Advanced Vehicle Technology Episode 1 Part 3 ppt

Smoothness of clutch engagement may be achieved by building into the driven plate some sort of cushioning device, which will be discussed later in the chapter, whilst rapid slowing down

the tractor unit, just sufficiently back to clear the

rear tractor road wheels when the trailer is coupled

and the combination is being manoeuvred

(Fig 1.28(a)) To provide additional support for

the legs, bracing stays are attached between the legs

and from the legs diagonally to the chassis

cross-member (Fig 1.28(b))

The legs consist of inner and outer high tensile

steel tubes of square section A jackscrew with a

bevel wheel attached at its top end supported by the

outer leg horizontal plate in a bronze bush bearing

The jawscrew fits into a nut which is mounted at

the top of the inner leg and a taper roller bearing

race is placed underneath the outer leg horizontal

support plate and the upper part of the jackscrew

to minimize friction when the screw is rotated (Fig

1.28(b)) The bottom ends of the inner legs may

support either twin wheels, which enable the trailer

to be manoeuvred, or simply flat feet The latter are

able to spread the load and so permit greater load

capacity

To extend or retract the inner legs, a winding

handle is attached to either the low or high speed

shaft protruding from the side of the gearbox The

upper high speed shaft supports a bevel pinion

which meshes with a vertically mounted bevel

wheel forming part of the jackscrew

Rotating the upper shaft imparts motion directly

to the jackscrew through the bevel gears If greater

leverage is required to raise or lower the front of the

trailer, the lower shaft is engaged and rotated

This provides a gear reduction through a

com-pound gear train to the upper shaft which then

drives the bevel pinion and wheel and hence the

jackscrew

1.6 Automatic chassis lubrication system 1.6.1 The need for automatic lubrication system (Fig 1.29)

Owing to the heavy loads they carry commercial vehicles still prefer to use metal to metal joints which are externally lubricated Such joints are kingpins and bushes, shackle pins and bushes, steering ball joints, fifth wheel coupling, parking brake linkage etc (Fig 1.29) These joints require lubricating in proportion to the amount of relative movement and the loads exerted If lubrication is to be effective in reducing wear between the moving parts, fresh oil must be pumped between the joints frequently This can best be achieved by incorporating an automatic lubrication system which pumps oil to the bearing's surfaces in accordance to the distance travelled by the vehicle

1.6.2 Description of airdromic automatic chassis lubrication system (Fig 1.30)

This lubrication system comprises four major com-ponents; a combined pump assembly, a power unit,

an oil unloader valve and an air control unit

Pumpassembly (Fig 1.30) The pump assembly consists of a circular housing containing a ratchet operated drive (cam) shaft upon which are mounted one, two or three single lobe cams (only one cam shown) Each cam operates a row of 20 pumping units disposed radially around the pump casing, the units being connected to the chassis bearings by nylon tubing

Power unit (Fig 1.30) This unit comprises a cylinder and spring-loaded air operated piston which is mounted on the front face of the pump assembly housing, the piston rod being connected indirectly to the drive shaft ratchet wheel by way of

a ratchet housing and pawl

Oil unloader valve (Fig 1.30) This consists of a shuttle valve mounted on the front of the pump assembly housing The oil unloader valve allows air pressure to flow to the power unit for the power stroke During the exhaust stroke, however, when air flow is reversed and the shuttle valve is lifted from its seat, any oil in the line between the power unit and the oil unloader valve is then discharged to atmosphere

Fig 1.27 Ball and socket caravan/trailer towing

attachment

Fig 1.28 (a and b) Semi-trailer landing gear

Air control unit (Fig 1.30) This unit is mounted

on the gearbox and is driven via the speedometer

take-off point It consists of a worm and wheel drive

which operates an air proportioning control

unit This air proportioning unit is operated by a

single lift face cam which actuates two poppet

valves, one controlling air supply to the power

unit, the other controlling the exhaust air from the

power unit

1.6.3 Operation of airdromic automatic chassis

lubrication system (Fig 1.30)

Air from the air brake auxiliary reservoir passes by

way of the safety valve to the air control

(propor-tioning) unit inlet valve Whilst the inlet valve is

held open by the continuously rotating face cam

lobe, air pressure is supplied via the oil unloader

valve to the power unit attached to the multipump

assembly housing The power unit cylinder is

sup-ported by a pivot to the pump assembly casing,

whilst the piston is linked to the ratchet and pawl

housing Because the pawl meshes with one of the ratchet teeth and the ratchet wheel forms part of the camshaft, air pressure in the power cylinder will partially rotate both the ratchet and pawl housing and the camshaft clockwise The cam (or cams) are

in contact with one or more pump unit, and so each partial rotation contributes to a proportion of the jerk plunger and barrel pumping cycle of each unit (Fig 1.30)

