STEERING SYSTEM In automobiles, steering wheel, gears, linkages, and other components are used to control the direction of a vehicle’s motion. Failure to any of these components will lead to fatal accidents. Steering system helps the student to understand the different types of steering systems ,its working, the operating principle of hydraulic power steering systems, MDPS (Motor Driven Power Steering ) and to service and diagnose these systems.
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ACKERMANN STEERING GEOMETRY Ackermann steering geometry is a geometric arrangement of linkages in the steering of a car or other vehicle designed to solve the problem of wheels on the inside and outside of a turn
Ackermann geometry needing to trace out circles of different radius. It was invented by the German Carriage Builder Georg Lankensperger in Munich in 1817, then patented by his agent in England, Rudolph Ackermann (1764–1834) in 1818 for horse drawn carriages. The intention of Ackermann geometry is to avoid the need for tyres to slip sideways when following the path around a curve. The geometrical solution to this is for all wheels to have their axles arranged as radii of a circle with a common centre point. As the rear wheels are fixed, this centre point must be on a line extended from the rear axle. Intersecting the axes of the front wheels on this line as well requires that the inside front wheel is turned, when steering, through a greater angle than the outside wheel. Rather than the preceding “turntable” steering, where both front wheels turned around a common pivot, each wheel gained its own pivot, close to its own hub. While more complex, this arrangement enhances controllability by avoiding large inputs from road surface variations being applied to the end of a long lever arm, as well as greatly reducing the fore-and-aft travel of the steered wheels. A linkage between these hubs moved the two wheels together, and by careful arrangement of the linkage dimensions the Ackermann geometry could be approximated. This was achieved by making the linkage not a simple parallelogram, but by making the length of the track rod (the moving link between the hubs) shorter than that of the axle, so that the steering arms of the hubs appeared to “toe out”. As the steering moved, the wheels turned according to Ackermann, with the inner wheel turning further. If the track rod is placed ahead of the axle, it should instead be longer in comparison, thus preserving this same “toe out”. Simple approximation for designing Ackermann geometry
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A simple approximation to perfect Ackermann steering geometry may be generated by moving the steering pivot points inward so as to lie on a line drawn between the steering kingpins and the centre of the rear axle. The steering pivot points are joined by a rigid bar called the tie rod which can also be part of the steering mechanism, in the form of a rack and pinion for instance. With perfect Ackermann, at any angle of steering, the centre point of all of the circles traced by all wheels will lie at a common point. Note that this may be difficult to arrange in practice with simple linkages, and designers are advised to draw or analyze their steering systems over the full range of steering angles. Modern cars do not use pure Ackermann steering, partly because it ignores important dynamic and compliant effects, but the principle is sound for low speed manoeuvres. Some race cars use reverse Ackermann geometry to compensate for the large difference in slip angle between the inner and outer front tyres while cornering at high speed. The use of such geometry helps reduce tyre temperatures during high-speed cornering but compromises performance in low speed maneuvers.
INTRODUCTION In automobiles, steering wheel, gears, linkages, and other components are used to control the direction of a vehicle’s motion. Because of friction between the front tires and the road, especially in parking, effort is required to turn the steering wheel. To lessen the effort required, the wheel is connected through a system of gears to components that position the front tires. The gears give the driver a mechanical advantage, i.e., they multiply the force he applies.
COMPONENTS OF STEERING SYSTEM. According to the type of steering gear box using the components of the steering system are varies. In modern days mainly two types of steering systems are mainly using.They are rack and pinion
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steering system and recirculating ball type steering system.There is also worm and nut worm and sector steering gear boxes are also using.
RACK AND PINION STEERING GEAR BOX UPPER MOUNTING PLATE & BEARING STEERING KNUCKLE
COIL SPRING MACPHERSON STRUT BELLOWS
ANTI-SWAY BAR
RACK & PINION UNIT
TIRE OUTER TIE-ROD END BALL JOINT
CONTROL ARM
CONTROL ARM BUSHINGS
RACK & PINION BUSHINGS
INMER SOCKET ASSEMBLY (INSIDE BELLOWS)
BACK & PINION STEERING
A rack-and-pinion gear set is enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack. The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle. The rackand-pinion gear set converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels and provides a gear reduction, making it easier to turn the wheels. The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle The rack-and-pinion gear set does two things: • It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels. • It provides a gear reduction, making it easier to turn the wheels. On most cars, it takes three to four complete revolutions of the steering wheel to make the wheels turn from lock to lock (from far left to far right). The steering ratio is the ratio of how far you turn the steering wheel to how far the wheels turn. For instance, if one complete revolution (360 degrees) of the steering wheel results in the wheels of the car turning 20 degrees, then the steering ratio is 360 divided by 20, or 18:1.
