditions of elasticity onbuses and height and tilt adjustabil-ity of the steering column.
• For reasons of comfort, the forcesfor tilt and height adjustment of thesteering wheel must be low.
• No maintenance during the servicelife of the vehicle.
Depending on the specific application,the mounting bracket is made of lightalloy or sheet steel. During the adjust-ment process, the necessary weightcompensation of the steering unit with steering wheel, steering column,switch and, if applicable, dashboard, ismade possible by a gas-filled springintegrated into the ball-track relayshaft or by a cylindrical compressionspring. It is also possible to fit the gas-filled spring between the mountingbracket and the inner panel.
Further design features and the advan-tages resulting from these can be seenfrom the table on page 32.
7
8
9
10
11
12
13
14
6
5
4
3
2
1
34 ZF Bevel boxes
ZF Bevel Boxes
ZF Bevel box, type 7860. Installation schematic with horizontally fitted steering gear.
ZF Bevel boxes are used on vehicleswhere a direct connection to the steer-ing column is not possible due to thearrangement of the steering gear.They can be flange-mounted directlyto the steering gear or be delivered forseparate installation. The standard bevel box has a shaft angle of 90° anda weight of only 2.1 kg. Special versionswith a 77° angle or a double outputdrive are also available.
The input and output shafts of thebevel boxes are carried in antifrictionbearings. Both shafts carry a straightbevel gear with the same number ofteeth, which means that the ratio is1:1. Especially beneficial effects onnoise and wear performance has thelifetime lubrication of the mechanicalratio system integrated in a sturdylight-alloy housing.
Figure at top:ZF Bevel box, type 7860. Standard design of light alloy and filler neck for lifetime lubrication.
1 Adjustable steering column2 Ball-track relay shaft3 Bevel box4 ZF Servocom steering gear
1
3 4
2
ZF Ball-Track Telescopic Shaftsand Ball-Track Relay Shafts
ZF Ball-track telescopic shafts, ball-track relay shaft 35
ZF Ball-track relay shaft, type 7025.
An important link between the steer-ing wheel and steering gear is the ball-track telescopic shaft or the ball-trackrelay shaft which is complemented byuniversal joints. Their installation hasthe advantage that the relative move-ments existing in driving between thedriver's cab and the chassis do notadversely affect steering performance.
The major components of the ball-track telescopic shaft / ball-track relayshaft developed by ZF LenksystemeGmbH are a drive shaft with internalball tracks and a profiled shaft withexternal ball tracks. Two axially ar-ranged rows of balls provide the noise-free connection. This design ensures aradially play-free, but axially easilymovable connection with an availablestroke of ± 28 mm.
The total length of the low-wear andmaintenance-free component can beadapted to the specific vehicle or com-plemented by an extension tube. Tosave weight, the extension tube andthe universal joints can alternatively bemade of light alloy. Also, a comfort-able weight compensation for installa-
tion in an adjustable steering column isachieved by integrating a gas-filledspring or cylindrical compressionspring into the ball-track telescopicshaft/ball-track relay shaft.
36 ZF Universal joints
ZF Universal Joints
ZF Universal joint, type 7026. Connection with cylindrical serrations.
The use of ZF universal joints ensures asafe transmission of torques and a highdegree of compensation for misalign-ment which exists between a separatesteering column and the steering gear or bevel box respectively. Highstrength reserves at an available flex-ing angle of 68° and a maximum diam-eter of rotation of only 76 mm are characteristic of this component.
All universal joints are carried in needlebearings, are play-free and mainten-ance-free and, thus, meet the require-ments of good steering returnability,low rotation torques and absolutelyminimal maintenance. Different pos-sible connections such as cylindricalserrations, internal thread or flangejoint design admit individual adap-tation of the installation. The variousjoint forks are made of wrought steeland can be combined optionally. For protection against corrosion thesurfaces of the universal joints areelectro-zinc-plated.
ZF Sensors
ZF Sensors 37
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.
The increasing use of electronicallyassisted or controlled steering systemsrequires the development and manu-facture of special adaptable sensors. Inorder to meet the high safety stand-ards for steering systems, our steeringangle and turning angle sensors use anon-contact inductive measurementprinciple. They can be used up to aneffective angle of 100°.
New methods are also available by theintegration of a linear sensor on powercylinders. The application of a non-contact magnetic principle of measure-ment means that stroke is measureddirectly where the movement is gener-ated. This possibility of comparing specified and actual values allows cal-culation and control of the piston rodstroke with millimeter accuracy.
