Double diaphragm chamber type actuator

(Fig. 12.28(b)) The double diaphragm chamber actuators are designed to be used when there are two separate air delivery circuits, known as the service line (foot) and the secondary (hand) line, systems operating on each foundation brake.

Service line application When the service (foot) brake is applied compressed air enters the service chamber via the intake ring. As the air pressure rises, the service diaphragm and push rod are forced outwards, applying the leverage to the slack adjuster.

Secondary application When the secondary (hand) brake is applied, compressed air enters the secondary chamber via the central pressure plate port. Rising air pressure forces both diaphragm and push rod outwards again, applying leverage to the slack adjuster and expanding the shoes against the brake drum.

Service and secondary brake line failure Should the service diaphragm puncture, the hand control valve secondary line can be used to operate the secondary diaphragm and push rod independently to apply the brakes.

Should the secondary diaphragm fail, the foot control valve service line will provide an alternative braking system. The air pressure in the service chamber automatically pushes the secondary diaphragm back against the central inlet port seal, preventing service line pressure escaping back through the secondary line due to a damaged or leaking secondary diaphragm.

If the secondary diaphragm and seal should fail together the service brake will still operate, provided the hand control valve is moved to the fully applied position because in this condition the exhaust valve exit for the secondary line is closed.

Triple diaphragm chamber type actuator (Fig. 12.28(c)) The triple diaphragm chamber actuator functions similarly to the double diaphragm actuator. Both types of actuators are designed to accommodate and merge both service and secondary braking systems into one integral wheel brake actuator, hence permitting both brake systems to operate independently of each other but applying their braking force and movement to one common slack adjuster lever.

Service line application When the service (foot) brake is depressed, compressed air passes through the inlet ring port between the central diaphragm and the service diaphragm nearest the push rod assembly. With increased air pressure, the secondary

Diaphragm Civil Engg

Fig. 12.28(a-d) Air brake actuator chambers

Brakes relea

Fig. 12.28(a-d) Air brake actuator chambers and central diaphragm react against the pressure plate chamber and the service diaphragm forces the push rod assembly outwards. This results in the rotation of the slack adjuster lever, camshaft and cam expander, causing the brake shoes to grip the drums and apply the brakes.

Secondary line application When the secondary (hand) brake is operated, compressed air is admitted through the central end port between the pressure plate and the secondary diaphragm, causing the diaphragm and push rod assembly to expand outwards, again applying the brakes similarly to the service brake application. Releasing the hand operated lever permits the compressed air to exhaust out of the chamber through the hand control valve into the atmosphere. The return spring is now able to move all three diaphragms in the opposite direction towards the pressure plate chamber.

Service and secondary brake line failure Should the service diaphragm or central diaphragm rupture, there will be no interference with operation of the secondary diaphragm chamber. Likewise, if the secondary diaphragm fails, the service line air pressure operates only between the central and service diaphragm so that they will apply the brakes as normal.

Note that with the triple diaphragm arrangement, the central diaphragm performs the same function as the central inlet port seal in the secondary diaphragm chamber so that there is no possibility of a back leakage problem as with the double diaphragm actuator.

Diaphragm/piston chamber type actuator (Fig. 12.28) This diaphragm/piston type actuator operates similarly to the double and triple diaphragm type actuators, but because the whole cross-sectional area of the piston is effective in applying air pressure thrust (as opposed to a variable effective diaphragm area) the service chamber has a high mechanical efficiency.

Pusher type piston diaphragm chamber (Fig. 12.28(d))

Service line application When the brake pedal is depressed and the foot valve opens, air is supplied to the service port of the brake chamber. Air pressure is then applied between the rear of the chamber and the piston face. The thrust produced on the piston moves the piston, diaphragm and push rod outwards against the resistance of the return spring. The force and movement of the push rod actuates the wedge type of brake shoe expander, normally used with this type of brake actuator, to apply the brakes. Releasing the foot pedal permits the compressed air in the chambers to exhaust through the foot control valve into the atmosphere.

Secondary line application When the secondary hand control valve is operated, air is delivered to the brake chamber secondary port where it enters the space made between the piston and the diaphragm. As the air pressure increases, the piston reacts against the brake chamber's rear wall. The diaphragm and push rod are forced outwards again to force the wedge between the brake shoe expander to apply the brakes.

If air failure occurs in either the service or secondary chambers the other braking system can be operated independently to apply the brakes.

Puller type piston-diaphragm chamber (Fig. 12.28(e))

Service line application When air is delivered to the service chamber port, the piston, diaphragm, central sleeve and the wedge expander rod are all pulled outwards from the rear wall of the brake chamber, actuating the wedges of the brake shoe expander to apply the brakes.

Secondary line application Similarly when the secondary hand control is operated, air passes through the secondary chamber port to the space between the piston and diaphragm, so that the diaphragm and the central sleeve assembly are moved outwards from the chamber rear wall. As before, the brakes are applied.

When either the service or secondary line air pressure operates the brake chamber actuator, the hand brake draw rod remains stationary. When the hand brake is operated the draw rod connected to the hand brake cable (not shown) operates the sleeve and pull rod applying the brakes.

Spring brake actuator (Fig. 12.29(a-d)) Spring brake actuators are designed for use in air pressure braking systems to produce the force and movement necessary to operate the foundation brakes. The actuator uses air pressure to respond and control both service and secondary braking. The conversion of air pressure into mechanical force and travel for the service braking systems is obtained by a conventional diaphragm-operated servo chamber. In contrast, secondary braking and parking is achieved through the strain energy stored in a powerful coil spring which, when permitted to expand, applies mechanical effort and displacement to the foundation brake shoe expander.


All brakes released (Fig. 12.29(a)) Under normal driving conditions both service and secondary/ park brake systems must be in the 'off' position. To release both braking systems, air pressure (above the low signal pressure) is supplied to the spring piston chamber which compresses the power spring and holds off the foundation brakes. At the same time air pressure is exhausted from the service line diaphragm chamber.

Service line applied (Fig. 12.29(b)) Air pressure controlled progressively by the foot valve is supplied to the service diaphragm chamber port. The cross-sectional air of the diaphragm exposed to the air pressure is subjected to a thrust forcing the push rod outwards. The combined movement and force is applied to the slack adjuster which then relays it to the camshaft, expander cam and shoes of the

Functions Brake Chambers Vehicles
Fig. 12.29(a-d) Spring brake actuator

foundation brakes. During service brake operation the power spring is held in compression by the secondary line air pressure so that it does not compound the service brake operational force.

Secondary/parking applied (Fig. 12.29(c)) Air pressure, controlled progressively by the hand valve, is released from the piston spring chamber, permitting the power spring to expand and push the plunger and push rod outwards against the resistance of the brake cam which is expanding the brake shoes up to the drum. When the vehicle is at a standstill, the hand lever is moved from secondary to park position, where any remaining air pressure in the secondary line and piston spring chamber is exhausted via the open hand control valve to the atmosphere. The brakes are then held on purely by the strain energy of the extended power spring and therefore are not dependent upon air pressure with its inherent leakage problem.

Manual release applied (Fig. 12.29(d)) Should there be a secondary line air supply failure, then the foundation brakes may be released for towing purposes or removal of the spring brake actuator by the readily accessible release bolt. Winding out the release bolt permits the plunger to move into the wind-off sleeve so that the push rod is able to return to the 'off' position, thereby releasing the brake shoes.

Do It Yourself Car Diagnosis

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