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A higher ratio means that you have to turn the steering wheel more to get the wheels to turn a given distance. However, less effort is required because of the higher gear ratio. Generally, lighter, sportier cars have lower steering ratios than larger cars and trucks. The lower ratio gives the steering a quicker response -- you don’t have to turn the steering wheel as much to get the wheels to turn a given distance -- which is a desirable trait in sports cars. These smaller cars are light enough that even with the lower ratio, the effort required to turn the steering wheel is not excessive. Some cars have variable-ratio steering, which uses a rack-and-pinion gear set that has a different tooth pitch (number of teeth per inch) in the center than it has on the outside. This makes the car respond quickly when starting a turn (the rack is near the center), and also reduces effort near the wheel’s turning limits.
RECIRCULATING BALL TYPE STEERING GEAR BOX
Construction Recirculating ball, also known as recirculating ball and nut is a steering mechanism commonly found in older automobiles, and some trucks. The recirculating ball steering mechanism contains a worm gear inside a block with a threaded hole in it; this block has gear teeth cut into the outside to engage the sector shaft (also called a sector gear) which moves the Pitman arm. The steering wheel connects to a shaft, which rotates the worm gear inside of the block. Instead of twisting further into the block, the worm gear is fixed so that when it spins, it moves the block, which transmits the motion through the gear to the pitman arm, causing the road wheels to turn. The worm gear is similar in design to a ball screw; the threads are filled with ball bearings that recirculate through the gear and rack as it turns. The balls serve to reduce friction and wear in the gear, and reduce slop. Slop, when the gears come out of contact with each other, would be felt when changing the direction of the steering wheel, causing the wheel to feel loose
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Working This is how mechanical steering, “worm and sector recirculating ball steering”, works. Any rotation of the steering wheel is transferred via the steering column to a steering worm. The steering worm is - in simple terms - rather like a screw with a thread. The counterpart to the screw is formed by the nut - here called the steering gear nut. As a link between the steering worm and the steering gear nut, steel balls are embedded in the thread, and these help to reduce friction during the turning motion. The steering worm and steering gear nut have, as it were, a thread with ball bearings. The steering gear nut is so fixed that is can not move “right round” during the turning process When the steering wheel is turned, the steering gear nut moves along the steering worm. The steering gear nut has a row of teeth on one side meshed directly with the sector on the Pitman shaft . All movements of the shaft are transferred to the wheels via the drop arm and the steering linkage. The advantages of worm and sector recirculating ball steering are based on two points. First of all, there is very little friction between the steering worm and steering gear nut due to the system of “recirculating balls”, thus making for light, positive steering. Furthermore, such steering has a constant reduction ratio within the steering box.
Adjustment Correctly adjusted worm bearings have no clearance and no pre-load. The Pitman shaft adjusting screw must be adjusted when the steering wheel is in the straight ahead position; tighten the adjusting screw until the spring is fully compressed and then back off approximately 3-4mm, measured at the circumference of the adjusting screw. The main components of steering system are tie rod, tie rod end, pitman arm, steering column, and steering wheel.
Recirculating Ball Gearbox
Tie Rod
Track Rod
Pitman Arm
Steering Arms
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Tie Rod
Pitman Arm STEERING SHAFT
CAM
HOUSING
CROSS SHAFT
STUDS
LEVER
BALL CUP BALLS PITMAN ARM
The Pitman arm is a linkage attached to the steering gear sector shaft, that converts the angular motion of the sector shaft into the linear motion needed to steer the wheels. The Pitman arm is supported by the sector shaft and supports the drag link or center link with a ball joint. It transmits the motion it receives from the steering box into the drag (or center) link, causing it to move left or right to turn the wheels in the appropriate direction. The idler arm is attached between the opposite side of the center link from the Pitman arm and the vehicle’s frame to hold the center or drag link at the proper height. A worn ball joint can cause play in the steering, and may get worse over time.
Tie Rod Tie-Rod Assemblies Two tie-rod assemblies are used to fasten the center link to the steering knuckles. Ball sockets are used on both ends of the tie-rod assembly. An adjustment sleeve connects the inner and outer tie rods. These sleeves are tubular in design and threaded over the inner and outer tie rods. The adjusting sleeves provide a location for toe adjustment. Clamps and clamp bolts are used to secure the sleeve. Some manufacturers require the clamps be placed in a certain position in relation to the tie rod top or front surface to prevent interference with other components.