Due to their high flexibility, these ver-sions offer ideal conditions for the in-stallation in rear axle steering systems.The best degree of integration isachieved by the direct adaptation ofthe steering angle sensor on the steering gear. The turning angle sensorcan be positioned in many suitableplaces on the vehicle and can also be
used for other applications. The leastloss of installation space is caused bythe linear sensor integrated on apower cylinder.
1
2
3
38 ZF Pressure filters
ZF Pressure Filters
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.
The use of electronically controlled valves increasingly requires additionalmeasures to meet with the high expect-ations regarding the reliability of steering systems. That's because majorparticles in the hydraulic fluid mayresult in limited control function. Thiscan be counteracted effectively by fitting a pressure filter from ZF Lenk-systeme GmbH.
The pressure filter which can be usedup to 210 bar rated pressure is of com-pact design and offers flexible solu-tions for attachment. The contamin-ation absorbing capacity can be con-trolled individually by means of differ-ent filter elements, depending onrequirements. Filter elements with filtration grades of 3, 5, 10 and 20 µmabsolute are available. Depending onthe filter element used, the maximumpermissible flow rate is between 32and 50 dm3/min.
Flow rate [dm3/min]Filtration grade 3 µm Filtration grade 5 µm Filtration grade 10 µm Filtration grade 20 µm
Dif
fere
nti
al p
ress
ure
[b
ar]
0 10 20 30 40 50 60
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Flow indicators
In hydraulic steering systems withemergency steering properties, greatimportance is attached to the moni-toring of the oil circuit for safety rea-sons. A flow indicator fitted betweenthe steering pumps and the steeringgear indicates to the driver the lack ofoil flow by a pilot lamp that goes on.
Pressure relief valves
They are fitted for maximum pressurerelief in hydraulic systems and to pro-tect the system from excessiveloads. Designs with their own housings or valve insertsdesigned for screwing into existing housings are available.
ZF Valves
ZF Valves 39
Figure showing various ZF valves.
1 Standby valve, type 77342 Valve block, type 77603 Valve insert, type 84704 Pressure relief valve, type 77535 Flow indicator, type 7780
Due to their different functions anddesigns, valves from ZF LenksystemeGmbH are perfectly suited for comple-menting the hydraulic management ofhydraulic steering systems ideally.That's why they are chiefly fitted oncommercial vehicles which are equip-ped with a dual-circuit steering systemor a ZF semi-integral power steeringgear. But they are also used in industry,in toolmaking as well as in machinebuilding and vehicle manufacture.
Standby valves
The standby valve coordinates the oilflow between two steering pumps anddirects it to the steering gear via ahydraulic line. In normal operation, avalve spool located in the housing isdisplaced by the main pump outputflow up to the stop position after it hasovercome a spring force and opens thesupply flow of this pump to the steer-ing gear. At the same time, this pos-ition of the valve spool determines thereturn to the oil reservoir of the oilflow generated by an emergency steer-ing pump. The oil delivered by theemergency steering pump can get tothe steering gear only if the valvespool is in its initial position.
Valve block
In this component, standby valve, flowindicator and – optionally – a pressurerelief valve or piston position indicatorare combined into a single unit. Theadvantage of this space-saving blockdesign compared to individual valves is easier assembly and installation ofpipes.
1
2
5
4
3
ZF Lenksysteme GmbH:the systems partner
ZF Lenksysteme GmbH is one ofthe largest independent manu-facturers of power steering sys-tems for passenger cars and com-mercial vehicles. Renowned auto-motive manufacturers from allover the world value us as a cre-ative and efficient systems part-ner for the development of newand innovative solutions.
As a joint venture of Robert BoschGmbH and ZF Friedrichshafen AG,ZF Lenksysteme GmbH offers itscustomers a unique source of ex-pertise when it comes to integra-ting a wide range of top technol-ogies in modules, system mod-ules or entire chassis systems.
The benefits for the manufactur-er are clear to see: even shorterdevelopment times and opti-mized production processes –with quality standards which justget better and better.
ZF Lenksysteme GmbHRichard-Bullinger-Strasse 77D-73527 Schwäbisch GmündGermanyPhone: +49 (0)71 71 / 31-0Telefax: +49 (0)71 71 / 31-32 22www.zf-lenksysteme.com
A Joint Venture of
Robert Bosch GmbH and
ZF Friedrichshafen AG
[Steering the right way]
Tech
nic
al m
od
ific
atio
ns
rese
rved
.
ZF
LS 8
090P
-MB
A 8
/02e
·Pri
nte
do
np
aper
ble
ach
edw
ith
ou
tu
seo
fch
lori
ne