LV39 steering systems (3)

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kap all phase & 6/11/03 11:37 am Page 31 Student Workbook LV39 Steering Systems (3) LV39/SWB Student Workbook for Technical Certificates in Light Vehicle Maintenance and Repair MODULE LV39 STEERING SYSTEMS (3) Contents Page Page … Introduction: Objectives Hydraulic Type Power Steering: Review of construction Hydraulic pump Exercise Control valve Exercise Hydraulic piston Principle of operation Speed sensitive steering Troubleshooting Diagnostic procedures Exercise Exercise A road speed sensitive system 3 3 4 6 10 11 12 13 14 Electronic Power Steering (EPS): Review of construction Principle of operation Troubleshooting Steering geometry ‘Ackerman’ steering Steering axis inclination (SAI) Camber angle Caster angle Toe angle Summary Progress check 15 15 16 18 18 19 20 22 23 24 25 26 Rear wheel steering 27 Active rear wheel steering – 4WS Progress check 27 29 -1Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue -2Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Introduction This is the third study module for power steering In this module we will focus on the troubleshooting techniques that you will need to master in order to resolve problems with power steering issues We will also introduce you to electronic power steering systems and how to troubleshoot these increasingly common systems We will also study how to assess the steering system for damage and alignment and review the steering geometry related to the steering system Finally, we will introduce four wheel steering system (4WS) and review the basic principles used in these systems Objectives After studying this module, you will be able to: • explain the common faults associated with power steering systems • explain the steering geometry associated with steering systems • describe the operating principles of a four wheel steering system (4WS) Hydraulic Type Power Steering The hydraulic type power steering system is currently the most common type used today, although the electronic type is quickly catching up and is likely to become the favoured choice in the years to come Hydraulic type steering systems require three parts to function: • hydraulic pump (pressure source) • control valve • hydraulic piston Review of Construction The detailed construction of the pump will vary from manufacturer to manufacturer but they will all follow the same basic layout -3Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Hydraulic pump The hydraulic pump consists of three main parts Exercise From your own knowledge, can you name the three parts labelled 1, and 3? You should of course, have identified the reservoir, vane pump and control valve The reservoir maintains a store of hydraulic fluid and feeds the vane pump The vane pump pressures the fluid and passes high-pressure fluid to the flow control valve The flow control valve regulates the flow rate of the pressurised fluid to the control valve In some designs, the flow control valve is designed to reduce the flow rate of the fluid in proportion to engine speed -4Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue The design shown uses the engine as its energy source The vane pump is driven by a belt from the engine For some vehicles, this can be difficult to achieve, for example mid engine vehicles An alternative to the engine driven pump is the electro hydraulic pump studied in Phase Steering Systems LV29 Here we show this type of pump but again the basic function is the same as before That is to provide a constant flow rate of fluid to the control valve A key difference between these two designs is the methods of flow control The electro hydraulic pump does not use a flow control valve Instead, the flow rate to the control valve is regulated by the speed of the pump The speed of the pump is controlled by a steering ECU -5Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Control valve The control valve is installed on the steering gear shaft on the steering gear assembly Its function is to distribute the regulated flow of fluid from the pump to the hydraulic piston The position of the control is determined by the input of the driver Three types have been used depending on the construction of the steering gear assembly Exercise Using your knowledge from your study of Phase Steering Systems LV29, can you identify the control valve as shown and identify the components marked 1, 2, and 4 -6Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Hydraulic piston The hydraulic piston is integrated with the steering rack The piston chamber is formed by the steering rack casing Hydraulic force is applied to both sides of the piston As the control valve moves pressure is increased on one side and decreased on the other The balance of the pressure forces the piston to move and therefore the steering rack moves -7Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Principle of operation The operating principle of the hydraulic power steering is based on controlling the direction of fluid flow into and out off the hydraulic piston The control valve is the key component that determines the passage of fluid The control valve is constructed of two parts, one attached to the steering column, the other attached to the pinion gear The shape of the two halves of the control valve create fluid passages which allow fluid to flow from the pump to each side of the piston and back to the reservoir Between them is a torsion bar When there is no input from the driver, the two parts of the control valve are forced to neutral position by the torsion bar The fluid passages in the control valve are equal and as a result the pressure on each side of the piston is equal When the driver turns the steering