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Four Wheel Drive - PowerPoint PPT Presentation


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Four Wheel Drive. Purpose. Provide additional traction for off-road or poor traction conditions May provide a low range for increased torque Some units may have provisions for PTO. 4WD VS. AWD. 4WD Generally is part-time Generally provides a low range AWD Generally is full-time

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Presentation Transcript
purpose
Purpose
  • Provide additional traction for off-road or poor traction conditions
  • May provide a low range for increased torque
  • Some units may have provisions for PTO
4wd vs awd
4WD VS. AWD
  • 4WD
    • Generally is part-time
    • Generally provides a low range
  • AWD
    • Generally is full-time
    • Generally does not provide a low range
    • Generally contains a front to rear differential
components
Components
  • Two drive axle assemblies
    • Front axle must be able to steer as well as drive
    • Gear ratios must match (front to rear)
    • Front drive axles have traditionally been solid (non-independent)
    • Independent front suspension is becoming much more popular
    • Some AWD vehicles are built off of a FWD platform
components1
Components
  • Transfer Case
    • An auxiliary transmission designed to split power between the front and rear drive axles
  • Lock out hubs – used to disengage the front wheels from the drive axle
    • Increases highway fuel economy and reduces wear
    • May be automatic or manual
      • Automatic hubs react to input torque when transfer case is shifted into 4WD
      • May not unlock after shifting out of 4WD until vehicle is backed up (30-50ft)
transfer cases
Transfer Cases
  • Transfer cases can be categorized by various characteristics
    • Style
      • In-line – Front and rear driveshaft will be offset. Rear driveshaft will be inline with the output shaft of the transmission
      • Offset – Both driveshafts exit from the transfer case directly across from each other
    • Ranges
      • Single range – only offers 1:1 4WD and 1:1 2WD
      • Dual range – also offers a gear reduction 4WD
transfer cases1
Transfer Cases
  • Drive
    • Chain
      • Uses a link type chain (similar to a timing chain)
      • If used with a dual range case, gear reduction takes place before the chain drive sprocket
      • Usually will use a planetary gearset for gear reduction
    • Gear
      • Uses a cluster gear between the transmission and the transfer case output shaft
gear selection
Gear Selection
  • Direct manual shift
    • Dog clutches lock gear or sprocket to shaft
    • No synchronizers
  • Syncronized manual shift
    • Allows shift from H2 to H4 and H4 to H2 while moving
    • Must stop to go to L4
      • Some may shift from L4 to H4 at very low speeds
gear selection1
Gear Selection
  • Vacuum shift
    • Manual or electric control opens vacuum valving to diaphragm which moves the shift mechanism
  • Electric
    • Uses an electric motor to shift
      • Uses a worm gear
    • Electromagnetic clutch spins up (synchronizes) front axle components before engagement
    • Allows “shift on the fly” at any road speed
    • L4 engagement still require the driver to stop and shift to neutral (or disengage clutch)
front axle
Front Axle
  • The front axle will have some provision for unlocking the wheels from the driveline (Part time 4WD)
    • Locking hubs
    • Front axle disconnect
      • Thermal linear actuator
      • Vacuum servo
      • Electric motor
slide16
AWD
  • All wheel drive require differentiation between the front and rear axles
    • Front and rear wheels follow different tracks
    • Prevents propshaft windup
  • Different types of differentiation are used
    • Open differential
    • Limited slip
      • Friction
      • Torsen
      • Viscous coupling
viscous coupling
Viscous Coupling
  • Uses a thick fluid between discs in a sealed unit
  • When a speed differential between the axles exists, fluid shears, heat builds up and forces the plates together until axle speeds match
service
Service
  • Change fluid at recommended intervals
    • Use only the recommended lubricant
      • Most gear types transfer cases use gear oil
      • Most chain type transfer cases use ATF
      • May use special lubricant (Syncromesh)
    • AWD with friction type limited slip is very sensitive to fluid type
  • Both drive axles must have the same gear ratio
    • Tire size and inflation
  • Warn customer about propshaft windup
    • Part time 4WD operating on concrete or asphalt
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