Steering Systems and Accessories for Commercial Vehicles

Transcription

1 Steering Systems and Accessories for Commercial Vehicles

2 Steering the right way. Many tonnes safely controlled and what's behind it. Contents Page ZF Servocom ZF Servocomtronic Dual-Circuit Steering Systems ZF Servocom RAS ZF Servocom RAS-EC Semi-Integral Power Steering Gears Steering Pumps Oil Reservoirs Power Cylinders Steering columns Bevel Boxes Ball-Track Telescopic Shafts/Ball-Track Relay Shafts Universal Joints Sensors Pressure Filters Valves The steering gear is something like the soul of the commercial vehicle. The driver is in permanent contact with it, via the steering gear he notices the condition of the road and the response of the vehicle. Here the quality of the steering gear is decided with light operation and comfort on the one hand and as much feedback from the road as possible on the other. Essential for this is a carefully balanced interaction of high-precision mechanics and a finely controllable hydraulic system, which we have perfected with our steering systems. These are preferred, by the vehicle manufacturers, because they are compact and reliable, and, by the drivers, because they are comfortable and precise. Such a high technical level is achieved only by someone who is master of the complete system. That s why we dedicate ourselves to all components. Steering columns, steering pumps, valves, oil reservoirs and other peripheral parts contribute their share to reliability. ZF Servocom and ZF Servocomtronic are registered trademarks of ZF. 2 Worth knowing

3 ZF Steering Systems. Steering toward the future. As a joint venture of Robert Bosch GmbH and ZF Friedrichshafen AG, ZF Lenksysteme GmbH has produced power steering systems for passenger cars and commercial vehicles for several decades. The products fulfill every specified requirement. With the ZF Servocom, the tried-and-tested recirculating-ball power steering principle has been continually further developed, protected by numerous patents, and adapted to suit specific vehicle requirements. And for a great variety of special applications and for compliance with legal safety criteria, it is logic to use our semi-integral and dual-circuit power steering systems. New courses were also taken in the field of rear axle steering systems. The ZF Servocom RAS (Rear Axle Steering), which is also suitable for retrofit, and the electronically controlled ZF Servocom RAS-EC (Rear Axle Steering Electronically Controlled) can completely do without the steering linkage known to date which is heavy and imprecise. Future-oriented innovative approaches regarding functions, design and safety are opened up with the ZFLS Steer-by-Wire Steering System. And as a member of a European consortium of vehicle manufacturers, suppliers, users, and institutions, we are preparing the implementation of automatic driving in goods traffic. An outstanding result of innovative further development is the ZF Servocomtronic. This steering gear designed on the basis of the Servocom is electronically controlled and speedsensitive. It is characterized by easy, comfortable steering in maneuvering as well as a safe road feel at increasing speed. Installation schematic of a ZF Servocom with height and tilt adjustable steering column, ball-track relay shaft, steering pump and oil reservoir. Worth knowing 3

4 ZF Servocom Mechanical construction The ZF Servocom, a compact-design recirculating-ball power steering gear, basically comprises a sturdy cast-iron housing (1) with integrated mechanical steering gear, control valve and power cylinder. A turning movement at the steering wheel is transmitted via the output shaft and the valve slide (3) in the control valve to the worm (4) and, via an endless ball chain (7), transformed into an axial movement of the piston (2). At the same time, the sector shaft (6), which is arranged at right angles to the longitudinal axis of the piston, is caused to rotate by the meshing of teeth. The drop arm mounted on the sector shaft moves the steering linkage which goes to the steering arms, thus causing the wheels to be turned. The basic components of the Servocom control valve are the valve slide, with six control grooves on its surface area, and a worm in whose valve bore axial grooves are provided which are matched to the control grooves. Centralizing the valve slide (neutral position) is done by a torsion bar (5) which provides at the same time the connection between the valve slide and the worm. The valve slide and the worm run in antifriction bearings to ensure the precision of operation and the functional safety of the control valve even at high pressures. If a torque is transmitted to the valve slide or the worm from the steering wheel or the steered wheels, a relative rotary motion influenced by the torsion bar will occur between the valve slide and the worm. The valve slide is thereby caused to change its position in relation to the worm valve bore surrounding it, so that the relative positions of the control grooves are changed, too. Pressurized oil can now flow through connecting bores to one of the two power cylinder chambers (ZL or ZR) and assists the axial movement of the piston. When the steering wheel is released, the action of the twisted torsion bar makes the control grooves return to the neutral position, and the same system pressure will exist in both of the power cylinder chambers. Basic hydraulic function of the Servocom control valve The hydraulic fluid delivered by an engine-driven steering pump (21) flows through a connecting bore in the housing bottom, via the feed oil radial groove (8) and transverse bores in the valve portion of the worm (4), onward to the three feed oil control grooves (9) of the valve slide (3). In the valve neutral position (see fig. on page 5), the oil flows, over the open feed oil control edges (10), to all axial grooves (11) of the worm head and from there, over the open return oil control edges (13), also to the return oil control grooves (12) of the valve slide. From Figure at top: ZF Servocom, type Figure on page 5: ZF Servocom, type 8098, control valve in neutral position. 1 Housing 2 Piston 3 Valve slide 4 Worm 5 Torsion bar 6 Sector shaft 7 Ball chain 8 Feed oil radial groove 9 Feed oil control groove 10 Feed oil control edge 11 Axial groove 12 Return oil control groove 13 Return oil control edge 14 Return oil chamber 15 Radial groove 16 Radial groove 17 Hydraulic steering limiter 18 Pressure relief valve 19 Replenishing valve 20 Flow limiting valve 21 Steering pump 22 Oil reservoir ZL Power cylinder, left ZR Power cylinder, right 4 ZF Servocom

