THE STEERING SYSTEM
The steering system enables the driver to guide the automobile or wheeled tractor down the road and turn right or left,as desired,by turning wheels，There are two types of steering systems.These are manual and power.
1.Power Steering
There are a couple of key components in power steering in addition to the rack-and-pinion or recirculating-ball mechanism.
2.Pump
The hydraulic power for the steering is provided by a rotary-vane pump(see diagram below).This pump is driven by the car's engine via a belt and pulley.It contains a set of retractable vanes that spin inside an oval chamber.
As the vanes spin,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.
3.Rotary Valve
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 torqu
e the driver applies to the steering wheel.
Animation showing what happens inside the rotary valve when you first start to turn 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.
It turns out that this type of power-steering system is pretty inefficient.Let's take
a look at some advances we'll see in coming years that will help improve efficiency.
4.The Future of Power Steering
Since the power-steering pump on most cars today runs constantly,pumping fluid all the time,it wastes horsepower.This wasted power translates into wasted fuel.
You can expect to see several innovations that will improve fuel economy.One of the coolest ideas o
n the drawing board is the"steer-by-wire"or"drive-by-wire" system.These systems would completely eliminate the mechanical connection between the steering wheel and the steering,replacing it with a purely electronic control system.Essentially,the steering wheel would work like the one you can buy for your home computer to play games.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.
General Motors has introduced a concept car,the Hy-wire,that features this type of driving system.One of the most exciting things about the drive-by-wire system in the GM Hy-wire is that you can fine-tune vehicle handling without changing anything in the car's mechanical components--all it takes to adjust the steering is some new computer software.In future drive-by-wire vehicles,you will most likely be able to configure the controls exactly to your liking by pressing a few buttons,just like you might adjust the seat position in a car today.It would also be possible in this sort of system to store distinct control preferences for each driver in the family.
In the past fifty years,car steering systems haven't changed much.But in the next decade,we'll see a
dvances in car steering that will result in more efficient cars and a more comfortable ride.
5.The Honda Insight uses a variable-assist rack and pinion electric power steering(EPS)system rather than a typical hydraulic power steering system.
A typical hydraulic power steering system is continually placing a small load on the engine,even when no steering assist is required.Because the EPS system only needs to draw electric power when steering assist is required,no extra energy is needed when cruising,improving fuel efficiency.
Electric power steering(EPS)is mechanically simpler than a hydraulic system, meaning that it should be more reliable.The EPS system is also designed to provide good road feel and responsiveness.The Insight's EPS system shares parts with the Honda S2000steering system.
The system's compactness and simplicity offer more design freedom in terms of placement within the chassis.The steering rack,electric drive and forged-aluminum tie rods are all mounted high on the bulkhead,and steer the wheels via steering links on each front suspension strut.This location was chosen in order to achieve a more compact engine compartment,while improving safety.
The system is also smoother operating,more responsive to driver input,and has minimal steering kickback.The overall steering ratio is16.4to1,and3.32turns lock-to-lock.
EPS Operation
The operating principle of the EPS is basically the same as hydraulic power steering except for the following:
•A torque sensor is used in place of the valve body unit
•An electric assist motor is used in place of the hydraulic power cylinder
•An EPS control unit is added
Mechanical Construction
The rack is unusual in that it is mounted high on the rear engine bulkhead,and that the tie rods engage the rack in the center.The high mount location is used for crash safety,as it keeps these components out of the Insight's crumple zone.
The tie rods are aluminum,and they connect to an ackerman arm that is mounted to the struts just below the spring seat.
The EPS control unit is mounted inside the car on the right side bulkhead, underneath the dash.It receives input from the vehicle speed sensor and torque sensor mounted on the steering pinion shaft.
The torque sensor is identical in construction to the unit on the S2000.The pinion shaft engages the pinion gear via a torsion bar,which twists slightly when there is a high amount of steering resistance.The amount of twist is in proportion to both the amount of resistance to wheel turning,and to the steering force applied.A pin on the torsion bar engages a diagonal slot in the sensor core,which moves up or down depending on the amount of torsion bar twist,and the direction of rotation.Two coils surrounding the core detect both the amount,and the direction of movement.
Using this information,the EPS control unit determines both the amount of steering assist required,and the direction.It then supplies current to the motor for steering assist.The amount of assist is also modified in proportion to vehicle speed to maintain good steering feel.
Torque Sensor
The torque sensor is a device to detect steering turning direction and read resistance.The sensing section of the torque sensor consists of two coils and a core (slider).The steering input shaft and pini
on gear are connected via a torsion bar.The slider is engaged with the pinion gear in a way that it turns together with the pinion gear but can move vertically.A guide pin is provided on the input shaft and the pin is in a slant groove on the slider.
When road resistance is low,the steering input shaft,pinion gear and slider turn together without the slider's vertical movement.
When road resistance is high,the torsion bar twists and causes a difference of steering angle between the input shaft and pinion gear.In other words,the turning angle of the guide pin and slider differ,and the guide pin forces the slider to move upward or downward.
Steering System
The steering system must deliver precise directional control.And it must do so requiring little driver effort at the steering wheel.Truck steering systems are either manual or power assisted,with power assist units using either hydraulic or air assist setups to make steering effort easier.
In addition to its vital role in vehicle control,the steering system is closely related to front suspension,axle,and wheel/tire components.Improper steering adjustment can lead to alignment and tire wear problems.Suspension,axle,and wheel problem can affect steering and handing.
The key components that make up the steering system are the steering wheel, steering column,steering shaft,steering gear,pitman arm,drag link,steering arm,ball joints,and tie-rod assembly.
Ball Joints
This ball-and-socket assembly consists of a forged steel ball with a threaded stud attached to it.A socket shell grips the ball.The ball stud moves around to provide the freedom of movement needed for various steering links to accommodate relative motion between the axle and the frame rail when the front axle springs flex.A ball stud is mounted in the end of each steering arm and provides the link between the drag link and the steering arm.
Tie-Rod aseembly
The steering arm or lever controls the movement of the driver’s side steering knuckle.There must be some method of transferring this steering motion to the opposite,passenger side steering knuckle.This is done through the use of a tie-rod assembly that links the two steering knuckles together and forces them to act in unison.The tie-rod assembly is also ealled a cross tube.
Steering gears
The steering gear converts the rotary motion of the steering wheel into straight-line motion.This moves the linkage to the steering arms on the steering knuckles,swinging the front wheels left to right.There are two basic types of steering gears.One type has a pitman arm attached to a shaft from the steering box.The second type is the rack-and-pinion steering gear.
retractableThe pitman-arm steering gear has two essential part.They are a worm gear on the end of the steering shaft and a matching sector gear or toothed roller attached to the sector shaft.
As the steering shaft and worm gear rotate,the sector gear must follow the worm gear.The sector gear is moved toward one end of the worm or the other,as the ball nut

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