Advanced Vehicle Technology Episode 2 Part 3 potx

7.1.7 Final drive axle noise and defects Noise is produced with all types of meshing gear teeth such as from spur, straight or helical gears and even more so with bevel gears where the o

pinion teeth when the transmission overruns the

engine or the vehicle is being reversed

Crownwheel and pinion backlash The free

clear-ance between meshing teeth is known as backlash

7.1.6 Checking crownwheel and pinion tooth

contact

Prepare crownwheel for examining tooth contact

marks (Fig 7.8) After setting the correct

back-lash, the crownwheel and pinion tooth alignment

should be checked for optimum contact This may

be achieved by applying a marking cream such as

Prussian blue, red lead, chrome yellow, red or

yellow ochre etc to three evenly spaced groups of

about six teeth round the crownwheel on both drive

coast sides of the teeth profiles Apply a load to the

meshing gears by holding the crownwheel and

allowing it to slip round while the pinion is turned

a few revolutions in both directions to secure a

good impression around the crownwheel Examine

the tooth contact pattern and compare it to the

recommended impression

Understanding tooth contact marks (Fig 7.8(a±f))

If the crownwheel to pinion tooth contact pattern

is incorrect, there are two adjustments that can be made to change the position of tooth contact These adjustments are of backlash and pinion depth Theadjustmentofbacklashmovesthecontactpatch lengthwise back and forth between the toe heel of the tooth Moving the crownwheel nearer the pinion decreases the backlash, causing the contact patch to shift towards the toe portion of the tooth Increasing backlash requires the crownwheel to be moved side-ways and away from the pinion This moves the con-tact patch nearer the heel portion of the tooth When adjusting pinion depth, the contact patch moves up and down the face±flank profile of the tooth With insufficient pinion depth (pinion too far out from crownwheel) the contact patch will be concentrated at the top (face zone) of the tooth Conversely, too much pinion depth (pinion too near crownwheel) will move the contact patch to the lower root (flank zone) of the tooth

Ideal tooth contact (Fig 7.8(b)) The area of tooth contact should be evenly distributed over the working depth of the tooth profile and should be nearer to the toe than the heel of the crownwheel tooth The setting of the tooth contact is initially slightly away from the heel and nearer the root to compensate for any deflection of the bearings, Fig 7.7 Setting differential cage bearing preload using adjusting nuts

crownwheel, pinion and final drive housing under

operating load conditions, so that the pressure

con-tact area will tend to spread towards the heel

towards a more central position

Heavy face (high) tooth contact (Fig 7.8(c))

Tooth contact area is above the centre line and on

the face of the tooth profile due to the pinion being

too far away from the crownwheel (insufficient

pinion depth) To rectify this condition, move the

pinion deeper into mesh by using a thicker pinion

head washer to lower the contact area and reset the

backlash

Heavy flank (low) tooth contact (Fig 7.8(d))

Tooth contact area is below the centre line and on

the flank of the tooth profile due to the pinion

being too far in mesh with the crownwheel (too

much pinion depth) To rectify this condition,

move the pinion away from the crownwheel using

a thinner washer between the pinion head and inner

bearing cone to raise the contact area and then reset the backlash

Heavy toe contact (Fig 7.8(e)) Tooth contact area is concentrated at the small end of the tooth (near the toe) To rectify this misalignment, increase backlash by moving the crownwheel and differential assembly away from the pinion, by transferring shims from the crownwheel side of the differential assembly to the opposite side, or slacken the adjusting nut on the crownwheel side

of the differential and screw in the nut on the opposite side an equal amount If the backlash is increased above the maximum specified, use a thicker washer (shim) behind the pinion head in order to keep the backlash within the correct limits Heavy heel contact (Fig 7.8(f)) Tooth contact area is concentrated at the large end of the tooth which is near the heel To rectify this misalignment, decrease backlash by moving the crownwheel nearer Fig 7.8 (a±e) Crownwheel tooth contact markings

the pinion (add shims to the crownwheel side of the

differential and remove an equal thickness of shims

from the opposite side) or slacken the differential

side adjusting nut and tighten the crownwheel side

nut an equal amount If the backlash is reduced

below the minimum specified, use a thinner washer

(shim) behind the pinion head

7.1.7 Final drive axle noise and defects

Noise is produced with all types of meshing gear

teeth such as from spur, straight or helical gears

and even more so with bevel gears where the output

is redirected at right angles to the input drive

Vehicle noises coming from tyres, transmission,

propellor shafts, universal joints and front or rear

wheel bearings are often mistaken for axle noise,

especially tyre to road surface rumbles which can

sound very similar to abnormal axle noise

Listen-ing for the noise at varyListen-ing speeds and road

surfaces, on drive and overrun conditions will assist

in locating the source of any abnormal sound

Once all other causes of noise have been

elimin-ated, axle noise may be suspected The source of

axle noise can be divided into gear teeth noises and

bearing noise

Gear noise Gear noise may be divided into two

kinds:

