1221 Truck air over hydraulic brake system Fig 121

Compressed air supply Air is drawn into the compressor and then discharged into and out of the wet tank where it is semi-dried; it then flows to the multi-circuit protection valve, here it divides to feed the two service reservoirs. At the same time, pressurized air from the reservoirs combine through internal passages in the multi-circuit protection valve to supply the remote spring brake actuator via the hand control valve.

Service line circuit (Fig. 12.1) There are two service lines feeding into a tandem power cylinder controlled by a dual foot valve, so that if a fault develops in one service line the air supply to the other circuit will not be interrupted. The air pressure is then converted to hydraulic pressure by the power piston push rod pushing the tandem master cylinder hydraulic piston forward. The hydraulic fluid supply is split into two circuits feeding the front and rear brake expander cylinders. To balance the proportion of braking provided by the rear axle according to the load carried, a hydraulic load sensing valve is installed on the tandem master cylinder rear axle output circuit. This therefore modifies the fluid pressure reaching the rear brake cylinders.

Secondary line circuit (Fig. 12.1) With the dual air and hydraulic lines, both systems operate independently and therefore provide a safeguard against failure of one or the other circuit. Thus the hand control valve is used only to park the vehicle.

Fig. 12.1 Truck air over hydraulic system

Moving the hand valve lever from 'off' to 'park' position exhausts air from the remote spring actuator chamber. This permits the power spring within the actuator to expand and exert its full pull to the mechanical parking brake rod linkage.

12.2.2 Tractor three line brake system (Fig. 12.2)

Compressed air supply (Fig. 12.2) The compressor in this arrangement is controlled by a separate unloader valve. An alcohol evaporator is installed in the air intake, so that in cold weather alcohol can be introduced into the airstream to lower the freezing point of any water which may be present. When the compressor is running light, a check valve built into the evaporator prevents alcohol entering the air intake. Pressurized air from the compressor is then delivered to both the service and secondary park reservoirs via the check valves on the inlet side of each reservoir.

Service line circuit (Fig. 12.2) When the foot pedal is depressed, air from the service reservoir is permitted to flow directly to the tractor's front and rear service line chambers in each of the double diaphragm actuators which are mounted on the tractor axles. At the same time, a pressure signal is passed to the relay valve piston. This opens the valve so that the service storage line pressure flows from the service reservoir to the service line coupling (yellow) via the pressure protection valve. The pressure protection valve in the service storage (emergency) line and the relay valve in the service line safeguard the tractor's air supply, should a large air leak develop in the flexible tractor to trailer coupling hose or if any other fault causes a loss of air pressure.

Secondary line circuit (Fig. 12.2) Applying the hand control valve lever delivers a controlled air pressure from the secondary park reservoir to the front wheel secondary chambers, which form part of the double diaphragm actuators, and to the secondary line (red) coupling, which then delivers pressurized air to the trailer brakes via a flexible hose. Note that there is no secondary braking to

Air Over Hydraulic Systems
Fig. 12.2 Tractor three line brake system

the tractor's rear axle to reduce the risk of jack-knifing during an emergency application.

Parking circuit (Fig. 12.2) Applying the hand brake lever opens the hand brake valve so that pressurized air flows to the rear axle parking line chambers within the double diaphragm actuators to apply the brakes. At the same time, the mechanical parking linkage locks the brake shoes in the applied position and then releases the air from the parking actuator chambers. This parking brake is therefore mechanical with air assistance.

12.2.3 Trailer three line brake system (Fig. 12.3) All trailer air braking systems have their own reservoir which is supplied through the emergency line from the tractor's service reservoir.

Service line circuit (Fig. 12.3) When applying the brakes, air pressure from the tractor's relay valve signals the emergency relay valve to open and supply air pressure from the trailer's own reservoir to the trailer's service line brake actuator chambers relative to that applied to the tractor brakes. The

Double diaphragm actual or

Fig. 12.3 Trailer three line brake system

Double diaphragm actual or

Fig. 12.3 Trailer three line brake system object of the separate reservoir and relay valve installed on the trailer is to speed up the application and release of the trailer brakes, which are at some distance from the driver's foot control valve. Should there be a reduction in emergency line pressure below some predetermined minimum, the emergency relay valve will sense this condition and will automatically apply the trailer service brakes.

Secondary line circuit (Fig. 12.3) The secondary braking system of the trailer is controlled by the hand control valve mounted in front of the driver. Moving the hand control valve lever towards the applied position delivers a graduable air pressure via the secondary lines to the secondary chamber within each double diaphragm actuator. A quick release valve incorporated at the junction between the trailer's front and rear brakes speeds up the exhausting of the secondary chambers and, therefore, the release of the secondary brakes.

To release the trailer brakes when the trailer is detached from the tractor caused by the exhausting of the emergency line, a reservoir release valve is provided which should be moved to the 'open' piston to release the trailer brakes.

12.2.4 Towing truck or tractor spring brake three line system (Fig. 12.4)

Compressed air supply (Fig. 12.4) Air pressure is supplied by a compressor driven off the engine. Built into the compressor head is an unloaded mechanism which is controlled by a governor valve and which senses pressure change through the wet tank. Installed on the intake side of the compressor is an alcohol evaporator which feeds in very small quantities of alcohol spray when the compressor is pumping. As a result, it lowers the freezing temperature of the wet air induced into the compressor cylinder. When the compressor is running light, a check valve prevents alcohol spray entering the airstream, thereby reducing the alcohol consumption. The compressor supplies pressurized air to both service and secondary/ park reservoirs via non-return check valves.

