Steering system is an integral part of every automobile. From the ordinary motor cycles to space vehicles, the easy manoeuvring & stability of the vehicle has always been the main subject of concern of the design engineers and research labs all over the world. In this scenario a new technology has emerged known as power steering- which must be a familiar word to every one who has at least once heard of something called a car which has enhanced features, enabling the driver to control the automobile with ease. The power steering helps the driver in manoeuvring the vehicle during sharp turns by multiplying the effort applied by the driver. This is done by increasing the sensitivity of the steering wheel to minute forces applied by the user. The two type of power steering systems that’s most commonly used are
1. Hydraulic power steering
2. Electric power steering
The report looks into the basic power steering mechanism used in the above two systems and their implications, advantages & limitations. The report starts with the steering basics, then goes into power steering technologies used in implementing the different power steering mechanisms and ends with a comparison of the two mechanisms so that an in depth analysis of each system can be very well understood.
The steering system is designed to allow the driver to move the front wheels to the right or left, with a minimum of effort, and without excessive movement of the steering wheel.
Basically there are two general types of steering systems
1. Manual steering.
2. Power steering.
In the manual steering system, the driver’s effort to turn the steering wheel is the primary force that causes the front wheels to swivel to left or right on the steering knuckles. In power steering, the turning efforts are multiplied by a hydraulic assist.
Linkage is necessary to connect the steering gear box pitman arm to the steering arms. The most commonly used arrangement today is parallelogram type of linkage.
Starting at the gear box, there is an arm called a pitman arm. This arm can be made to swing from side to side (cross steering) or from back (fore-and-aft steering). In the case of parallelogram linkage train, it swings from side to side. The pitman arm is moved by the gear box cross shaft.
The steering gear shaft turns in direct relation to input from the driver. The pitman arm attaches to the steering gear shaft and acts as a lever, converting torque from the steering gear to mechanical force for movement of the steering linkage.
There are mainly three types of manual steering systems commonly used.
1. Rack & Pinion steering.
2. Worm & sector type steering.
3. Re-circulating Ball steering.
RACK & PINION STEERING
A typical rack and pinion steering gear assembly consists of a pinion shaft and bearing assembly, rack gear, gear housing, two tie rod assemblies, an adjuster assembly, dust boots and boot clamps, and grommet mountings and bolts. When the steering wheel is turned, this manual movement is relayed to the steering shaft and shaft joint, and then to the pinion shaft. Since the pinion teeth mesh with the teeth on the rack gear, the rotary motion is changed to transverse movement of the rack gear. The tie rods and tie rod ends then transmit this movement to the steering knuckles and wheels.
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.
WORM& SECTOR TYPE STEERING
The manual worm and sector steering gear assembly uses a steering shaft with a three-turn worm gear supported and straddled by ball bearing assemblies. The worm meshes with a 14-tooth sector attached to the top end of the pitman arm shaft. In operation, a turn of the steering wheel causes the worm gear to rotate the sector and the pitman arm shaft. This movement is transmitted to the pitman arm and throughout the steering train to the wheel spindles.
RE-CIRCULATING BALL STEERING
Re-circulating ball steering works on the principle of turning forces transmitted through ball bearings from a worm gear on the steering shaft to a sector gear on the pitman arm shaft. In operation, a ball nut assembly is filled with ball bearings which roll along groves between the worm teeth and grooves inside the ball nut.
When the steering wheel is turned, the worm gear on the end of the steering shaft rotates, and movement of the re-circulating ball causes the ball nut to move up and down along the worm. Movement of the ball nut is carried to the sector gear by teeth on the side of ball nut. The sector gear, in turn, moves with the ball nut to rotate the pitman arm shaft and activate the steering linkage. The balls re-circulate from one end of the ball nut to the other through a pair of ball return guides.
Power steering is designed to reduce the steering wheel turning effort by utilizing hydraulic pressure to bolster or strengthen the normal torque developed by the steering gear box. It should ease steering wheel manipulation.
Power steering is generally divided into two
1. Hydraulic power steering
2. Electric power steering
HYDRAULIC POWER STEERING
Hydraulic power steering systems are of two general types. One type controls and utilizes the hydraulic pressure directly with in the steering gear box housing. The other type uses a hydraulic cylinder and control valve attached to the linkage system. This second type uses a conventional standard gear box. Both systems use a hydraulic pump generally belt driven by the engine.
