Advanced Vehicle Technology 2 Episode 10 pdf

the epicyclic gear set does not operate in the fourth quadrant even under full steering lock conditions. 9.5 Variable-ratio rack and pinion (Fig. 9.37(a±d)) Variable-ratio rack and pinion can be made to improve both manual and power assisted steering operating characteristics. For a manual rack and pinion steering system it is desirable to have a moderately high steering ratio to provide an almost direct steering response while the steering wheel is in the normally `central position' for straight ahead driving and for very small steering wheel angular correction movement. Conversely for parking manoeuvres requiring a greater force to turn the steering wheel on either lock, a more indirect lower steering ratio is called for to reduce the steering wheel turning effort. However, with power assisted steering the situation is different; the steering wheel response in the straight ahead driving position still needs to be very slightly indirect with a relatively high steering ratio, but with the power assistance provided the off-centre steering response for manoeuvring the vehicle can be made more direct compared with a manual steering with a slightly higher steering ratio. The use of a more direct low steering ratio when the road wheels are being turned on either lock is made possible by the servo action of the hydraulic operated power cylinder and piston which can easily overcome the extra tyre scrub and swivel-pin inclination resisting force. The variable-ratio rack is achieved by having tooth profiles of different inclination along the length of the rack, accordingly the pitch of the teeth will also vary over the tooth span. With racks designed for manual steering the centre region of the rack has wide pitched teeth with a 40  flank inclination, whereas the teeth on either side of the centre region of the rack have a closer pitch with a 20  flank inclination. Con- versely, power assisted steering with variable-ratio rack and pinion (see Fig. 9.37(c)) has narrow pitch teeth with 20  flank inclination in the cen- tral region; the tooth profile then changes to a wider pitch with 40  flank inclination away from the central region of the rack for both steering locks. Fig. 9.36 (a±d) Principle of rear steering box mechanism 352 Pressure angle 20° Pressure angle 40° (a) Central rack teeth (b) Off-centre rack teeth Wide pitch (P) Narrow pitch (p) Wide pitch (P) (c) Variable-ratio tooth rack Large p.c.d. more direct Transition Small p.c.d. Transition Large p.c.d. 30 25 20 15 5 0 480 180 120 60 30 0 30 60 120 180 480 Turning steering wheel to left Turning steering wheel to right Steering wheel and pinion rotation (deg) (d) Rack and pinion movement ratio from lock to lock of the steering wheel PP R r Movement ratio Fig. 9.37 (a±d) Variable ratio rack and pinion steering suitable for power assisted steering 353 With variable-ratio rack and pinion involute teeth the rack has straight sided teeth. The sides of the teeth are normal to the line of action, therefore, they are inclined to the vertical at the pressure angle. If the rack has narrow pitch `p' 20  pressure-angle teeth, the pitch circle diameter (2R) of the pinion will be small, that is, the point of contact of the meshing teeth will be close to the tip of the rack teeth (Fig. 9.37(a)), whereas with wide pitched `P' 40  pressure-angle tooth contact between teeth will be near the root of the rack teeth (Fig. 9.37(b)) so its pitch circle diameter (2R) will be larger. The ratio of steering wheel radius to pinion pitch circle radius (tooth contact radius) determines the movement ratio. Thus the smaller the pitch circle radius of the pinion for a given steering wheel size, the greater will be the movement ratio (see Fig. 9.37(d)), that is, a smaller input effort will be needed to steer the vehicle, but inversely, greater will be the steering wheel movement relative to the vehicle road wheel steer angle. This design of rack and pinion tooth profile can provide a movement-ratio variation of up to 35% with the number of steering wheel turns limited to 2.8 from lock to lock. 9.6 Speed sensitive rack and pinion power assisted steering 9.6.1 Steering desirability To meet all the steering requirements the rack and pinion steering must be precise and direct under normal driving conditions, to provide a sense of feel at the steering wheel and for the steering wheel to freely return to the straight ahead position after the steering has been turned to one lock or the other. The conventional power assisted steering does not take into account the effort needed to perform a steering function relative to the vehicle speed, particularly it does not allow for the extra effort needed to turn the road wheels when man- oeuvring the vehicle for parking. The `ZF Servotronic' power assisted steering is designed to respond to vehicle speed requirements, `not engine speed', thus it provides more steering assistance when the vehicle is at a standstill or moving very slowly than when travelling at speed; at high speed the amount of steering assistance may be tuned to be minimal, so that the steering becomes almost direct as with a conventional man- ual steering system. 