5 ZL ZR ZF Servocom 5

6 ZL ZR ZF Servocom, type 8098, control valve in working position. Steering wheel turned clockwise. 1 Housing 2 Piston 3 Valve slide 4 Worm 5 Torsion bar 6 Sector shaft 7 Ball chain 8 Feed oil radial groove 9 Feed oil control groove 10 Feed oil control edge 11 Axial groove 12 Return oil control groove 13 Return oil control edge 14 Return oil chamber 15 Radial groove 16 Radial groove 17 Hydraulic steering limiter 18 Pressure relief valve 19 Replenishing valve 20 Flow limiting valve 21 Steering pump 22 Oil reservoir ZL Power cylinder, left ZR Power cylinder, right 6 ZF Servocom

7 these grooves the oil flows back, via bores, to the return oil chamber (14) inside the worm and from there to the oil reservoir (22). At the same time, the radial grooves (15 and 16) of the control valve and their associated connections provide for a connection between the right-hand (ZR) and lefthand power cylinder chambers (ZL). When turning the steering wheel clockwise (fig. on page 6), the piston (2) will move to the right in the piston bore if it is a right-hand ball thread. Due to the simultaneous rotation of the valve slide (3) to the right, the pressurized oil is directed, over the further opened feed oil control edges (10), to the three associated axial grooves (11), via bores to the radial groove (16), and, via a connection, to the left-hand cylinder chamber (ZL), whereby the piston movement is hydraulically assisted. An individually adaptable pressure build-up is achieved by the fact that the partially or fully closed feed oil control edges (10) restrict or prevent a connection between the pressure oil inlet and the other three axial grooves (11) connected to the radial groove (15). At the same time, the pressure oil outlet toward the pressurized axial grooves is restricted or prevented, too, by the closing return oil control edges (13). The oil displaced by the piston (2) from the right-hand power cylinder chamber (ZR) first flows through a connection to the radial groove (15) and, through transverse bores, to the associated axial grooves and onward to the return oil control grooves (12) over the further opened return oil control edges (13). From here, the further return flow of the oil to the oil reservoir (21) takes places via the connecting bores leading to the return oil chamber (14). When the steering wheel is turned counterclockwise, the operating sequence will be analogous to the above. Hydraulic steering limiter To protect the steering linkage, the wheel lock stops and the steering pump from excessive loads at the maximum wheel lock angles, the ZF Servocom has a mechanically adjustable or automatically on the vehicle adjusting hydraulic steering limiter (17). This device integrated in the piston (2) is always closed due to the oil pressure in the right-hand or lefthand power cylinder chamber. It is only just before each piston end position that the steering limiter valve is opened by the contact of the valve pin with the adjusting screw or sleeve. Thus, that cylinder chamber which is under high pressure is connected with the opposite return oil chamber. As a result of the pressure drop, hydraulic assistance is reduced heavily. The steering wheel can only be turned up to the road wheel lock stop by using increased effort. Further features The ZF Servocom is fitted with a pressure relief valve (18) which limits the delivery pressure of the steering pump at the maximum specified pressure. Also, a replenishing valve (19) can be fitted to the housing or the valve slide, if required. This valve allows to suck oil from the return flow if the vehicle must be steered without hydraulic assistance. Mechanically adjustable (top) and automatically adjusting steering limiter (bottom). Steering limiter valve opened, oil pressure greatly reduced. ZF Servocom 7

8 ZF Servocomtronic Pressure p [bar] 0 km/h 20 km/h 50 km/h 100 km/h Torque required at the steering wheel [Nm] Design and function The ZF Servocomtronic is a speed-sensitive recirculating-ball power steering gear for trucks and buses. The use of advanced electronics, of an electro-hydraulic transducer, and of a hydraulic reaction device makes it possible to vary steering effort with vehicle speed. Application of the ZF Servocomtronic requires either an electronic speedometer (1) or a suitable ABS control unit. The speed signals coming from one of these units are transmitted to the electronic control unit (2) which can either be a separate component or integrated in the existing vehicle electronics. The signals are analyzed by the microprocessor of the Servocomtronic control unit and converted into a controlled electric current which actuates the electro-hydraulic transducer. On the basis of this influence, the transducer, which is directly attached to the housing cover, determines the hydraulic reaction of the control valve and, thus, the amount of input torque at the steering wheel. This speed-dependent influencing of steering ensures that static steering as well as steering at low vehicle speeds, e.g. in parking maneuvers, require minimal effort. As the hydraulic reaction changes in proportion to the vehicle speed, the steering effort increases as the vehicle goes faster (see fig. at top). At higher speeds the driver thus has particularly good road contact and is able to steer the vehicle precisely and with directional stability. Figure at top: ZF Servocomtronic diagram of characteristic curves. It shows the change in pressure and steering wheel torque as a function of vehicle speed. The course of the characteristics can be matched to the specific character of the vehicle. Figure on page 9: Schematic representation of the ZF Servocomtronic, type Electronic speedometer on the vehicle 2 Electronic control unit (microprocessor, 12 V) 3 Electro-hydraulic transducer 4 ZF Servocomtronic 5 Steering pump 6 Oil reservoir with fine filter 7 Ball-track relay shaft 8 Height and tilt adjustable steering column 8 ZF Servocomtronic

