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Student Workbook

LV13 Final Drive Systems (1)

kap all covers 6/9/03 9:49 am Page 25

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and repair

MODULE LV13 FINAL DRIVE SYSTEMS (1)

Contents

Front engine rear wheel drive 3

Front engine front wheel drive 4 Front and rear wheel drive 18

Operating Principles of Final Drive Systems

Rear axle assembly

A vehicle’s final drive system is, in effect, two assemblies combined – the final drive gear set and the differential

Front engine rear wheel drive arrangement

The final drive gear’s primary function is to further multiply torque received

from the gearbox and direct this to the differential The differential distributes this torque to the driven wheels, but also allows a difference in wheel speed to occur under certain conditions

Front engine front wheel drive arrangement

So, it can be seen from this that the final drive differential assembly performs two distinct tasks and should therefore be referred to using the full name,

rather than just ‘diff’ i.e final drive and differential

FINAL DRIVE RING GEAR

FINAL DRIVE PINION GEAR OUTPUT SHAFT

Final drive ring gear Output shaft

INPUT SHAFT

Input shaft

Final drive pinion gear

As the final drive pinion receives its torque from the main gearbox ratios via a shaft, it compounds the selected gear ratio

The ring gear then transfers this drive torque to the differential unit

Further details on gear trains can be found in (Manual Transmission Systems LV14)

Final Drives

There are two distinct types of final drive – bevel gear and helical gear The type used is dictated by the vehicle’s drive configuration A front engine rear wheel drive vehicle will use a bevel gear type (as long as the engine is

longitutinally mounted)

The drive from the gearbox runs from front to rear on the vehicle, so there is a requirement to change the drive direction through 90 degrees out to the driven wheels A bevel gear assembly lends itself to this task perfectly

If the vehicle is a front engine front wheel drive or perhaps rear/ mid engine rear wheel drive, it is most likely that a helical gear set will be used as there is

no requirement to achieve a drive direction change (transversely mounted

Gear sets suitable for the afore mentioned drive arrangements have a number

of variations

The spiral bevel gear assembly is a standard bevel gear arrangement Its

teeth have been cut with a pronouced helix (spiral shape) This arrangement increases the area of tooth contact between the pinion and the ring gear

(sometimes referred to as a crown wheel), therefore increasing its ability to

transfer torque without the risk of damage or excessive wear The rolling

action of the teeth also reduces noise considerably

The hypoid spiral bevel gear assembly is now used almost universally if the drive layout dictates the need for drive directional change It is identical to the spiral bevel with one exception – the pinion gear is lowered to a point beneath the centre-line of the ring gear This further increases tooth contact area, with all of the advantages that we can now associate with this

In both instances, the final drive gear set compounds the selected gear ratio

to further increase torque (and reduce driven speed) All drive from the

gearbox has to pass through the final drive assembly, so it can be seen that every gear ratio available will be affected by the final drive ratio Motor sport teams often exploit this fact to bring about a quick and relatively easy change

of ratios throughout the vehicle’s transmission system It suits various

conditions such as:

High speed tracks – smaller final drive ratio for increased top speed

Low speed tracks – higher final drive ratio for faster acceleration

Helical gear

A helical gear is one with teeth that have been cut at an angle Again, this

has the effect of increasing tooth contact area and reducing noise

Unfortunately, it produces end-thrust as a result of this angular cut (the gears try to force themeselves out of mesh in the axial direction) This end-thrust

has to be contained by the gearbox casing and if the torque is considerable, this casing will have to be very strong It will, therefore, be heavy A point is often reached where the torque (and end-thrust associated with this) is so

large that straight cut gears (spur gears) have to be used

These are very noisy, but the kind of torque figures that we are talking about are normally found only in the motorsport arena and on load carrying vehicles where noise isn’t such an issue!