As the control unit face cam continues to rotate, the inlet poppet inlet valve is closed and the exhaust poppet valve opens Compressed air in the air con-trol unit and above the oil concon-trol shuttle valve will now escape through the air control unit exhaust port to the atmosphere Consequently the com-pressed air underneath the oil unloader shuttle valve will be able to lift it and any trapped air and oil in the power cylinder will now be released via the hole under the exhaust port The power unit piston will be returned to its innermost position by the spring and in doing so will rotate the ratchet and pawl housing anti-clockwise The pawl is thus Fig 1.29 Tractor unit automatic lubrication system

Fig 1.30 Airdromic automatic chassis lubrication system

able to slip over one or more of the ratchet teeth to

take up a new position The net result of the power

cylinder being charged and discharged with

com-pressed air is a slow but progressive rotation of the

camshaft (Fig 1.30)

A typical worm drive shaft to distance travelled

relationship is 500 revolutions per 1 km For 900

worm drive shaft revolutions the pumping cam

revolves once Therefore, every chassis lubrication

point will receive one shot of lubricant in this

distance

When the individual lubrication pump unit's primary plunger is in its outermost position, oil surrounding the barrel will enter the inlet port, filling the space between the two plungers As the cam rotates and the lobe lifts the primary plunger,

it cuts off the inlet port Further plunger rise will partially push out the secondary plunger and so open the check valve Pressurised oil will then pass between the loose fitting secondary plunger and barrel to lubricate the chassis moving part it services (Fig 1.30)

2 Friction clutch

2.1 Clutch fundamentals

Clutches are designed to engage and disengage the

transmission system from the engine when a vehicle

is being driven away from a standstill and when the

gearbox gear changes are necessary The gradual

increase in the transfer of engine torque to the

transmission must be smooth Once the vehicle is

in motion, separation and take-up of the drive for

gear selection must be carried out rapidly without

any fierceness, snatch or shock

2.1.1 Driven plate inertia

To enable the clutch to be operated effectively, the

driven plate must be as light as possible so that

when the clutch is disengaged, it will have the

mini-mum of spin, i.e very little flywheel effect Spin

prevention is of the utmost importance if the

vari-ous pairs of dog teeth of the gearbox gears, be they

constant mesh or synchromesh, are to align in the

shortest time without causing excessive pressure,

wear and noise between the initial chamfer of the

dog teeth during the engagement phase

Smoothness of clutch engagement may be

achieved by building into the driven plate some

sort of cushioning device, which will be discussed

later in the chapter, whilst rapid slowing down of

the driven plate is obtained by keeping the diameter,

centre of gravity and weight of the driven plate to

the minimum for a given torque carrying capacity

2.1.2 Driven plate transmitted torque capacity

The torque capacity of a friction clutch can be

raised by increasing the coefficient of friction of

the rubbing materials, the diameter and/or the

spring thrust sandwiching the driven plate The

friction lining materials now available limit the

coefficient of friction to something of the order of

0.35 There are materials which have higher

coeffi-cient of friction values, but these tend to be

unstable and to snatch during take-up Increasing

the diameter of the driven plate unfortunately

raises its inertia, its tendency to continue spinning

when the driven plate is freed while the clutch is in

the disengaged position, and there is also a limit to

the clamping pressure to which the friction lining

material may be subjected if it is to maintain its

friction properties over a long period of time

2.1.3 Multi-pairs of rubbing surfaces (Fig 2.1)

An alternative approach to raising the transmitted torque capacity of the clutch is to increase the number of pairs of rubbing surfaces Theoretically the torque capacity of a clutch is directly propor-tional to the number of pairs of surfaces for a given clamping load Thus the conventional single driven plate has two pairs of friction faces so that a twin

or triple driven plate clutch for the same spring thrust would ideally have twice or three times the torque transmitting capacity respectively of that of the single driven plate unit (Fig 2.1) However, because it is very difficult to dissipate the extra heat generated in a clutch unit, a larger safety factor

is necessary per driven plate so that the torque capacity is generally only of the order 80% per pair

of surfaces relative to the single driven plate clutch 2.1.4 Driven plate wear (Fig 2.1)