9 A further advantage of the ZF Servocomtronic is that oil pressure and flow rate are never reduced and can therefore be utilized immediately in emergencies where sudden and unexpected steering corrections may become necessary. These features bring about extraordinary precision of steering, together with a high safety standard and optimum steering comfort Mechanical construction The basic steering gear for the ZF Servocomtronic is the well-proven ZF Servocom millions of which have already been fitted based on the design and operating principle described on pages 4 to 7. The difference from the description there is that in the axial bore of the worm a torsionally resistant bellows (5, see fig. on page 10) is arranged whose lower end is connected with the worm such that rotation is prevented. The upper part of the bellows forms a centering bush (26) and presses, with an axial spring force which is matched to the specific vehicle, against a prism centering device with two rollers (27). When driving straight ahead, this has especially positive effects on the exact centralizing of the control valve. When subjected to hydraulic pressure, a reaction piston (28) located in the cylinder cover concentrically with the worm also applies, via a spacer tube (29), load on the prism centering device and makes necessary, in addition to the spring force of the bellows, further force for the displacement of the control valve from its neutral position. The amount of that hydraulic reaction is determined by the indicated instantaneous vehicle speed and the opening position of the electro-hydraulic transducer resulting from this ZF Servocomtronic 9

10 ZL ZR ZF Servocomtronic, type Control valve in working position. Steering wheel turned clockwise, driving at high speed, transducer valve fully opened, maximum reaction limited by cut-off valve. 1 Housing 2 Piston 3 Valve slide 4 Worm 5 Bellows 6 Sector shaft 7 Ball chain 8 Feed oil radial groove 9 Feed oil control groove 10 Feed oil control edge 11 Axial groove 12 Return oil control groove 13 Return oil control edge 14 Return oil chamber 15 Radial groove 16 Radial groove 17 Hydraulic steering limiter 18 Pressure relief valve 19 Replenishing valve 20 Flow limiting valve 21 Steering pump 22 Oil reservoir 23 Electronic speedometer 24 Electronic control unit (12 V) 25 Electro-hydraulic transducer 26 Centering bush 27 Roller 28 Reaction piston 29 Tube 30 Reaction chamber 31 Orifice 32 Cut-off valve ZL Power cylinder, left ZR Power cylinder, right 10 ZF Servocomtronic

11 Function of the ZF Servocomtronic At low speeds, e.g. in maneuvering, the electronic speedometer (23) or the ABS control unit transmit very few signals to the microprocessor integrated into the electronic control unit (24). The microprocessor analyzes the signals and passes them to the electrohydraulic transducer (25) in the form of a correspondingly adapted control current. Due to the maximum current existing in that driving mode, the transducer valve closes and prevents pressure build-up in the reaction chamber (30). An orifice (31) ensures that there is also return pressure level in the reaction chamber. Thus, owing to the elimination of reaction, the steering is light in operation and can be handled with little effort. As the driving speed increases, the speed signals become more frequent and, after having been converted by the microprocessor, cause a reduction in the amount of control current transmitted to the electro-hydraulic transducer. As a result, the transducer valve takes up an opening position adapted to the instantaneous vehicle speed and allows a limited oil supply from the feed oil radial groove (8), via a tube, to the reaction chamber (30). Via the spacer tube, the reaction piston now presses against the prism centering device and makes necessary more force for the displacement of the control valve. Thus, this mode of operation of the hydraulic reaction requires an individually established higher steering wheel torque until a determined hydraulic assistance is raised in the right-hand (ZR) or left-hand cylinder chamber (ZL). At high driving speeds (fig. on page 10), for instance on the motorway, the transducer valve is fully open owing to a very low or non-existing control current for the actuation of the transducer. This enables maximum pressure supply from the feed oil radial groove (8) to the reaction device. When the steering wheel is turned clockwise, the reaction pressure increases in accordance with the existing operating pressure and pressurizes the reaction piston from the reaction chamber (30). As soon as the reaction pressure determined for a specific vehicle reaches its upper limit, the oil is discharged to the return oil chamber (14) through the cut-off valve (32) to avoid a further increase in reaction pressure. The input torque at the steering wheel thus achieved will not now rise any more and gives a safe driving feel owing to optimum road contact. Safety of the ZF Servocomtronic Even in the event of a failure of the vehicle electrical system or any other electrical fault, the steering gear remains fully operational. In such exceptional cases the ZF Servocomtronic will work at maximum hydraulic reaction (high-speed characteristic) due to the mechanically forced opening of the transducer valve. When speed signals all of a sudden are not transmitted any more during driving, for instance due to lack of cable contact or a defective speedometer, the highly advanced microprocessor in the electronic control unit is in a position to derive a constant control current from the last speed signals evaluated. This ensures a constant steering performance until the vehicle engine is turned off. When the engine is started again, maximum hydraulic reaction conforming to the high-speed characteristic will develop again. ZF Servocomtronic, type ZF Servocomtronic 11

