28 Air and endurance brake systems

28.1 PRINCIPLES OF AIR BRAKES Advantages of air brakes

General background For medium to heavy commercial vehicles, compressed air
braking systems, or air brakes as they are generally known,
It was as long ago as 1868 that the American engineer offer the following advantages:
George Westinghouse first patented his invention for an
automatic compressed air brake for railway trains, which 1 As an operating medium, air costs nothing and is always
soon proved superior to other types of braking system available.
including his own earlier development of a vacuum- 2 The system will tolerate a certain amount of air leakage
operated brake. Although compressed air brakes for railway without failing completely.
trains were soon adopted in America and many other 3 Large operating forces to expand the brake shoes can
countries, another one hundred years passed before they readily be generated.
appeared to any extent on railway trains in Britain. The 4 A supply of compressed air is a convenient source of
change from vacuum to compressed air braking in fact energy to operate various ancillary equipment on the
coincided with the demise of the steam engine and the vehicle.
introduction of the diesel. With a steam engine the vacuum
was created by an ejector, wherein the action of a jet of Basic installation of air brakes
steam was made to exhaust air from the pipes, cylinders Commercial vehicle air brake installations constitute a true
and reser- voirs of the system. Therefore with a diesel power as distinct from power-assisted braking systems. This
engine either a vacuum pump was necessary, or it could be is because there is no direct connection in any mechanical
just as economi- cal to fit a pressure pump for compressed or hydraulic sense between the brake treadle and the wheel
air braking, with its greater power. brake chambers, although the driver is provided with a cer-
Rather interestingly a somewhat similar pattern of tain degree of feel related to system air pressure during
progress occurred with heavy vehicles on the road brak- ing. The functions of an air brake system are to
(although much more rapidly, of course) when the petrol compress, store, meter and deliver a volume of air under
engine was super- seded by the diesel. With the petrol- pressure to the wheel brake actuating chambers.
engined vehicle the once popular vacuum-operated system In simple terms, the operation of a basic single-circuit air
utilized the depression existing in the intake manifold, but brake system (Figure 28.1) is such that when the brake
since the diesel engine could not offer this facility it treadle is depressed one of two related control valves is
similarly became advantageous to mount an engine-driven opened, so that air under pressure from the reservoir can
compressor, rather than an exhauster, and install pass through the control valve and into each wheel brake
compressed air brakes, again with their greater power. actuating chamber. Here the compressed air acts against a
Both railway and heavy-vehicle engineers have therefore diaphragm, its resulting movement being transmitted via a
considered it preferable to utilize a source of compressed push-rod to either the oper- ating lever of the brake
air, generally at 700 kN/m2 (100 lbf/in2) or more, rather than camshaft, or the wedge of a brake expander unit, which
a source of vacuum that must always be less than forces the shoes against the brake drum. As the brake treadle
atmospheric pressure, or to put it another way a negative is released, the previously mentioned con- trol valve closes
pressure no greater than 105 kN/m2 (15 lbf/in2). More and the other one is opened, thereby allowing the air under
specifically, the higher operating pressures that are made pressure in the brake actuating chambers to be exhausted to
possible with com- pressed air brakes allow a reduction in the atmosphere and the shoe return springs to release the
size of the system components, accompanied by quicker brakes. In the event of the system air pressure falling below
application and release characteristics. a safe working minimum, either a warning light or a buzzer
More recent years have seen the operation of heavy vehi- is automatically activated in the driving compartment.
cles where both gross weight and speed capability are much The components of an air brake system are most conve-
greater than ever before. This in turn has led to an niently considered under the following headings:
increasing degree of sophisticated engineering in air brake
systems, not least to meet the safety directives of the EC Compression and storage
(European Community). Indeed, it may be said that air System control
brake technol- ogy has virtually become a separate branch System actuation.
of motor vehicle engineering.
591
 Warning
signal
Battery

Filter Driver
Air Pressure effort
in regulator Low pressure
Safety indicator switch
valve valve
To ancillaries