Ball sockets Ball sockets are like small ball joints; they provide for motion in all directions between two connected components. Ball sockets are needed so the steering linkage is NOT damaged or bent when the wheels turn or move up and down over rough roads. Ball sockets are filled with grease to reduce friction and wear. Some have a grease fitting that allows chassis grease to be inserted with a grease gun. Others are sealed by the manufacturer and cannot be serviced
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Steering Column Steering column assembly
Intermediate shaft
Power-assisted steering system
Universal joint
Steering column
The steering column assembly is bolted to the bulkhead. The steering column shaft is mounted in two needle bearings suspended in rubber mountings in the steering column assembly. A jointed intermediate shaft connects the steering column to the steering gear. For reasons of safety the steering column assembly incorporates a collapsible steel cage, a telescopic steering column shaft and an intermediate shaft with a deformation zone designed to crumple progressively in the event of a head-on collision. In addition, the joint configuration is such that the shaft will be directed away from the driver in a collision. Adjustment of the position of the steering wheel spokes is achieved by adjusting the toe-in on both sides of the car.
Steering Wheel The steering wheel is the part of the steering system that is manipulated by the driver; the rest of the steering system responds to such driver inputs. This can be through direct mechanical contact as in recirculating ball or rack and pinion steering gears, without or with the assistance of hydraulic power steering.
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HYDRAULIC POWER STEERING. Introduction Power steering helps drivers steer vehicles by augmenting steering effort of the steering wheel. It does this by adding controlled energy to the steering mechanism, so the driver needs to provide only modest effort regardless of conditions. In particular, power steering helps considerably when a vehicle is stopped or moving slowly. The hydraulic power steering system uses a hydraulic pressure which is generated by the power steering pump to reduce the effort required to turn the steering wheel. The power steering pump is mounted on the front of the engine. The pump is driven by the crankshaft through a drive belt. Power steering uses hydraulic pressure for reduction of steering effort, enabling the driver to easily operate the steering wheel. Steering effort is generally 20N to 39N. In addition to that, power steering systems offer higher stability during driving and prevention of shock from road surface irregularities that may otherwise be transmitted to the steering wheel.
Components of rack-and-pinion Power Steering • The rack-and-pinion power steering system consists of: • Rack and pinion steering gear box • Pressure and Flow Control Valve • Power steering oil pump • Oil reservoir
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• Tubes
Cylinder tube right chamber
Hydraulic control unit Cylinder tube left chamber
Rack and pinion steering gear box The cylinder is part of the steering gear housing. The rack is equipped with a piston complete with seals. For the flow of power steering fluid to and from the control valve there are two connections on the servo cylinder, one on each side of the piston. When turning right, power steering fluid is pumped to the right hand section of the servo cylinder. Piston and rack are forced to the left and power steering fluid is discharged from the left hand section of the servo cylinder. The rubber gaiter on the left hand side is distended at the same time as the one on the right side is compressed. Movement of the rack is transferred via the inner ball joints (3.), track rod and outer track rod ends to the steering arms of the steering swivel member. Both the inner ball joints and the outer track rod ends are lubricated for life and self-adjusting, with no further lubrication or adjustment being necessary or possible.
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Oil Pump Vane Cam ring
Cover Rotor
Pully Oil seal
O-ring
O-ring
Flow control valve
The hydraulic power for the steering is provided by a rotary-vane pump. This pump is driven by the car’s engine via a belt and pulley. The pump element consists of a rotor with a number of slits, a vane for each slit, a pump ring and two end plates with inlet and outlet ports for power steering fluid. Due to the oval shape of the pump ring, the volume between the vanes increases and decreases twice during each revolution of the rotor. Inlet ports lead to the areas in which the volume increases and outlet ports lead from those in which the volume decreases, thereby producing a pumping effect. Apart from being forced outwards by centrifugal force, the vanes are also pressed outwards against the pump ring by the pressure of the fluid. The fluid is directed into the slots inside the vanes.
Pressure and Flow Control Valve
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The purpose of the control valve is to regulate the flow from the pump so that it remains constant, regardless of engine/pump rpm. The control valve sits on one side, directly connected to the pump flow . At the outlet passage of the pump, a restrictor is situated from which a connecting passage leads to the other side of the valve, which contains a spring .When not actuated, the valve presses against the outlet side. When pressure is high, an overflow valve housed in the control valve is actuated by the power steering fluid pressure on the spring loaded control valve. For the control valve to operate, a certain amount of power steering fluid must circulate through it continuously ,although not when the steering wheel is at full lock. Steering and parking at low engine speed
The pressure produced by the pump is slightly reduced over the restrictor at the pump outlet. The reduced pressure is led to the spring loaded side of the control valve, at which time there is a minor pressure difference between the two sides of the valve. Due to the low pump speed, however, this pressure difference is not enough to actuate the valve. Steering at high engine speed (pump in flow control mode)
The flow of power steering fluid inside the pump increases with increasing engine rpm and owing to the restrictor in the pump outlet the flow velocity also increases. This reduces the pressure in the connecting passage, with the result that the pressure on the spring loaded side of the control valve will be lower than that acting on the outlet side of the valve. The valve therefore overcomes the force of the spring, opening a port to the suction side of the pump
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and allowing a certain amount of internal recirculation of the fluid to take place so that the flow from the pump is maintained at constant rate, regardless of engine /pump rpm. Steering wheel turned to full lock
Pump speed in this case is often low. When the steering wheel is turned to full lock, the control valve of the steering gear closes. The flow of fluid from the pump will then be zero. The resulting high pressure is directed via the connection passage to the spring loaded side of the control valve. The pressure opens the overflow valve and allows the fluid to pass to the inlet side of the pump. The pressure difference across the control valve forces it to move against the spring and thus open the port for recirculation of the full delivery flow from the pump. The predetermined maximum pressure is maintained as long as the control valve remains closed.