12 ZF Dual-Circuit Steering System with 2 Pumps Design and function The installation of dual-circuit steering systems is required for vehicles with high steered axle loads or with more than one steered axle in order to meet statutory safety criteria. These requirements are met by the modified dualcircuit steering system with 2 pumps which builds on the basic concept of the ZF Servocom (see pages 4 to 7). A crucial component in this system is the change-over valve (6) adapted at the input stub shaft end. In normal operation it monitors the readiness for operation of the engine-driven steering pump 1 (1) and ensures the connection to a power cylinder (5) which is necessary for additional comfortable steering assistance. The change-over valve delivers the oil flow generated by the wheel-driven steering pump 2 (2) directly, i.e. without being utilized, back to the second oil reservoir 2 (4). In an emergency, which is very rare, for instance when the engine has stopped or the performance of the enginedriven pump is reduced, the two switching pistons (9 and 10) are pressed with spring force against the associated contact switches (7), owing to the insufficient pump pressure. At the same time, this makes one switching piston (9) direct the pressure oil from the wheel-driven steering pump 2 to the control valve of the steering system. The second switching piston (10) shuts off the connection to the power cylinder, which means that the lower output flow from the wheeldriven pump is available for the safe operation of the steering system. This condition is indicated to the driver by means of a pilot lamp. Also, the steering wheel effort is increased in extreme steering situations, due to the inactive power cylinder. ZF Servocom, type Modified dual-circuit steering system. Fig. on page 13: Schematic representation of the modified dual-circuit steering system, ZF Servocom, type 8099 (right-hand). Normal function for right-hand turning. Both pumps deliver oil. Power cylinder with pressure assistance. 1 Steering pump 1 2 Steering pump 2 3 Oil reservoir 1 4 Oil reservoir 2 5 Power cylinder 6 Change-over valve 7 Contact switches 8 Feed oil radial groove 9 Switching piston (inside) 10 Switching piston (outside) 11 Check valve 12 Check valve ZL Power cylinder, left ZR Power cylinder, right 12 ZF Dual-circuit steering system

13 ZR ZL 10 5 ZL ZR 8 ZF Dual-circuit steering system 13

14 ZR 9 6 ZL 10 5 ZL ZR 8 Schematic representation of the modified dual-circuit steering system, ZF Servocom, type 8099 (right-hand). Emergency function for right-hand turning. Only the wheel-driven steering pump delivers oil. Power cylinder without pressure assistance. 1 Steering pump 1 2 Steering pump 2 3 Oil reservoir 1 4 Oil reservoir 2 5 Power cylinder 6 Change-over valve 7 Contact switches 8 Feed oil radial groove 9 Switching piston (inside) 10 Switching piston (outside) 11 Check valve 12 Check valve ZL Power cylinder, left ZR Power cylinder, right 14 ZF Dual-circuit steering system

15 Basic hydraulic function of the change-over valve The oil flow generated by the enginedriven steering pump 1 (1) after engine start will pressurize the inner (9) and outer switching piston (10) in the change-over valve (6) and displace both in a direction contrary to the contact switches (7) and against a specifically determined spring force. At that position of the inner switching piston, the pressure oil can get to the feed oil radial groove (8) on the control valve via an internal connection as is the case on a standard ZF Servocom. Via the opened radial groove, the return oil from the cylinder chambers (ZL/ZR) flows back to the oil reservoir 1 (3). The oil flow from the wheel-driven steering pump (2) is directed back to the oil reservoir 2 (4), without being utilized, via the opened radial groove. The position of the outer switching piston allows the free exchange of pressure oil between the associated cylinder chambers (ZL/ZR) and the separate power cylinder (5). This position of the inner switching piston allows the supply flow of the pressure oil generated by the wheeldriven steering pump, via a connection, to the feed oil radial groove (8). The direct return flow to the oil reservoir 2 (4) which takes place in normal operation is prevented. Additionally, a check valve (11) prevents the outflow of oil to the oil reservoir 1 (1) via the enginedriven steering pump. Now the return oil flowing from the cylinder chambers (ZL/ZR) passes back to the oil reservoir 2 (4) via the opened radial groove. In this switching mode, an outflow of the return oil to the oil reservoir 1 (3) is not possible. At that position, the outer switching piston (10) completely shuts off the connection between the cylinder chambers (ZL/ZR) and the power cylinder. This means that the steering gear pressure or return oil respectively, which is radially available at the switching piston, cannot pass through to the two radial grooves on the switching piston. But, via a channel system, these radial grooves permit the mutual exchange of oil in the now inactive hydraulic circuit of the power cylinder. Check valves (12) between the shut-off hydraulic circuit and the cylinder chambers provide a balanced pressure level. When the engine has stopped or a specifically determined amount of oil (fig. on page 14) is not reached, both switching pistons are pressed against the contact switches by spring force. ZF Servocom, type Modified dual-circuit steering system. ZF Dual-circuit steering system 15