Foot
Reservoir control
valve
Compressor Unloader Check
valve valve Drain tap

Brake actuators

Brake camshaft
Exhaust air operating levers

Figure 28.1 Layout of a basic single-circuit air brake system

28.2 COMPRESSION AND STORAGE

Air compressor
This generates a supply of compressed air to operate the
braking system and also the vehicle ancillary services. In
modern installations it is usually mounted on, and either
belt or gear driven from the engine. It takes the form of a
recip- rocating piston pump that may have a single or twin
in-line cylinders. The cylinder head is provided with inlet
and deliv- ery valves of the spring-loaded disc type, which
are arranged to open in opposite directions. Of rugged
construction, the main body of the air compressor is
typically cast from high- grade iron. An air-cooled
(a)
compressor can be recognized by cooling fins integrally
cast into the cylinder block and head, (b)
while a water-cooled compressor is jacketed and has a Figure 28.2 Operating principle of air compressor: (a) intake
smooth exterior with connections to the engine cooling stroke (b) delivery stroke
system. The main and big-end bearings of the compressor
receive oil under pressure from the engine lubrication
system, the small-end bearing and cylinder wall being
splash lubricated. Oil return and breathing is into the engine cylinder, until it becomes sufficient to overcome the spring
sump. loading acting on the delivery valve (Figure 28.2b). Air is
In operation, the compressor receives a clean supply of then discharged under pressure through this valve for
air either via the engine intake manifold or from a separate delivery to a storage reservoir. As the piston commences its
filter mounted on the engine cylinder block. During the next down- stroke, and simultaneous with the reopening of
down-stroke of the piston a partial vacuum or depression is the inlet valve, the increasing depression in the cylinder
created in the cylinder space above the piston, so that allows the delivery valve to close under the influence of its
atmospheric pressure acting above the inlet valve forces it spring loading, thus preventing the compressed air from
to open against its spring loading and allows air to enter the returning to the cylinder.
cylinder (Figure 28.2a). As the piston commences its up-
stroke, the increasing pressure of air in the cylinder allows Air reservoirs
the inlet valve to close under the influ- ence of its spring Several reservoirs are necessary to serve the separate main
loading. The continued movement of the pis- ton on its up- service, emergency secondary and, where applicable, trailer
stroke then further increases the air pressure in the
brake circuits, which comprise the modern comprehensive
air brake system. Their purpose is to provide storage cap- under pressure from the compressor, so that it is carried by
acity of compressed air at the required pressure for immedi- the air stream to the rest of the system. Its purpose is to
ate effective braking and other ancillary services, and also lower the freezing point of any water vapour entrained in
to maintain the pressure supply after the engine has the com- pressed air supply, which reduces the risk of brake
stopped. The total air storage capacity of the reservoirs is misbe- haviour through moisture freezing in the system
subject to legislative requirements. It is decided on such during low-temperature operation of the vehicle. An alcohol
factors as the air delivery rate of the compressor, the size injector essentially comprises a simple spring-returned
and number of the wheel brake actuating chambers, the plunger pump with an alcohol reservoir. The injection
frequency of brake application as related to vehicle type process is accom- plished by depression of the plunger over
and operation, and the further demands of the ancillary its working stroke, which occurs whenever the reservoir
services. air pressure signals the governor to unload the compressor,
Since they act as pressure vessels, air reservoir tanks are since air from the unloader line is also directed above the
designed with a large factor of safety and constructed from enlarged head of the plunger. A non-return valve at the
sheet steel with welded seams. Each reservoir is provided outlet of the injector isol- ates the entire unit from system
with either a plug or a drain valve at its lowest point, so that air pressure when the pump is not being activated.
any condensation or sediment that accumulates may be Provision is made to lock the plunger out of action when
regu- larly drained off. This is necessary because, the injector is no longer required after winter use, and, of
depending on humidity, the air entering the compressor course, the reservoir is then allowed to remain empty.
contains a certain amount of water vapour, and as it passes Mineralized methylated spirit or ethyl alcohol are used in
through the com- pressor it can also acquire oil mist from the injector unit, the latter generally being pre- ferred. In
cylinder wall lubri- cation. The emulsion of water and oil either case it should be recognized that these sub- stances
can have adverse effects on air brake components, while are highly flammable and give off toxic fumes.
any freezing of the water content in could weather could
seriously affect the operation of the braking system. Governor valve
Two related items of protective equipment that may be
included in the air brake compression and storage system Also known as an air limiter valve, this is mounted in a
are the air dryer and alcohol injector. An air dryer is fitted return line between the compressor and reservoirs to
in the compressor air discharge line to the reservoirs and is maintain air storage pressure between normal operating
mounted external to the engine compartment, such that the limits; typical cut- out and cut-in pressures for a governor
motion of the vehicle induces a cooling air flow over the valve are 735 kN/m2 (105 lbf/in2) and 630 kN/m2 (90 lbf/in2)
dryer body. Although the main purposes of an air dryer will respectively. The gov- ernor valve exerts this control by
be self-evident, it is also intended to exclude oil droplets, directing air at cut-out pres- sure to an unloader plunger in
carbon particles and any other contaminants from the the compressor head, which holds the inlet valve off its seat
compressed air before it enters the system reservoirs, so so that no further air compres- sion is possible in the
that the provi- sion of cool, clean and dry air for the control cylinder. Otherwise, the compressor would pump air
and actuation systems will increase both service life and continuously to the reservoirs regardless of operating
operating effi- ciency of their components. An air dryer is a pressure requirements. When the compressor is ren- dered
basically simple unit and operates in three stages that inactive in this way it is said to run light.
involve cooling, filter- ing and drying. The heated air The governor valve is intended to be progressive in its
entering from the compressor is first cooled by contact with operation, which depends upon the opposing forces created
the dryer body interior and deposits water and oil by a control or pressure setting spring and air storage
condensate into a lower sump. The cooled air then passes pressure acting against either a sliding piston or a flexible
through an oil filter to remove any remaining liquid droplets diaphragm. We take the latter construction as an example.
or solid contaminants, before enter- ing a desiccant The diaphragm is clamped at its outer edges by the valve
chamber. This contains a cartridge of micro- crystalline body and at its centre by the head of a hollow valve
pellets that possess a strong affinity for water and present plunger, above which acts the control spring. A
an exceedingly large surface area to the air flowing over combination inlet and exhaust disc valve is initially spring
them. By this means the water vapour still in the air is loaded into contact with the lower tip of the valve plunger.
adsorbed by the desiccator, the air being progressively In operation, a supply of air at storage pressure is
dried before finally entering an upper purge chamber from returned to the governor valve and enters a chamber
whence it flows, via an outlet check valve, to the reservoirs. beneath the diaphragm. As the pressure increases against the
Dried air remaining in the purge chamber is utilized to diaphragm it flexes to lift the valve plunger and compress
expel sediment from the sump of the dryer and to remove the control spring. Continued lifting of the valve plunger
adsorbed moisture from the desiccant pellets. This process will first allow the inlet and exhaust valve to cover the
occurs whenever the reservoir air pressure signals the exhaust passage seat, following which the tip of the valve
governor to unload the com- pressor, because air from the plunger will lose contact with the valve altogether. When
unloader line is then allowed to open a purge valve in the this happens the compressed air acting beneath the
base of the dryer, which causes a sudden decompression diaphragm passes through the hollow valve plunger and is
and reverse flow of air through the unit. directed to the unloader plunger in the compressor (Figure
The function of an alcohol injector is to deliver a positive 28.3). Simultaneous with this action, the air pressure exerts
and metered quantity of alcohol into the air being an additional upward force on the control spring by virtue
discharged of acting on the area of the plunger itself. It is the presence
of this additional force that allows a substantial
Governor valve Adjus table control spring
 Diaphragm Unloader plunger brake circuits show a pressure of 735 kN/m2 (105 lbf/in2) or
Air from 7 bar and stop the engine, then observe that the pressure
reservoir Compressor inlet valve
held open
does not drop more than 52 kN/m2 (7.5 lbf/in2) or 0.5 bar
Valve plunger
over a period of 4 minutes. Second, run the engine until the
air gauges again show a pressure of 735 kN/m2 (105 lbf/in2)
Spring-loaded
No air delivery or 7 bar, next fully depress the brake pedal and stop the
through exhaust
valve
engine, then keeping the pedal depressed for 2 minutes
observe that the pressure does not drop more than 41 kN/m 2
(6.0 lbf/in2) or 0.4 bar. If these drops in pressure are
exceeded then the
inlet and exhaust Storage air at
valve cut-out pressure system should receive urgent investigation.
Exhaust port The fitting of anti-lock brake systems to heavy vehicles
(Section 29.5) is now a requirement of European Union
Figure 28.3 Schematic arrangement and operation of governor legis- lation. This has made it necessary for the driver to
valve
check the functioning of the system before commencing a
journey. For this purpose the satisfactory operation of the
system is indi- cated by a warning lamp on the dashboard, a
drop in pressure of the storage air before charging recom- second warning lamp being provided for a trailer
mences. In other words, it determines the required cut-out combination. The signal that appears after switching on
and cut-in pressures. When the storage air pressure drops should extinguish once the vehicle reaches a speed of about
below the cut-in point of the governor valve, the control 6 mile/h (10 km/h), when the anti- lock facility normally
spring first restores contact between the plunger tip and the becomes operative.
inlet and exhaust valve to isolate the air supply to the
unloader plunger. With reducing air pressure it then causes
the plunger to unseat the inlet and exhaust valve and allow 28.3 SYSTEM CONTROL
the air supply to the unloader plunger to be vented to
atmosphere. The air com- pressor now resumes its normal Foot-operated brake valves
operation of charging the reservoirs. These are treadle operated. Their function is to provide con-
trol of system air pressure, during application and release of
Unloader valve the brakes which is precise and proportional to driver effort.
Consider first a single foot brake valve. This essentially
An unloader valve is installed in the supply line between
com- prises a treadle-operated telescopic plunger and piston
the compressor and reservoir where it performs a similar
assembly, which acts upon a combination inlet and exhaust
function to that of a governor valve. The essential difference
valve at its foot. The plunger acts upon the piston through
between these alternative types of pressure control valve is
the medium of what is called a graduating spring, this being
that, with an unloader valve, the governor valve and
pro- vided with a retaining collar through which can slide
unloader plunger are combined into a single unit. Its
the stem of the piston. Below the piston is a lighter return
operation is such that instead of rendering the compressor
spring. The lower body of the piston is made both hollow
inactive when cut-out pressure is reached, the compressor
and ported, so that not only does it serve as an exhaust
continues pumping air that is simply vented to the
valve seating on brake application, but also it can allow the
atmosphere via the unloader valve.
passage of exhaust air from the actuating chambers on
brake release. In the nor- mal position, the combination
Safety valve inlet and exhaust valve is spring loaded against the inlet
This provides a safeguard against any excessive build-up of valve seat, the foot of the pis- ton remaining clear of the
air pressure in the storage system in the event of either a upper exhaust valve.
governor, or an unloader valve, failing to operate. It may be When the driver depresses the brake treadle, a force is
located at either the compressor, the unloader valve or the transmitted through an interposed roller to the spring-
reservoir, and simply comprises a ball check valve that is loaded telescopic plunger and piston assembly, the
spring loaded on to an orifice seating formed in a brass downward motion of which causes the exhaust seat of the
housing. If the storage air pressure rises above a preset piston to close on to the exhaust side of the combination
limit, which is typically in the range of 896 kN/m2 (130 valve. Further depression of the treadle causes the piston to
lbf/in2) to 1103 kN/m2 (160 lbf/in2), the ball check valve is unseat the inlet side of the combination valve, which then
lifted against its spring loading and allows excess air to be allows compressed air from the reservoir to pass through
vented to the atmosphere through the exhaust port. The the valve and on to the wheel brake actuators (Figure
safety valve remains closed during all nor- mal operation of 28.4a). This compressed air is also allowed access to the
the air brake system. underside of the piston, via a bleed hole in the valve body.
Therefore when this upward pressure just exceeds the
downward one being exerted on the piston through the
Safety tests medium of the plunger and graduating spring, the piston
A periodic check for any abnormal leakage from the air lifts just sufficiently to allow the inlet side of the com-
brake system typically involves the following procedure. bination valve to close into its seating. At the same time the
First, run the engine until the driver air gauges for the piston still maintains a closed exhaust passage against the
front and rear exhaust side of the combination valve (Figure 28.4b). In
this
 To atmosphere

To brakes From brakes

Pressure balance
providing feel
From reservoir at treadle

(a) (b) (c)

