Transfer case and 4x4 system - Lesson 6

Transfer case and 4×4 system – Lesson 6

Today we talk about Transfer case and 4×4 system. It is Lesson 6 of Manual Transmission and Drivetrain

Transfer case and 4x4 system - Lesson 6

Manual Transmission and Drivetrain : Lesson 6 – Transfer case and 4×4 system

General

Objectives

Upon completion of this lesson, you will be able to:

! Explain the purpose and function of a transfer case and 4×4 system.

! Describe a transfer case and 4×4 system.

! Identify the components of a transfer case and 4×4 system.

! Explain the theory and operation of a transfer case and 4×4 system

 

At a glance

4×4 drivetrain

 

 

4×4 drivetrain components

1      Rear axle

2      Rear driveshaft

3      Transfer case

4      Front driveshaft

5      Locking hub

6      Front differential

7      Engine

8      Front axle

9      Transmission

10  Slip yoke

11  Rear differential

The purpose of the 4WD system is provide torque to all four wheels of the vehicle rather than just two of the wheels.

The 4×4 drivetrain includes all the same components as a rear-wheel drive drivetrain as well as a transfer case, a front driveshaft, and a front drive axle (with or without locking hubs).

Some vehicles use an AWD system. The AWD system is a system that uses all four wheels as drive wheels all the time. AWD systems can be made up of many different components. The AWD system has a transmission or transaxle, a transfer case, a front and rear differential.

 

Introduction

Transfer case

Just as in a rear-wheel drive system, the engine supplies power to the transmission through the driveshaft and rear axle to the wheels. The transfer case is the gear assembly used to control powerflow to the front axle and somewhat to the rear axle. The transfer case transfers the power through one input shaft and two output shafts, one to each drive axle. The input shaft of the transfer case is connected to the transmission output shaft.

Many transfer cases include a gear reduction mechanism to provide a lower range, usually at a 2:1 ratio. Low range is limited to 4-wheel drive (4WD) operation because it doubles the torque to the drive axles, and this increased torque is often more than one axle is designed to handle.

 

 

Typical transfer case

 

Types

Types of transfer cases

The two types of transfer cases are part-time 4-wheel drive and full-time 4-wheel drive.

 

Part-time transfer case

Part-time transfer case systems may be manually shifted, electronically shifted, or automatically shifted into 4WD. Vehicles with part-time transfer cases operate the same as 2-Wheel Drive (2WD) vehicles when 4WD is not engaged. The system is only operated in 4WD mode when commanded by the driver, or in the case of an Automatic 4-Wheel Drive (A4WD) system, commanded by the module.

Full-time transfer cases

 

 

Controls

4×4 controls

Manual shift lever

The manual shift transfer case allows the driver to manually shift the transfer case into N (Neutral – no power to either axle), 2WD, 4WD, or 4WD Low. A shift cam in the transfer case operates the forks, which move the reduction hub and 2W-4W lockup assembly into position.

To shift from N to 2WD or 4WD, the driver must place the transmission in the P (Park) position (automatic transmission) or depress the clutch pedal while the engine is idling (manual transmission) and move the transfer case shift lever to 2WD, 4WD, or

4WD Low.

 

 

Typical manual shift lever

 

Controls

Transfer case shift lever

Shifts from 2WD to 4WD can be made while the vehicle is moving in either direction at any speed, as long as the front wheel hubs are locked. Shifts from

4WD to 2WD can be made at any time. Shifts to 4WD Low must be made while at a complete stop. Vehicles with an automatic transmission require that the transmission be placed in the P position, pausing momentarily in the R (Reverse) position, then moving the transfer case shift lever to 4WD Low.

If the transfer case shift lever becomes difficult to shift due to drivetrain windup, move the vehicle in the opposite direction for a few feet to release the windup. If the transfer case shift lever is still difficult to shift, move the vehicle forward a few feet, shift the transmission to N, and while the vehicle is moving 3 mph or less, shift the transfer case to the desired position.

The shift range selector switch, located on the instrument panel, signals to the control module the driver’s shift selection. The shift range selector switch and instrument panel indicator illuminate when the shift motor is moved to the 4WD High or 4WD Low position.