Progress check 1

Answer the following questions:

1 Calculate the gear ratio of the final drive assembly shown here

Include all working out

2 Calculate the compound gear ratio for the selected gear shown below

(first gear compounded with the final drive) Include all working out

FINAL DRIVE RING GEAR

FINAL DRIVE PINION GEAR

OUTPUT SHAFT

INPUT SHAFT

FINAL DRIVE RING GEAR

FINAL DRIVE PINION GEAR

Differential

The requirement

When cornering, the inside wheels rotate at a slower speed than the outside wheels This is because the inside wheels ‘cut the corner’ and therefore have less distance to travel All wheels are, of course, attached to the vehicle

Therefore, in order to keep up with the vehicle, the outside wheels will have to turn at an increased speed because of the difference in distance Try this

analogy Imagine you are in a line of people ice skating, who all join hands

and rotate around the first person, who is used as a pivot The further

towards the end of the line of people you are, the faster you have to go, even though you are all part of the same overall body of people

With non-driven wheels, the difference in speed is not a problem as these

wheels are independent of each other However, with driven wheels this is

more of a problem as they are connected together courtesy of the drive shaft and final drive assembly The differential allows for this difference in speed with no loss of drive It should be noted that differences in rotational speed

have to be acommodated in instances other than cornering Uneven tyre

wear/ tyre pressures across the driven axle will result in differences in rolling radii; imperfect road surfaces and road camber – these will cause differences

in rotational speed

All wheel drive vehicles (4WD)

All wheel drive vehicles require a differential assembly for both axles as both are driven We no longer have a ‘dead axle’ that is able to absorb differences

in rotational speed In addition to this requirement, a third differential is

required to allow for a difference in speed between the two axles This

differential is often referred to as the third differential but is also known as the centre differential

The centre differential relays the drive to the transfer gear set, where the

primary role is to turn the drive through 90 degrees in order for that drive to be sent to the rear of the vehicle

The diagram above shows a part-time four wheel drive (4WD) vehicle These

vehicles provide a 4WD option when the vehicle is to be used off-road As

these conditions would suggest that the vehicle is on slippy ground, a centre differential is seldom provided as any difference in rotational speed can be

absorbed through natural slippage of the driven wheels These vehicles

should not be driven on hard standing (such as metalled roads) with 4WD

engaged

The tyres will struggle to slip with the large amount of grip and will result in

transmission ‘wind-up’ Wind-up is a condition where the transmission shafts absorb the difference in rotational speed at the road wheels by twisting, this can damage transmission components and also make the vehicle very difficult

to steer

On a point of safety, these vehicles should never be jacked up without first

checking that the 4WD is disengaged Wind-up will cause the first wheel that

is lifted clear of the ground to ‘whip’ violently

The principle

The diagram shows the principle of differential action The central gear is

meshed to two gear racks These racks are free to slide up and down in their respective runners The diagram to the left shows the racks with equal mass acting upon them As the central gear is pulled up, the upward motion is

transferred directly and evenly to each rack Each rack moves a distance

equal to each other and the central gear The diagram on the right shows the racks with unequal mass acting upon them The rack on the left has a far

greater mass acting upon it When the shackle is now pulled, the central gear will rotate to enable the rack on the right to lift further (and more quickly) than the rack on the left The course of least resistance is taken

A vehicle’s differential works in a very similar fashion As the vehicle corners, the resistance to rotation of the inside wheel increases (the equivalent of a

greater mass acting on it) and the resistance to rotation on the outside wheel reduces (the equivalent of less mass acting on it) The differential takes the course of least resistance and sends more of the drive (speed) to the outside wheel The drive split is proportionate The speed of the outer wheel will

increase by an amount equal to the reduction of speed on the inside wheel

Application

The diagram above shows a final drive differential assembly as used on a

vehicle with front engine rear wheel drive It shows this assembly in a

condition of ‘no difference in rotational speed’ Drive flow through the

assembly will be as follows: Drive from the gearbox is received via the

propshaft onto the drive pinion The drive pinion transfers this drive to the ring gear (crown wheel) Mounted directly onto the ring gear is the differential

case (cage) As the ring gear rotates, so does the differential case The

differential drive pin is mounted directly into the differential case and rotates with it, taking around the differential pinion gears