Two general types are
1. Self contained steering
2. Linkage type steering
In both, a control valve is actuated by driver steering effort. This valve admits oil, under heavy pressure to one side or other of a hydraulic piston. The pressure the oil creates against the piston is transferred to either the pitman shaft or to a direct connection to the steering linkage which assists the driver in manipulating the front wheels.
SELF – CONTAINED POWER STEERING
The self – contained unit places the control valve mechanism, power piston, and gears in an integral unit. Pressure developed by the unit is applied to the pitman shaft.
There are several different models of the self – contained type, but all share many basic design principles. They may be divided into two general categories.
1. In-line power steering 2. Offset power steering
The above figure represents self-contained offset type. The additional force offered by the pressurized oil is applied to the pitman shaft by a power piston rod working in separate power cylinder. The power piston has a rack of gear teeth cut in one side that mesh with a separate set of pitman sector teeth. The pitman shaft sector teeth’s are built on both sides. The ball nut presses on one sector and the power piston rack engages the other set of sector teeth.
LINKAGE TYPE POWER STEEIRNG
The linkage type power steering system employs a power cylinder and control valve to provide the power assist. This system uses a conventional manual gear box. A pitman arm actuates the power cylinder control valve.
One end of the power cylinder is attached to the frame; the other end is connected to the steering linkage relay rod. The relay rod is attached to the control valve, which is connected to the power cylinder by high pressure hoses. The end of the pitman arm is formed into a ball, which is placed in the control valve assembly in a ball socket arrangement. Pressure of the pitman shaft ball, either to right or left, actuates the control valve. The control valve then transmits oil pressure to one side or the other of the power cylinder piston.
POWER RACK & PINION STEERING
Power rack and pinion steering assemblies are hydraulic/ mechanical unit with an integral piston and rack assembly. An internal rotary valve directs power steering fluid flow and controls pressure to reduce steering effort. The rack and pinion is used to steer the car in the event of power steering failure, or if the engine (which drives the pump) stalls. When the steering wheel is turned, resistance is created by the weight of the car and tire-to-road friction, causing a torsion bar in the rotary valve to deflect. This changes the position of the valve spool and sleeve, thereby directing fluid under pressure to the proper end of the power cylinder. The difference in pressure on either side of the piston (which is attached to the rack) helps move the rack to reduce turning effort. The fluid in the other end of the power cylinder is forced to the control valve and back to the pump reservoir. When the steering effort stops, the control valve is centered by the twisting force of the torsion bar, pressure is equalized on both sides of the piston, and the front wheels return to a straight ahead position
POWER STEERING PUMP
The hydraulic power for the steering is provided by a pump. The figure given below represents a rotary-vane pump. The pump is driven by the cars engine via a belt and pulley. It contains a set of retractable vanes that spins inside an oval chamber.
As the vanes spins, they pull hydraulic fluid from the return line at low pressure and force it into the outlet at high pressure. The amount of flow provided by the pump depends on the car's engine speed. The pump must be designed to provide adequate flow when the engine is idling. As a result, the pump moves much more fluid than necessary when the engine is running at faster speeds. The pump contains a pressure-relief valve to make sure that the pressure does not get too high, especially at high engine speeds when so much fluid is being pumped.
A power-steering system should assist the driver only when he is exerting force on the steering wheel (such as when starting a turn). When the driver is not exerting force (such as when driving in a straight line), the system shouldn't provide any assist. The device that senses the force on the steering wheel is called the rotary valve. The key to
the rotary valve is a torsion bar. The torsion bar is a thin rod of metal that twists when torque is applied to it. The top of the bar is connected to the steering wheel, and the bottom of the bar is connected to the pinion or worm gear (which turns the wheels), so the amount of torque in the torsion bar is equal to the amount of torque the driver is using to turn the wheels. The more torque the driver uses to turn the wheels, the more the bar twists.
