The Motor Vehicle 2010 Part 16 pot

1076 The Motor Vehicle Fig. 40.47, now has a variable pitch so that when the roller follower B is in the central position shown, the ratio angular motion of cam : angular motion of drop arm (C), for a very small motion, is about 21 : 1 but when the drop arm and follower have moved about 12° away from the central position that ratio has fallen to 13 : 1 and thereafter it remains constant at that value. The valve which controls the servo action now operates by the rotational displacement of its two main components instead of by their axial displacements as in the Ross system. The cam is carried in the casing on two angular contact ball bearings and is coupled at the left-hand side to the torsion bar D by a cross-pin. At the right-hand end the torsion bar is coupled, also by a cross-pin, to the sleeve E which is splined to the steering-wheel shaft and which forms the inner member to the valve. The left-hand end of the sleeve E is formed with splines F which engage splines formed in the right-hand end of the cam A but these splines are machined so as to allow 7° of freedom of rotation and are only to provide a safeguard against over-stressing of the torsion bar when the steering is operated without servo assistance. Pipes connect the pump, which is driven by the engine, to the valve and the latter to the outer end of the servo cylinder; the inner end of the servo cylinder forms the casing which houses the cam A and follower assembly and the valve is directly connected to that space. The outer member G of the valve is coupled by the ball-ended pin H to the right-hand end of the cam A. The servo piston J is integral with a stem on which rack teeth are formed and these teeth engage teeth machined on the end of the drop arm forging C. The principle of operation of the valve is shown by the diagrams Figs 40.48(a) and (b). It is really three valves in parallel, parts relative to which are denoted by suffixes 1, 2 and 3 – following the letters P and S in the diagrams – but the action will be described in relation to one of them. The ports P are connected to the delivery of the pump and when the valve sleeve E is centrally placed, as shown at (a), fluid flows equally to each of the pockets S 1 and S 2 which are connected at their ends to the return pipe to the pump. The pressure drops across the apertures b and a are equal to those across the apertures c and d and so there is no pressure difference between the spaces C L and C R which are connected to the ends of the servo cylinder. Hence there is no net hydraulic force acting on the servo piston. But when a torque is applied to the steering wheel to overcome a resistance to a steering motion of the road wheels the torsion bar is twisted and relative motion occurs between the inner valve sleeve E and the outer one G, as is shown at (b). The passage c is thereby increased while the passage d is decreased and so the pressure in the space C R is raised. Conversely, the passage b is decreased and a is increased so that the pressure in the space C L is lowered. A pressure difference is thus established across the servo piston and the drop arm is rotated. As this occurs the cam and outer sleeve of the valve rotate so as to follow up the inner valve sleeve and bring the valve to a central position. The drop arm having thus been rotated the required amount the servo action ceases and the system remains in equilibrium. The use of three sets of ports provides a valve in which the radial hydraulic pressures are balanced and the required port areas are obtained with a valve only one-third the length that would be needed if only one set was provided. Provision is made for adjusting the mesh of the roller B with the cam A; 1077Front axle and steering mechanism this is done by means of the screw L which, when turned, will move the drop arm shaft C in or out and thus bring the roller into closer or looser mesh. Similarly, the mesh of the rack teeth on the servo piston stem can be adjusted by means of the screwed plug M which bears on the underside of the stem through the spherically seated pad N. 40.30 Electrically powered systems Electrical is much more amenable than hydraulic power to electronic control. Consequently, many more variables can be taken into consideration for the provision of assistance appropriate for differing conditions, such as the speed of the vehicle, condition of the road, rate of change of speed of rotation of the steering wheel and degree of braking or acceleration, if any. The advantages of electric control therefore are as follows: 1. Increased safety at high speeds because, in these conditions, the provision of power is independent of engine speed so assistance can be applied more sensitively. 