steering system(1)

8/12/2019 Steering System(1)

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Steering Systems Any mode of transportation used by people must have some means of control. For

the automobile, two primary control systems are at the driver's disposal: (1) thesteering system, and (2) the braking system.

The steering mechanism converts the driver's rotational input at the steering wheelinto a change in the steering angle of the vehicle's steering road wheels.

For a car to turn smoothly, each wheel must follow a different circle. Since the insidewheel is following a circle with a smaller radius, it is actually making a tighter turnthan the outside wheel. If you draw a line perpendicular to each wheel, the lines will

intersect at the center point of the turn. The geometry of the steering linkage makesthe inside wheel turn more than the outside wheel.


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Steering behavior

The requirements in terms of steering behavior can be summarized as follows:

1. Jolts from irregularities in the road surface must be damped as much as possible

during transmission to the steering wheel. However, such damping must not cause

the driver to lose contact with the road. 2. The basic design of the steering kinematics must satisfy the Ackermann

conditions: the extensions of the wheel axes of the left and right front wheels,

when at an angle, intersect on an extension of the rear axle.


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3. When the steering wheel is released, the wheels must return

automatically to the straight-ahead position and must remain stable in this

position.

4. The steering should have as low ratio as possible (number of steering-

wheel turns from lock to lock) in order to obtain ease of handling. The

steering forces involved are determined not only by the steering ratio but

also by the front suspension load, the turning circle, the suspension

geometry (caster angle, kingpin angle, kingpin offset), the properties of

the tire tread and the road surface.

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. 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.


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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 wheelas 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.


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Steering SystemLinkage Steering System (Worm Gear) Parts

Steering Wheelused by the driver to rotate a

steering shaft that passes through the steering

column.

Steering Shaft transfers turning motion from

the steering wheel to the steering gearbox.

Steering Column supports the steering column

and steering shaft.
http://www.perrinefamily.org/photogallery/Igor/column/914%206%20steering%20column.JPG


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Steering gears are enclosed in a casing known as steering gear box.

A steering box must have the following qualities:

- no play in the straight-ahead position,

-low friction, resulting in high efficiency,

- high rigidity,

- readjustability.

For these reasons, two types have become established:


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Rack-and-pinion Steering Basically, as the name implies, the rack-and-pinion

steering consists of a rack and a pinion, The steeringratio is defined by the ratio of pinion revolutions(steering-wheel revolutions) to rack travel. Suitabletoothing of the rack allows the ratio to be madevariable over the travel. This lowers the actuating force

or reduces the travel for steering corrections. Rack-and-pinion steeringis quickly becoming the most

common type of steering on cars, small trucks. It isactually a pretty simple mechanism. A rack-and-piniongear set is enclosed in a metal tube, with each end of

the rack protruding from the tube. A rod, called a tierod, connects to each end of the rack.

The pinion gearis attached to the steering shaft. Whenyou turn the steering wheel, the gear spins, moving the

rack.


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The rack-and-pinion gearset 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 primary components of the rack and pinion steering system are:

Rubber bellows

Pinion

Rack

Inner ball joint or socket Tie-rod


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Pinion

The pinion is connected to the steering column. As the driver turns the

steering wheel, the forces are transferred to the pinion and it then causes

the rack to move in either direction. This is achieved by having the pinion

in constant mesh with the rack.


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Rack

The rack slides in the housing and is moved by the action of the

meshed pinion into the teeth of the rack. It normally has an

adjustable bush opposite the pinion to control their meshing, and anylon bush at the other end.


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Tie-rod

A tie rod end is attached to the tie-rod shaft. These pivot as the rack is

extended or retracted when the vehicle is negotiating turns. Some tie-rods

and tie-rod ends are left or right hand threaded. This allows toe-in or toe-

out to be adjusted to the manufacturer's specifications.


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Recirculating-ball steering The forces generated between steering worm and steering nut are

transmitted via a low-friction recirculating row of balls. The steering nutacts on the steering shaft via gear teeth. A variable ratio is possible with

this steering box,


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Recirculating-ball steering is used on many trucks and SUVs today. The

linkage that turns the wheels is slightly different than on a rack-and-pinion

system.

The recirculating-ball steering gear contains a worm gear. The first part is ablock of metal with a threaded hole in it. This block has gear teeth cut into

the outside of it, which engage a gear that moves the pitman arm (see

diagram above). The steering wheel connects to a threaded rod, similar to a

bolt, that sticks into the hole in the block. When the steering wheel turns, it

turns the bolt. Instead of twisting further into the block the way a regularbolt would, this bolt is held fixed so that when it spins, it moves the block,

which moves the gear that turns the wheels.


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Instead of the bolt directly engaging the threads in the block, all of the

threads are filled with ball bearings that recirculate through the gear as it

turns. The balls actually serve two purposes: First, they reduce friction and

wear in the gear; second, they reduce slop in the gear. Slop would be feltwhen you change the direction of the steering wheel -- without the balls

in the steering gear, the teeth would come out of contact with each other

for a moment, making the steering wheel feel loose.

Power steering in a recirculating-ball system works similarly to a rack-and-

pinion system. Assist is provided by supplying higher-pressure fluid to oneside of the block.