Figure 28.4 Schematic arrangement and operation of foot-operated brake valve: (a) brakes applied (b) brakes held (c) brakes released

balance or lapped condition the brakes are held on with a

Service brakes Dual footbrake Service brakes
force that is proportional to the effort being applied by the valves
driver, who is therefore provided with a sense of feel as to
the amount of braking being used.
Rear circuit
Any change in the braking effort exerted by the driver on Front circuit
Signal pressure
the treadle will alter the balance point. If the treadle is
depressed further, transmitting additional force through the Reservoir Reservoir
graduating spring, the downward movement of the piston
will again unseat the inlet side of the combination valve, Relay valve
thereby increasing the air pressure delivered to the brake
actuators. Similarly, this additional pressure will act against
the underside of the piston until it exceeds that imposed by
the graduating spring force. The piston will then rise to Figure 28.5 Layout of a basic dual-circuit air brake system
allow the inlet side of the combination valve to close on to for a six-wheel rigid vehicle
its seat- ing with the exhaust side remaining closed.
Another balance point is thus established with the brakes
being more firmly held on, in accordance with the increased
braking effort exerted by the driver. air pressure from the brake actuators to release the brakes
Conversely, if the driver partially releases the treadle (Figure 28.4c).
then the reduced force transmitted by the graduating spring The air braking systems installed in modern heavy
will allow the piston to rise, owing to the air pressure vehicles in Britain must be designed to meet The Road
beneath it and its return spring. When this occurs the inlet Vehicles (Construction and Use) Regulations and also EC
side of the combination valve remains closed and the piston Directives. One result of this has been the fitting of dual-
retracts from the exhaust side, which allows some air to be circuit braking systems for rigid heavy vehicles, which was
exhausted from the brake actuators until the drop in first encouraged during the mid 1960s and then some ten
pressure allows the piston to sink just sufficiently to close years later became mandatory. For example, the foot-
off the exhaust side of the combination valve. Once again a operated service brake of a six-wheeled vehicle is split into
new point of balance is reached, but this time at a lower a front service brake which is operative on the front axle
pressure as reflected by the reduced braking effort exerted brake assemblies, and a rear service brake which is
by the driver. Of course, when the driver fully releases the operative on the foremost and rear- most rear axle brake
treadle the piston returns to its highest position, so that the assemblies (Figure 28.5). If a fault occurs in either of the
inlet side of the combination valve remains seated to cut off two systems the other one must operate inde- pendently, so
compressed air supply from the reservoirs, and the exhaust it becomes necessary to provide dual foot brake valves with
side is unseated to exhaust all each receiving air from its own reservoir.
13 Adjustment for
spring load
14 12
 15
2 Regulating
spring

Diaphragm-controlled
delivery valve

16 Pressure-protected
3
brake circuit
10
1
4
9 Ancillary circuit
6
7 Figure 28.7 Schematic arrangement and operation of pressure
regulating valve
5
8
closes. This prevents any further loss of air under pressure
from the circuit being protected.
11
Figure 28.6 Section of Bendix Westinghouse dual foot-operated Single and double check valves
brake valves (Seddon Atkinson) A single check valve is also known as a non-return valve,
1 body 9 return spring and its purpose is to allow the flow of air in one direction
2 plunger 10 spring retainer only. This type of valve is fitted at the entry to the air reser-
3 graduating spring 11 valve spring voirs according to system requirements, and prevents loss
4 piston 12 treadle of pressure in the event of an air pipe or coupling suffering
5 inlet/exhaust valve 13 roller a fracture. The body of the check valve is screwed directly
6 exhaust port 14 link
into the entry port of a reservoir, and its internal
7 supply port 15 pivot pin
8 inlet valve 16 treadle stop
components comprise no more than a spring-loaded rubber
valve with suitable guidance for the stem and a seating for
the valve.
The valve is intended to operate when the pressure of air
The valves may be arranged either in line with each other at the outlet side is equal to, or less than, the pressure at the
and known as a dual-concentric foot valve, or side by side inlet side. Air is therefore allowed to flow through the unit
and straddled by a balance beam. The latter is acted upon at once the pressure at the inlet port forces the valve off its
its centre by the treadle roller to provide matched air pres- seat, but it cannot flow in the reverse direction because the
sures for the two circuits (Figure 28.6). com- bination of pressure and spring load will return the
valve to its seating. To meet EEC requirements a more
sophisticated version of check valve is now used, this being
Pressure regulating valve known as cir- cuit protection valve. It has the characteristic
This type of valve is used to ensure that adequate air of automat- ically isolating a defective circuit from the air
pressure exists in that portion of the system preceding the supply line, so that air pressure in the remaining circuits is
valve, before any pressure is allowed to build up in the kept at a level high enough to maintain acceptable braking
system beyond it. For example, it can ensure that a brake efficiency.
reservoir receives charging priority over a reservoir that A double check valve performs a change-over function in
supplies the ancillary services. When this type of valve is allowing air under pressure from either one of two control
installed between reservoirs it is also known as a pressure valve systems to supply a brake actuator, while isolating the
protection valve. other in the event of failure. This type of valve simply com-
A pressure regulating valve is of simple construction and prises a body with two inlets and one outlet, the inlet
comprises a valve body, a regulating spring and either a passage being sleeved to guide a rubber shuttle valve,
diaphragm- or a piston-controlled delivery valve. In which is pro- vided with a seating at each end of the
operation, the valve remains closed until reservoir air passage.
pressure acting on the underside of either its diaphragm or In operation, the shuttle action of the double check valve
piston builds up suffi- ciently to overcome the regulating ensures that only air at the higher of the two pressures
spring load. Air under pres- sure can then flow to the delivered to its inlet connections will pass to the outlet and
ancillary services (Figure 28.7). In the event of a system on to the brake actuator. This is because the air delivered at
failure beyond the valve, the air pressure will drop until it higher pressure will compel the valve to move over and seal
reaches the cut-out pressure at which the valve off the opposite inlet connection where the air pressure is
lower (Figure 28.8).
Air delivery at Initial application of brakes
higher pressure Spring-loaded 'Signal' pressure from distant
relay piston brake control valve

Guide sleeve Valve seat r-pressure air

Highe
Shuttle valve Exhaust passage
Valve seat
Pressurized air to brake actuators Pressurized air from
nearby reservoir
Lower-
pressure air Spring-loaded
inlet and exhaust valve
Figure 28.9 Schematic arrangement and operation of relay
Figure 28.8 Schematic arrangement and operation of double valve
check valve

combinations with a single-line connection. It was so

arranged that air passed through it to the trailer reservoir
Relay valves and when storage pressure fell with brake application it
triggered the relay valve, thereby admitting air to the trailer
For the effective operation of an air brake system it is
brakes from their nearby reservoir. This system had the
import- ant to obtain a rapid pneumatic balance or
disadvan- tage that no air could be supplied to the trailer
equalization of air pressure at all brake actuators.
reservoir dur- ing prolonged periods of downhill braking.
Otherwise, there would be an unacceptable delay in the
Another example is the relay emergency valve, which is
application of the rear brakes of a rigid long-wheelbase
associated with mod- ern tractive unit and trailer
vehicle. To overcome this difficulty it can be arranged for
combinations. Apart from speed- ing up the application and
the rear brakes to receive their pressur- ized air supply more
release of the trailer brakes, it also has an emergency valve
directly from a separate nearby reservoir via a relay valve.
that allows automatic application of the trailer brakes
This valve can then be rapidly triggered by a signal pressure
should either the air pressure in the feed or emergency line
from a brake control valve, since only a small volume of air
to the trailer reservoir fall too low, or in the event of the
need be passed through a narrow-bore pipeline to provide
trailer breaking loose from the tractive unit. For this reason
the signal, and the apply pressure of the brakes remains
it is known to American engineers as a break- away valve.
proportional to the signal pressure.
The relay valve comprises an upper air chamber with a
Differential protection valve
signal pressure port and delivery ports in the chamber base.
Sliding within the chamber is a spring-loaded relay piston Also known as an anti-compounding valve, its purpose is
with a valve seat carried at the lower end of its stem. The to prevent simultaneous application of both service air and
relay valve lower body contains the by now familiar secondary spring brakes, which otherwise could result in
combin- ation inlet and exhaust valve, which is made mechanical overstressing and damage to the actuating
hollow and is normally spring loaded against the valve inlet mech- anism between the brake chambers and the brake
port at the base of the air chamber. Valve inlet or reservoir shoes. This type of valve is installed after the relay valve
ports are also provided in the lower body. associated with the hand control valve for the spring brake
In operation, the large area of the relay piston ensures system, which will be described later.
that there is a quick response to signal pressure from a The valve body is provided with inlet connections from
brake con- trol valve. The resulting movement of the piston the hand control and footbrake valves, and outlet connec-
against its spring first closes the exhaust passage through tions to the ports of the spring brake chambers. In a typical
the combin- ation inlet and exhaust valve, and then construction the valve assembly comprises two concentric
depresses this valve so that the inlet seat is uncovered. pistons, the outer one being spring loaded against a seating
Storage air from the reservoir can now pass through the that controls the inlet port for service line air, and the inner
inlet port of the valve body into the air chamber beneath the one being spring loaded away from a seating that controls
piston, and leave via the delivery ports to supply the brake the inlet port for secondary line air. The latter arrangement
actuators (Figure 28.9). When the combined force of the is necessary to ensure that during normal driving a
piston and valve return springs, together with the air pressurized supply of air is made available to hold off the
pressure acting beneath the piston, just exceeds the signal spring brakes (Figure 28.10). This secondary pressurized
pressure acting above the piston it will lift sufficiently to air also acts in conjunction with the spring loading on both
allow reseating of the combination valve. With the inlet seat pistons, so that the outer piston covers the service air inlet
now covered by the combination valve and the exhaust port.
passage through this valve sealed by contact with the piston The differential protection valve will be activated should
stem, the relay valve is in a state of balance. That is, those the driver not only apply the service air brakes from the
brakes operated by air supplied via the relay valve are foot treadle, but also operate the hand control valve to
applied just as quickly and held on with the same force as release air from the secondary spring brakes and apply
the other brakes, in all cases this force being them as well. This unwanted application of the spring
proportional to the effort exerted by the driver on the brakes is prevented because, as the pressure in the
control valve. secondary line falls, the pres- sure in the service line
Other more elaborate versions of relay valve may be overcomes the spring load on the inner piston so that it
used for certain applications. One example is the inverse moves to cover its seating and prevents
relay valve that was once used with tractive unit and trailer
To spring brake actuator Valv
Pressurized air when supplied
Seating for spring-loaded e
outer piston seati
ng
 Diaphragm valve lifted
Secondary line Service line air
air pressure for pressure held in
brake release normal operation