2. By virtue of direct mechanical application of assistance by an electric motor and the potential for using irreversible gearing, the kickback felt at the steering wheel, when driving off-road for example, can be even less pronounced than with hydraulic power assistance (Fig. 40.49) 3. Lower energy consumption, hence reduced emissions, and improved acceleration of cars with small engines. Claims of up to 5% reduction in fuel consumption by comparison with engine-driven hydraulically powered systems have been claimed, although it seems likely that an overall figure of about 2–3% would be more realistic. 4. The elimination of hydraulics leads to compactness, fewer components, reduced weight and maintenance, no fluid to be kept topped up, no leaks, and installation costs are reduced. 5. Steering assistance is maintained if the engine stalls. 6. Much better performance under very cold conditions, when hydraulics would be adversely affected by increased viscosity of the fluid. 7. With electronic control, it is easier to provide failure warning, self- diagnosis and self-protection systems, and it is possible to design systems that can be easily and rapidly tuned to suit individual applications and can be integrated with other electronic systems such as ABS and integrated vehicle stability control (IVSC). It is even practicable to design driver- selected feel into the system. 40.31 TRW systems In addition to producing electrohydraulic power steering systems, TRW, which now embraces Lucas Varity Electric Power Steering Systems, offers three electric power steering arrangements: rack drive, column drive and pinion drive. All three are of what the company terms integrated architecture, in other words, they are self-contained. The benefits of a fully integrated design are as follows: 1. All components are contained as a single sub-assembly: that is, without harnesses between the ECU, motor and sensor. 1078 The Motor Vehicle 2. Such units can be fully tested by their manufacturers, ready for installation in the vehicle. 3. Because there is only one item per vehicle, the inventory is reduced to the minimum. 4. The ECU can store unit-specific data, including serial numbers and calibration data, such as torque sensor offset, torque sensor gain, etc. 5. Numbers of parts reduced to a minimum. 6. Motor-to-sensor connections and protected circuits are unnecessary. 7. The single housing forms an effective screen for obviating both radio interference and protection against radio frequency emissions from, for example, roadside installations. 8. No connections liable to be made incorrectly or damaged during vehicle assembly. 9. Fault diagnosis is simplified. 10. Such installations are very compact and of light weight. 40.32 TRW rack drive system The rack drive units, which weighs 16 kg, has four main components. Power is provided by a 93 mm diameter brushless reversible AC motor with rare earth permanent magnets and copper windings in the stator. The rotary motion of this motor, which is extremely compact, is converted into linear motion by a recirculating ball-nut rotating in grooves around the rack, which is coaxial with both these components, Fig. 40.50. There are two sensors, one for input torque and the other, on the motor, indicates with high precision the position of the rotor. At a rotational speed of 360° per sec, the rack force is 7650 N. The output power is 360 W at 65 A. Steering feel is programmable. Fuel efficiency is claimed to be between about 0.3 to 1.5 miles/gal better than with hydraulic power steering. At the time of writing, units are being developed for handling rack forces of up to 11 kN. As can be seen from Fig. 40.51, the ECU is mounted on top of the tubular housing of the motor. At one end of the motor is the recirculating ball-nut Transducer housing Torque sensor Pinion Motor coil Motor stator laminations ECU Motor rotor laminations Vehicle connector Battery connector Outboard housing DX rack bushing Preload device Wave spring Integral end-mounted ball-nut Motor bearing Motor tube Stator shield Motor position sensor Rack Pinion housing Fig. 40.50 The TRW electric power steering unit is extremely compact. Its high efficiency motor is coaxial with the rack 1079Front axle and steering mechanism Pinion Manually applied torque Manually applied force Total force to steering arms Power assistance force Torque sensor ECU Torque signals Sensors for other functions Motor Position sensor Motor position Reference signal Ball-nut and ball screw assembly Assist torque Reference signal Drive currents Rack Fig. 40.51 The ECU is mounted on top of the motor and rack-and-pinion gear drive for the rack and, at the other, the motor position sensor. Outboard of the last mentioned is the rack-and-pinion in a housing bolted on to that of the motor. A torque sensor is fitted immediately above the pinion. In common with all the variants of the rack drive system, the input torque sensor is of the inductive type, Fig. 40.52. This has three pick-ups from the torsion bar, one being the median, or datum, reading and the other two the end readings. Consequently, electronic noise is cancelled out because it is duplicated in a positive and a negative mode, and thus cancelled out. There are two tracks for transmission of the signals to the ECU. Alternative sensors that might have been used are: strain gauge on torsion bar, with overtorque protection, which produces only small signals; magnetostrictive, which is entirely new technology with unknown reliability; potentiometric, with torsion bar and overtorque protection; or optical, which requires a stiff torsion bar with overtorque protection, Fig. 40.53. However, the last mentioned is the only alternative which offers intrinsic position sensing. 