9.6.2 Design and construction (Fig. 9.38(a±d)) The `ZF Servotronic' speed-sensitive power assisted steering uses a conventional rotary control valve, with the addition of a reaction-piston device which modi- fies the servo assistance to match the driving mode. The piston and rotary control valve assembly comprises a pinion shaft, valve rotor shaft with six external longitudinal groove slots, valve sleeve with six matching internal longitudinal groove slots, torsion bar, reaction-piston device and an electro-hydraulic transducer. The reaction-piston device is supported between the rotary valve rotor and valve sleeve, and guided internally by the valve rotor via three axially arranged ball grooves and externally guided by the valve sleeve through a multi-ball helix thread. The function of the reaction-piston device is to modify the fluid flow gap formed between the valve rotor and sleeve longitudinal groove control edges for different vehicle driving conditions. An electronic control unit microprocessor takes in speed frequency signals from the electronic speedometer, this information is then continuously evaluated, computed and converted to an output signal which is then transmitted to the hydraulic transducer mounted on the rotary control valve casing. The purpose of this transducer is to control the amount of hydraulic pressure reaching the reaction-piston device based on the information supplied to the electronic control unit. 9.6.3 Operation of the rotary control valve and power cylinder Neutral position (Figs 9.38(a) and 9.39(a)) With the steering wheel in its central free position, pres- surized fluid from the pump enters the valve sleeve, passes though the gaps formed between the long- itudinal groove control edges of both sleeve and rotor, then passes to both sides of the power cylin- der. At the same time fluid will be expelled via corresponding exit `sleeve/rotor groove' control- edge gaps to return to the reservoir. The circulation of the majority of fluid from the pump to the reservoir via the control valve prevents any build- up of fluid pressure in the divided power cylinder, and the equalization of the existing pressure on both sides of the power piston neutralizes any `servo' action. Anticlockwise rotation of the steering wheel (turning left Ðlowspeed)(Figs 9.38(b) and 9.39(b)) Rotating 354 Rack Pinion shaft Reservoir Pump Valve sleeve Inner check valve Outer check valve Inner reaction chamber Outer reaction chamber Torsion bar Reaction piston (RP) Valve rotor shaft Outer orifice Inner orifice Teflon ring seal Electronic speedometer Electronic control unit (ECU) Power piston Power cylinder Electro-hydraulic transducer (EHT) Left hand side Right hand side Cut-off valve (CO-V) (2) (3) (a) Neutral position (4) (1) 6 5 7 6 Fig. 9.38 (a±d) Speed sensitive rack and pinion power assisted steering with rotary reaction control valve 355 the steering wheel in an anticlockwise direction twists the control valve rotor against the resistance of the torsion bar until the corresponding leading edges of the elongated groove in the valve rotor and sleeve align. At this point the return path to the exit port `4' is blocked by control edges `2' while fluid from the pump enters port `1'; it then passes in between the enlarged control-edge gaps to come out of port `3', and finally it flows into the right- hand power cylinder chamber. Left hand side R P (4) (1) Inner check valve Outer check valve RP 6 5 7 Speedo ECU (3)(2) EHT CO-V Right hand side (b) Turning left anticlockwise (low speed) 6 Fig. 9.38 contd 356 Left hand side R P (4) (1) Inner check valve Outer check valve RP 6 5 7 6 CO-V (2) (3) EHT ECU Speedo Right hand side Ball guide grooves Ball thread grooves Reaction piston (c) Turning left anticlockwise (high speed) Fig. 9.38 contd 357 Conversely fluid from the left hand side power cylinder chamber is pushed towards port `2' where it is expelled via the enlarged trailing con- trol-edge gap to the exit port `4', then is returned to the reservoir. The greater the effort by the driver to turn the steering wheel, the larger will be the control-edge gap made between the valve sleeve and rotor and greater will be the pressure imposed on the right hand side of the power piston. Left hand side (4) (1) P R Right hand side ECU Speedo (3) EHT (2) co-v 6 5 Inner check valve Outer check valve RP (d) Turning right clockwise (high speed) 7 6 Fig. 9.38 