wheel the torsion bar allows the outer section of the control to rotate relative to the inner section attached to the pinion In this condition the passages inside the control are no longer equal One side of the hydraulic piston is opened to the reservoir and the other is opened to the pump supply The pressure difference between each side of the hydraulic piston causes the rack to move In summary, to operate the power assistance the driver has to move one half of the control valve relative to the other The torsion bar determines the force required by the driver to operate the power assistance and the amount of power assistance is determined by the difference between the two parts of the control valve -8Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Speed sensitive steering As you may recall there are two types of speed sensitive steering used: • engine speed • road speed Engine speed sensitive steering is the simplest form The flow control valve in the pump is designed to reduce the flow rate of the fluid when the engine speed is above certain value The assumption is that the vehicle will be travelling at higher speed when the engine speed is higher The road speed sensitive type is more sophisticated and will use an ECU to modify the flow rate in direct proportion to the road speed In either case the advantage of reducing the level of assistance when the vehicle is travelling at higher road speeds is that the vehicle stability is improved and a high level of assistance is maintained at parking speeds -9Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Electronic Power Steering (EPS) EPS systems have become increasing popular with manufacturers They are more compact and can be easily adapted to many different vehicles by adapting the control software Review of construction Turning Torque Signal Vehicle Spd Engine Data Power Torsion Bar Motor Control Motor Motor Position Signal The EPS system uses an electric motor acting on the steering column The steering column is in two halves linked with a torsion bar The torque sensor measures the relative position of the two sections of steering column The example shown uses optical type sensors An LED and phototransistor are arranged either side of a shadow plate fixed to the steering column The outputs from the phototransistors are processed by the sensor circuit to produce a turning torque signal The ECU calculates the required assistance level and controls the motor It also provides a diagnostic system -15Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Principle of operation The EPS system provides the following control functions: • road speed sensitive power assistance • assisted steering return Both of these functions are the result of the motor voltage control by the ECU The ECU determines the voltage supply to the motor based on the following data: • force applied to the steering wheel by the driver • vehicle speed • steering angle position • speed of steering angle change Torque sensor measures the input force applied by the driver When the driver turns the steering wheel the torsion bar between the two halves of the steering column will twist The amount of twist is proportional to the force applied The output signal of the two phototransistors, will move out of phase The size of the phase change will be proportional to the twist in the torsion bar and therefore to the force applied by the driver -16Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue The torque sensor will also measure the steering angle position and the speed of angle change The steering angle position is calculated by counting the number of pulses from the phototransistor The speed of steering angle change is proportional to the frequency of the signal from the phototransistor The torque sensor circuit measures these three parameters and then outputs a digital signal to the EPS ECU The EPS ECU combines this data with the road speed to calculate the direction and speed of the motor -17Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Troubleshooting Troubleshooting EPS systems is actually much simpler than you may think Invariably the motor, gear housing and torque sensor will be integrated with the steering column as a single assembly Diagnosis of a system defect becomes a simple matter of confirming that the input signals, power source and ground are correct at the ECU Then confirm that the ECU connections between the torque sensor and the motor are correct The operation of the system will also be supported by diagnostic codes from the ECU allowing you to identify the trouble area very easily Steering Geometry Correct steering geometry is essential to maintaining safe and consistent handling characteristic Faulty steering geometry can at best lead to increased tyre wear and at worst dangerous handling Diagnosing steering geometry faults is an important skill for any vehicle technician and a basic understanding of the geometry angles and how to check them will ensure that you can successfully diagnose and repair this type of problem -18Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue ‘Ackerman’ steering 20o 22.5o When a vehicle travels around a bend the inside wheel must follow a tighter curve than the outside wheel To achieve this the geometry of the steering must be arranged to turn the inside wheel through a larger angle than the outside wheel The ‘Ackerman’ steering geometry provides a simple solution to this problem Shown is the ‘Ackerman’ geometry First, draw two lines from the front swivel joints so that they intersect the centre line of the vehicle between the rear wheels The track rod ends are then located along these lines This geometry results in the inside wheel turning through a larger angle than