40.33 The column and pinion drive variants For the column-mounted, Fig. 40.54, and pinion-mounted, Fig. 40.55, variants of the system a brushless, rare earth magnet type AC motor drives respectively the column or pinion through a worm-and-wheel gear, Figs 40.56 and 40.57. In both variants the motor is used in association with an optical torque-and- position sensor, Fig. 40.53. The motor is electronically controlled, quiet, and is driven by a 3-phase power bridge in the ECU. It is not only simple and without rotating contacts but also relatively quiet and has low inertia. Alternative power units would have been: the variable reluctance motor, but this would have been unnecessarily complex and costly; a brushless DC motor would have been less costly but noisier and its output would have been a square, instead of sinusoidal, wave; a brushed motor with an H-bridge and without a position sensor would have been simpler still to control, but its inertia and friction would have been higher. Incidentally, square waves have 1080 The Motor Vehicle Input Outputs A & B Input shaft from steering wheel Output shaft disc Detector B Output shaft to motor and rack Detector A Input shaft disc Light source A Fig. 40.53 The optical sensor has the advantage of being capable of monitoring intrinsic position as well as degrees of rotation, or torque Fig. 40.52 An inductive type sensor signals the input torque to the ECU Light source B 1081Front axle and steering mechanism IN-CAB Under-bonnet ECU Gearbox Sensor Motor Fig. 40.54 With the TRW column mounted system, all the sensitive components are in the saloon or cab Fig. 40.55 The pinion mounted layout is the more compact, and installation can be easier IN-CAB Under-bonnet Motor Sensor ECU Gearbox the disadvantage of sending a strong ripple feedback to the steering wheel. With sinusoidal, or near sinusoidal, waves, the reversals are not only inherently less sharp, but also their timing can be such that, as each successive wave is ascending the next is descending so, to a major degree, they tend to cancel out. Although with the pinion mounted variant, the ECU can be better protected by mounting it remotely, in the cab or body, the simpler solution is to integrate it with the pinion drive, as in Fig. 40.55. If space around the pinion is at a premium, it may have to be mounted remotely under the bonnet, although this introduces parts that are vulnerable to damage, and assembly into the vehicle costs more. Moreover, particular attention has to be devoted to sealing and, since underbonnet temperatures can be high, especially in summer, costly heat resistant materials may be needed for some of the components. Because of the high level of radio frequency radiation, screening is required. Extra protection against short circuiting is needed and the electrical resistance will be higher than that of the integrated version. 40.34 ZF Servolectric system The ZF Servolectric system is powered directly by an electronically controlled motor. Thus, it dispenses with hydraulics and the consequent complexity of 1082 The Motor Vehicle Fig. 40.56 Motor and worm drive of the TRW system control needed to regulate the supply between the pump and the hydraulic accumulator. Because it is controlled electronically, the system can be adapted to suit precise requirements such as speed-related assistance, condition of the road and changes in circumstances, for instance when braking or accelerating. Damping characteristics can be programmed to suit changing conditions, including on- and off-road driving. There is even a possibility of linking the electronic control to a satellite navigation system. By virtue of the fact that energy is consumed only when the vehicle is being steered, the overall fuel consumption of the vehicle may be as little as 0.01% more than that of its manually steered equivalent. Indeed, the energy consumption of this electrical system is claimed to approach 80% of that of a hydraulic system, thus reducing the fuel consumption of a medium size car by up to about 0.25 litres per 100 km. Output Torque sensor Input ECU Fig. 40.57 Section through the worm drive, showing the torsion bar and torque sensor 1083Front axle and steering mechanism Three versions of the Servolectric unit are available: in the first, which is for light cars with a maximum load on the steered wheels of 600 kg, the electro-servo