Advanced Vehicle Technology Episode 3 Part 5 ppt

11.19 When the foot brake is applied generated hydraulic pressure pushes the piston and inboard pad against their adjacent disc face.. 11.21 The assembled disc brake caliper unit compris

one end on it supporting carrier bracket The disc is

driven by the transmission drive shaft hub on

which it is mounted and the lining pads are

posi-tioned and supported on either side of the disc by

the rectangular aperature in the yoke frame

Operation (Fig 11.19) When the foot brake is

applied generated hydraulic pressure pushes the

piston and inboard pad against their adjacent disc

face Simultaneously, the hydraulic reaction will

move the cylinder in the opposite direction away

from the disc Consequently, as the outboard pad

and cylinder body are bridged by the yoke, the

latter will pivot, forcing the outboard pad against

the opposite disc face to that of the inboard pad

As the pads wear, the yoke will move through an

arc about its pivot, and to compensate for this tilt

the lining pads are taper shaped During the wear

life of the pad friction material, the amount of

taper gradually reduces so that in a fully worn

state the remaining friction material is

approxi-mately parallel to the steel backing plate

The operating clearance between the pads and

disc is maintained roughly constant by the inherent

distortional stretch and retraction of the pressure

seals as the hydraulic pressure is increased and

reduced respectively, which accordingly moves the piston forwards and back

11.4.2 Sliding yoke type brake caliper (Fig 11.20)

With this type of caliper unit, the cylinder body is rigidly attached to the suspension hub carrier, whereas the yoke steel pressing fits over the cylin-der body and is permitted to slide between parallel grooves formed in the cylinder casting

Operation (Fig 11.20) When the foot brake is applied, hydraulic pressure is generated between the two pistons The hydraulic pressure pushes the piston apart, the direct piston forces the direct pad against the disc whilst the indirect piston forces the yoke to slide in the cylinder in the opposite direction until the indirect pad contacts the out-standing disc face

Further pressure build-up causes an equal but opposing force to sandwich the disc between the friction pads This pressure increase continues until the desired retardation force is achieved

During the pressure increase the pressure seals dis-tort as the pistons move apart When the hydraulic pressure collapses the rubber pressure seals retract

Fig 11.19 Swing yoke type brake caliper

and withdraw the pistons and pads from the disc

surface so that friction pad drag is eliminated

Yoke rattle between the cylinder and yoke frame

is reduced to a minimum by inserting either a wire

or leaf type spring between the sliding joints

11.4.3 Sliding pin type brake caliper (Fig 11.21)

The assembled disc brake caliper unit comprises

the following; a disc, a carrier bracket, a cylinder

caliper bridge, piston and seals, friction pads and

a pair of support guide pins

The carrier bracket is bolted onto the suspension

hub carrier, its function being to support the

cylin-der caliper bridge and to absorb the brake torque

reaction

The cylinder caliper bridge is mounted on a pair

of guide pins sliding in matched holes machined in

the carrier bracket The guide pins are sealed

against dirt and moisture by dust covers so that

equal frictional sliding loads will be maintained at

all times On some models a rubber bush sleeve is

fitted to one of the guide pins to prevent noise and

to take up brake deflection

Frictional drag of the pads is not taken by the

guide pins, but is absorbed by the carrier bracket

Therefore the pins only support and guide the

caliper cylinder bridge

As with all other types of caliper units, pad to disc free clearance is obtained by the pressure seals which are fitted inside recesses in the cylinder wall and grip the piston when hydraulic pressure forces the piston outwards, causing the seal to distort When the brakes are released and the pressure is removed from the piston crown, the strain energy

of the elastic rubber pulls back the piston until the pressure seal has been restored to its original shape

Operation (Fig 11.21) When the foot brake is applied, the hydraulic pressure generated pushes the piston and cylinder apart Accordingly the inboard pad moves up to the inner disc face, whereas the cylinder and bridge react in the oppo-site sense by sliding the guide pins out from their supporting holes until the outboard pad touches the outside disc face Further generated hydraulic pressure will impose equal but opposing forces against the disc faces via the pads