Seating for spring-loaded

inner piston
From brake From brake
To spring brake actuator actuator actuator

Figure 28.10 Schematic arrangement and operation of

differential protection valve
Air exhausted from
brake actuators
escape rapidly to the atmosphere through the valve body
exhaust port (Figure 28.11).
further loss of air through the secondary inlet port. Further
increase in service line pressure then overcomes the spring Load sensing valve
load on the outer piston, which moves to uncover its seating The purpose of fitting a load sensing valve is to ensure that
so that service line air is admitted to the spring brake cham- the braking force generated at a particular axle is propor-
bers. This supply of pressurized air from the service line tional to the load borne by that axle. With an air brake sys-
thus replaces that exhausted from the secondary line, tem the valve will therefore control the air pressure in the
thereby ensuring that the spring brakes remain released brake actuators for the regulated axle, according to the load
during appli- cation of the service brakes. When the service on that axle and brake line pressure. The valve senses the
brakes are released, the reduction in service line pressure extent of rear axle loading by being made responsive to
allows first the outer piston to reseat and cover the service deflection of the road springs. It is therefore mounted on
inlet port and next the inner piston to return and uncover the chassis frame and connects to the axle through a shock
the secondary inlet port. The secondary spring brakes will absorbing linkage, so that it can ignore the effects of spring
then be either applied or released according to driver deflection caused merely by road surface irregularities.
operation of the hand control valve. In one type of load sensing valve a balance beam is con-
nected at one end to a control piston and at the other end to
Quick release valves a balance piston, the fulcrum of the beam being afforded by
These valves are installed adjacent to the wheel brake actu- a pair of slidably mounted rollers. These rollers are
ators, so that air can be rapidly evacuated from the automat- ically repositioned along the beam by a control
actuators when the line pressure is released. This avoids the rod, which is responsive to variations in axle load. The
delay in response associated with air from all the actuators control piston is spring loaded towards the balance beam
being evacu- ated simultaneously through long runs of air and incorporates a combination inlet and exhaust valve,
line, before reaching the atmosphere via the exhaust port of while the balance piston is also spring loaded towards the
the control valve. Quick release valves can therefore balance beam and engages its end through a lost-motion
perform two func- tions in an air brake system, since they fork and pin arrangement.
can allow quicker release of the service air brakes and When the driver applies the brakes, the line pressure
quicker application of the secondary spring brakes. received by the load sensing valve acts against the control
This type of valve is of simple construction, and typically piston to force it downwards. This movement of the control
comprises a valve body with two ports connecting to a pair piston allows the combination inlet and exhaust valve to
of brake actuators and a third port that exhausts to the cover the exhaust port in the valve body and uncover the
atmos- phere. Another port is included in the cover of the inlet passage in the piston. Pressurized air can then flow
valve body and connects to the brake control valve. through the control piston to the brake actuators and also
Sandwiched between the valve body and cover is a flat act against the top of the balance piston. The latter is
rubber diaphragm that can seal against either an upper or a likewise forced downwards and tends to raise the control
lower seating. When the service air brakes are applied, piston owing
pressurized air is supplied to the port in the valve cover,
which presses the diaphragm on to its lower seating to seal
off the exhaust port. The incoming air now flexes the outer
edge of the diaphragm and passes out of the ports
connecting to the brake actuators. During release of the
brakes the air pressure in the actuators will be greater than
the reducing line pressure at the valve
Figure 28.11 Schematic arrangement and operation of quick
release valve

cover port, so that the diaphragm is lifted away from its

lower seating and allows the air from the actuators to
 Balance beam

Slotted
fork

Line pressure
Increased axle
Control load
piston
Piston return
To brakes spring

Balance piston
Inlet and
exhaust valve Exhaust
passage Piston return
spring

Figure 28.12 Schematic arrangement and operation of load sensing valve

to the see-saw effect of the balance beam. This upwards or

its torsionally flexible operating linkage. This isolates the
return movement of the control piston continues until a
action of the valve against minor suspension disturbances
point is reached where the combination valve covers both
during the process of braking, while avoiding unnecessary
the inlet and exhaust passages (Figure 28.12). This point
wear in the connection by allowing it greater freedom of
corresponds to a state of equilibrium between the control
movement when the brakes are released.
and balance pis- tons, with the air pressure in the brake
When a brake application is made, the line pressure
actuators remaining constant. The brakes are therefore
received by the load sensing valve therefore establishes a
being held on with a force that is proportional not only to
positive connection with the operating linkage, via an exter-
the effort being applied by the driver, but also to the load
nal pipe and the stem valve plunger, and also forces down
on the axle. Now if we increase the load on the regulated
the control piston until its spring-loaded combination inlet
axle, the control rod of the load sensing valve will
and exhaust valve is unseated by contact with the mouth of
withdraw to reposition the fulcrum rollers closer to the
the stem valve. This results in the exhaust passage through
connecting fork of the balance piston. It thus follows that
the stem valve being closed off and the inlet passage in the
when the brakes are applied, there will be a greater build-up
con- trol piston being opened up, so that air under pressure
of pressure in the actuators before it is held constant. This is
is not only directed through the load sensing valve to the
because more pressure must be applied to the balance
brake actuators, but also gains access to the underside of the
piston before a state of equilibrium is reached, owing to the
control piston diaphragm. When the force acting upwards
less favourable leverage exerted by the balance piston on
on the diaphragm becomes equal to that acting downwards
the beam.
on the piston, the latter achieves a state of balance and the
In the Bendix type of load sensing valve the two-piece
combined inlet and exhaust valve assumes a lapped
housing contains an upper assembly of sliding control
condition with the air pressure in the brake actuators
piston, diaphragm and the familiar combination inlet and
remaining constant. The effect of the vehicle being heavily
exhaust valve, which operates in conjunction with a lower
laden will be to increase the height to which the stem valve
assembly of a hollow sliding stem valve that is made
rises in the housing. This means that the control piston will
responsive to sus- pension deflection and hence vehicle
also be working in a higher position and by the same token
loading (Figure 28.13). There are two ingenious features
giving less support to its diaphragm. Since this will reduce
associated with the oper- ation of this type of load sensing
its effective area closer to that of the control piston itself,
valve. The first of these is an intermeshing arrangement of
there needs to be a greater air pressure acting on the
tapered radial fins for control- ling the effective area of the
diaphragm before the opposing forces on the piston become
piston diaphragm. One set of fins is fixed in the housing
balanced and the combination inlet and exhaust valve
and the other set moves with the con- trol piston, so that the
assumes a lapped condition. In other words, the output
effective area of the diaphragm and therefore the upthrust
pressure to the brake actuators is now less reduced in
due to air pressure experienced by the piston depend upon
relation to the input line pressure and thus corresponds with
the extent to which the fins of the latter lend support for the
the heavier loading of the vehicle. When the brakes are
flexing diaphragm. Secondly, there is an air-pressure-
released, the air pressure in the brake actuator line will lift
controlled plunger that can temporarily clamp the ball-pin
the control piston and unseat the combination inlet and
connection between the sliding stem valve and
exhaust
 5