is incorporated in the steering column; in the second, for mid- range cars, maximum steering axle load 900 kg, it drives the pinion; in the third, for larger cars and light commercial vehicles, the servo acts upon the steering rack. 40.35 Honda EPS and VGR systems Honda first applied a mechanically driven hydraulic pump type power assisted steering rack and pinion gear to its NSX car in 1990. In 1991, to save energy, an electrically driven hydraulic pump replaced the engine-driven one and a rotary spool type valve was introduced for the system installed on the Honda Civic. Their first electrically powered system was introduced in 1993, and speed dependent power assistance in 1995. The latter system combined reduced steering effort at low speeds with increased stability at high speeds. In 1997, Honda announced the first variable ratio, electronically controlled, electrically actuated (EPS + VGR) rack-and-pinion steering gear. As can be seen from Fig. 40.58, the variable ratio is obtained by pitching the central gear teeth of the rack more closely than towards its ends. Consequently, manoeuvring for parking is easier and steering during, for example lane changes, at higher speeds can be effected more smoothly. Overall, the rack is longer than normal. Near one end, the variable ratio teeth mesh with a spiral toothed pinion. Mounted coaxially around the rack near its other end is the reversible electric motor. By rotating a ball-nut assembly running in spiral grooves around that end of the rack, this motor moves the rack axially in a direction determined by which way the steering wheel is turned. Spherical sockets at each end of the rack receive the ball ends of the steering rods. Signals indicating the torque applied by the driver to the steering wheel are transmitted, from a sensor interposed between the steering shaft and the Centre of rack Steering column High ratio Standard ratio High ratio Fig. 40.58 The teeth near the centre of the rack of the Honda EPS + VGR system are more closely pitched than those nearer its ends 1084 The Motor Vehicle pinion, to the electronic control unit (ECU). The differentials of these signals represent the speed of rotation of the steering wheel. Other inputs from sensors to the ECU include the torque being delivered by the electric motor, the resistance torque from the road surface, kickback torque, vibration due to wheel and tyre imbalance, and self-alignment torque. A current of up to about 10 A is required by the motor but, of course, only when power assistance is actually being applied. The motor and ECU are mutually adjacent, to minimise losses in the cables. 1085 Chapter 41 Wheels and tyres Over many decades, the wheel and tyre assembly has become increasingly regarded as an integral part of the suspension system Section 42.2 and Fig. 42.1. Consequently, it is appropriate at this juncture to examine it in detail. Since wheels and tyres are largely standard components used throughout the industry, frequent reference has to be made in this chapter to BS AU 50 Parts 1 and 2. These Standards comprise respectively about six and eight sections and many subsections, taking up something like 200 pages. Since all are subject periodically to revision, readers needing more detailed information are advised to write to the British Standards Institution, 2 Park Street, London W1A 2BS for the latest details. First, some historical notes. While wheels date back prehistoric times, the pneumatic tyre, Fig. 41.1, was invented by R.W. Thomson, of Edinburgh, in 1845, and produced in the Macintosh Works in Manchester. It comprised an inner tube of rubberised canvas enclosed in a leather casing. This was long before rubber of adequate durability became available for use in tyre production. It was John Boyde Dunlop, a Scottish veterinary surgeon living in Belfast, who introduced the first pneumatic tyre for cycles. Prior to this, solid rubber tyres were employed for a wide variety of vehicles. Indeed, it was not until 1912 that the first pneumatic tyred truck appeared, and solid rubber tyred heavy vehicles were still to be seen on the roads up to the early 1920s. Even so, it was Michelin who, in 1891, first manufactured detachable pneumatic tyres for bicycles in quantity. By 1895, this company was providing tyres for the rapidly expanding motor manufacturers in France. The early tyres produced in quantity comprised an outer casing designed to embrace, reinforce and protect an inflated air-tight inner tube, and to provide the essential wear resistance and grip on the road. Isolation of the vehicle structure from the high frequency vibrations applied to the wheel as it rolled along the road was, and remains, the basic function of the wheel and tyre assembly. Nowadays, vibrations at frequencies below 20 Hz are absorbed mainly by the suspension springs, while the tyres absorb vertical and longitudinal vibrations up to about 400 Hz. In general, early tyres were narrow and inflated to about twice the pressures common today. The high pressures were necessary to reduce the chafing, due to the scissor-like movements of the warps and wefts of the woven canvas reinforcement, which tended to cause failure by overheating the carcass of [...]