While the shift motor may be in position with the indicator lamp on, the reduction hub or 2WD-4WD lockup assembly may not yet be engaged. This could occur if their splines are not aligned with those of the planetary carrier or clutch housing. Because the reduction hub and lockup assembly are spring loaded, they will move into position when conditions allow. Lamp illumination does not guarantee that the selected shift mode has been engaged.

Some vehicles use a three-position switch to control the system operation, while other vehicles use a fourposition switch. The available switch positions are 2WD, A4WD, 4WD High, and 4WD Low.

 

Mechanical components

Shift forks

 

 

 

Typical shift fork assembly (electronic shift)

  • Reduction shift fork
  • Lock-up shift fork
  • Shift motor
  • Shift cam assembly

 

Some transfer cases use a reduction shift fork to shift between low and high ranges. The reduction shift fork usually moves a component that engages the planetary gearset.

Some transfer cases use a lock-up shift fork to shift between 2WD and 4WD. The lock-up shift fork usually moves a component that engages the drive sprocket and chain.

Most transfer cases use a shift lever or motor and shift cam assembly to shift the reduction shift fork and lock-up shift fork.

 

Planetary gearsets

 

 

Typical planetary gearset

1      Planetary pinion gears

2      Reduction hub

3      Planet carrier assembly

4      Bearing

5      Sun gear

6      Ring gear

Some transfer cases use a simple planetary gearset for torque multiplication in 4WD Low. Planetary gearsets function in several ways. In part-time transfer cases, the planetary gears are driven by the input shaft, or sun gear. The planetary gears “walk” around the inside of the ring gear, which is held stationary in the case.

The result is that the planetary gear carrier turns more slowly than the input shaft. Because the planetary gear carrier is attached directly to the output shaft through the reduction hub (high-low shift collar), the output shaft rotates at a lower speed than the input shaft. This action increases the engine torque available to the wheels. In all positions other than 4WD Low, the reduction hub joins the sun gear to the output shaft.

 

Mechanical components (continued)

Chain and sprockets

The transfer case contains a high-efficiency drive chain that provides power transfer to the front output shaft. The chain is driven by a drive sprocket on the rear output shaft and drives the driven sprocket on the front output shaft. On manual shift transfer cases, the 2WD to 4WD shift is accomplished when the 2W-4W shift fork moves the lock-up collar to engage the drive sprocket on the rear output shaft splines. The drive sprocket turns the chain, which turns the driven sprocket on the front output shaft.

On AWD transfer cases, the output from the planetary gearset enters the viscous coupling, which has one output to the rear output shaft and another to the sprocket of the drive chain for the front output shaft.

 

 

Drive Chain and Sprockets

  • Chain
  • Rear output shaft
  • Drive sprocket
  • Driven sprocket

 

Lock-up collar

 

 

Lock-up collar assembly

  • Lock-up collar
  • Drive sprocket
  • Rear output shaft

Some transfer cases use a lock-up collar assembly to mechanically lock the rear output shaft to the drive sprocket in 4WD High and 4WD Low. The lock-up collar is not applied in 2WD and A4WD.

Pump

Most transfer cases use a fluid pump for lubrication. Fluid pumps are usually driven by the rear output shaft.

 

 

Typical fluid pump

 

Transfer case clutch types

Clutch pack and cam/coil housing assembly

 

 

Basic clutch pack and cam/coil housing assembly

  • Pressure plate
  • Cam
  • Cam/coil housing assembly
  • Clutch coil
  • Balls
  • Armature

Some automatic transfer cases use a clutch pack and cam/coil housing assembly. The clutch pack and cam/ coil housing assembly allows torque to be transferred to the front output shaft, as needed, to meet driving conditions. The clutch is module controlled through a relay, and applies when the module senses speed variances between the front and rear output shafts. In some vehicles, the clutch is used in place of a lock-up component to engage all four wheels.

  • Externally splined clutch discs
  • Clutch hub
  • Clutch housing
  • Drive sprocket
  • Internally splined clutch discs The clutch hub is splined to the rear output shaft and to the internally splined clutch discs. The clutch hub rotates at rear output shaft speed.

The clutch housing is splined to the drive sprocket and to the externally splined clutch discs. The clutch housing rotates at front output shaft speed.

When the module activates the relay, the relay provides battery power to the clutch coil. The clutch coil magnetically pulls the armature against the cam/ coil housing assembly. Since the armature is splined to the clutch housing, it turns at the front output shaft speed and the cam/coil housing assembly starts to turn at the same speed.