16 ZF Dual-Circuit Steering System with 3 Pumps Design and function The installation of dual-circuit steering systems with 3 pumps is required for commercial vehicles with especially high steering axle loads or with several steered axles in order to meet the statutory safety criteria and to ensure the necessary steering comfort. The steering gear used is the basic concept of the ZF Servocom (see pages 4 to 7). The high safety potential of this steering system is based on the existence of two steering circuits which are completely separate from each other and complement each other favorably in normal operation. Steering circuit I is supplied, on a priority basis, with pressure oil from the an engine-driven steering pump 1 (1). From a determined oil flow, a standby valve (2) in the valve block (3) controls the supply of the oil from that pump, via the control valve 1 (4), to the steering gear and to the parallel-connected power cylinder (5) that may be fitted in the particular case. At the described switching position of the standby valve, the oil flow generated by the wheel-driven steering pump 2 (6) is, at the same time, delivered back to the common oil reservoir 1 (8), without being utilized, but monitored by a flow indicator (7). In steering circuit II, the oil pressurizing, via the control valve 2 (12), the power cylinder (13) fitted downstream is also supplied by an engine-driven pump 3 (9). A flow indicator (11) between the control valve 2 and the separate oil reservoir 2 (10) monitors the flowing oil. Figure on page 17: Schematic representation of the dualcircuit steering system ZF Servocom, type 8099 (right-hand). Normal function for right-hand turning. Both control valves in working position. Presssure buildup in steering circuit II delayed. All pumps deliver oil. 1 Steering pump 1 2 Standby valve 3 Valve block 4 Control valve 1 5 Power cylinder 6 Steering pump 2 7 Flow indicator 8 Oil reservoir 1 9 Steering pump 3 10 Oil reservoir 2 11 Flow indicator 12 Control valve 2 13 Power cylinder 14 Steering limiter valve 15 Radial groove 16 Radial groove 17 Piston position indicator 16 ZF Dual-circuit steering system

17 Steering circuit I Steering circuit II ZF Dual-circuit steering system 17

18 Steering circuit I Steering circuit II Schematic representation of the dual-circuit steering system ZF Servocom, type 8099 (right-hand). Emergency function for left-hand turning. Both control valves in working position. Only the wheel-driven steering pump delivers oil. 1 Steering pump 1 2 Standby valve 3 Valve block 4 Control valve 1 5 Power cylinder 6 Steering pump 2 7 Flow indicator 8 Oil reservoir 1 9 Steering pump 3 10 Oil reservoir 2 11 Flow indicator 12 Control valve 2 13 Power cylinder 14 Steering limiter valve 15 Radial groove 16 Radial groove 17 Piston position indicator 18 ZF Dual-circuit steering system

19 Only in an emergency, which is very rare, for instance when the engine has stopped, the system is limited to the safe operation of steering circuit I. Due to the lack of oil flow from the stopped steering pump 1, the standby valve is not moved, which means that the pressure oil flow generated by the wheeldriven steering pump 2 is now directed, through the standby valve, to the control valve 1 and can thus be effective in the steering gear and power cylinder. At the same time, the standby valve prevents the outflow of oil through the flow indicator and indicates that condition to the driver by means of a pilot lamp. Additionally, a piston position indicator (17) monitors the function of the standby valve. A flow indicator between the control valve 2 and the oil reservoir 2 notifies the driver, by means of a further pilot lamp, of the lack of oil flow. When the pilot lamps go on, an early check and maintenance of the steeering system will be necessary for safety reasons. in the same manner as on the standard single-circuit steering gear (see pages 4 to 7). In steering circuit II, however, the individually determinable reduction of pressure in the power cylinder is controlled by two adjustable steering limiter valves (14). These are arranged in the housing cover of the steering gear and are opened by a cam of the sector shaft. Via pipes, both valves are connected to the respective radial grooves (15 and 16) of control valve 2 and thus, indirectly, also to the lines to the power cylinder. Hydraulic steering limiter To protect the steering linkage, wheel lock stops and steering pump from excessive loads, both steering circuits are usually equipped with a hydraulic steering limiter. The hydraulic steering assistance in steering circuit I is limited Installation schematic of a ZF Servocom dual-circuit steering system with 3 pumps, 2 oil reservoirs, valve block, flow indicator, power cylinder, steering column and ball-track relay shaft. ZF Dual-circuit steering system 19

20 1 ZF Servocom RAS Rear Axle Steering System 5 2 System description 3 The rear axle steering system ZF Servocom RAS (Rear Axle Steering) is optimized both with regard to cost and weight and is particularly suitable for the forced steering of nondriven rear axles. The steering work is done by the front axle steering system (1) and a power cylinder portion in the master cylinder (2). 4 Up to now, comparable systems have predominantly been implemented according to the mechanical principle, consisting of drag links, intermediate arms, etc. Such mechanical transmission systems need a lot of installation space because of the swivelling ranges of the levers and drag links. In addition, a fairly heavy weight is typical of such systems. Another disadvantage is the inadequate steering stiffness, i.e. because of the elasticity of the transmission parts the steered rear axle is not stabilized enough and is prone to oscillations during straight line driving. Compared to mechanical steering systems, the rear axle steering system ZF Servocom RAS achieves better results in many respects. The content of the RAS system is basically made up of 2 special power cylinders and a hydraulic accumulator (3). In conformity with the steering angle movement, the master cylinder (2) fitted at the front axle pumps oil into the associated cylinder chamber of the centering cylinder (4) arranged at the the steered rear axle. In this way, the steering movement of the front axle is hydrostatically transmitted to the rear axle. The steering angle ratio between front axle and rear axle is determined by the vehicle manufacturer by rating the length of the steering arms appropriately. The master cylinder is fitted with an automatic synchronizer system and a pressure relief valve. In a definable steering angle range of the front axle, e.g. up to about ± 5, both cylinder chambers of the master cylinder are shortcircuited. Within this small steer- Schematic representation of the rear axle steering system ZF Servocom RAS, type ZF Servocom 2 Master cylinder 3 Hydraulic accumulator 4 Centering cylinder 5 Additional lines 20 ZF Servocom RAS