11.4.4 Sliding cylinder body type brake caliper (Fig 11.22)

This type of caliper unit consists of a carrier bracket bolted to the suspension hub carrier and

a single piston cylinder bridge caliper which straddles Fig 11.20 Sliding yoke type brake caliper

the disc and is allowed to slide laterally on guide

keys positioned in wedge-shaped grooves machined

in the carrier bracket

Operation (Fig 11.22) When the foot brake is

applied, the generated hydraulic pressure enters

the cylinder, pushing the piston with the direct

acting pad onto the inside disc face The cylinder

body caliper bridge is pushed in the opposite

direc-tion As a result, the caliper bridge reacts and slides

in its guide groove at right angles to the disc until

the indirect pad contacts the outside disc face,

thereby equalling the forces acting on both sides

of the disc

A pad to disc face working clearance is provided

when the brakes are released by the retraction of

the pressure seal, drawing the piston a small

amount back into the cylinder after the hydraulic

pressure collapes

To avoid vibration and noise caused by relative

movement between the bridge caliper and carrier

bracket sliding joint, anti-rattle springs are

nor-mally incorporated alongside each of the

two-edge-shaped grooves

11.4.5 Twin floating piston caliper disc brake with hand brake mechanism (Fig 11.23) This disc brake unit has a pair of opposing pistons housed in each split caliper The inboard half-caliper is mounted on a flanged suspension hub carrier, whereas the other half straddles the disc and is secured to the rotating wheel hub Lining pads bonded to steel plates are inserted on each side of the disc between the pistons and disc rub-bing face and are held in position by a pair of steel pins and clips which span the two half-calipers Brake fluid is prevented from escaping between the pistons and cylinder walls by rubber pressure seals which also serve as piston retraction springs, while dirt and moisture are kept out by flexible rubber dust covers

Foot brake application (Fig 11.23) Hydraulic pressure, generated when the foot brake is applied,

is transferred from the inlet port to the central half-caliper joint, where it is then transmitted along passages to the rear of each piston

As each piston moves forward to take the clear-ance between the lining pads and disc, the piston Fig 11.21 Slide pin type brake caliper

Fig 11.22 Slide cylinder body brake caliper

Fig 11.23 Twin floating piston caliper disc brake with hand brake mechanism

pressure seals are distorted Further pressure

build-up then applies an equal but opposite force by way

of the lining pads to both faces of the disc, thereby

creating a frictional retarding drag to the rotating

disc Should the disc be slightly off-centre, the

pis-tons will compensate by moving laterally relative to

the rubbing faces of the disc

Releasing the brakes causes the hydraulic pressure

to collapse so that the elasticity within the distorted

rubber pressure seals retracts the pistons and pads

until the seals convert to their original shape

The large surface area which is swept on each

side of the disc by the lining pads is exposed to

the cooling airstream so that heat dissipation is

maximized

Hand brake application (Fig 11.23) The hand

brake mechanism has a long and short clamping

lever fitted with friction pads on either side of the

disc and pivots from the lower part of the caliper A

tie rod with an adjusting nut links the two clamping

levers and, via an operating lever, provides the

means to clamp the disc between the friction

pads Applying the hand brake pulls the operating

lever outwards via the hand brake cable, causing

the tie rod to pull the short clamp lever and pad

towards the adjacent disc face, whilst the long

clamp and pad is pushed in the opposite direction

against the other disc face As a result, the lining

pads grip the disc with sufficient force to prevent

the car wheels rolling on relatively steep slopes

To compensate for pad wear, the adjustment nut

should be tightened periodically to give a maximum

pad to disc clearance of 0.1 mm

11.4.6 Combined foot and hand brake caliper

with automatic screw adjustment (Bendix)