8
7

Figure 28.13 Section of Bendix Westinghouse load sensing valve (Seddon Atkinson)
1 operating lever 5 external pipe
2 stem valve 6 plunger
3 ball-pin 7 piston
4 operating shaft 8 inlet/exhaust valve

valve from the mouth of the stem valve, which allows the tractor and trailer decelerations can likewise be sensed at
air to escape down through its exhaust passage and thence the fifth-wheel coupling of an articulated vehicle or at the
to the atmosphere, via a rubber check valve in the unit cou- pling arrangements of a drawbar combination and
body. monitored by the ECU, which then issues command signals
to the trailer electro-pneumatic relay valve that regulates
Introduction to electronically-controlled air brakes the air pressure accordingly. Electronic braking systems
are, of course, com- patible with established anti-lock brake
A new development in air brake system control for heavy and traction control systems.
vehicles is that in which a quicker acting electronic control
is superimposed on the traditional pneumatic control, so
that the latter essentially performs a safety backup function, 28.4 SYSTEM ACTUATION
although there are long-term development aims to make
this feature redundant if legislation permits. Systems of this Air brake actuators
type are now designated as ‘electronic braking systems’
These are also known as brake chambers. One is mounted
(EBS) or, in popular jargon, ‘braking-by-wire systems’.
externally to each wheel brake. Through the medium of a
In basic principle the foot-operated brake valves
diaphragm element they convert the energy stored in the
assembly is also equipped with a potentiometer device, so
com- pressed air into the mechanical force and movement
that the extent to which the driver applies the brakes can be
required to actuate the brake shoes. Owing to their bulky
signalled to an electronic control unit (ECU). This then
nature they cannot be accommodated within the brake
issues appro- priate command signals to electro-pneumatic
drums, and there- fore act upon either lever and cam or
relay valves, which serve their nearby brake actuators. The
wedge and tappet shoe expanders instead of directly on the
function of these relay valves is first to isolate the slower
brake shoes.
acting pneu- matic back-up control, second to admit air
Reference has been made in Section 27.1 to the S-cam
more rapidly at a regulated pressure to the brake actuators,
and rollers type of brake expander and its use on air-braked
and third to release this pressure and restore the back-up
heavy vehicles. A further development of the fixed-cam
control when braking is no longer required.
brake takes the form of a cam and struts type of expander
By virtue of the brakes responding more quickly to the
mechanism. Here the shoe tip rollers associated with S-cam
driver pressing the pedal, it will be evident that shorter
operation (Figure 27.4) are replaced by ball-ended struts
stopping distances and hence improved vehicle safety is the
and sliding tappets. Each strut or push-rod locates at one
primary objective of an electronic braking system. This type
end in a spher- ical recess within the cam and at the other
of control system can also be extended to confer other
end in a spherical recess inside its sliding tappet (Figure
advantages, because the addition of load sensors can make
28.14). Rotation of the camshaft therefore causes the struts
it responsive to rear axle loading and load transfer during
to separate the tappets and expand the shoe tips. The
brak- ing, so that overbraking on any particular axle can be
advantages claimed for this par- ticular construction are that
avoided to the benefit of reduced liner or pad wear.
it provides a sealed and lubricated enclosure for the
Imbalance between
expander mechanism, and that the shoe tip
 17 4 10 12 13 9

1 4

B
3 6

6
19
Figure 28.14 Air brake cam and struts expander (Seddon
Atkinson) 8

11
18
forces are always constrained to act in-line with the sliding 3
tappets, whereas with an S-cam and rollers the angularity of
the shoe tip forces is such that their action is not 5
independent of the direction of drum rotation. 7
Reference has also been made in Section 27.1 to the con-
tinuing use of wedge-type brake expanders for commercial 2
vehicles. Those operated by compressed air for heavy
vehicles were originally developed in America during the
mid 1960s. The advantages of replacing the lever and cam
by a wedge and tappets were perceived as reducing Figure 28.15 Air brake wedge expander (Lucas Girling)
unsprung weight by virtue of eliminating the camshaft and 1 wedge 12 manual override socket
slack adjuster; greater structural rigidity resulting from the 2 roller 13 manual override pinion head
brake actuator directly attacking the wedge; shorter 3 plain tappet 14 manual override stem
actuating stroke with smaller air chamber, thereby 4 roller tappet 15 spring retainer
minimizing air consumption and pro- moting quicker 5 adjuster screw 16 pin/circlip
application and release; ready accommoda- tion of both air 6 input tappet 17 expander cover
brake and spring brake actuators; and allowing the use of 7 adjuster pinion 18 tappet stop pin
dual-circuit air systems by incorporating twin wedge 8 drive cone 19 gasket
expanders in each brake. Hardly a disadvantage but more 9 overload spring 20 seal
10 cone spring 21 wedge push-rod
an essential requirement for an air-operated wedge brake is
11 housing
an in-built automatic adjuster, which ensures that the wedge
travel between the sliding tappets never becomes excessive
or runs out.
Various shoe arrangements may be employed with air
brake wedge expanders. A single wedge expander may be During normal brake application the wedge is urged
used in conjunction with either a simple arrangement of inwards, via its push-rod, by the air brake actuator. It there-
lead- ing and trailing shoes to give a floating-cam fore imposes a separating force on the sliding tappets
characteristic or, via a bell-crank and struts linkage, an through the interposed rollers. Since there are two wedge
arrangement that pro- vides two leading shoes in the expanders, each shoe receives movement at both ends and
forwards direction and leading and trailing shoes in reverse, is bodily brought into contact with the rotating drum. Once
while the use of twin wedge expanders confers a two this occurs both shoes move round slightly, until the
leading shoe action in both the for- wards and reverse trailing end of each is arrested by its adjacent sliding tappet,
directions. Here it should be noted that in some countries which has been forced back against an abutment in the
these different shoe arrangements are better known as expander housing. The opposing tappets then continue to
simplex, duplex and duo-duplex brakes respect- ively. A transmit a shoe tip force to the leading ends of the shoes, so
modern example of the latter type of air wedge brake is the that a two leading shoe action is obtained. This same
Girling Twinstop. In this design the air brake actuators are sequence of events does, of course, occur in the opposite
stem mounted from the wedge expanders, their mounting sense with reverse rotation of the drum and again confers a
tubes being screwed directly into the wedge expander two leading shoe action on the brake.
bodies. Each expander incorporates an integral push-rod Of simple construction, a single-diaphragm brake actu-
and wedge with the usual rollers interposed between the ator comprises a reinforced rubber diaphragm that is sand-
wedge and the inclined faces of its sliding tappets (Figure wiched between a two-piece pressed steel casing, which is
28.15). The wedge also provides a positive caged guidance held together by a clamping ring and provided with suitable
for the rollers, so as properly to control their movements mounting studs. The diaphragm is furnished with a push-
during brake actuation. An automatic adjustment facility rod connection to the shoe expander mechanism and is
with a manual override is embodied in one of the sliding spring returned to the brakes released position (Figure
tappets in each expander, the opposing sliding tappet 28.16).
remaining solid.
 Spring brake actuators
Another improvement that was introduced into air brake
sys- tems during the mid 1960s was the secondary and
1 14 parking spring brake, this type of brake having previously
20
been used in American heavy-vehicle braking systems. A
spring brake actuator utilizes the stored potential energy of
a powerful compression spring to apply the wheel brake.
During normal driving the spring must therefore be held in
a compressed state to ensure that the brake remains
released. For this pur- pose the spring brake actuator is
provided with a supply of pressurized air via a hand control
valve, the latter being described later. Since air pressure is
reduced to apply a spring brake actuator and increased to
apply an air brake actuator, the air brake engineer
distinguishes between the two forms of air supply by
referring to them as inverse air and upright air respectively.
Although air pressure is released from a spring brake for
brake application, there still remains sufficient compression
in the expanded spring to exert the required force for actu-
Figure 28.16 Air brake actuator and slack adjuster ation of the brake. A spring brake may be either applied
(Bendix Westinghouse) grad- ually by the hand control valve for the purpose of
secondary or emergency braking, or applied fully to hold
the brakes on for parking, thereby replacing the once
conventional hand- brake that had direct mechanical
In some installations where a rear brake actuator operates in linkage to the wheel brakes. The spring brake also
conjunction with a mechanical handbrake, the diaphragm uti- possesses an important fail-safe fea- ture, because the
lizes a pull-rod connection to the shoe expander mechanism. brakes will be automatically applied should a failure occur
In operation, the pressurized air enters the inlet port of the in the air pressure circuit for the secondary and parking
brake actuator and compels the diaphragm and pushrod brake functions.
assembly to move against the return spring and apply the A spring brake is typically mounted in tandem with a
brakes. The apply force developed is proportional to the con- ventional single-diaphragm service air brake, each
effective area of the diaphragm and the pressure of the air brake operating independently of the other (Figure 28.17).
admitted to the actuator. As the brakes are released and the The ser- vice brake chamber is therefore supplied with
air pressure reduces, the diaphragm and push-rod assembly upright air from the footbrake valve and the spring brake
is returned to its original position by the actuator return chamber with inverse air from the hand control valve;
spring and the pull-off springs for the brake shoes. simultaneous application of service and spring brakes is
During the mid 1960s the legally permitted gross weights prevented by the differential pro- tection valve described
of heavy vehicles were increased, and this led to various earlier. In construction the spring brake chamber contains
improvements being incorporated in their air brake systems either a diaphragm or a sliding piston with air seal, so that
to meet more stringent regulations. Included among these when these are subject to increasing air pressure the
improvements was the use in some systems of double- powerful spring is compressed to release the brake. When
diaphragm brake actuators for improved safety. With this air pressure is reduced, either the diaphragm or the piston
arrangement the two diaphragms are separated at their moves in the opposite direction under the influence of
clamp- ing edges by a spacer ring, so that pressurized air spring load. Its central stem then bears against the back of
can be admitted through a rear inlet port to the secondary the service brake diaphragm, thereby actuating the brake
diaphragm and a side inlet port to the service diaphragm. through the usual push-rod connection. Some form of screw
The secondary diaphragm has a forward thickening of its release mechanism is provided for manually relieving the
central portion to complement the thickness of the spacer brake assemblies of spring brake load, which allows the
ring, and it is also provided with a small-diameter rear lip brake assemblies to be safely serviced, or if necessary the
seal that covers the rear inlet port. In the event of the vehicle to be moved in the absence of air pressure.
service diaphragm leaking then clearly the brake can still be
operated by using the sec- ondary system, since this part of
the chamber remains self- contained. The situation is a little Lock actuators
different with a leaking secondary diaphragm, because in
this case the return spring or pressurized air acting on the Introduced as an alternative to the spring brake, the lock
central portion of the diaphragm forces its sealing lip to actuator was intended to simplify the parking brake
cover the rear inlet port and prevents the escape of air into function of heavy vehicles. When signalled to do so from a
the secondary system. Since this then provides the hand con- trol valve, the lock actuator will arrest the return
equivalent of a self-contained chamber, normal operation of movement of the guided stem shaft of an air brake
the service diaphragm is restored. diaphragm to hold the brakes applied. Similar to the spring
brake, air pressure is released from a lock actuator to hold
the brake applied (Figure 28.18).
 21 Air pressure released to
hold brake fully applied
15