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From the 1950s to the 1980s, the conventional recirculating

ball steering gear was the dominant system. The 1980s sawthe introduction of the front-wheel-drive passenger car with

rack and pinion steering. Rack and pinion systems weigh less

and use fewer parts. Also, the manufacturers were able to

bring the cost of rack and pinion systems down due to

increased automation in the final machining process.

Today, most passenger cars and light trucks are equipped with

rack and pinion steering.


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Power Steering

Power steering helps drivers steer vehicles by increasing steering effort of the

steering wheel. Hydraulic or electric actuators add controlled energy to the steeringmechanism, so the driver needs to provide only slight effort regardless of

conditions. Power steering helps considerably when a vehicle is stopped or moving

slowly. As well, power steering provides some feedback of forces acting on the front

wheels to give an ongoing sense of how the wheels are interacting with the road;

this is typically called "radfeel"


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Representative power steering systems for cars increase steering effort via

an actuator, a hydraulic cylinder, which is part of a servo system. Thesesystems have a direct mechanical connection between the steering wheel

and the linkage that steers the wheels. This means that power-steering

system failure still permits the vehicle to be steered using manual effort

alone.

In other power steering systems, electric motors provide the assistanceinstead of hydraulic systems. As with hydraulic types, power to the

actuator (motor, in this case) is controlled by the rest of the power-

steering system.

Some construction vehicles have a two-part frame with a rugged hinge in

the middle; this hinge allows the front and rear axles to become non-parallel to steer the vehicle. Opposing hydraulic cylinders move the halves

of the frame relative to each other to steer.


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Hydraulic power-assisted steering Energy source

The energy source consists of a vane pump (generally driven by the engine) with an integral

oil-flow regulator, an oil reservoir and connecting hoses and pipes.

The pump must be dimensioned so that it generates sufficient pressure to enable rotation of

the steering wheel at a speed of at least 15 m/s even when the engine is only idling.

The compulsory pressure-limiting valve required on hydraulic systems is usually integrated. .

The pump and the system components must be designed such that the operating

temperature of the hydraulic fluid does not rise to an excessive level (


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Control valve

All power steering pumps have a flow-control valve to vary fluid flow and power steering

system pressures. A pressure relief valve prevents excessive pressures developing when the

steering is on full-lock, and held against its stops. The flow control valve is located at theoutlet fitting of the pump.

During slow cornering, or when parking, pump speeds are normally low. There is less demand

for fluid flow, but to provide the required assistance, high pressure is needed. Discharge ports

direct the fluid to the outlet, and then to the steering gear. The outlet fluid pressure is slightly

lower than the internal high pressure coming from the pump.


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Pump The hydraulic power for the steering is provided by a rotary-vane

pump. This pump is driven by the car's engine with a belt and pulley. Itcontains a set of retractable vanes that spin inside an oval chamber.

As the vanes spin, they pull hydraulic fluid from the return line at lowpressure and force it into the outlet at high pressure. The amount of flowprovided by the pump depends on the car's engine speed. The pump mustbe designed to provide adequate flow when the engine is idling. As aresult, the pump moves much more fluid than necessary when the engineis running at faster speeds.

The pump contains a pressure-relief valve to make sure that the pressuredoes not get too high, especially at high engine speeds when so muchfluid is being pumped.


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Toe Toe is defined as the difference of the distance between the leading edge of the wheels

and the distance between the trailing edge of the wheels when viewed from above. Toe-in

means the front of the wheels are closer than the rear; toe-out implies the opposite.Figure 7.20 shows both cases.

For a rear-wheel-drive vehicle, the front wheels normally have a slight amount of toe-in. .When the vehicle begins to roll, rolling resistance produces a force through the tirecontact patch perpendicular to the rolling axis. This force produces a torque around thesteering axis that tends to cause the wheels to toe-out. The slight toe-in allows for this,and when rolling, the wheels align along the axis of the vehicle. Conversely, front-wheel-

drive vehicles require slight toe out. In this case, the reactive force of the front wheelsproduces a moment about the steering axis that tends to toe the wheels inward. In thiscase, proper toe-out absorbs this motion and allows the wheels to parallel the direction ofmotion of the vehicle.

R t V l


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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 thedriver 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 onthe steering wheel is called the rotary valve.


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Electric Power Assist Steering (EPAS) with Pull-Drift Compensation

Pull-Drift Compensation starts with EPAS technology, which replaces thetraditional hydraulic-assist powersteering pump with an electric motor. This

increases fuel economy because the electric motor operates only when steering

assistance is required.

Sensors constantly measure steering wheel torque applied by the driver to

maintain the vehiclespath. Continuous adjustments are made as the system resets

to adapt to changing road conditions or maneuvers, such as the vehicle turning acorner.

When the system detects a pulling or drifting condition, such as a crowned

road surface, it provides torque assistance to help make steering easier. For drivers,

this assistance is seamless and imperceptible.

EPAS technology can be fine-tuned by engineers to fit the drivingcharacteristics of varying products, whether itsa luxury sedan or sporty compact

SUV.


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A steering mechanism as a machine

The force required to steer a vehicle is often considerably larger than adriver can comfortably exert. The steering mechanism is a machine that

allows the driver of a vehicle to operate the steering without having to exert

a large force at the steering wheel. The rack and pinion steering mechanism

that is widely used on light vehicles is a convenient example of such a

machine


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steering system(1)

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