This unit provides automatic adjustment for the

freeplay in the caliper's hand brake mechanism

caused by pad wear It therefore keeps the hand

brake travel constant during the service life of the

pads

The adjustment mechanism consists of a

shoul-dered nut which is screwed onto a coarsely

threaded shaft Surrounding the nut on one side

of the shoulder or flange is a coiled spring which is

anchored at its outer end via a hole in the piston

On the other side of the shouldered nut is a ball

bearing thrust race The whole assembly is enclosed

in the hollow piston and is prevented from moving

out by a thrust washer which reacts against the

thrust bearing and is secured by a circlip to the

interior of the piston

Foot brake application (Fig 11.24(a)) When the hydraulic brakes are applied, the piston outward movement is approximately equal to the predeter-mined clearance between the piston and nut with the brakes off, but as the pads wear, the piston takes up a new position further outwards, so that the normal piston to nut clearance is exceeded

If there is very little pad wear, hydraulic pressure will move the piston forward until the pads grip the disc without the thrust washer touching the ball race However, as the pads wear, the piston moves forward until the thrust washer contacts the ball race Further outward movement of the piston then forces the thrust washer ball race and shouldered nut together in an outward direction Since the threaded shaft is prevented from rotating

by the strut and cam, the only way the nut can move forward is by unwinding on the screw shaft Immediately the nut attempts to turn, the coil spring uncoils and loses its grip on the nut, permit-ting the nut to screw out in proportion to the piston movement

On releasing the foot brake, the collapse of the hydraulic pressure enables the pressure seals to withdraw the pads from the disc Because the axial load has been removed from the nut, there is

no tendency for it to rotate and the coil spring therefore contracts, gripping the nut so that it can-not rotate Note that the outward movement of the nut relative to the threaded shaft takes up part of the slack in the mechanical linkage so that the hand brake lever movement remains approximately con-stant throughout the life of the pads The threaded shaft and nut device does not influence the operat-ing pad to disc clearance when the hydraulic brakes are applied as this is controlled only by the pressure seal distortion and elasticity

Hand brake application (Fig 11.24(b)) Applying the hand brake causes the cable to rotate the cam-shaft via the cam lever, which in turn transfers force from the cam to the threaded shaft through the strut The first part of the screwed shaft travel takes up the piston to nut end-clearance With further screw shaft movement the piston is pushed outwards until the pad on the piston contacts the adjacent disc face At the same time an equal and opposite reaction causes the caliper cylinder to move in the opposite direction until the outside pad and disc face touch Any further outward movement of the threaded shaft subsequently clamps the disc in between the pads Releasing the hand brake lever relaxes the pad grip on the disc

with the assistance of the Belleville washers which

draws back the threaded shaft to the `off' position

to avoid the pads binding on the disc

11.5 Dual- or split-line braking systems

Dual- or split-line braking systems are used on all

cars and vans to continue to provide some degree

of braking if one of the two hydraulic circuits

should fail A tandem master cylinder is

incor-porated in the dual-line braking system, which is

in effect two separate master cylinder circuits

placed together end on so that it can be operated

by a common push rod and foot pedal Thus, if

there is a fault in one of the hydraulic circuits, the

other pipe line will be unaffected and therefore will

still actuate the caliper or drum brake cylinders it

supplies

11.5.1 Front to rear brake line split (Fig 11.25(a))

With this arrangement, the two separate hydraulic pipe lines of the tandem master cylinder are in circuit with either both the front or rear caliper or shoe expander cylinders The weakness with this pipe line split is that roughly two-thirds of the braking power is designed to be absorbed by the front calipers, and only one-third by the rear brakes Therefore if the front brakes malfunction, the rear brake can provide only one-third of the original braking capacity