1 Broken, bent or forcibly damaged gear teeth which

produce an abnormal audible sound which is easily

recognised over the whole speed range

a) Broken or damaged teeth may be due to

abnormally high shock loading causing

sud-den tooth failure

b) Extended overloading of both crownwheel

and pinion teeth can be responsible for

even-tual fatigue failure

c) Gear teeth scoring may eventually lead to

tooth profile damage The causes of surface

scoring can be due to the following:

i) Insufficient lubrication or incorrect grade

of oil

ii) Insufficient care whilst running in a new

final drive

iii) Insufficient crownwheel and pinion

back-lash

iv) Distorted differential housing

v) Crownwheel and pinion misalignment

vi) Loose pinion nut removing the pinion

bearing preload

2 Incorrect meshing of crownwheel and pinion

teeth Abnormal noises produced by poorly

meshed teeth generate a very pronounced cyclic pitch whine in the speed range at which it occurs whilst the vehicle is operating on either drive or overrun conditions

Noise on drive If a harsh cyclic pitch noise is heard when the engine is driving the transmission

it indicates that the pinion needs to be moved slightly out of mesh

Noise on overrun If a pronounced humming noise

is heard when the vehicle's transmission overruns the engine, this indicates that the pinion needs to be moved further into mesh

Slackness in the drive A pronounced time lag in taking the drive up accompanied by a knock when either accelerating or decelerating may be traced

to end play in the pinion assembly due possibly to defective bearings or incorrectly set up bearing spacer and shim pack

Bearing noise Bearings which are defective pro-duce a rough growling sound that is approximately constant in volume over a narrow speed range Driving the vehicle on a smooth road and listening for rough transmission sounds is the best method

of identifying bearing failure

A distinction between defective pinion bearings

or differential cage bearings can be made by listen-ing for any constant rough sound A fast frequency growl indicates a failed pinion bearing, while a much slower repetition growl points to a defective differential bearing The difference in sound is because the pinion revolves at about four times the speed of the differential assembly

To distinguish between differential bearing and half shaft bearing defects, drive the vehicle on a smooth road and turn the steering sharply right and left If the half shaft bearings are at fault, the increased axle load imposed on the bearing will cause a rise in the noise level, conversely if there is

no change in the abnormal rough sound the differ-ential bearings should be suspect

Defective differential planet and sun gears The sun and planet gears of the differential unit very rarely develop faults When differential failure does occur, it is usually caused by shock loading, extended overloading and seizure of the differential planet gears to the cross-shaft resulting from exces-sive wheel spin and consequently lubrication breakdown

A roughness in the final drive transmission when

the vehicle is cornering may indicate defective

planet/sun gears

7.2 Differential locks

A differential lock is desirable, and in some cases

essential, if the vehicle is going to operate on low

traction surfaces such as sand, mud, wet or

water-logged ground, worn slippery roads, ice bound

roads etc at relatively low speeds

Drive axle differential locks are incorporated on

heavy duty on/off highway and cross-country

vehi-cles to provide a positive drive between axle half

shafts when poor tyre to ground traction on one

wheel would produce wheel spin through

differen-tial bevel gear action

The differential lock has to be engaged manually

by cable or compressed air, whereas the limited

slip or viscous coupling differential automatically

operates as conditions demand

All differential locks are designed to lock

together two or more parts of the differential gear

cluster by engaging adjacent sets of dog clutch

teeth By this method, all available power

trans-mitted to the final drive will be supplied to the

wheels Even if one wheel loses grip, the opposite

wheel will still receive power enabling it to produce

torque and therefore tractive effect up to the limit

of the tyres' ability to grip the road Axle wind-up will

be dissipated by wheel bounce, slippage or scuffing

These unwanted reactions will occur when travelling over slippery soft or rough ground where true rolling will be difficult Since the tyre tread cannot exactly follow the contour of the surface it is rolling over, for very brief periodic intervals there will be very little tyre to ground adhesion As a result, any build up

of torsional strain between the half shafts will be continuously released

7.2.1 Differential lockmechanism (Figs 7.9 and 7.10)

One example of a differential lock is shown in Fig 7.9 In this layout a hardened and toughened flanged side toothed dog clutch member is clamped and secured by dowls between the crownwheel and differential cage flanges The other dog clutch member is comprised of a sleeve internally splined

to slot over the extended splines on one half shaft This sleeve has dog teeth cut at one end and the double flange formed at the end to provide a guide groove for the actuating fork arm