contd 358 When the vehicle is stationary or moving very slowly and the steering wheel is turned to man- oeuvre it into a parking space or to pull out from a kerb, the electronic speedometer sends out its minimal frequency signal to the electronic control unit. This signal is processed and a corresponding control current is transmitted to the electro- hydraulic transducer. With very little vehicle move- ment, the control current will be at its maximum; this closes the transducer valve thus preventing fluid pressure from the pump reaching the reaction valve piston device and for fluid flowing to and through the cut-off valve. In effect, the speed sen- sitive rotary control valve under these conditions now acts similarly to the conventional power assisted steering; using only the basic rotary con- trol valve, it therefore is able to exert relatively more servo assistance. Anticlockwise rotation of the steering wheel (turning left Ð high speed) (Figs 9.38(c) and 9.39(b)) With increasing vehicle speed the frequency of the elec- tronic speedometer signal is received by the electro- nic control unit; it is then processed and converted to a control current and relayed to the electro- hydraulic transducer. The magnitude of this con- trol current decreases with rising vehicle speed, Return long slot Sleeve Rotor Torsion bar Supply short slot Reservoir Pump Right hand Left hand Power cylinder and piston (a) Neutral position (4) (2) (1) (3) (4) Fig. 9.39 (a±c) Rack and pinion power assisted steering sectional end views of rotary reaction control valve 359 correspondingly the electro-hydraulic transducer valve progressively opens thus permitting fluid to reach the reaction piston at a pressure determined by the transducer-valve orifice opening. If the steer- ing wheel is turned anticlockwise to the left (Fig 3.38(c)), the fluid from the pump enters radial groove `5', passes along the upper longitudinal groove to radial groove `7', where it circulates and comes out at port `3' to supply the right hand side of the power cylinder chamber with fluid. Conversely, to allow the right hand side cylinder chamber to expand, fluid will be pushed out from the left hand side cylinder chamber; it then enters port `2' and radial groove `6', passing through the lower longitudinal groove and hollow core of the rotor valve, finally returning to the reservoir via port `4'. Fluid under pressure also flows from radial groove `7' to the outer chamber check valve to hold the ball valve firmly on its seat. With the electro-hydraulic transducer open fluid under pump pressure will now flow from radial grooves `5' to the inner and outer reaction-piston device orifices. Fluid passing though the inner orifice cir- culates around the reaction piston and then passes to the inner reaction chamber check valve where it pushes the ball off its seat. Fluid then escapes through this open check valve back to the reservoir by way of the radial groove `6' through the centre of the valve rotor and out via port `4'. At the same time fluid flows to the outer piston Left hand (b) Turning left – anticlockwise rotation of the steering wheel (4) (2) (1) (3) (4) R P Sleeve Rotor Torsion bar Supply short slot Return long slot Fig. 9.39 contd 360 reaction chamber and to the right hand side of the outer check valve via the outer orifice, but slightly higher fluid pressure from port `7' acting on the opposite side of the outer check valve pre- vents the valve opening. However, the fluid pres- sure build-up in the outer piston reaction chamber will tend to push the reaction piston to the left hand side, consequently due to the pitch of the ball- groove helix, there will be a clockwise opposing twist of the reaction piston which will be trans- mitted to the valve rotor shaft. Accordingly this reaction counter twist will tend to reduce the fluid gap made between the valve sleeve and rotor long- itudinal control edges; it therefore brings about a corresponding reaction in terms of fluid pressure reaching the left hand side of the power piston and likewise the amount of servo assistance. In the high speed driving range the electro- hydraulic transducer control current will be very small or even nil; it therefore causes the transducer valve to be fully open so that maximum fluid pres- sure will be applied to the outer reaction piston. The resulting axial movement of the reaction pis- ton will cause fluid to be displaced from the inner reaction chamber through the open inner reaction chamber check valve, to the reservoir via the radial groove `6', lower longitudinal groove, hollow rotor and finally the exit port `4'. As a precaution to overloading the power steer- ing, when the reaction piston fluid pressure reaches (c) Turning right – clockwise rotation of the steering wheel Right hand (4) (2) (1) (3) (4) R P Fig. 9.39 contd 361 [...]