Service line circuit (Fig. 12.4) When the driver depresses the dual foot valve, air flows from the service reservoir through the service delivery line (yellow) directly to the front wheel service line actuator chamber, and indirectly via a variable load valve which regulates the air pressure,

Fig. 12.4 Towing truck or tractor spring brake three line system

according to the loading imposed on the rear axle, to the rear wheel service chamber actuators. Compressed air is also delivered to both the service and the emergency line couplings via the relay valve and the pressure protection valve. This therefore safeguards the tractor air supply should there be a hose failure between the tractor and trailer. A differential protection valve is installed between the service line and the secondary/park line to prevent both systems operating simultaneously which would overload the foundation brakes.

Secondary/park line circuit (Fig. 12.4) Air is supplied from the secondary/park reservoir to the hand control valve and to a pair of relay valves. One relay valve controls the air delivered to the spring brake actuator, the other controls the service line air supply to the trailer brakes. With the hand control valve in the 'off' position, air is delivered through the secondary/park line relay valve to the spring brakes. The secondary/park spring brakes are held in the released position due to the compression of each power spring within the actuator. As the spring brakes are being released, the secondary line to the trailer is exhausted of compressed air via its relay valve. Moving the hand control valve lever to the 'on' position progressively reduces the secondary/park line pressure going to the spring brake. The secondary line pressure going to the trailer coupling increases, thereby providing a tractor to trailer brake match. Moving the hand control valve to the 'park' position exhausts the air from the trailer secondary line and the spring brake secondary/park line. The tractor foundation brakes are then applied by the thrust exerted by the power spring within the actuator alone. The release of the parking brake is achieved by delivering air to the spring brake when the hand control valve is moved to the 'off position again.

12.2.5 Towing truck or tractor spring brake two line system (Fig. 12.5)

Compressed air supply (Fig. 12.5) The air supply from the compressor passes through the air dryer on its way to the multi-circuit protection. The output air supply is then shared between four reservoirs; two service, one trailer and one secondary/ park reservoirs.

Service line circuit (Fig. 12.5) The air delivered to the service line wheel actuator chambers is

Fig. 12.5 Towing truck or spring brake two line system

provided by a dual foot valve which splits the service line circuits between the tractor's front and rear wheels. Therefore, if one or other service line circuit should develop a fault, the other circuit with its own reservoir will still function. At the same time as the tractor service brakes are applied, a signal pressure from the foot valve passes to the multi-relay valve. This opens an inlet valve which permits air from the trailer reservoir to flow to the control line (service line — yellow) trailer coupling.

To prevent both service line and secondary/park line supplies compounding, that is, operating at the same time, and overloading the foundation brakes, a differential protection valve is included for both the front and rear axle brakes.

Secondary/park line circuit (Fig. 12.5) A secondary braking system which incorporates a parking brake is provided by spring brakes which are installed on both front and rear axles. Control of the spring brakes is through a hand valve which provides an inverse signal to the multi-relay valve so that the trailer brakes can also be applied by the hand control valve.

With the hand control valve in the 'off' position the secondary line from the hand valve to the multi-relay valve, and the secondary/park line, also from the hand valve, going to the spring brake actuators via the differential protection valves, are both pressurized. This compresses the power springs, thereby releasing the spring brakes. During this period no secondary line pressure signal is passed to the trailer brakes via the multi-relay valve.

When the hand valve is moved towards the 'applied' position, the secondary line feeding the multi-relay valve and the secondary/park line going to the spring brakes reduces their pressures so that both the tractor's spring brakes and the trailer brakes are applied together in the required tractor to trailer proportions.

Moving the hand valve lever to the 'park' position exhausts the secondary/park line going to the spring brakes and pressurizes the secondary line going to the multi-relay valve. As a result, the power springs within the spring actuators exert their full thrust against the foundation brake cam lever and at the same time the trailer control line (service line) is exhausted of compressed air. Thus the vehicle is held stationary solely by the spring brakes.

Multi-relay valve (Fig. 12.25(a-d)) The purpose of the multi-relay valve is to enable each of the two service line circuits to operate independently should one malfunction, so that trailer braking is still provided. The multi-relay valve also enables the hand control valve to operate the trailer brakes so that the valve is designed to cope with three separate signals; the two service line pressure signals controlled by the dual foot valve and the hand valve secondary pressure signal.

Supply dump valve (Fig. 12.26(a, b and c)) The purpose of the supply dump valve is to automatically reduce the trailer emergency line pressure to 1.5 bar should the trailer service brake line fail after the next full service brake application within two seconds. This collapse of emergency line pressure signals to the trailer emergency valve to apply the trailer brakes from the trailer reservoir air supply, overriding the driver's response.

12.2.6 Trailer two line brake system (Fig. 12.6) The difference with the two and three line trailer braking systems is that the two line only has a single control service line, whereas the three line has both a service line and a secondary line.

Control (service) line circuit (Fig. 12.6) On making a brake application, a pressure signal from the tractor control (service) line actuates the relay

Trailer Air Brake System Diagram
Fig. 12.6 Trailer two line brake system

portion of the emergency relay valve to deliver air pressure from the trailer reservoir to each of the single diaphragm actuator chambers. In order to provide the appropriate braking power according to the trailer payload, a variable load sensing valve is installed in the control line ahead of the emergency relay valve. This valve modifies the control line signal pressure so that the emergency relay valve only supplies the brake actuators with sufficient air pressure to retard the vehicle but not to lock the wheels. A quick-release valve may be included in the brake actuator feed line to speed up the emptying of the actuator chambers to release the brakes but usually the emergency relay valve exhaust valve provides this function adequately. If the supply (emergency) line pressure drops below a predetermined value, then the emergency portion of the emergency relay valve automatically passes air from the trailer reservoir to the brake actuators to stop the vehicle.

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