21 ing angle range of the front axle, the rear axle is put and kept in straight ahead position with the help of a hydraulic centering device in the centering cylinder. Owing to this automatic synchronization, hydrostatic mismatches in the cylinders, which seldom occur, can be compensated for. At steering angles of more than about ± 5 at the front axle, the cylinder chambers of the master cylinder are separated from each other and sealed hermetically. This means that at steering angles at the front axle exceeding ± 5 the hydrostatic steering system is in operation. A control system in the master cylinder prevents that in straight ahead position of the front axle and at high actuating forces at the steered rear axle, caused for example by driving over obstacles in forward or reverse, the kinematic conditions change considerably. An accumulator is assigned to the hydrostatic system. The function of this accumulator is to optimize the stiffness of the hydostatic transmission system by preloading the transmission lines with the centering pressure. Besides, the accumulator pressure acts permanently on the two hydraulic centering pistons which are integrated into the centering cylinder. Experience has shown that the centering and preload pressure is approximately 15 bar and thus meets the safety criteria required for driving without hydraulic assistance. If due to the extra steering work performed by the rear axle the steering comfort becomes too low, i.e. the hydraulic pressure reserve becomes too small, the master cylinder can be subjected to steering pressure via additional lines (5) in order to provide sufficient steering comfort, e.g. during static steering. The ZF Servocom RAS concept does not need additional electric and electronic components and is also suitable for later installation because of a small variety of parts. What has to be stressed particularly is the improvement in vehicle dynamics during straight line driving due to the hydraulically centered rear axle. Better maneuverability, less tire wear, and fuel saving are further assets of the system. 2 4 ZF Servocom RAS 21

22 ZF Servocom RAS-EC Rear Axle Steering System Application Modern commercial vehicles have to correspond more and more to the requirements of the clients for economy, environmental protection, legislation and special applications. To this, the electronically controlled rear axle steering system ZF Servocom RAS-EC (Rear Axle Steering Electronically Controlled) makes an important contribution and at the same time it offers a basis for entirely new solutions for the development of commercial vehicles. That's because the ZF RAS-EC is particularly suitable for commercial vehicles with a very big wheelbase and several rear axles. The increasingly heavy traffic as well as modern city planning concepts require commercial vehicles with a big loading capacity and good maneuverability in order to supply and dispose of the goods for the city centers. The active rear axle steering system increases the maneuverability of the vehicle, particularly during parking maneuvers, because here, in contrast to adhesionsteered axles, steering operations are also carried out when the vehicle is reversed. Owing to the optimum kinematic conditions, tire wear is reduced and the traction at this axle is increased. The forced steering of rear axles offers the additional advantage of an immediate build-up of side force and therefore leads to an increase in vehicle stability and driving safety. The RAS-EC does not require a mechanical connection between the front and rear axles, which makes the adaptation in series production as well as retrofitting a lot easier. The possibility of endof-assembly-line programming of the electronic control opens up an adaptation to changed geometric quantities (wheelbase, steering angle) or to a particular application pattern of the vehicle. Because of that, no modification of mechanical power transmission components is necessary. Due to the electronic control, the steering angle at the rear axle can be altered as a function of whatever driving parameters are chosen. This means for example that by opposite steering motions of rear and front wheels in the low speed range maximum maneuverability can be achieved. Same-direction rear/front wheel steering motions at medium and high speeds lead to exact straight line driving and vehicle stabilization during lane changes. Special steering programs, such as rear swing-out minimization or constant ratio with same-direction rear/front wheel steering motions in the low speed range, additionally increase the ZF Servocom RAS-EC. Basic diagram for vehicle 6x2x4 with steered trailing axle (nondriven). 22 ZF Servocom RAS-EC