... situated relatively low between the two front wheels Fig 10 .20 wishbone Fig 10 .21 wishbone Outward converging transverse double Fig 10 .22 10 .2. 4 Transverse double wishbone suspension (Figs 10 .20 , 10 .21 and 10 .22 ) If lines are drawn through the upper and lower wishbone arms and extended until they meet either inwards (Fig 10 .20 ) or outwards (Fig 10 .21 ), their intersection point becomes a virtual instantaneous... (3 Nm in total) the actuating pressure can reach 94 bar For a vehicle speed of 20 km/h the rise in servo pressure is less steep, thus for an input effort torque of 2 Nm the actuating pressure has only risen to 3 62 100 80 Km/h 160 Km/h 94/3 /h Km 60 20 0 Km/h Fluid pressure (bar) 80 40 /2 40/6 40 30/3 14 /2 20 18/3 17/6 10 /2 0 8 6 /2 6 4 2 0 2 8.7/3 4 6 8 Steering wheel torque (Nm) Fig 9.40 speeds Speed... increases with both wheel traction and vehicle speed 10. 1.5 Swivel joint positive and negative offset (Figs 10. 10 10. 15) When one of the front wheels slips during a brake application, the inertia of the moving mass will tend to swing the vehicle about the effective wheel which is bringing about the retardation because 3 72 Fig 10. 11 Swivel pin inclination negative offset Fig 10. 10 Swivel pin inclination positive... pivot axes or the vertical sliding pillar axis enables the roll centre height to be varied proportionally 10 .2. 5 Parallel trailing double arm and vertical pillar strut suspension (Figs 10 .23 and 10 .24 ) In both examples of parallel double trailing arm (Fig 10 .23 ) and vertical pillar strut (Fig 10 .24 ) suspensions their construction geometry becomes similar to the parallel transverse double wishbone layout,... tyre stability base tw 10 .2. 7 Semi-trailing arm rear suspension (Fig 10 .26 ) A semi-trailing arm suspension has the rear wheel hubs supported by a wishbone arm pivoted on an inclined axis across the body (Fig 10 .26 (a)) If lines are projected through the wishbone arm pivot axis and the wheel hub axis they will intersect at the virtual instantaneous centres IBW1 and IBW2 (Fig 10 .26 (a and b)) The distance... axles of the vehicle 1 The swing arm pivot instantaneous centres IWB1 and IWB2 rotate about their instantaneous centres IWG1 and IWG2 in proportion to the amount of body roll 2 The swing arm pivot instantaneous centres IWB1 and IWB2 move on a circular path which has a centre derived by the intersecting projection lines drawn through the tyre to ground instantaneous centres IWG1 and IWG2 10 .2. 1 Determination... members moving parallel to the body as they deflect up and down Hence looking at the suspension from the front, neither the double trailing arms (Fig 10 .23 ) nor the sliding pillar (Fig 10 .24 ) layout has any trans- 10 .2. 6 MacPherson strut suspension (Fig 10 .25 ) To establish the body roll centre height of any suspension, two of the three instantaneous centres, the tyre contact centre and the swing arm virtual... strut alone 10 .2 Suspension roll centres Roll centres (Fig 10 .29 ) The roll centre of a suspension system refers to that centre relative to the ground about which the body will instantaneously Fig 10. 16 Concentric coil spring and swivel pin axes permit bending moment reaction 375 rotate The actual position of the roll centre varies with the geometry of the suspension and the angle of roll 10 .2. 2 Short swing... transverse distance between both steering wheel contact centres Fig 10. 1 Wheel camber geometry 368 Fig 10 .2 10. 1.3 Swivel or kingpin inclination (Figs 10. 3 10. 7) Swivel pin or kingpin inclination is the lateral inward tilt (inclination) from the top between the upper and lower swivel ball joints or the kingpin to the vertical (Fig 10. 3) If the swivel ball or pin axis is vertical (perpendicular) to... commonly referred to as the body roll centre 10 .2. 8 High load beam axle leaf spring sprung body roll stability (Fig 10 .27 ) The factors which influence the resistance to body roll (Fig 10 .27 ) are as follows: a) The centrifugal force acting through the centre of gravity of the body load b) The arm length from the centre of load to the effective roll centre h1 or h2 c) The spring stiffness in Newtons/metre . the output pinion shaft. 100 80 60 40 20 0 86 42 02 46 Steering wheel torque (Nm) Fluid pressure (bar) 94/3 40 /2 0 Km/h 20 /hKm 80 Km/h 160 Km/h 30/3 18/3 14 /2 10 /2 6 /2 17/6 40/6 8 8.7/3 Fig wheel contact centres. 10. 1 .2 Wheel camber angle (Figs 10. 1 and 10 .2) Wheel camber is the lateral tilt or sideway inclin- ation of the wheel relative to the vertical (Fig. 10. 1). When the top of. Note the engine and vehicle speeds are monitored by the tachometer and anti- lock brake sensors respectively. 9.7 .2 Operating principle (Figs 9. 42( a±c)) Neutral position (Fig. 9. 42( b)) When the input and