Book Rear

Transmission

Rear Axle

Preface

This training module introduces the concepts of rearaxle. The training module is part of a series of modulesdesigned for the Transmission Basic Training.

This module should be studied right after the modules:General Information, Clutch, and Gearbox.

While studying this module, you will have theopportunity to learn the rear axle function, its maincomponents, as well as how these componentswork, individually and in the rear axle assembly.

ContedoIntroduction 3

Main components of the rear axle - housings 4

Function of differential. 5

Final drive components 6

Thrust bolt 7

Crown wheel and pinion set 8

Crown wheel and pinion combinations. 9

Crown wheel and pinion relation. 10

Differential. 11

Differential assembly operation 13

Function of the Differential 14

Function of the Differential 15

Differential lock, general information 17

Differential lock, main component 18

Differential lock, operation 19

Hub reduction, general information 20

Hub reduction, main component 21

Tandem drive 22

Differential with a transfer gearbox 23

Global TrainingTP95850 3

Introduction

The main task of the rear axle is to transfer driving power from the engine out to the wheels.The rear axle consists of:

Rear axle-housing (1), which is made of cast iron and is to support and protect all axlecomponents.

It is bolted on the vehicle suspension.

Final drive (2) with helical gears that turn the driving power through 900.

Driving shafts (3) that transfer the driving power on to the wheels.


Main components of the rear axle - housings

Rear axle-housing (1):

The rear axle housing contains all the components of the rear axle assembly.The differential case is mounted on the front of the rear axle housing, supported on four points.

Differential case (2):

The differential case is made of a one-piece cast iron to withstand great dynamic stress fromthe weight of the rear axle components during vehicle movement. This means that only thebearing cap and the pinion seat are separated from the case.


Function of differential.

The differential has the job of adjusting the speed of the individual drive wheels while retaining thetotal driving power. This is something that is very necessary because when the vehicle turns acorner, the outside wheel has a longer distance to roll than the inside wheel, which means that ithas to rotate more faster.

If there were no differential and the two wheels where fixed together and rotated at same speedwhen the vehicle turned, one wheel would have to slip to compensate for the difference in rollingdistance.


Final drive components

The final drive that is bolted to the axle housing consist of:

Pinion (1), is the input shaft of the rear axle assembly. It is supported by two taper roller bearingsin front and in a straight roller bearing at the rear.

The rear bearing keeps the pinion engaged against the crown wheel.

The crown wheel (2), the driving power is transferred from the pinion to the crown wheel, whichis fitted to the differential housing. Due to the fact that the pinion and crown wheel rotate at 90angle to each other, the driving power can be transferred to the driving wheels via the driveshafts.

The differential (3), is fitted in the differential housing and consist of differential gears, foursmaller gears fitted on a spider and 2 bigger gears, which in its turn are connected to the driveshafts.


Thrust bolt

The final drive has a thrust bolt which prevents the crown wheel from being pressed outwards(away from the pinion) under loading. Under normal operation, the thrust bolt is at distance fromthe crown wheel. It only makes contact with the crown wheel when pressed outwards inconnection with heavy loading.


Crown wheel and pinion set

The pinion is supported in two taper roller bearings in front (1) and by a straight roller bearing (2)at the rear.

For smooth and efficient operation, the crown wheel and pinion are matched duringmanufacture. The crown wheel and pinion are then marked with a number (3) to unsure alignedfit. They must always be installed together when assembling the final drive.


Crown wheel and pinion combinations.

The crown wheel and pinion work together to turn the torque through an angel of 90. Thefunction of this angel operation is to transfer the driving power from engine out to the driveshafts.

The crown wheel and pinion can be found in two different designs:

Spiral-bevel, helical teeth (1), the crown wheel and pinion are on same centreline.

Hypoid, helical teeth (2), the pinion centreline is below that of crown wheel.This allows that more teeth are in contact and larger torque could be transferred from pinion tocrown wheel. The hypoid design is usually used in Volvo finals drives.


Crown wheel and pinion relation.

The input speed from the propeller shaft is quite high but the speed is reduced by the final drive.This to make the wheels turn at a suitable speed.

The speed is reduced by the crown wheel (1) and the pinion (2) and at the same time increases thetorque to the wheels.

The ratio which the speed is reduced is as follows:

No. of teeth on crown wheel No of teeth on pinion

Example:

30 = reduction 3:1 10

If the crown wheel has 30 teeth and the pinion 10 teeth the pinion makes 3 turns for each crownwheel revolution.As a consequence this will make the torque to increase 3 times.


Differential.

The differential consist of:Satellite (smaller) gear (1) 4 pcs, journalled on the spider (3)Sun (larger) gear (2) 2 pcs, which run on two independent, drive shafts.


