4WD ppt

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41
FourFour- and AllAllWheel Drive

Prepared by

Martin Restoule Algonquin College
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-1

Objectives
‡ Identify the advantages of four- and allwheel drive. ‡ Name the major components of a conventional four-wheel drive system. ‡ Name the components of a transfer case. ‡ State the difference between a transfer, open, and limited slip differential.
continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-2

Objectives
‡ State the major purpose of locking/unlocking hubs. ‡ Name the five shift lever positions on a typical 4WD vehicle. ‡ Understand the difference between four- and all-wheel drive. ‡ Know the purpose of a viscous clutch in all-wheel drive.
continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-3

4WD Systems
‡ Have a separate transfer case. ‡ Give the driver a choice between twoand four-wheel drive. ‡ Can have manual or automatic locking hubs. ‡ May or may not use an interaxle differential or viscous clutch.
continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-4

4WD Systems
A transfer case

continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-5

AWD Systems
‡ Do not have a separate transfer case. ‡ Use a front-wheel drive transaxle equipped with a viscous clutch, center differential, or transfer (Haldex) clutch. ‡ Operate continuously in four-wheel drive.

continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-6

AWD Systems
A viscous clutch assembly

continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-7

A Typical 4WD Vehicle

continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-8

4WD Operational Modes
‡ 2-high (2H)
± Vehicle operates like a normal two-wheel drive vehicle.

‡ 4-high (4H)
± The front axle is connected to the driveline.

‡ 4-low (4L)
± The transfer case provides a lower gear ratio.
continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-9

The Transfer Case
‡ The transfer case itself is constructed similarly to a standard transmission. ‡ It uses shift forks to select the operating mode, plus splines, gears, shims, bearings, and other components found in manual and automatic transmissions. ‡ The outer case of the unit is made of cast iron, magnesium, or aluminum.
continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-10

The Transfer Case
‡ The front and rear drivelines are connected to a interaxle differential inside the transfer case. ‡ The interaxle differential allows for different front and rear driveline shaft speeds. ‡ Driveline windup, developed as a result of different front and rear axle gear ratios, is dissipated by the interaxle differential. continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-11

The Transfer Case
‡ The power flow through transfer case in 2H, 4H, and 4L modes.

Chapter 41

Copyright © 2007 by Nelson, a division of Thomson Canada Limited.

continued

41-12

Axle Disconnects
‡ Rather than using locking wheel hubs some 4WD vehicles use a vacuum or electric motor axle disconnect system
± The motor moves a collar that connects or disconnects one front axle from the front differential. ± When the collar is disconnected, the front ring gear and pinion remain stationary, reducing wear.
continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-13

Axle Disconnects
A vacuum operated axle disconnect

continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-14

Axle Disconnects
An electric motor axle disconnect

continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-15

4WD Design Variations
‡ Open centre differential
± Uses differential action to compensate for differences in speed between the axles.

‡ Limited-slip centre differential
± Allows more torque to be delivered to the axle with the most traction.

continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-16

Viscous Clutches or Couplings
‡ Typically used in AWD vehicles. ‡ Are self contained and not serviceable. ‡ Can be installed inside or outside the transaxle housing. ‡ Consist of two sets of steel plates in a drum of thick fluid. ‡ Split the engine torque according to the needs of each axle.
continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-17

Viscous Clutches or Couplings
A typical viscous clutch assembly

continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-18

Haldex Clutch
‡ Has three main components: ± Hydraulic pump. ± Wet multi-disc clutch. ± Electronically controlled valve. ‡ Uses hydraulic pressure to engage the clutches to engage the rear wheels.

continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-19

Haldex Clutch
A Haldex clutch assembly

Chapter 41

Copyright © 2007 by Nelson, a division of Thomson Canada Limited.

continued

41-20

Summary
‡ The heart of most conventional 4WD systems is the transfer case. ‡ The interaxle is placed in the transfer case to operate in the same fashion as the differentials at the axles. ‡ Many vehicles require that the front hubs be in a locked condition to operate as 4WD vehicles.
continued
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-21

Summary
‡ Components of 4WD vehicles can be serviced in basically the same manner as the same components on a 2WD vehicle. ‡ All-wheel drive vehicles may use a viscous clutch, rather than a transfer case, to drive the axle. ‡ Some AWD vehicles have a third differential, called an interaxle differential, instead of a transfer case.
Chapter 41 Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 41-22

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