Automotive Steering Systems Quiz

Automotive Engineering Course after 1st Term

• Topics covered for Gear
  • Description of sliding mesh and constant mesh gearboxes
  • Synchro-mesh gearboxes
  • Semi-automatic transmissions
  • Final drive and differentials, rear axles, and overdrive

Introduction to Gear Box (Transmission)

• Gearbox acts according to running conditions

  • Reduces engine speed and increases torque when power is needed
  • Transmits high speed and low torque to wheels when high speed is desired
  • Reverses vehicle by meshing gears

• Gears used are helical gears

  • Helical teeth are set at an angle to the gear centreline
  • Wiping action improves contact and lubrication

Gear Ratio

• Gear ratio is determined by the number of teeth on the two gears

  • Gear ratio = (Number of teeth on Driven gear) / (Number of teeth on Driving gear)
  • If smaller gear (with 15 teeth) drives larger gear (with 30 teeth), the ratio is 2:1 (2 to 1)
  • A 15-tooth gear driving a 45-tooth gear results in a 3:1 (3 to 1) gear ratio

• Typical gear box ratios in a small car with a four-speed gearbox

  • 3.5:1 (3.5 to 1) for first gear
  • 2:1 (2 to 1) for second gear
  • 1.4:1 (1.4 to 1) for third gear
  • 1:1 (1 to 1) for top gear

• These ratios are multiplied by the axle ratio (4:1) to determine the corresponding ratios between engine speed and road wheel speed

• Power available from the engine is directly proportional to the engine torque (T) and the gear box ratio (G)

Torque and Gear Ratio

• When smaller gear drives larger gear, the gear ratio is 2:1, but the torque ratio is 1:2
• Larger gear has twice the torque of smaller gear
• Speed reduction in gear systems increases the torque

Types of Gear Boxes

• Selective type:
  • Sliding mesh
  • Constant mesh
  • Synchromesh
• Progressive type
• Epicyclic or planetary type

Selective Type Gear Boxes

• Any speed can be selected from the neutral position
• Neutral position needed before selecting forward or reverse
• Advantages: simple construction, relatively trouble-free, light and small, low production costs
• Disadvantages: gear ratios in steps (3 to 5 steps), noisy operation

Sliding Mesh Gear Box

• Gears on splined main shaft move left or right to mesh with appropriate gears on the lay shaft for different speeds
• Gears mesh by sliding or crashing onto each other
• Also known as crash-type gearbox

Constant Mesh Gear Box

• All gears are in constant mesh with each other
• Helical gears for easier gear changing.
• Main shaft is splined and gears are free
• Two dog clutches provided on the main shaft

Synchro Mesh Gear Box

• Sliding synchronizing units to replace sliding dog clutches
• Synchronizing unit synchronizes driving and driven shafts before engagement
• Arrangements of power flow for gears remain the same as in constant mesh gearbox

Synchromesh Gear Devices

• Two gears brought into frictional contact to equalize speed before engagement
• Two types: pin type and synchronizer ring type

Conical Surfaces in Synchromesh

• Cone 1 part of collar; Cone 2 part of gear wheel
• Cones revolve at different speeds
• Friction slows or speeds up gear wheel, then both revolve at the same speed

Synchromesh Figure

• Collar and gear wheel rotate at the same speed
• Spring-loaded outer ring of collar pushed forward
• Dogs slide smoothly into mesh without clashing

Gearbox Components:

• Sun gear, planet gears, planet carrier, arm.
• Simplest epicyclic gear train provides three different gear ratios when arm is locked, or the larger gear or the smaller pinion is locked.

Overdrive

• Interposed between transmission (gearbox) and propeller shaft

• Enables propeller shaft to turn faster than the transmission main shaft

• Called overdrive because it provides a higher speed ratio than the high-speed ratio

• Permits engine operation at about 70% of propeller shaft speed in high-speed ranges

• Suited for high-powered cars with three-speed gearboxes

• Provides super-top gear

• Ratio of engine crankshaft speed /road speed equal to 1 in top gear and less than 1 in super-top gear

Advantages of Overdrive

• Attain cruising speed on highways and expressways at low engine speeds
• Reduced engine part wear due to low speeds
• Decreased vibrations and noise
• Lesser frictional losses in the engine
• Fuel saving due to lower engine speeds

Differential

• Mechanism of epicyclic gear train, located between final drive and rear axles
• Purposes: avoid rear wheel skidding during turning; reduce inner wheel speed and increase outer wheel speed while negotiating curves; maintain equal wheel speeds when moving straight

Differential: Necessity of Differential

• Rear wheels roll-down the road at equal speeds when in straight motion
• Outer wheels cover a longer distance than inner wheels during turning
• Without a differential, outer wheels would skid during turns

Differential: Consequence of No Differential

• Outer wheels skid; inner wheels exert excessive pressure on road surface

Differential Construction and Working

• Consists of bevel pinion & crown wheel; cage; two sun gears; two planet pinions, cross-pin or spider
• Sun gears always in mesh with planet pinions
• Half shaft (rear axle) is splined to allow movement in sun gear
• Outer half-shaft ends connected to wheel hub
• Crown wheel rotates freely on bush mounted over one half-shaft

Steering Systems

• Assembly of linkages to control the vehicle's direction
• Driver input to steering wheel results in lateral movement of wheels
• Efficient control achieved with pure rolling motion
• Steering systems: Front-wheel or all-wheel
• Front-wheel steering systems are popular; power steering provides assisted steering

Front Axle

• Holds front wheels, allowing their rotation and obeying steering commands
• Rigid axle beam type (stationary): Used to connect stub axles and sustain bending/torsion; I-section for bending resistance; circular ends for torsion resistance
• Stub axle type (intermediate): Short shaft between road wheel and front (dead) axle; connected to axle beam by kingpins

Steering Gear

• Transforms steering wheel rotary motion into reciprocating drag link motion
• Multiplies small steering wheel effort to greater force on the drag link Different types: worm and worm wheel; worm and sector; recircuting steel balls

Steering Gear Ratio and Overall Steering Ratio

• Steering gear ratio (SGR): Ratio of steering wheel turn angle to steering gear cross-shaft turn angle -Lower value = faster steering, useful for small, fast vehicles
• Overall steering ratio (OSR): Includes the mechanical advantage of linkages too, making the ratio slightly larger than SGR, useful for cars with higher speed.

Wheel Alignment

• Setting front wheels and steering mechanism for easy turn control, minimizing tyre wear and maintaining straight-line stability
• Toe-in / Toe-out: Front wheels set closer at the front, or further apart than their rear, in the stationary state
• Camber: Angular tilt of the wheel
• Caster: Tilting of the kingpin from the vertical axis

Ackermann's Steering Mechanism

• Mechanism design to ensure that wheel turn angles are different for inner and outer wheels during cornering
• Equal angles of inclination for two links connecting the steering arms are achieved for correct cornering.

Layout of Steering System

• Key components and linkages of a steering system in a car