11.5.2 Diagonally front to rear brake split (Fig 11.25(b))

To enable the braking effort to be more equally shared between each hydraulic circuit (if a fault should occur in one of these lines), the one front Fig 11.24 (a and b) Combined foot and hand brake caliper with automatic screw adjustment

and one diagonally opposite rear wheel are

con-nected together Each hydraulic circuit therefore

has the same amount of braking capacity and the

ratio of front to rear braking proportions do not

influence the ability to stop A diagonal split also

tends to retard a vehicle on a relatively straight line

on a dry road

11.5.3 Triangular front to rear brake split

(Fig 11.25(b))

This hydraulic pipe line system uses front calipers

which have two independent pairs of cylinders,

and at the rear conventional calipers or drum

brakes Each fluid pipe line circuit supplies half

of each front caliper and one rear caliper or

drum brake cylinder Thus a leakage in one or

the other hydraulic circuits will cause the other

three pairs of calipers or cylinders or two pairs of

caliper cylinders and one rear drum brake

cylin-der to provide braking equal to about 80% of

that which is possible when both circuits are

operating When one circuit is defective, braking

is provided on three wheels; it is then known as

a triangular split

11.5.4 Compensating port type tandem master cylinder (Fig 11.26(a±d))

Tandem master cylinders are employed to operate dual-line hydraulic braking systems The master cylinder is composed of a pair of pistons function-ing within a sfunction-ingle cylinder This enables two inde-pendent hydraulic cylinder chambers to operate Consequently, if one of these cylinder chambers

or part of its hydraulic circuit develops a fault, the other cylinder chamber and circuit will still continue to effectively operate

Brakes off (Fig 11.26(a)) With brakes in the `off' position, both primary and secondary pistons are pushed outwards by the return springs to their respective stops Under these conditions fluid is permitted to circulate between the pressure cham-bers and the respective piston recesses via the small compensating port, reservoir supply outlet and the large feed ports for both primary and secondary brake circuits

Brakes applied (Fig 11.26(b)) When the foot pedal is depressed, the primary piston moves inwards and, at the same time, compresses both the intermediate and secondary return springs so that the secondary piston is pushed towards the cylinder's blanked end

Initial movement of both pistons causes their respective recuperating seals to sweep past each compensating port Fluid is trapped and, with increased piston travel, is pressurized in both the primary and secondary chambers and their pipe line circuits, supplying the front and rear brake cylinders During the braking phase, fluid from the reservoir gravitates and fills both of the annular piston recesses

Brakes released (Fig 11.26(a)) When the foot pedal effort is removed, the return springs rapidly expand, pushing both pistons outwards The speed

at which the swept volume of the pressure cham-bers increases will be greater than the rate at which the fluid returns from the brake cylinders and pipe lines Therefore a vacuum is created within both primary and secondary pressure chambers

As a result of the vacuum created, each recuper-ating seal momentarily collapses Fluid from the annular piston recess is then able to flow through the horizontal holes in the piston head, around the inwardly distorted recuperating seals and into their respective pressure chambers This extra fluid Fig 11.25 (a±c) Dual- or split-line braking systems

entering both pressure chambers compensates for

any fluid loss within the brake pipe line circuits or

for excessive shoe to drum clearance But, if too

much fluid is induced in the chambers, some of this

fluid will pass back to the reservoir via the

com-pensating ports after the return springs have fully

retracted both pistons

Failure in the primary circuit (Fig 11.26(c))

Should a failure (leakage) occur in the primary

circuit, there will be no hydraulic pressure

gener-ated in the primary chamber When the brake pedal

is depressed, the push rod and primary piston will

move inwards until the primary piston abuts the secondary spring retainer Further pedal effort will move the secondary piston recuperating seal beyond the compensating port, thereby pressuriz-ing the fluid in the secondary chamber and subse-quently transmitting this pressure to the secondary circuit pipe line and the respective brake cylinders