Shaft locked against

return movement
Apply spring

(a)

Brake reaction
force

Lock collar
Release piston

Brake actuator chamber

Figure 28.18 Schematic arrangement and operation of lock

(b) actuator

With a lock actuator the stem shaft of the air brake

diaphragm is encircled by hourglass-shaped rollers, which
can be either spring loaded against the conical bore of a
lock collar so their wedging effect prevents return
movement of the stem shaft, or forced away from the
conical bore of the lock collar by air pressure acting against
a release piston and thereby restoring free movement of the
stem shaft. In a typ- ical installation for a double-diaphragm
air brake, the lock actuator is released by first applying the
secondary brake to allow the rollers to unwedge
(c) themselves, following which they remain free on the shaft
as long as air pressure is main- tained against their release
piston. The secondary brake is then, of course, released to
drive away the vehicle.

Air brake adjusters

Maintaining correct adjustment in an air brake system is a
matter of considerable importance, because it is related to
what is sometimes termed brake force build-up time, during
which the braking force at the wheels is building up to a
max- imum value. If adjustment is neglected and the
clearance between the shoe linings and the drum becomes
excessive, the amount of air that must pass into the actuator
(d) chamber to apply the brakes is necessarily greater, which
therefore length- ens the brake force buildup time and
increases the stopping distance.
The adjustment of cam-operated air brakes is accom-
plished by what is known as a slack adjuster, the
mechanism of which is embodied in the lever arm used
between each brake actuator push-rod and expander cam
for the shoes. By means of a lockable worm and wheel
Figure 28.17 Spring brake actuator operation (Seddon mechanism the rota- tional position of the expander cam
Atkinson): (a) normal driving (b) service braking (c) secondary/ may be manually adjusted relative to that of the lever arm
park braking (d) mechanical release (Figure 28.16). Hence the most favourable operating
geometry for the actu- ating linkage may be preserved
throughout the life of the shoe linings, and by the same
token the actuator diaphragm
 Figure 28.20 Identifying the angular movements of an
air brake slack adjuster (Haldex Division, Garphyttan
Ltd)
Figure 28.19 Cut-away view showing internal mechanism (C) – Clearance angle (Ce) – Excess clearance angle
of Haldex automatic slack adjuster (Haldex Division, (E) – Elasticity angle
Garphyttan Ltd )
which is necessary to prevent any brake drag and to allow
cooling between the friction surfaces.
is never over-extended. During normal brake operation the Excess clearance angle (Ce) This is additional to the nor-
entire lever arm cum slack adjuster does, of course, rotate mal clearance angle and appears as the brake linings
bodily with the brake camshaft. continue to wear in service, prior to the normal running
Automatic slack adjusters have increased in popularity clearance being restored by automatic adjustment.
since the late 1970s, because they can maintain almost Elasticity angle (E) this occurs as the brake shoes are
constant the running clearance between the shoe linings and expanded with increasing force against the brake drum tem-
their drums, whilst making allowance for thermal porarily dilating it, the effect of which must be excluded
expansion of the drums during heavy braking by not taking from the adjustment process to prevent over-adjustment and
up all the perceived excess slack. In more recent years they brake drag.
have become especially rele- vant to the responsive
operation of anti-lock air brake systems, and indeed since The take-up of the Haldex automatic slack adjuster is
late 1994 automatic slack adjusters have been a mandatory determined by the gear ratio of the lever arm worm and the
fitment under EC legislation for all newly regis- tered heavy camshaft wheel. It is, of course, most important that the
vehicles and trailers. Automatic slack adjusters basically manufacturer’s advice be sought in relation to the correct
incorporate a lost-motion ratchet and pinion drive to a installation and maintenance of an automatic slack adjuster.
spring-loaded worm shaft and friction clutch arrangement. As earlier mentioned, automatic shoe adjustment is an
Since the ratchet reacts against a fixed link that serves as a essential feature of the air-operated wedge brake. For mod-
datum point, any excess clearance due to lining wear is ern duo-duplex versions it is accomplished by incorporating
taken up on the return stroke of the brake actuator. A visual a jacking screw device within one of the sliding tappets in
indication of lining wear may also be incorporated in the each expander unit. This mechanism takes the form of an
mechanism of these slack adjusters. A long-established and adjuster screw that can be either turned directly within a
widely used type of automatic slack adjuster for cam- threaded tappet, or translated by a threaded sleeve that can
operated air brakes is that known as the Haldex (Figure be turned within a counterbored tappet, the latter in both
28.19), which confers almost con- stant and correct cases being restrained from rotation. Automatic provision is
clearance between the brake linings and the drum. To then made for either the screw itself, or its threaded sleeve,
accomplish this the action of the slack adjuster is able to to receive a small degree of turning when the return move-
distinguish between three phases of angular movement ment of the tappet increases owing to wear of the shoe lin-
(Figure 28.20) as follows: ing. This results in the shoe tip being jacked further away
from the end of the sliding tappet, so that upon release of
Clearance angle (C) This corresponds to the normal run- the brake the correct shoe to drum clearance is restored.
ning clearance between the brake linings and the drum, The
automatic means of turning either the adjuster screw, or its
threaded sleeve, basically relies upon an additional helical
connection being established between the sliding tappet and
its adjuster, combined with a lost-motion pawl and ratchet
or friction clutch control system, so that excess return
move- ment of the sliding tappet causes a turning moment
to be exerted on either the screw or its sleeve. An example
of the former system is shown in Figure 28.15, where the
required helical connection is established between a skew
gear that turns the adjuster screw and a meshing adjuster
pinion, which also provides a manual override adjustment.
When the outward movement of the sliding tappet exceeds
the prede- termined backlash between the gear and pinion,
the reaction from their helical teeth lifts and declutches the
pinion against its spring loading, thereby allowing it
freedom to turn; but on the return movement of the tappet
the pinion drops and is held clutched, so that the helical
sliding between their teeth becomes sufficient to turn the
gear wheel and hence the adjuster screw. The manual
override adjusters serve the three- fold purpose of initially
setting the shoes to drum clearance, retracting the shoes to
facilitate drum removal and retracting the expanders when
fitting new shoes.