the outside wheel, allowing the vehicle to travel around a curve without scrubbing on of the tyres The ‘Ackerman’ principle defines the relationship between the connection points of the top and bottom swivels and the track rod end connection point Damage in this area will lead to increased tyre wear usually in the form of edge wear To diagnose this type of damage check the toe change between the inside and outside wheel as the steering angle increases This is often called the toe out on turns -19Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Steering axis inclination (SAI) The steering axis inclination is the angle formed by a line drawn through the upper and lower swivel joint or steering axis The inclination of the steering axis is necessary to allow the steering axis line and the contact point of the tyre to intersect close to road surface The advantage of this arrangement is to reduce the offset between the steering axis line and the contact point of the tyre The size of the offset, also known as the scrub radius, affects the effort required to turn the steering A larger offset increases the steering effort Reducing the offset will reduce the loading on the stub axle External forces applied to the wheel will attempt to change the direction of the wheel When the offset is large, the lever ratio is large and the tendency for the wheel direction to change will increase A reduced offset will limit the effect of bumps, braking forces and acceleration forces on the steering making it easier for the driver to control the direction of the vehicle A small toe in effect under braking can increase stability This is usually achieved by creating a small negative offset -20Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue The above diagram shows the effect of the braking forces on different offsets The upward arrow is the driving force, the downward arrow shows the braking force and the arrows at the very top of the diagram show the resulting effect on the toe angle The SAI also contributes to the directional stability of the vehicle This effect is because the inclined steering axis causes the body to lift when the steering is turned The weight acting on steering axis will always force the wheel to the straight-ahead position to the steering -21Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Camber angle The more sophisticated road speed sensitive type will use ECU control to modify the flow rate in direct proportion to the road speed In either case the advantage of reducing the level of assistance when the vehicle is travelling at higher road speeds is that the vehicle stability is improved and a high level of assistance is maintained at parking speeds The camber angle is the inward or outward lean of the wheel relative to the vertical reference Originally, the camber angle was used in a similar way to the SAI Shown are the three possible options for camber As you can see the positive camber reduces the offset when combined with the SAI The downside to positive camber is the tyre thrust generated at the contact with the road In order for the tyre to sit on the road surface some deformation of the tyre must occur A reaction thrust is generated by the tyre, tending to force the tyre to move outwards For example, when a vehicle is travelling round a left hand bend the positive camber will tend to force the tyre to the right and reduce the cornering ability of the vehicle -22Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue If you also consider that the body roll experienced during cornering will increase the positive camber on the outside wheel this effect is not very desirable To reduce this effect high performance vehicles will use a negative camber arrangement Again this will generate a camber thrust but this time in the same direction as the corner Both negative and positive camber will increase the tyre wear due to the deformation that occurs with this set-up As a result most modern vehicles will tend to be set close to zero camber This reduces wear and rolling resistance generated by the tyre deformation Castor angle The castor angle is the rearward lean of the steering axis relative to the vertical reference The main purpose of the castor angle is to create a selfcentring effect in the steering Tilting the steering axis in this way means that the driving force acts at the point where the castor angle intersects the road The resistance between the tyre and the road creates an opposite force that acts along the axis of the tyre The effect is to generate side force pushing the tyre back in line with the driving force The further away from the straight ahead position the greater the side force -23Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue The difference between the vertical reference and the point at which the castor angle intersects the road is called the castor trail The larger the castor trail the greater the self centring effect Increasing the castor angle will increase the weight of the steering Generally, larger castor angles are used on higher performance vehicles to maximize stability at speed The trade off for increasing the castor angle is increased steering effort and tyre wear Toe angle The toe describes the angle of each wheel relative to the centre line of the vehicle when viewed from above The ideal toe angle should ensure that the front and rear wheels are parallel as the vehicle is driving along the road To achieve this the static toe angle will have to be set to accommodate the movement in suspension linkages and steering joints -24Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue The toe angle can be described as an angle in terms of the difference measured between the front and the rear of the wheels on