Failure in the secondary circuit (Fig 11.26(d)) If there is a failure (leakage) in the secondary circuit, the push rod will move the primary piston inwards until its recuperating seal sweeps past the com-pensating port, thus trapping the existing fluid Fig 11.26 (a±d) Tandem master cylinder

in the primary chamber Further pedal effort

increases the pressure in the primary chamber and

at the same time both pistons, separated by the

primary chamber fluid, move inwards unopposed

until the secondary piston end stop contacts the

cylinder's blanked end Any more increase in

brak-ing effort raises the primary chamber pressure,

which accordingly pressurizes the primary circuit

brake cylinders

The consequence of a failure in the primary or

secondary brake circuit is that the effective push

rod travel increases and a greater pedal effort will

need to be applied for a given vehicle retardation

compared to a braking system which has both

primary and secondary circuits operating

11.5.5 Mecanindus (roll) pin type tandem

master cylinder incorporating a pressure

differential warning actuator (Fig 11.27(a±d))

The tandem or split master cylinder is designed to

provide two separate hydraulic cylinder pressure

chambers operated by a single input push rod

Each cylinder chamber is able to generate its own

fluid pressure which is delivered to two

indepen-dent brake pipe line circuits Thus if one hydraulic

circuit malfunctions, the other one is unaffected

and will provide braking to the wheel cylinders

forming part of its system

Operation of tandem master cylinder

Brakes off (Fig 11.27(a)) With the push rod fully

withdrawn, both primary and secondary pistons

are forced outwards by the return springs This

outward movement continues until the central

poppet valve stems contact their respective

Mecanindus (roll) pins With further withdrawal

the poppet valves start opening until the front end

of each elongated slot also contacts their respective

roll pins, at which point the valves are fully open

With both valves open, fluid is free to flow between

the primary and secondary chambers and their

respective reservoirs via the elongated slot and

vertical passage in the roll pins

Brakes applied (Fig 11.27(b)) When the brake

pedal is applied, the push rod and the primary

return spring pushes both pistons towards the

cylinder's blank end Immediately both

recuperat-ing poppet valves are able to snap closed The fluid

trapped in both primary and secondary chambers

is then squeezed, causing the pressure in the

primary and secondary pipe line circuits to rise and operate the brake cylinders

Brakes released (Fig 11.27(a)) Removing the foot from the brake pedal permits the return spring

to push both pistons to their outermost position The poppet valve stem instantly contacts their respective roll pins, causing both valves to open Since the return springs rapidly push back their pistons, the volume increase in both the primary and secondary chambers exceeds the speed of the returning fluid from the much smaller pipe line bore, with the result that a depression is created

in both chambers Fluid from the reservoir flows via the elongated slot and open poppet valve into the primary and secondary chambers to compen-sate for any loss of fluid or excessive shoe to drum

or pad to disc clearance This method of transfer-ring fluid from the reservoir to the pressure cham-ber is more dynamic than relying on the collapse and distortion of the rubber pressure seals as in the conventional master cylinder

Within a very short time the depression dis-appears and fluid is allowed to flow freely to and fro from the pressure chambers to compensate for fluid losses or fluid expansion and contraction caused by large temperature changes

11.5.6 Operation of the pressure differential warning actuator

As a warning to the driver that there is a fault in either the primary or secondary hydraulic braking circuits of a dual-line braking system, a pressure differential warning actuator is usually incorpo-rated as an integral part of the master cylinder or

it may be installed as a separate unit (Fig 11.27) The switch unit consists of a pair of opposing balance pistons spring loaded at either end so that they are normally centrally positioned Mounted centrally and protruding at right angles into the cylinder is an electrical conducting prod, insulated from the housing with a terminal formed at its outer end The terminal is connected to a dash-board warning light and the electrical circuit is completed by the earth return made by the master cylinder

Operation (Fig 11.27(b)) If, when braking, both hydraulic circuits operate correctly, the opposing fluid pressure imposed on the outer ends of the balance piston will maintain the pistons in their equilibrium central position

Fig 11.27 (a±d) Tandem master cylinder with pressure differential warning actuator