... ratio and tyre markings The aspect ratio is the inverse of the ratio of the width to the height of the crown of the tyre above the edge of the bead that seats on the rim Modern 1102 The Motor Vehicle tyres are all wider than they are high Before World War II, Avon, for example, produced their Super Safety range, which were Super Balloon type tyres with an aspect ratio of 95% By the time that radial tyres... such that the frequency of the disturbance that they generate coincides with the natural frequency of the suspension system The second is imbalance of the wheel, the out-of-balance force of which will increase as the square of the speed of rotation Of the two different frequencies: one is that of the sprung mass on the suspension spring system, and the second is that of the unsprung mass – the wheel... For example, the tyres must be capable of supporting both the static and dynamic vertical loads applied to the contact patches between their peripheries and the road Only 5% of this load is taken by the sidewalls of the tyres, the rest being taken by the air pressure acting on the inner surface of the tyre in the contact patch Therefore, if a fourwheel vehicle weighs 2000 lb (56.63 kg), and the tyres... data and the fact that recommended tyre pressures are a function of, among other things, the load on the tyre and the area of its contact patch with the road Following a great deal of research with road vehicles, the need became apparent for the spaces between the tread blocks and ribs to be increased and, in some instances, the blocks and spaces to be tapered from their roots to the periphery of the tread,... than 1.6 mm depth of tread Among the other requirements is that there be no damage to the plies in the side walls, nor bulges on them The latter indicate that the plies have separated from the rubber of the carcass Exposed plies too are illegal regardless of their position on the tyre 41.9 Off-road vehicle tyres Off-road tyres, such as those fitted to many 4-wheel drive vehicles, generally have deeply... to right, in which case the attachment face to the brake drum is that on the other side of the nave to provide a wider track Whereas in the former case the brake drum is enveloped by the wheel, in the latter it is not and therefore is better cooled Fig 41.10 This is a layout widely employed for axles with two wheels at each end; these two are bolted back to back 1094 The Motor Vehicle Consequently, rim... reasonably accurately if the inflation pressure falls The aim is at preventing the tyre from leaving the wheel Devices ranging from a simple band tightened round the rim to cover the well in the rim after the tyre has been fitted but before it is inflated, to others such as spacers to be expanded between the two beads to lock them in their seatings The simple band device is among the lightest, but it does... 1955–65 Obviously the diameter of the tyre, size of contact patch between tyre and road, the rate of the tyre acting as a spring, and weight of wheel and axle assembly affect the magnitude of the shock transmitted to the axle, while the amplitude of wheel motion is influenced by all these factors plus the rate of the suspension springs, damping effect of the shock absorbers, and the weights of the unsprung... masses The unsprung mass can be loosely defined as that between the road and the main suspension springs, while the sprung mass is that supported on these suspension springs, though both may also include the weights of parts of the springs and linkages 1109 1110 The Motor Vehicle Two entirely different types of shock are applied to the wheel: that due to the wheel’s striking a bump, and that caused by the. .. bump, and that caused by the wheel’s falling into a pothole The former will be influenced to a major extent by the geometry of the bump and the speed of the vehicle, while the major influence on the latter, apart from the geometry of the hole, is the unsprung masses and spring rates, speed being an incidental influencing factor Human sensitivity to these disturbances is very complex, and a more detailed . between the ECU, motor and sensor. 1078 The Motor Vehicle 2. Such units can be fully tested by their manufacturers, ready for installation in the vehicle. 3. Because there is only one item per vehicle, . of the motor and rack-and-pinion gear drive for the rack and, at the other, the motor position sensor. Outboard of the last mentioned is the rack-and-pinion in a housing bolted on to that of the motor. . around the rack, which is coaxial with both these components, Fig. 40.50. There are two sensors, one for input torque and the other, on the motor, indicates with high precision the position of the