The input from the steering shaft forms the inner part of a spool-valve assembly. It also connects to the top end of the torsion bar. The bottom of the torsion bar connects to the outer part of the spool valve. The torsion bar also turns the output of the steering gear, connecting to either the pinion gear or the worm gear depending on which type of steering the car has. As the bar twists, it rotates the inside of the spool valve relative to the outside. Since the inner part of the spool valve is also connected to the steering shaft (and therefore to the steering wheel), the amount of rotation between the inner and outer parts of the spool valve depends on how much torque the driver applies to the steering wheel. When the steering wheel is not being turned, both hydraulic lines provide the same amount of pressure to the steering gear. But if the spool valve is turned one way or the other, ports open up to provide high-pressure fluid to the appropriate line.
ELECTRIC POWER STEERING
Electric Power Steering was first introduced in the mid 1970s to prevent the sudden loss of control caused by the loss of the hydraulic assistance if the engine were to stall with the car still moving. The Electric Power Steering works independently from the engine, taking its power from the car's battery. Electric Power Steering functions in a very similar way, in that a power source, this time electric only, kicks in when driver turn the wheel. This time however the power source is an electric motor. The system is powered by electrical current being drawn from the vehicle's electrics. They have a built-in safety device so that if the electrical power supply were to fail, then it will default to a conventional un-assisted steering system. Additionally, if the engine was to stop whilst the car is moving then the electrically assisted steering system will still operates, unlike a conventional hydraulic assisted system.
The figure below shows the working principle of electric power steering.
There are two types of EPS systems being mass produced:
(1) A column type where the motor and reduction gear are mounted on the steering column, just below the steering wheel.
(2) A pinion type where the motor and reduction gear are mounted on the pinion
of the rack and pinion assembly.
PINION TYPE ELECRIC POWER STEERING
Figure given shows the construction of pinion type EPS. The main parts are defined below.
Torque sensor: detects the driver’s input torque of the steering wheel, as well as the movement of the vehicle.
Electronic control unit (ECU): performs turning force calculations based on signals from the torque sensor.
Motor: generates turning force according to output from the ECU.
Reduction gear: reduces the rotary speed of the motor and amplifies the turning force.
Vehicle and engine speed information are relayed to the ECU, which is then used for vehicle speed-reactive type EPS systems.
ADVANTAGE OF EPS OVER HYDRAULIC POWER STEERING
(1) Saving energy
EPS system consumes power only when the driver turns the steering wheel. 5% of the energy that a conventional hydraulic power steering system uses is all that is required for the EPS system. As a result, fuel consumption is 3 to 5% less for EPS-equipped vehicles. Therefore, EPS is regarded as an energy saving power steering system.
(2) Environmentally friendly
Since hydraulic fluid is eliminated in the EPS system, there is no hydraulic fluid related environmental pollution at both the production and disposal stages of the vehicle and steering system.
FUTURE OF POWER STEERING
A “by-wire” denotes a control system that replaces traditional mechanical or hydraulic linkages with electronic connections between control units that drive electromechanical actuators. Originally used in the aerospace industry, by-wire technology is making its way into the ground transportation sector. Automotive by-wire includes three categories: throttle by-wire, steer by-wire, and brake by-wire.
These systems would completely eliminate the mechanical connection between the steering wheel and the steering, replacing it with a purely electronic control system. It would contain sensors that tell the car what the driver is doing with the wheel, and have some motors in it to provide the driver with feedback on what the car is doing. The output of these sensors would be used to control a motorized steering system. This would free up space in the engine compartment by eliminating the steering shaft. It would also reduce vibration inside the car.
A steer by-wire system replaces the steering column with control units linked by a fault-tolerant network. The driver's steering controller is connected through the network to motors that are connected to the steering rack or individual corners. Steer by-wire systems enhance safety, increase fuel economy, provide varying levels of “road feel”, and allow car designers more flexibility.
CONCEPTUAL DESIGN OF STEER BY WIRE
Different types of steering systems have been discussed above and it can be conclude that EPS(Electric power steering) is the best among them.
In the past fifty years, car steering systems haven't changed much. But in the next decade, we'll see advances in car steering that will result in more efficient cars and a more comfortable ride. Most of the future systems will be using “ By – wire “ technologies. Research is currently going on in this “ Steer wire “ mechanism in various parts of world.
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