The drive shafts are the part of the power transfer system that is subjected to the greateststresses provided the vehicle is not equipped with hub reduction.

The drive shafts are made of forged hardened steel. They are so tough and elastic that then canbe twisted nearly a whole turn before breaking off.

The inside end of the shaft is equipped with splines to enable it to mesh with the sun gears (thelarge ones) and its outside end has a flange with holes for the wheel hub studs.

If the rear axle is fitted with hub reduction, the outside end of the shaft is equipped with splines(1) that fit into the planetary gear in the hub reduction unit.


Differential assembly operation

For a better understanding on how the differential adapts the speed, lets get to know its gearingsystem.

Each drive shaft is connected to one of the wheels by a flange in its outer end (1).

On the other end, the drive shaft is connected to the differential planetary gear (2).

Each drive shaft is connected to a planetary gear.


Function of the Differential

When the vehicle is running straight ahead, the smaller planetary gears (1) are stationary on thespider but are rotating with (2) the crown wheel.This makes the larger sun gears rotate (3) so that they both drive their drive shafts at the samespeed (4).


Function of the Differential

When turning the vehicle the inside wheel slows down (1), the smaller satellite gears (2) start torotate on the spider.

Thanks to this rotation the speed reduction is taken up from the inside wheel and transferred tothe outside wheel, which then speeds up (3).


It means that when a vehicle makes a turn, the inner wheel is subjected to a higher torque,which reduces the inner drive shaft speed slightly.

This causes the satellite gears to start working and turn on their own axis, which causes theother drive shaft to turn with a slightly faster speed.

This movement prevents the inner wheel to slipping when the vehicle makes a turn.


Differential lock, general information

Should one of the driving wheels slip, on an icy incline for instance, the differential can be put ofaction by locking both drive shafts together with a differential lock.With the differential lock activated both wheels rotates at same speed.

The differential lock should only be used when there is a risk of wheel slipping. If using it whendriving on hard ground, the axle components will be exposed to high stress and risk damagesthe components in the axle.


Differential lock, main component

The differential lock consists of a coupling sleeve (1) that is fastened on the right-hand side ofthe differential housing and a sliding coupling sleeve (2) for the right-hand drive shaft.The diaphragm (3) pushes on a sleeve (4) connected to a shift fork (5). The shift fork isconnected to the sliding coupling sleeve.

The differential lock is engaged by a switch (7) on the instrument panel, and switch (6) closesand light up the differential lock warning lamp in the instrument panel.


Differential lock, operation

To activate the differential lock, the driver just needs to press on the differential lock switch on theinstrument panel.The switch activates a solenoid valve, which applies compressed air to the diaphragm in therear axle. The diaphragm presses on the sleeve connected to the shift fork.The shift fork causes the sliding coupling sleeve to move against engagement with the couplingsleeve on the differential housing.

When the two coupling sleeves are engaged, the differential is forced to drive both drive shaftswith the same speed. When differential lock is engaged the differential lock warning lamp in theinstrument panel lights up and an alarm sound.


Hub reduction, general information

When the vehicle is used in extremely heavy-duty condition operation, constructions site workfor example, stresses on the drive shafts and differential increases.By reducing the ration of the driving power out to driving wheels even further (the first step is thefinal drive), stresses and consequently wear, can be reduced.

This downshift is carried out with the aid of a hub reduction unit (1) that consists of a planetarygear.


Hub reduction, main component

The hub reduction consists of a planetary gear system.

The planetary system consist of a sun gear (1), three or four planetary gears (depends on hubreduction application) and a ring gear (3).

When the sun gear, which is connected to the drive shaft, begins to rotate the rotation istransferred to the planetary gears, which are mounted in the wheel hub. When the planetarygears rotate against the ring gear, which is attached in rear axle housing, we will have reductionof ratio to the driving wheels.


Tandem drive

A heavy-duty truck is often equipped with a tandem drive, that is, two driven rear axles.

This results in less slip and enhanced grip. Tandem drive is most suited to operation thatrequires towing of heavy trailers and for construction-site duties.

The front-most driven axle is equipped with a transfer gear while the final drive in the rear-mostaxle is a standard single gear as described previously.


Differential with a transfer gearbox

On trucks with tandem drive, the front-most rear axle is equipped with a transfer gear, whichdistributes power between the two rear axles.Power from the gearbox passes via the transfer boxs input shaft (1), the transfer gearsdifferential (2) to the output shaft (3), which drive the rearmost axle via a short propeller shaft.The transfer gearbox as a gear wheel (4), which uses an additional gear (5) to transfer power tothe final drive (6) on the front-most rear axle.

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Book Rear