On the inside of the cam/coil housing assembly is half of a ball and cam assembly. The other half of the ball and cam assembly is splined to the rear output shaft and the balls are seated between the two cams. Since the two cams are turning at different speeds, it causes the balls to “ramp up” on the cams. This ramp-up action pushes the cam against the pressure plate and squeezes the clutch pack. By squeezing the clutch pack together, the clutch hub is linked to the clutch housing and transfers torque to the front output shaft.

 

Transfer case clutch types (continued)

Magnetic clutch assembly

 

 

Typical magnetic clutch assembly

  • Output shaft
  • Clutch housing assembly 3 Clutch coil

Electronic shift transfer cases are equipped with a magnetic clutch, which is similar in operation to an air conditioning compressor clutch. The magnetic clutch allows the transfer case to be shifted into 4WD High at any safe speed. The clutch is controlled by a module through a solid state relay.

The clutch is used to spin up the front drive system from zero to vehicle speed in milliseconds. The spinup engages the front locking hubs. After the hubs are locked, the control module allows the shift motor to execute the shift. When the transfer case rear and front output shafts reach synchronous speed, the spring-loaded synchronizer sleeve mechanically engages the mainshaft lockup hub to the chain-drive sprocket and the magnetic clutch is then deactivated.

 

Viscous coupling

 

 

Viscous coupling transfer case

  • Housing
  • Clutch discs

The function of the viscous coupling is similar to that of a limited slip differential. The viscous coupling provides torque to the wheels regardless of the traction available to the tires. The viscous coupling automatically performs this function to improve vehicle traction when the rotation speed of the front and rear wheels varies.

The viscous coupling is similar to multi-disc clutches used in automatic transmissions. The viscous coupling has a sealed housing that contains two sets of discs. The discs have a ferric nitro-carburized coating. One set of discs is connected to the coupling housing and the other set is connected to the internal The viscous coupling housing is a sealed unit and is filled with a combination of a special silicone based fluid (85%) and air (15%). The rotation of the housing is transmitted to the hub by the action of the fluid.

As the housing (drive) plates rotate, they cut through, or “shear,” the fluid. The fluid quickly heats up and expands in the housing. As the fluid expands, it forces the air contained in the housing to press tightly against the hub (driven) plates and to rotate at the same speed as the drive plates. Torque is efficiently transferred between the drive and driven plates without direct contact between the drive and driven plates.

hub of the coupling.

 

Powerflow

Basic transfer case powerflow

 

 

Part-time transfer case

  • Input shaft (sun gear)
  • Planetary pinion gears
  • Planetary carrier
  • Ring gear
  • Reduction hub
  • Rear output shaft
  • Inner lock-up hub
  • Lock-up collar 9 Drive sprocket
  • Clutch housing
  • Drive chain
  • Driven sprocket
  • Front output shaft

 

The transfer case uses a large chain to transfer power to the front output shaft. There are three ways for power to flow through this transfer case:

! 4×4 high

! 4×4 low

! 2-wheel drive

Low range is provided by a set of planetary gears. A lock-up collar is used to control the powerflow for 4WD to the drive chain sprocket.

The current transfer cases offer several advantages over systems used in the past, including ease of shifting, quiet operation, and reduced component wear. Unlike older systems, current transfer cases allow all internal 4WD components to be disengaged from the drivetrain when 2WD is selected. They create no additional drag and do not adversely affect fuel economy.

 

Powerflow

Basic transfer case powerflow (continued)

4WD high range

 

 

4H powerflow

  • Input shaft (sun gear)
  • Reduction hub
  • Rear output shaft
  • Inner lock-up hub
  • Lock-up collar
  • Drive sprocket
  • Clutch housing
  • Drive chain
  • Driven sprocket
  • Front output shaft

 

When the vehicle is put into gear, torque is transmitted to the transfer case input shaft from the transmission output shaft. The reduction hub connects the input shaft to the rear output shaft, and torque is transmitted to the rear axle. When 4WD is selected, the lock-up collar moves and locks the inner lock-up hub to the drive sprocket.