Engagement of the differential lock is obtained when the sleeve sliding on the extended external splines of the half shaft is pushed in to mesh with corresponding dog teeth formed on the flanged member mounted on the crownwheel and cage Locking one half shaft to the differential cage pre-vents the bevel gears from revolving independently within the cage Therefore, the half shafts and cage

Fig 7.9 Differential lock mechanism

will be compelled to revolve with the final drive

crownwheel as one The lock should be applied

when the vehicle is just in motion to enable the

tooth to align, but not so fast as to cause the

crash-ing of misaligned teeth The engagement of the lock

can be by cable, vacuum or compressed air,

depend-ing on the type of vehicle usdepend-ing the facility An

alternative differential lock arrangement is shown

in Fig 7.10 where the lock is actuated by

com-pressed air operating on an annulus shaped piston

positioned over one half shaft When air pressure is

supplied to the cylinder, the piston is pushed

out-wards so that the sliding dog clutch member teeth

engage the fixed dog clutch member teeth, thereby

locking out the differential gear action

When the differential lock is engaged, the vehicle

should not be driven fast on good road surfaces to

prevent excessive tyre scrub and wear With no

dif-ferential action, relative speed differences between

inner and outer drive wheels can only partially be compensated by the tyre tread having sufficient time

to distort and give way in the form of minute hops

or by permitting the tread to skid or bounce while rolling in slippery or rough ground conditions 7.3 Skid reducing differentials

7.3.1 Salisbury Powr-Loklimited slip differential (Fig 7.11)

This type of limited slip differential is produced under licence from the American Thornton Axle Co

The Powr-Lok limited slip differential essentially consists of an ordinary bevel gear differential arranged so that the torque from the engine engages friction clutches locking the half shafts to the differential cage The larger the torque, the greater the locking effect (Fig 7.11)

Fig 7.10 Differential lock mechanism with air control

Fig 7.11 Multiclutch limited slip differential

There are three stages of friction clutch loading:

1 Belleville spring action,

2 Bevel gear separating force action,

3 Vee slot wedging action

Belleville spring action (Fig 7.11) This is achieved

by having one of the clutch plates dished to form a

Belleville spring so that there is always some spring

axial loading in the clutch plates This then produces

a small amount of friction which tends to lock the

half shaft to the differential cage when the torque

transmitted is very low The spring thus ensures that

when adhesion is so low that hardly any torque can

be transmitted, some drive will still be applied to the

wheel which is not spinning

Bevel gear separating force action (Fig 7.11) This

arises from the tendency of the bevel planet pinions

in the differential cage to force the bevel sun gears

outwards Each bevel sun gear forms part of a hub

which is internally splined to the half shaft so that it

is free to move outwards The sun gear hub is also

splined externally to align with one set of clutch

plates, the other set being attached by splines to the

differential cage Thus the extra outward force

exerted by the bevel pinions when one wheel tends

to spin is transmitted via cup thrust plates to the

clutches, causing both sets of plates to be camped

together and thereby preventing relative movement

between the half shaft and cage

Vee slot wedging action (Fig 7.11(a and b)) When

the torque is increased still further, a third stage of

friction clutch loading comes into being The bevel

pinions are not mounted directly in the differential

cage but rotate on two separate arms which cross at

right angles and are cranked to avoid each other

The ends of these arms are machined to the shape of

a vee wedge and are located in vee-shaped slots in

the differential cage With engine torque applied, the

drag reaction of the bevel planet pinion cross-pin

arms relative to the cage will force them to slide

inwards along the ramps framed by the vee-shaped

slots in the direction of the wedge (Fig 7.11(a and b))

The abutment shoulder of the bevel planet pinions

press against the cup thrust plates and each set of

clutch plates are therefore squeezed further together,

increasing the multiclutch locking effect

Speed differential and traction control (Fig 7.12)

Normal differential speed adjustment takes place

continuously, provided the friction of the multi-plate clutches can be overcome When one wheel spins the traction of the other wheel is increased by

an amount equal to the friction torque generated

by the clutch plates until wheel traction is restored

A comparison of a conventional differential and

a limited slip differential tractive effort response against varying tyre to road adhesion is shown in Fig 7.12

7.3.2 Torsen worm and wheel differential

Differential construction (Figs 7.13 and 7.14) The Torsen differential has a pair of worm gears, the left hand half shaft is splined to one of these worm gears while the right hand half shaft is splined to the other hand (Fig 7.13) Meshing with each worm gear on each side is a pair of worm wheels (for large units triple worm wheels on each side) At both ends of each worm wheel are spur gears which mesh with adjacent spur gears so that both worm gear and half shafts are indirectly coupled together Normally with a worm gear and worm wheel combination the worm wheel is larger than the worm gear, but with the Torsen system the worm gear is made larger than the worm wheel The important feature of any worm gear and worm wheel is that the teeth are cut at a helix angle such that the worm gear can turn the worm wheel but the worm wheel cannot rotate the worm gear This is achieved with the Torsen differential by giving the Fig 7.12 Comparison of tractive effort and tyre to road adhesion for both conventional and limited slip differential