Hydraulic Control Valve The hydraulic control valve consists of a valve spool (1.), a sleeve (2.) , a torsion bar (3.) and a pinion (4.). The steering column’s intermediate shaft is connected to the valve by means of a universal joint. The torsion bar is connected to the upper end of the valve by means of a pin (5.). The other end of the torsion bar is press fit in the pinion. The sleeve is connected to the pinion by a pin (6.) and follows the rotation of the pinion exactly. There is also a fail safe connection between the valve spool and the pinion required to maintain steer ability in case the torsion bar is broken. The sleeve has three radial grooves (7.), the power steering fluid being pumped to the middle one. When the steering wheel is in straight ahead position, the control valve is open and the fluid flows up through the valve and back to the power steering reservoir via the chamber above the sleeve. The upper end of the pinion is mounted in a needle bearing while the lower end is mounted in a ball bearing. A spring loaded plunger presses the rack against the pinion.
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When the steering wheel is turned, the movement is transferred via the torsion bar to the pinion. Since the torsion bar is somewhat elastic, there will be a difference between the degree of rotation of the valve spool (which follows the rotation of the intermediate shaft) and the sleeve which is fixed to the pinion. As a result, the fluid can no longer flow through the control valve and back to the power steering fluid reservoir directly. Instead, delivery and return passages open for the servo cylinder.
Operating Principle
Input shaft Torisioning bar
Rotary valve
In straight ahead position.
Pressurized oil is delivered through port „a“ and to the right (port b) and left (port „c) servo cylinder. The pressure in both, the left and right servo cylinders are equal, and the oil is drained through the open port „d“ back to the reservoir.
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When turning left
Power steering fluid is pumped to the left hand side of the servo cylinder via the upper radial groove (port „b“) of the sleeve. At the same time, the left hand side of the servo cylinder is emptied via the lower radial groove (port “c“) of the sleeve. Power steering fluid is led up through the valve to the chamber above the spool and on back to the power steering fluid reservoir.
When turning right. The process is reversed. As long as the torsion bar is twisted, power steering fluid presses on the rack so that servo effect is obtained. The difference between the valve spool and the sleeve reduces when the power steering fluid actuates the rack in the same direction as the pinion. When there is no longer a difference, the valve opens the passage returning power steering fluid to the reservoir. Some power steering fluid continually circulates in the valve except when the steering wheel is turned to an end position. This makes it possible for the control valve in the power steering pump to work while circulation cools the power steering fluid.
MDPS Motor Driven Power Steering(MDPS), which is generally called Electrical Power Steering(EPS), is developed to assist a steering force by using an electric motor without the help of engine power. It controls motor torque according to the steering conditions resulting in optimal steering characteristics and less fuel consumption. Besides, it is an environment friendly technology not to use steering oil and is able to reduce system weight as well as better
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service access owing to the removal of oil lines. Recently, EPS equipped cars are increasing and EPS is expected to replace hydraulic power steering system. Electrical Power Steering is divided into three types according to the motor location, Column type, Pinion type, Rack type. Advantages • Better fuel consumption: 2~3 % • Environment friendly: Power steering oil & oil leak free • Enhanced steering performance: Exact manipulation - Basically the steering force is controlled depends on the vehicle speed. - In addition to basic factor (vehicle speed), several logic such as damping control, friction • control are adopted for optimizing steering ability. - Required steering force is decreased in case of low vehicle speed for easy driving feeling. - Required steering force is increased in case of high vehicle speed for safety. • Weight reduction: by 2.4 Kg • Driving performance: Engine power is not used for steering so vehicles acceleration • performance will be improved. • System status can be checked due to the communication with Hi-scan and warni lamp. • NVH : Hydraulic noise may be eliminated and electrical motor has a function to absorb the • vibration from the steering column and it results the reduced vibration on steeri wheel.
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