With the inner lock-up hub locked to the drive sprocket, the drive sprocket is locked to the rear output shaft. With the rear output shaft locked to the drive sprocket, torque is also transmitted to the front axle through the drive chain, driven sprocket, and front output shaft.

 

Powerflow

4WD low range

 

 

4L powerflow

  • Input shaft (sun gear)
  • Planetary pinion gears
  • Planetary carrier
  • Ring gear
  • Reduction hub
  • Rear output shaft
  • Inner lock-up hub

When 4L is selected, the reduction hub moves onto the planetary carrier, locking the planetary carrier to the rear output shaft. When the vehicle is put into gear, torque is routed through the sun gear to the planetary pinion gears. The planetary pinion gears walk around the inside of the ring gear, which causes the planetary carrier and rear output shaft to rotate in reduction to the input shaft. More torque is now transferred through the transfer case.

  • Lock-up collar 9 Drive sprocket
  • Clutch housing
  • Drive chain
  • Driven sprocket
  • Front output shaft

As in the 4H position, the inner lock-up hub remains locked to the drive sprocket. With the inner lock-up hub locked to the drive sprocket, torque is also transmitted to the front differential through the drive chain, driven sprocket, and front output shaft.

 

Front drivetrain

Front drivetrain components

Front driveshaft

 

 

Typical front driveshaft

  • Front U-joint
  •  Boot
  • Driveshaft
  • Rear U-joint

During 4WD operation, the front driveshaft delivers power to the front drive axle. A typical front driveshaft has a front single Cardan U-joint, a rear single or double Cardan U-joint, a rear slip yoke, and a boot (to cover the slip yoke splines).

Front drivetrain

Front axle

 

Monobeam front axle

  • Locking wheel hub
  • Brake rotor
  • Axle housing
  • Steering knuckle
  • Shaft and joint assembly
  • Pinion yoke

 

A front-drive axle is required to deliver power from the differential to the front wheels. The front-drive axle operates the same as a rear-wheel drive axle. The front-drive axle directs powerflow through the axle shafts to both front wheels when the system is in 4WD mode. Most axle shafts consist of two pieces, with an inner axle that connects to the axle gear in the differential, the U-joint, or CV joint, and an outer axle that connects to the wheel hub.

There are two types of front axles used on 4WD vehicles: Monobeam and Independent Front Suspension (IFS).

Monobeam axles have a one-piece design. Monobeam axles are used in applications where substantial amounts of weight are expected to be added to the front of the vehicle, such as winches, snow plows, etc.

IFS axles are designed to allow the front axle to become an integral part of the vehicle suspension. IFS axles use halfshafts with constant velocity (CV) joints to allow the axles to flex in relation to ground conditions and to the differential carrier, which is part of the axle housing.

 

Front drivetrain

Front drivetrain components (continued)

Locking wheel hubs

 

transfer case

 

Manual locking hubs

  • Knob
  • Hub
  • Cam
  • Spring mount
  • Axle shaft sleeve ring 6 Spring mount
  • Bearing and retainer
  • Inner spring
  • Inner clutch ring
  • Outer spring

 

There are three types of locking hubs: manual, which require the driver to operate them; automatic, which engage based on axle shaft rotation; and vacuum, which are engaged by an externally supplied vacuum source. Manual hubs contain a clutch that is shifted by rotating a portion of the hub unit to manually engage the clutch (lock the hub), or disengage it. The hub engages when an internal cam moves a spline on the clutch hub into mesh with a spline in the wheel hub.

Automatic hubs have a feature in which rotation of the axle causes the cam in the hub to operate. This feature allows the driver to engage 4WD without having to go to the hubs to lock them.

The pulse vacuum hub lock (PVH) system is a wheelend disconnect front axle system that engages or disengages the hubs from the front axle halfshafts by means of vacuum from the engine manifold. When the driver makes a 4×4 engagement or disengagement, the electronic module energizes the PVH solenoid to apply one of two vacuum pressure levels downstream to the hub locks. The lower pressure level is regulated by the solenoid and acts to disengage each hub lock.

The high level acts to engage the hub locks. If vacuum is lost, the system remains in whichever mode it is in at that time. The PVH system is designed in such a manner that there is no overlap between hub lock engagement/disengagement levels and solenoid engagement/disengagement levels. This ensures proper 4×4 function during shift-on-the-fly at all altitude and temperature extremes.

 

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