23 1 2 advantages of the RAS-EC. The electronic control of the rear axle steering system is provided with a diagnostic interface (KWP 2000). Dectected errors are stored in the memory and can be read out for service and/or for troubleshooting purposes. System description The requirements for rear axle steering systems are described in the directive EC 70/311. When an error in the steering system occurs, no safety critical driving situation is permissible. Derived from this, the safety concept for the RAS-EC is as follows: For nondriven axles, with self-centering characteristics the axle free safe mode is recommended, i.e. the axle will then work like a trailing axle. Precondition for this safety philosophy is a redundant electronic system so that system errors are detected and the appropriate safety reactions are initiated. Mechanical construction Hydraulic components On a ZF RAS-EC, the operating circuit is supplied with pressure oil from an engine-driven pump. The required steering movement of the rear axle is introduced by a proportional valve connected to the electric control unit. The necessary steering forces are generated by a hydraulic power cylinder at the rear axle. By means of a pressure relief valve the components are protected from overload. If an error occurs, the pressurization of the operating circuit can be removed with a cut-off valve which is switched through in the zero-current condition. Sensors For the measurement of the instantaneous values at the front and rear axles non-contact sensors are used. These sensors have a long service life, operate over a large temperature range and are accommodated in a watertight housing. To make it possible to check the proper function of these sensors, they are of redundant design and intrinsically safe. 1 ZF Servocom 2 Steering angle sensor 3 CAN-Bus 4 Diagnosis 5 Oil reservoir 6 Steering pump 7 Pressure filter 8 Control valve 9 Electronic control unit 10 24V supply 11 Power cylinder 12 Turning angle sensor 13 Power cylinder with linear sensor ZF Servocom RAS-EC. Basic diagram for vehicle 6x2x4 with steered leading axle (nondriven). ZF Servocom RAS-EC 23

24 I Steering angle I rear axle [ ] VA_LW_max[ ]* max. steering angle rear axle* 0-15 km/h V [km/h] km/h 4 25 km/h Steering angle * These parameters can be programmed at the end of the assembly line. front axle [ ] Electrical structure The control unit is a system with 2 channels with mutual monitoring of the readiness for operation of the channels and includes all the components necessary for sensor evaluation, nominal-value calculation and valve actuation. The control circuit basically consists of a 16-bit high-capacity microcontroller. This computer acquires all incoming data and carries out a plausibility check. Using a graph of steering angle characteristics, the nominal value of the rear axle is generated from the vehicle speed and the front axle steering angle data. Then, from the nominal value and the steering angle of the rear axle, the quantity for the actuation of the control valve is computed. The valves are actuated via short-circuit-proof outputs and their function is then reread and checked by the microcontroller. In order to adapt the function of the rear axle steering system to the vehicle conditions additional sensors can be Vneutral [km/h]* read in. Thus, due to the axle geometry, actuation of the rear axle is not possible when the vehicle is at a standstill and the brake is applied at the same time. This for example can be noticed when the control unit also reads in the brake pressure. In this case, the rear axle is not actuated until either the brake has been released or a speed is measured. The safety circuit includes an 8-bit microcontroller. This device also acquires all incoming data and carries out a plausibility check. The nominal value of the rear axle calculated by it is compared with the actual value. The control computer of the operating circuit and the monitoring computer of the safety circuit are connected with each other via an interface and can thus exchange and compare input, intermediate and output quantities. Additionally, status information is exchanged via this interface in order to guarantee and check the correct function of the rear axle steering gear. If an error is detected, both the control computer and the monitoring computer are in a position to automatically switch off the outputs and thus initiate the safe mode. For the localization of errors and/or for the maintenance of the rear axle steering system, the control unit has full diagnostic capability. Thus, the values from the sensors for instance can be read out and checked via the diagnostic interface (KWP 2000). An integrated CAN interface allows data exchange with other control units during operation. By means of the integrated diagnostic interface the control unit can be programmed at the end of the assembly line, and during vehicle production it can be adapted to the conditions on the target vehicle. If necessary, this can for instance be used to carry out an electronic alignment of the sensors. Special functions The system allows the processing of numerous signals such as those from brake, door switch and other switches. Together with the speed signal, the front axle steering angle etc. it s possible to implement a variety of special functions. ZF Servocom RAS-EC. Basic diagram of a steering angle characteristic curve. 24 ZF Servocom RAS-EC

25 ZF Semi-Integral Power Steering Gears Application Steering gears of this type are used on vehicles which require high steering effort due to their high steering axle load and on which the required piston displacement exceeds the volume which can economically be accommodated in the power cylinder of a steering gear of integral design. Application begins at steering axle loads of approx. 8 tonnes on vehicles with king pin steering. Another possibility for the use of semiintegral power steering gears exists when, due to its length or offset, the drag link is unable to transmit the required steering forces. The number and size of the power cylinders used can be chosen such that at the maximum steering forces which occur and at the required steering wheel turning rate the vehicle can be steered with full hydraulic assistance. Design and function turning movements, thus causing the sector shaft to rotate. The control valve is located in the valve housing concentrically with the input shaft. When the worm shaft is rotated, the control valve is moved axially back and forth. This will displace the control edges such that the pressure oil gets from the steering pump to one power cylinder chamber. When the steering wheel is released, the valve is returned to its neutral position by spring force; the return flow is thus maintained. The valve housing is also provided with the connections for the pressure and return lines as well as for the lines to the power cylinder. Mechanical steering limiter Depending on customers requests, the semi-integral power steering gears can be fitted with mechanical steering limiter. It will avoid that the wheels are turned up to the wheel lock stops with full hydraulic pressure. This protects the steering linkage components from excessive loads. The semi-integral power steering gear (single-circuit design) comprises a complete mechanical steering gear. In it, the steering effort is transmitted from the input shaft, via a ball screw thread, to the steering nut and from there, via a tooth system, to the sector shaft. The steering nut is moved up and down by Installation schematic of a semi-integral power steering gear type 7421, dualcircuit design on a mobile crane. With engine-driven vane pump, wheel-driven radial piston pump and with one power cylinder each at the steering axles. ZF Semi-integral power steering gears 25