Lining life is also improved by increasing the number of pairs of rubbing surfaces because wear

is directly related to the energy dissipation per unit area of contact surface Ideally, by doubling the surface area as in a twin plate clutch, the energy input per unit lining area will be halved for a given slip time which would result in a 50% decrease in facing wear In practice, however, this rarely occurs (Fig 2.1) as the wear rate is also greatly influenced

by the peak surface rubbing temperature and the intermediate plate of a twin plate clutch operates at

a higher working temperature than either the fly-wheel or pressure plate which can be more effect-ively cooled Thus in a twin plate clutch, half the energy generated whilst slipping must be absorbed

by the intermediate plate and only a quarter each

by the flywheel and pressure plate This is usually borne out by the appearance of the intermediate plate and its corresponding lining faces showing evidence of high temperatures and increased wear compared to the linings facing the flywheel and pressure plate Nevertheless, multiplate clutches

do have a life expectancy which is more or less related to the number of pairs of friction faces for

a given diameter of clutch

For heavy duty applications such as those required for large trucks, twin driven plates are used, while for high performance cars where very

rapid gear changes are necessary and large

amounts of power are to be developed, small

diameter multiplate clutches are preferred

2.2 Angular driven plate cushioning and torsional

damping (Figs 2.2±2.8)

2.2.1 Axial driven plate friction lining cushioning

(Figs 2.2, 2.3 and 2.4)

In its simplest form the driven plate consists of

a central splined hub Mounted on this hub is a

thin steel disc which in turn supports, by means of

a ring of rivets, both halves of the annular friction

linings (Figs 2.2 and 2.3)

Axial cushioning between the friction lining

faces may be achieved by forming a series of evenly

spaced `T' slots around the outer rim of the disc

This then divides the rim into a number of

seg-ments (Arcuate) (Fig 2.4(a)) A horseshoe shape

is further punched out of each segment The central

portion or blade of each horseshoe is given a

per-manent set to one side and consecutive segments

have opposite sets so that every second segment is

riveted to the same friction lining The alternative

set of these central blades formed by the horseshoe

punch-out spreads the two half friction linings apart

An improved version usesseparately attached, very

thin spring steel segments (borglite) (Fig 2.4(b)),

pos-itioned end-on around a slightly thicker disc plate

These segments are provided with a wavy `set' so as

to distance the two half annular friction linings

Both forms of crimped spring steel segments

situated between the friction linings provide

Fig 2.1 Relationship of torque capacity wear rate and pairs of rubbing faces for multiplate clutch

Fig 2.2 Clutch driven centre plate (pictorial view)

progressive take-up over a greater pedal travel and

prevent snatch The separately attached spring

segments are thinner than the segments formed out

of the single piece driven plate, so that the squeeze

take-up is generally softer and the spin inertia of the

thinner segments is noticeably reduced

A further benefit created by the spring segments

ensures satisfactory bedding of the facing material

and a more even distribution of the work load In

addition, cooling between the friction linings occurs

when the clutch is disengaged which helps to

sta-bilise the frictional properties of the face material

The advantages of axial cushioning of the face

linings provide the following:

a) Better clutch engagement control, allowing

lower engine speeds to be used at take-up thus

prolonging the life of the friction faces

b) Improved distribution of the friction work over

the lining faces reduces peak operating

tempera-tures and prevents lining fade, with the resulting

reduction in coefficient of friction and

subse-quent clutch slip

The spring take-up characteristics of the driven plate are such that when the clutch is initially engaged, the segments are progressively flattened so that the rate of increase in clamping load is provided

by the rate of reaction offered by the spring segments (Fig 2.5) This first low rate take-up period is followed by a second high rate engage-ment, caused by the effects of the pressure plate springs exerting their clamping thrust as they are allowed to expand against the pressure plate and

so sandwich the friction lining between the flywheel and pressure plate faces

2.2.2 Torsional damping of driven plate Crankshaft torsional vibration (Fig 2.6) Engine crankshafts are subjected to torsional wind-up and vibration at certain speeds due to the power impulses Superimposed onto some steady mean rotational speed of the crankshaft will be additional fluctuating torques which will accelerate and decel-erate the crankshaft, particularly at the front pulley Fig 2.3 Clutch driven centre plate (sectional view)