28.5 HAND-OPERATED BRAKE VALVES

Figure 28.21 Section of Bendix Westinghouse hand control
AND OTHER EQUIPMENT
valve (Seddon Atinson)
Hand-operated brake valves 1 cam follower 9 valve chamber
2 cam 10 delivery port
There are several types of hand-operated brake valves used
3 spring 11 adjusting ring
in heavy-vehicle air brake systems, their detail design vary- 4 piston 12 piston return spring
ing in accordance with system requirements, but usually 5 inlet/exhaust valve 13 piston vent
they are associated with the secondary and parking 6 inlet 14 valve chamber
functions of spring brake actuators. Although the secondary 7 exhaust 15 exhaust port
brake sys- tem is only used if there is a failure in the foot- 8 inlet port
operated ser- vice brake system, it is still desirable that the
hand-operated brake valve should exert a progressive
controlling action over the spring brakes. In the language of
is governed by the preset load exerted by the control spring,
the air brake engin- eer, this means that such a valve must
this pressure being sufficient to hold off the brakes for nor-
provide a graduable inverse air pressure, since in applying
mal driving.
the spring brakes for secondary braking we are concerned
When the hand lever is moved from the off position
with gradually reducing air pressure. For the parking
(spring brakes released) and held towards the
function all the air pressure is exhausted from the spring
secondary/park pos- ition (spring brakes being applied), the
brakes, the handle of the valve being made self-locking in
rotary cam allows the control springs to extend and thus
the parked position.
reduce the load on the pis- ton. This permits the piston to
In construction a hand-operated brake valve typically
rise under the influence of the air pressure and return spring
comprises a control piston that is acted upon by a lower
force beneath it, until the spring-loaded combination valve
return spring and an upper control spring. The amount of
covers the body inlet port and uncovers the piston exhaust
compression in the latter is varied by a rotary cam arrange-
port. Excess air then escapes through the piston into the
ment that is positioned by movement of the valve handle,
chamber above, and from there is released to the
which has a friction-damped motion for sensitive control.
atmosphere via the exhaust port of the valve body itself.
When the spring brakes are released the control spring pres-
The reduction in air pressure and therefore appli- cation of
sure is greatest and it is least when they are applied. Spring
the spring brakes continues until the control spring moves
loaded against the inlet port of the valve body and towards
the piston down sufficiently to cover both inlet and exhaust
the exhaust port of the control piston is again the familiar
ports, following which the forces above and below the
combination inlet and exhaust valve (Figure 28.21). With
valve become equal and it assumes a balanced or lapped
the hand lever in the off position, the increase in control
condition. A constant braking effect is then obtained until
spring pressure causes the piston to bear down on the
there is further movement of the hand lever. The operating
combination inlet and exhaust valve, which uncovers the
characteristics of this type of valve are therefore similar to
inlet port and covers the exhaust port. This allows air under
those of a foot-operated brake valve, but without the same
pressure from the secondary/park reservoir to pass through
sense of feel, although this feature is present in some
the valve and on to the spring brake actuators. Their
designs of hand-operated brake valve.
delivery air pressure
 16

Released
Compressed air pressure

Applied

Fluid supply tank

Air chamber

Treadle valve

Puller
transverse wheel cylinder
Handbrake rod connection

Figure 28.22 Basic layout of an air-over-hydraulic braking system (Girling)

Trailer connecting hoses Air-over-hydraulic brakes

Since the early 1960s, precoiled nylon air lines, known as For heavy vehicles in the medium weight range, hydraulic
‘Susie’ hoses, have generally replaced rubber air lines actuation of the brakes may be employed in conjunction
between tractive units and either their semi-trailers or with a compressed air source of power operation. Systems
drawbar trailers. The quick release end couplings for the of this type are therefore known as air-over-hydraulic
hoses may be of the earlier male and female bayonet type or brakes, and can more readily generate the higher operating
the later inter- nationally used palm type, these also being forces at the hydraulic wheel cylinders, which otherwise
known aptly as gladhand couplings in America because of would demand unacceptably large vacuum servo units.
their resemblance to clasped hands. Automatic shut-off Furthermore, the smaller compressed air chamber together
valves may be incorp- orated in both types of coupling, so with hydraulic master cylinder can be located wherever
that air pressure is retained when the feed line to the trailer convenient on the chassis frame.
reservoir is disconnected. The end couplings of the hoses are In a typical air-over-hydraulic brake system (Figure
designed so that they cannot be wrongly connected and the 28.22) an engine-driven compressor supplies air that is
hoses themselves are colour coded for ready identification. stored in the usual reservoir. When the brake pedal or
The colours normally used are as follows: treadle is depressed to operate the control valve, pressure
builds up in the air chamber and acts against a diaphragm.
Yellow Service or control line, which supplies a signal air Movement of the diaphragm and its associated push-rod
pressure to the relay valve on the trailer for normal then directly forces the piston of the hydraulic master
operation of the trailer brakes. cylinder to displace fluid at equal pressure to the wheel
Blue Secondary or auxiliary line, which supplies direct air cylinders and thus apply the brakes.
pressure for secondary operation of the trailer brakes in the
event of the service brake failing.
Red Feed or emergency line, which supplies air under 28.6 AIR DISC BRAKES
pressure to the trailer reservoir through a relay emergency
valve. General background
Caution It is very important that the correct procedure for Following the successful adoption of hydraulically operated
safely connecting and disconnecting these brake line hoses disc brakes for passenger cars that began in the mid 1950s,
is carefully followed, and this forms part of the driver train- uprated versions of this type of brake were later developed
ing for goods vehicles. Reference should be made to The and initially installed on public service vehicles, a notable
Official Goods Vehicle Driving Manual, published by The example of their use being on the motorway coaches of the
Stationery Office for the Driving Standards Agency. Midland Red Bus Company in Britain in the early 1960s.
Operator
experience at the time had tended to show that, so far as suited Straddle mounted
public carrier member
service vehicles were concerned, the disc brake was best
Input from air chamber
for long-distance hauls where brake temperatures were Actuator mechanism
lower than those encountered in city running. Hence it was Straddle mounted
soon rec- ognized that the key to successful operation of Disc pads
reaction beam
disc brakes on commercial vehicles was ensuring adequate
dissipation of heat from their discs, otherwise both pad and
disc life would become unacceptably short. To meet this
Guide pins
requirement in prac- tice meant that the disc had to be of
ventilated construction, and designed with the maximum
outer and minimum inner diameters together with the Brake disc
maximum thickness which could be installed within the
existing wheel space, there being greater difficulties posed
by the axle arrangements of commer- cial vehicles than had (a) Torque plate
previously been the case for passenger cars. It also meant
that further research and development had to be undertaken
on disc pad materials to provide an increased
life. For these reasons it was not until the early 1970s, Input torque Input torque
follow- ing a renewed interest in their development in
America, that manufacturers elsewhere began to introduce
improved designs of disc brake which, together with more Apply Apply
suitable friction mate- rials, has led to their gradual force force
acceptance on light to medium commercial vehicles. Multi-lobe Helical ramp
(b) face cam face cam
The introduction of air-operated disc brakes for heavy
vehicles is of more recent origin, because in their case add-
itional design requirements had to be taken into account,
not the least of these being the need for a force multiplying Input
force
mechanism between the disc pads and the air brake actuator
chamber. This requirement arises from the relatively low
Apply
operating pressures of an air brake system, as compared force
with those of a hydraulic one, which would otherwise
demand
unrealistic increases in the size of the caliper piston and the Multiplying
amount of air to be moved. That is, the caliper could not (c) lever
accommodate the size of piston required and the brake
force
build-up time would be unacceptable. Since the presence of Figure 28.23 Basic arrangement of an air-operated disc
a force multiplying mechanism implies a large increase in brake: (a) straddle-mounted caliper (b) and (c) force
brake actuator stroke for a small wear reduction in pad multiplying actuators
thick- ness, there is an essential requirement for an
automatic adjuster that must necessarily be of the
mechanical type. caliper reaction beam or bridge and an opposite load
However, the modern air disc brake can offer important spreader plate, which is urged towards the disc by the
advantages over those of an equivalent drum brake, which caliper actuating mechanism when the brake is applied.
in general terms are greater structural rigidity, more Equality of clamping loads is achieved by virtue of the
consistent brake torque, better fade resistance and easier friction drag forces on both disc pads being reacted against
access for maintenance. locating lugs, which are formed integral with a straddle-
mounted carrier member (Figure 28.24). This is bolted
either directly or indir- ectly to an axle flange, an
Basic construction of air disc brakes intermediate torque reaction plate being used in the latter
Air disc brakes for heavy vehicles have a basically similar case. The carrier member also incorp- orates the sealed low-
clamping action to later versions of the floating caliper and friction guide or slide pins, which support the freely
cylinder, or first-type, hydraulic disc brake used on passen- floating caliper assembly (Figure 28.24). Quadruple slide
ger cars (Section 27.2). However, the construction of the pins are employed in the established Haldex air disc brake,
sliding caliper is modified to incorporate a more robust these minimize any tendency for brake drag while
straddle-mounted reaction beam or bridge, instead of a improving the durability of the sliding components. The
simple claw, to provide an abutment for the outboard pad. avoidance of brake drag with its adverse effects on both
The reaction beam is extended chordally across the face of fuel economy and brake wear, is an important consideration
the disc and straddles it at each end, where it unites with the to the transport engineer.
caliper housing (Figure 28.23a). This structural change to
the caliper ensures that the outboard disc pad receives a Actuators and automatic adjusters for air disc brakes
cen- trally applied clamping load. The backing plates for The actuator housing for the force multiplying mechanism
the out- board and inboard disc pads therefore reside is either integrated with, or separately bolted to, the
against the inboard
 Brake Manual
chamber override
stem
Mounting Protection
Compression spring
bracket ring
Dust Wrap spring
cover Drive ring
Pinch Guide Actuator Circlip
Guide pin pin Clutch plate
bolt assembly
bolt
Operating Helical spring
Seal
lever Clutch
Plug plates
Shroud
End cap Seal Bush Housing
Fixed Thrust
ramp Operating
race
Spring shaft/free
plate ramp Needle Tappet
race
Pad Adjuster Gasket
retaining nut Wavy
pin Retaining washer Retaining
Spreader clip plate Dust
plate cover
Bridge Capscrew