26 Semi-integral power steering gear, dual-circuit design Extra-heavy and special-purpose vehicles with very high steering axle loads and speeds in excess of 62 km/h cannot usually be steered without exceeding the steering effort limits prescribed by law if hydraulic assistance fails. For such applications our dual-circuit steering gears of semi-integral design with two independent steering valves for the control of the pressure oil in two entirely separate circuits are available. Vehicles thus equipped can still be steered fully even if pressure oil is lost, for instance due to pipe breakage in one steering circuit. Normally, one circuit of the steering system is fed by an engine-driven pump and the other by a wheel-driven steering pump. the steering circuits is provided with a wheel-driven emergency steering pump whose output flow is directed, under normal operating conditions of the two engine-driven pumps, into the return flow to the oil reservoir. If the engine-driven pump fails, the pressurized oil from the emergency steering pump is automatically supplied to the steering gear via a standby valve, with the result that vehicle steerability is maintained in any event. Thus, dualcircuit semi-integral power steering gears are an important contribution to safety in road traffic. To allow the wheels to be turned even if the engine is stopped, pressure oil supply to the two circuits by one engine-driven steering pump each has been provided for. Additionally, one of Figure at top: Semi-integral power steering gear, type 7421, single-circuit design with mechanical steering limiter and flange-mounted bevel box. Semi-integral power steering gear, type 7421, dual-circuit design with mechanical steering limiter. 26 ZF Semi-integral power steering gears

27 ZF Steering Pumps Application The main function of a steering pump is to generate a sufficient amount of the oil flow required for operating a hydraulic steering system. Convincing advantages which suggest the consistent installation of steering pumps from ZF Lenksysteme GmbH on commercial vehicles are in particular the compact design, the high efficiency at low weight as well as the possibilities of individual adaptation due to modular system configuration. Depending on the type of pump, such pumps can be attached to the vehicle engine or the compressor. For driving them, different elements of transmission such as V-belt pulleys, cross-slotted discs or gears are utilized. Radial piston pumps which act as emergency steering pumps are wheel-driven, i.e. by an axle or the gearbox output. pressure plate. These sealing faces have two suction and pressure zones opposite to each other. Thus, during each revolution the ten chambers formed by the vanes located in the rotor deliver an amount of oil which is twice their chamber volume. Also, this double arrangement of the suction and pressure zones neutralizes the radial hydraulic loads acting on the rotor. The pressure between the vanes and the cam ring, which is decisive for function, is produced by the radial centrifugal force of the vanes when the ZF Vane pump FN 4 On this type of pump, the pumping element which basically consists of a rotor (3), ten vanes (4), a cam ring (5) and the pressure plate (6) is accommodated in the light-alloy housing (1). The axial end is formed by a cover (7), which is also made of light alloy, with a sealing face made to conform to the Representation of the function of the ZF vane pump FN 4, type Housing 2 Shaft 3 Rotor 4 Vane 5 Cam ring 6 Pressure plate 7 Cover 8 Pressure relief and flow limiting valve ZF Steering Pumps 27

28 drive shaft is rotated. Additional pressure is achieved by pressurizing the inside faces of the vanes with pressure oil. The oil flow generated in the crescentshaped pressure chambers is supplied to the flow limiting valve (8) positioned longitudinally to the shaft (2) and limited at a set value. If relief of the type-specific maximum pressure of the pump (165 or 180 bar respectively) is not carried out within the steering gear as is recommended, this can in a specific instance be done by means of a pressure relief valve integrated in the flow limiting valve. The design of the vane pump FN 4 is mainly intended for attachment to the air compressor or to the generator by means of a cross-slotted disc for the transmission of torque free from radial load. With a view to technical necessity and economic volume of production, it is also possible to integrate into this pump type design features which allow radial drive loads, for example via a gear, and a pressure level of 200 bar maximum. It is also possible to mount the oil reservoir directly on the pump. This saves a hose and assembly costs at the vehicle manufacturer. ZF Vane pump FN 31 The basis for the modular-design pump is the short-length light-alloy cover. It includes a pumping element which features two pressure plates and is equal in its principle of hydraulic operation to the vane pump FN 4 described above. The cover also incorporates the flow limiting valve (if required, with integrated pressure relief), arranged at right angles to the drive shaft, and the suction and pressure ports. The pressure port is optionally feasible on the right or left. The symmetrical flange bolt pattern of housing and cover allows fitting in any one of 4x90 positions. The variable use of this pump is also based on the rugged shaft bearing system in the light-alloy housing. The pump is perfectly suitable to safely absorb the axial and radial drive loads if a clutch disc, V-belt pulley or gear is used. In a gear drive, the drive-side antifriction bearing can be lubricated by engine oil. Figure at top: ZF Vane pump FN 4, type Longitudinally fitted pressure relief and flow limiting valve. Right-hand figure: ZF Vane pump FN 31, type With short-length housing and transversely fitted flow limiting valve. 28 ZF Steering pumps