an axle Measuring the toe would appear to be quite simple and if you measure the toe angle it is However, if the toe valve is specified in millimetres the method of measuring must also be specified There are three methods used for measuring the toe in millimetres If you use a different method of measurement to adjust the toe to that used by the manufacturer then the toe on the vehicle will be incorrect leading to increased tyre wear The three methods are European, American and Japanese Shown are the three different methods You can see from the diagram that the measured toe in millimetres is very different for each method but the angle is always the same Correct toe adjustment is necessary to ensure that the vehicle drives in a straight line Incorrect toe can lead to pulling problems and increased tyre wear Summary Most problems related to alignment result in either tyre wear or pulling Assessing the cause of an alignment problem requires an understanding of the effect of each angle on the way the vehicle performs It is important to remember that the balance between the left and right of the vehicle is more important than simply achieving the manufacturers’ specification For example, both the left and right camber could be within the tolerance set by the manufacturer but if the difference between them is too large then a pulling problem may occur We often think of pulling problems being caused by faults in the front alignment In fact, the direction of travel is determined more by the rear axle geometry In particular, the alignment of the rear toe relative to the centre line of the body is the most common cause of pulling complaints The relationship between the rear toe and the centre of the vehicle body is called the thrust axis -25Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Progress check Test your knowledge and understanding by completing the following summary table State the function of each angle and the effect it would have on the two primary alignment problems Angle Function Pulling Tyre Wear Camber Reduces steering load or improves cornering performance Small effect unless the left right difference is large Large effect if the angle is increased a small amount Castor Provides self centring force and directional stability Small effect unless the left right difference is large SAI Small effect unless Provides directional the left right stability difference is large Front Toe Rear Toe Determines the direction of travel for the front of the vehicle Determines the direction of travel for the rear of the vehicle Increased angle will increase tyre scrub when cornering Increased angle will increase tyre scrub when cornering Large effect Increased tyre scrub Large effect if alignment will body incorrect Increased tyre scrub -26Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Rear Wheel Steering If the driver wishes to change the direction of travel, the steering wheel is turned generating a side force at the front tyre This generates a force on the vehicle body and changes the direction As the rear wheels are attached to the body, they will also change direction and the vehicle has changed direction This process occurs every time that the driver moves the steering wheel The delay between the steering input by the driver and the rear of the vehicle changing direction, determines the speed a vehicle can turn into a corner Rear steering systems have been developed by a number of manufacturers to improve the speed that a vehicle takes to change direction This can be achieved by applying lateral force to both the front and rear tyres at the same time which will reduce the time it takes a vehicle to change direction Active Rear Wheel Steering - 4WS The example used in this section is an electronic control type The basic components required to operate the system are shown Notice that the system uses two sensors for each of the critical inputs This ensures that the system can safely detect a failure in one of the sensors and revert to a failsafe condition The rear actuator will be designed to achieve a neutral position in the event of a failure The system calculates the steering angle at the rear based on: • road speed • front steering angle • speed of front steering angle change -27- Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue The system can turn the wheels in the same direction as the front wheels to improve high speed cornering or in the opposite direction at low speed providing increased manoeuvrability Shown here is how the system controls the rear wheel steering -28Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Progress check Answer the following questions: What two advantages does the 4WS system offer over a conventional steering system? Why you think the system uses auxiliary steering angle sensors on the front and the back? If a vehicle is travelling at 60 km/h and the steering is turned through 100 degrees which way will the rear wheels turn? If a vehicle is travelling at 100 Km/h is it possible for the rear wheels to be turned in the opposite direction to the front wheels? -29Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue ... with power steering systems • explain the steering geometry associated with steering systems • describe the operating principles of a four wheel steering system (4WS) Hydraulic Type Power Steering. .. steering ECU -5Copyright © Automotive Skills Limited 2003 All Rights Reserved LV39: Steering Systems (3) Issue Control valve The control valve is installed on the steering gear shaft on the steering. .. Reserved LV39: Steering Systems (3) Issue Speed sensitive steering As you may recall there are two types of speed sensitive steering used: • engine speed • road speed Engine speed sensitive steering