Fig 2.4 (a and b) Driven plate cushion take-up

end and to a lesser extent the rear flywheel end

(Fig 2.6) If the flywheel end of the crankshaft

were allowed to twist in one direction and then the

other while rotating at certain critical speeds, the

oscillating angular movements would take up the

backlash between meshing gear teeth in the

transmis-sion system Consequently, the teeth of the driving

gears would be moving between the drive (pressure

side) and non-drive tooth profiles of the driven gears

This would result in repeated shockloads imposed on

the gear teeth, wear, and noise in the form of gear clatter To overcome the effects of crankshaft torsional vibrations a torsion damping device is normally incorporated within the driven plate hub assembly which will now be described and explained Construction and operation of torsional damper springs (Figs 2.2, 2.3 and 2.7) To transmit torque more smoothly and progressively during take-up of normal driving and to reduce torsional oscillations being transmitted from the crankshaft to the trans-mission, compressed springs are generally arranged circumferentially around the hub of the driven plate (Figs 2.2 and 2.3) These springs are inserted

in elongated slots formed in both the flange of the splined hub and the side plates which enclose the hub's flange (Fig 2.3) These side plates are riveted together by either three or six rivet posts which pass through the flanged hub limit slots This thus provides a degree of relative angular movement between hub and side plates The ends of the helical coil springs bear against both central hub flange and the side plates Engine torque is therefore transmitted from the friction face linings and side plates through the springs to the hub flange, so that any fluctuation of torque will cause the springs to compress and rebound accordingly

Multistage driven plate torsional spring dampers may be incorporated by using a range of different springs having various stiffnesses and spring loca-tion slots of different lengths to produce a variety

of parabolic torsional load±deflection characteris-tics (Fig 2.7) to suit specific vehicle applications The amount of torsional deflection necessary varies for each particular application For example, with a front mounted engine and rear wheel drive vehicle, a moderate driven plate angular movement

is necessary, say six degrees, since the normal trans-mission elastic wind-up is almost adequate, but with

an integral engine, gearbox and final drive arrange-ment, the short transmission drive length necessit-ates considerably more relative angular deflection, say twelve degrees, within the driven plate hub assembly to produce the same quality of take-up Construction and operation of torsional damper washers (Figs 2.2, 2.3 and 2.8) The torsional energy created by the oscillating crankshaft is partially absorbed and damped by the friction washer clutch situated on either side of the hub flange (Figs 2.2 and 2.3) Axial damping load is achieved by a Belleville dished washer spring mounted between one of the side plates and a four lug thrust washer

Fig 2.5 Characteristics of driven plate axial clamping

load to deflection take-up

Fig 2.6 Characteristics of crankshaft torsional

vibrations undamped and damped

The outer diameter of this dished spring presses

against the side plate and the inner diameter pushes

onto the lugged thrust washer In its free state

the Belleville spring is conical in shape but when

assembled it is compressed almost flat As the

fric-tion washers wear, the dished spring cone angle

increases This exerts a greater axial thrust, but

since the distance between the side plate and lugged

thrust washer has increased, the resultant clamping

thrust remains almost constant (Fig 2.8)

2.3 Clutch friction materials

Clutch friction linings or buttons are subjected to

severe rubbing and generation of heat for relatively

short periods Therefore it is desirable that they

have a combination of these properties:

a) Relatively high coefficient of friction under

operating conditions,

b) capability of maintaining friction properties

over its working life,

c) relatively high energy absorption capacity for

short periods,

d) capability of withstanding high pressure plate

compressive loads,

e) capability of withstanding bursts of centrifugal

force when gear changing,

f) adequate shear strength to transmit engine

torque,

g) high level of cyclic working endurance without

the deterioration in friction properties,

h) good compatibility with cast iron facings over

the normal operating temperature range,

i) a high degree of interface contamination toler-ance without affecting its friction take-up and grip characteristics

2.3.1 Asbestos-based linings (Figs 2.2 and 2.3) Generally, clutch driven plate asbestos-based lin-ings are of the woven variety These woven linlin-ings are made from asbestos fibre spun around lengths

of brass or zinc wire to make lengths of threads which are both heat resistant and strong The woven cloth can be processed in one of two ways: a) The fibre wire thread is woven into a cloth and pressed out into discs of the required diameter, followed by stitching several of these discs together to obtain the desired thickness The resultant disc is then dipped into resin to bond the woven asbestos threads together

b) The asbestos fibre wire is woven in three dimen-sions in the form of a disc to obtain in a single stage the desired thickness It is then pressed into shape and bonded together by again dip-ping it into a resin solution Finally, the rigid lining is machined and drilled ready for riveting

to the driven plate

Development in weaving techniques has, in certain cases, eliminated the use of wire coring so that asbestos woven lining may be offered as either non- or semi-metallic to match a variety of working conditions

Asbestos is a condensate produced by the solidi-fication of rock masses which cool at differential Fig 2.7 Characteristics of driven plate torsional spring

torques to deflection take-up Fig 2.8 Characteristics of driven plate torsionaldamper thrust spring