The differential side gears are directly meshed to these pinions and therefore rotate with them The side gears are splined to the drive shafts / half shafts and therefore the drive is sent directly to the road wheels There is equal

resistance to rotation at both of these wheels, so the differential pinions will

rotate with the differential case but not about their own axis (the drive pin)

This shows the differential compensating for a difference in required driven

wheel speed The flow of drive is identical up to the differential drive pin The differential pinions rotate with this drive pin, but as the differential finds it

difficult to rotate the left hand wheel (in this example) due to the larger

resistance experienced during a left hand corner manoeuvre, the differential pinions now start to rotate about their own axis In the process the speed of the right hand wheel increases proportionately

It should be noted that differentials have no gear ratio as such; only the final drive has this If you turn a driven wheel by hand with the vehicle on a wheel-free lift, you will often find that the other wheel turns in the opposite direction!

If you follow the drive through the assembly, as it would be if you had turned one wheel, you will see why!

Progress check 2

Answer the following question:

1 Complete the labels by naming the parts correctly on the diagram

below:

Centre differential – 4WD vehicles

The centre differential assembly as found on permanent four-wheel drive

vehicles

Features:

• construction is simple

• reliability is high

The diagram above shows a simple bevel gear arrangement Its action is

identical to that of a conventional differential, but the flow of torque is different: torque is input to the assembly via a side gear and this drive is received from the front differential Drive to the rear differential is output from the centre

differential via the other side gear Any required difference in axle speed is

then proportionate via the rotating action of the centre differential pinion gears

Bearings

Rear Wheel Drive

Front Wheel Drive

The bearings that are used to support the major rotating components in a final

aper roller bearing

taper roller bearing i tanding massive radial loads (as are

they are severely loaded in all directions!

drive differential assembly are taper roller bearings Their location, for rear

wheel drive and front wheel drive vehicles respectively, can be seen in the

figures above and their design on the page 18

T

all bearings) but their tapered design enables them to withstand huge axial

loadings, also This is critical for every final drive differential assembly, as

Seals

keep the lubricant in and keep moisture and dirt out It is important to note

eir ability to seal increases This is because of the tapered design The

is

f seal can be referred to as a dynamic seal, as it is designed to

eal directly against a moving component as opposed to static seals, which do

L e commonly used in final drive systems Their pr

to

that these seals are asymmetric and must be fitted the correct way around The seals are designed so that as the pressure acting upon them increases, th

increased pressure forces the main sealing lip of the seal more firmly into

contact with the moving surface If fitted the wrong way around, this effect

Lubrication

Front and rear wheel drive

Filler / level plug

Seals

Lubrication of the final drive differential assembly, as fitted to a front wheel

drive vehicle, is through splash (non-forced) via the transaxle Most

manufacturers adopt what is known as a unitary system (the gearbox casing and the final drive (differential casing are open to each other internally) In

this instance, there will be a single filler / level plug as the oil is common to

both assemblies This level should be checked periodically and the oil

changed periodically according to the manufacturer’s guidelines

Four wheel drive (4WD)

Most manufacturers use forced (pressure fed via a pump) lubrication systems for their permanent 4WD vehicles It can be seen that the oil pump receives its drive from the final drive ring gear

Therefore, pressure is only produced when the vehicle is in motion This is

quite deliberate To run the pump-off assemblies that rotate while in neutral would suggest a pointless waste of hard earned mechanical energy and would also increase wear

The pump supplies oil under pressure to all of the key components via oil

drillings, including the transfer drive, front and centre differentials

Progress check 3

Answer the following questions:

1 What is the purpose of a differential on a two wheel drive vehicle?

2 List the tasks performed by a final drive on a rear wheel drive vehicle:

3 What is the purpose of a centre differential as fitted to a 4WD vehicle?