worm gear teeth a fine pitch while the worm wheel

has a coarse pitch

Note that with the conventional meshing spur

gear, be it straight or helical teeth, the input and

output drivers can be applied to either gear The

reversibility and irreversibility of the conventional

bevel gear differential and the worm and worm

wheel differential is illustrated in Fig 7.14 by the

high and low mechanical efficiencies of the two

types of differential

Differential action when moving straight ahead

(Fig 7.15) When the vehicle is moving straight

ahead power is transferred from the propellor shaft

to the bevel pinion and crownwheel The

crown-wheel and differential cage therefore revolve as one

unit (Fig 7.15) Power is divided between the left

and right hand worm wheel by way of the spur gear

pins which are attached to the differential cage It

then flows to the pair of meshing worm gears, where

it finally passes to each splined half shaft Under

these conditions, the drive in terms of speed and

torque is proportioned equally to both half shafts

and road wheels Note that there is no relative

rotary motion between the half shafts and the

differ-ential cage so that they all revolve as a single unit

Differential action when cornering (Fig 7.15) When

cornering, the outside wheel of the driven axle will

tend to rotate faster than the inside wheel due to its turning circle being larger than that of the inside wheel It follows that the outside wheel will have to rotate relatively faster than the differential cage, say

by ‡20 rev/min, and conversely the inside wheel has

to reduce its speed in the same proportion, of say

20 rev/min

Fig 7.13 Pictorial view of Torsen worm and spur gear differential

Fig 7.14 Comparison of internal friction expressed in terms of mechanical efficiency of both bevel pinion type and worm and spur type differentials

When there is a difference in speed between the

two half shafts, the faster turning half shaft via the

splined worm gears drives its worm wheels about

their axes (pins) in one direction of rotation The

corresponding slower turning half shaft on the

other side drives its worm wheels about their axes

(pins) in the opposite direction but at the same

speed (Fig 7.15)

Since the worm wheels on opposite sides will be

revolving at the same speed but in the opposite sense

while the vehicle is cornering they can be simply

interlinked by pairs of meshing spur gears without

interfering with the independent road speed

require-ments for both inner and outer driving road wheels

Differential torque distribution (Fig 7.15) When

one wheel loses traction and attempts to spin, it

transmits drive from its set of worm gears to the worm wheels The drive is then transferred from the worm wheels on the spinning side to the opposite (good traction wheel) side worm wheels

by way of the bridging spur gears (Fig 7.15) At this point the engaging teeth of the worm wheel with the corresponding worm gear teeth jam Thus the wheel which has lost its traction locks

up the gear mechanism on the other side every time there is a tendency for it to spin As a result

of the low traction wheel being prevented from spinning, the transmission of torque from the engine will be concentrated on the wheel which has traction

Another feature of this mechanism is that speed differentiation between both road wheels is main-tained even when the wheel traction differs con-siderably between wheels

Fig 7.15 Sectioned views of Torsen worm and spur gear differential

7.3.3 Viscous coupling differential

Description of differential and viscous coupling

(Figs 7.16 and 7.17) The crownwheel is bolted to

the differential bevel gearing and multiplate

hous-ing Speed differentiation is achieved in the normal

manner by a pair of bevel sun (side) gears, each

splined to a half shaft Bridging these two bevel sun

gears are a pair of bevel planet pinions supported

on a cross-pin mounted on the housing cage

A multiplate back assembly is situated around

the left hand half shaft slightly outboard from the

corresponding sun gear (Fig 7.16)

The viscous coupling consists of a series of

spaced interleaved multiplates which are

alterna-tively splined to a half shaft hub and the outer

differential cage The cage plates have pierced

holes but the hub plates have radial slots Both

sets of plates are separated from each other by a

0.25 mm gap Thus the free gap between adjacent

plates and the interruption of their surface areas

with slots and holes ensures there is an adequate

storage of fluid between plates after the sealed plate

unit has been filled and that the necessary

progres-sive viscous fluid torque characteristics will be

obtained when relative movement of the plates

takes place

When one set of plates rotate relative to the

other, the fluid will be sheared between each pair

of adjacent plate faces and in so doing will generate

an opposing torque The magnitude of this

resist-ing torque will be proportional to the fluid viscosity and the relative speed difference between the sets of plates The dilatent silicon compound fluid which has been developed for this type of application has the ability to maintain a constant level of viscosity throughout the operating temperature range and life expectancy of the coupling (Fig 7.17)

Fig 7.16 Viscous coupling differential

Fig 7.17 Comparison of torque transmitted to wheel having the greater adhesion with respect to speed difference between half shafts for both limited slip and viscous coupling