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Steering Systems

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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 ... – PowerPoint PPT presentation

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Title: Steering Systems


1
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) the steering system, and (2) the
    braking system.
  • The steering mechanism converts the driver's
    rotational input at the steering wheel into 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 inside wheel
    is following a circle with a smaller radius, it
    is actually making a tighter turn than 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 makes the inside wheel turn more
    than the outside wheel.

2
  • 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.

3
  • 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 Iow 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 181.
    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.

4
  • 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 gearset 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.

5
Steering System
Linkage Steering System (Worm Gear) Parts
  • Steering Wheel used 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.

6
  • 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

7
Rack-and-pinion Steering
  • Basically, as the name implies, the
    rack-and-pinion steering consists of a rack and a
    pinion, The steering ratio is defined by the
    ratio of pinion revolutions (steering-wheel
    revolutions) to rack travel. Suitable toothing of
    the rack allows the ratio to be made variable
    over the travel. This lowers the actuating force
    or reduces the travel for steering corrections.
  • Rack-and-pinion steering is quickly becoming the
    most common type of steering on cars, small
    trucks. It is actually a pretty simple mechanism.
    A rack-and-pinion gearset 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.

8
  • 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

9
Rubber bellows
  • This rubber bellows is attached to the Rack and
    Pinion housing. It protects the inner joints from
    dirt and contaminants. In addition, it retains
    the grease lubricant inside the rack and pinion
    housing. There is an identical bellows on the
    other end of the rack for the opposite side
    connection.

10
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.

11
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
    a nylon bush at the other end.

12
Inner ball joint or socket
  • The inner ball joint is attached to the tie-rod,
    to allow for suspension movement and slight
    changes in steering angles

13
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.

14
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 tire contact patch
    perpendicular to the rolling axis. This force
    produces a torque around the steering 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 tractive force
    of the front wheels produces a moment about the
    steering axis that tends to toe the wheels
    inward. In this case, proper toe-out absorbs this
    motion and allows the wheels to parallel the
    direction of motion of the vehicle.

15
Power Rack-and-pinion
  • When the rack-and-pinion is in a power-steering
    system, the rack has a slightly different design.
  • Part of the rack contains a cylinder with a
    piston in the middle. The piston is connected to
    the rack. There are two fluid ports, one on
    either side of the piston. Supplying
    higher-pressure fluid to one side of the piston
    forces the piston to move, which in turn moves
    the rack, providing the power assist.

16
Recirculating-ball steering
  • The forces generated between steering worm and
    steering nut are transmitted via a low-friction
    recirculating row of balls. The steering nut acts
    on the steering shaft via gear teeth. A variable
    ratio is possible with this steering box,

17
  • 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 a block 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 regular bolt would, this bolt
    is held fixed so that when it spins, it moves the
    block, which moves the gear that turns the wheels.

18
  • 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 felt when 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 one side of the block.

19
  • From the 1950s to the 1980s, the conventional
    recirculating ball steering gear was the dominant
    system. The 1980s saw the 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.

20
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 steering mechanism, 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 "r?ad
    feel"

21
  • Representative power steering systems for cars
    increase steering effort via an actuator, a
    hydraulic cylinder, which is part of a servo
    system. These systems 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 assistance instead 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.

22
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 (lt100C) and such that no noise is
    generated and the oil does not foam.

23
  • 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 the
    outlet 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.

24
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. 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.

25
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.

26
Electric Power Assist Steering (EPAS) with
Pull-Drift Compensation
  • Pull-Drift Compensation starts with EPAS
    technology, which replaces the traditional
    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
    vehicles path. Continuous adjustments are made
    as the system resets to adapt to changing road
    conditions or maneuvers, such as the vehicle
    turning a corner.
  • 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 driving characteristics of varying
    products, whether its a luxury sedan or sporty
    compact SUV.

27
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28
A steering mechanism as a machine
  • The force required to steer a vehicle is often
    considerably larger than a driver 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

29
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