Pad
Carrier

Figure 28.24 Exploded view of Girling reaction beam caliper brake (Lucas Industries)

side of the floating caliper and also provides a mounting for expanders for the shoes (Section 28.4). Likewise, the more
the air chamber (Figures 28.24 and 28.25). This is positioned direct actuation of the disc pads can make for a compact
either parallel with, or normal to, the disc face according to design of brake with a reduction in unsprung weight and a
the type of force multiplying mechanism used. The former minimum of wearing parts. A modern example of this type
position may be likened to that used for operating rotary of construction is the previously mentioned Haldex DB20
cam air drum brakes (Section 28.4), and similarly requires air disc brake (Figure 28.25). Before describing the force
the intervention of a clevis joint where the air chamber multi- plying mechanism of this heavy vehicle brake, it
push-rod connects to the brake operating lever, the cross- may be of interest to quote a few items of dimensional data
shaft of which transmits a torque input to the force to put its size into perspective, as follows:
multiplying mechanism (Figure 28.24). This therefore
serves not only to convert the partial rotation of the input Road wheel size 22.5 in
shaft into a linear motion for the caliper tappet External diameter of brake disc 430 mm (16.9
(corresponding to the piston in a hydraulic disc brake), but in) Thickness of brake disc (new) 45 mm (1.8 in)
also to multiply further the actuating force sup- plied by the Effective radius of brake disc 172.6 mm (6.8 in)
air chamber, so that the clamping load imposed on the disc Swept area of brake disc 1808 cm2 (280 in2)
pads generates the required brake torque. Lining area of each pad (2) 187 cm2 (29 in2)
The force multiplying mechanism itself may comprise a The air chamber is offset above the centre-line of the disc
pair of opposing face cams with either multi-lobes, or pads, so that its spherically-ended push-rod can engage an
helical ramps, and rolling bearings interposed between their upstanding lever. This acts eccentrically upon a wide cross
thrust surfaces (Figure 28.23b). An equivalent mechanism bar, to confer a 15.8:1 mechanical advantage for the apply
used in an American design of brake features a multi-start force at the disc pads. To reduce internal friction partially
screw thread on the cross-shaft that engages an externally caged needle roller bearings are used at the pivot points for
splined sliding nut. Hence, when the cross-shaft is turned the straddle-mounted lever and its associated cross bar
relative to the rota- tionally fixed cam of the pair, or the (Figure 28.25). The force multiplying mechanism is designed
sliding nut, the resulting linear movement creates an end- to accept a maximum brake chamber force of 13.9 kN (3127
thrust on the load spreader plate for the inboard disc pad. lbf ). A spring brake chamber is, of course, used in cases
Then by virtue of the caliper being free to slide, this end where the disc brake has a parking function.
thrust is also transmitted to the out- board disc pad, so that
both pads engage the disc for braking. Brake application (Figure 28.26) The upstanding lever
The alternative construction for air disc brakes, where (44) is actuated by the air pressure in the brake chamber
the air chamber is mounted normal to the disc with a lever (25/26). Since the external and internal radii of the lever (44)
type force multiplying mechanism (Figure 28.23c) may
similarly be compared to air drum brakes with wedge
instead of cam
Figure 28.25 Installation and general arrangement of Haldex DB20 air-operated disc brake (Haldex Brake Products AB, Sweden)

do not have a common centre, their eccentricity means that

the cross bar (41) is forced to move axially in the direction
of the brake disc (A). This force is then transmitted from
the cross bar (41), which also incorporates twin adjusting
screws for setting the predetermined clearance between
pads and disc, to the caliper tappets (28) for the inner pad
(5). Once this pad (5) comes into contact with the brake
disc (A), the caliper (2) is moved on its slide pins so that
the outer pad
(3) likewise comes into contact with the brake disc.

Brake release (Figure 28.26) The return spring (38) forces

the cross bar (41) back into its rest position, so that the
design clearance between the pads (5) and the brake disk
(A) is restored.
In principle there are two types of automatic adjuster,
which may be classified as ‘stroke sensing’ and ‘clearance
sensing’. The former type ignores the clearance existing
between the friction elements, and adjusts the brake when
the air chamber stroke exceeds a predetermined limit. In
contrast, the latter type adjusts the brake when the actual
clearance between the friction elements exceeds a predeter-
mined limit. Hence, it follows that the possibility of the
brakes being over-, or under-, adjusted is less likely to
occur where a clearance sensing system is used.
The Haldex DB20 air disc brake operates according to
the clearance sensing principle. Similar to identifying the
angu- lar movements of their automatic slack adjuster for
air drum brakes (Figure 28.20), the braking sequence is
divided into three phases (Figure 28.26). Namely, Design Figure 28.26 Application, release and automatic adjustment
of Haldex DB20 disc brake (Haldex Brake Products AB,
clearance ‘C’, Excess clearance ‘Ce’ (which is to be
Sweden)
adjusted out) and Elasticity ‘E’. Basically, the housing of
the adjuster (62) is held in position against the internal
radius of the upstanding lever (44) by a guide pin (47). This
compels the adjuster housing (62) to follow the movement cross bar (41), which then restore the correct clearance
of the lever (44). The rotational motion is transferred from between the pads and disc.
housing (62) to adjust- ment spring (63), which in turn,
after passing the design clearance, transmits the motion to Caution It is, vitally important to consult the brake and
the companion sleeve (65), friction spring (66) and hub vehicle manufacturers’ instructions when servicing the air
(67). If excess clearance is present, the rotation of the hub disc brakes of heavy vehicles, which should only be per-
actuates a synchronized bevel gear drive for the twin formed by trained personnel.
adjustment screws incorporated in the