The Brake System:
Operation Of The Air-Hydraulic Trailer Brake System
Several different applications of air-over-hydraulic brake systems are used on trailers, but they can be broken down into two general types--brake systems with automatic breakaway protection, and brake systems that do not have breakaway protection.
Trailer air-over-hydraulic brake systems that do not have breakaway protection have only one air hose coupling which must be connected to the service trailer coupling on the truck. When either the brake foot pedal or the hand control valve on the truck is applied, brake controlled air pressure flows through the service air line to the trailer.
On the trailer, the air line carries the controlled air to an air filter and then to the brake air chamber. The brake air chamber changes the air pressure to the mechanical motion needed to work the hydraulic master cylinder. As the air chamber pushrod moves the master cylinder piston, brake fluid is forced into the wheel cylinders to apply the wheel brakes.
When the brakes are released by the driver, air pressure in the trailer brake air chamber flows back to the truck through the service line and is exhausted through the hand control valve or the air-hydraulic cylinder control unit.
Trailers with automatic breakaway protection have an emergency air hose coupling, emergency relay valve, and an air reservoir plus all the parts used on trailer brake systems without breakaway protection. The action of this brake system is controlled by the emergency relay valve. Let's discuss its operation.
With the air hose couplings connected to the truck and the air shutoff cocks open, compressed air from the truck reservoirs flows through the emergency line to the supply air inlet opening of the emergency relay valve. The air flows through the check valve in the center of the emergency valve diaphragm, through the air supply passage, and out the reservoir line opening to the trailer reservoir. The airflow continues until the pressure in the trailer reservoir is equal to the pressure in the truck reservoir. Under these conditions the air pressure above and below the emergency diaphragm is equal, so the emergency diaphragm spring holds the diaphragm in the raised position.
When the truck driver applies the brakes, controlled air from the truck flows through the service line to the control line inlet at the top of the emergency relay valve. An internal passage carries the controlled pressure to the cavity above the relay valve diaphragm. Because this cavity is small, the controlled air fills it quickly and moves the diaphragm and relay valve plunger down, compressing the relay valve diaphragm spring. The valve plunger closes the exhaust valve and then compresses the valve spring, opening the air inlet valve. Compressed air now flows from the trailer reservoir through the reservoir line opening, past the open air inlet valve, and out the outlet opening to the brake chamber. The brake chamber rod extends working the master cylinder and applying the brakes.
When the driver releases the brakes, controlled line pressure is released from the top of the relay valve diaphragm and the relay valve plunger. The diaphragm spring pushes the relay valve plunger up, opening the exhaust valve and permitting the valve spring to close the air inlet valve. Brake applied air pressure in the brake chamber is released to the open air by passing through the open center of the relay valve plunger to a cavity that is connected to the exhaust port of the emergency relay valve.
The emergency relay valve will maintain an air applied pressure in the brake chamber that corresponds with the air controlled pressure from the truck. This is known as the balanced or holding position and occurs when brake control pressure from the truck pushing down on the relay diaphragm is equal to the upward pressure on the diaphragm. Upward pressure is exerted by the diaphragm spring and brake applied air pressure. The applied pressure is admitted to the bottom side of the relay diaphragm through an internal passage from the air applied passage. When pressures on both sides of the diaphragm are equal, the diaphragm is held in the mid position so that both the air inlet valve and the air exhaust valve are closed, which maintains a definite amount of brake applied air pressure. If controlled pressure is increased, it pushes the valve plunger down opening the air inlet valve to increase applied pressure. If controlled pressure is decreased, the valve plunger moves up exhausting some applied pressure.
Now let's see how the emergency relay valve provides automatic breakaway protection. Since the emergency air line is connected to the air supply inlet opening, air pressure from the truck reservoir is present on the bottom side of the emergency valve diaphragm. Air pressure from the trailer reservoir is present on the top side of the emergency valve diaphragm from the air supply passage and the reservoir line inlet opening. Air can pass from the truck reservoir to the trailer reservoir through the one-way check valve in the emergency valve diaphragm, so that during normal operation the air pressure above and below the diaphragm is equal. At this time the emergency valve diaphragm release spring will hold the diaphragm in the raised position.
Assume that there is a loss of air pressure in the emergency line. This could be caused by the trailer breaking away from the truck and breaking the line or by a leaking or disconnected emergency air hose coupling. This removes the air pressure on the bottom side of the emergency valve diaphragm. Air pressure from the trailer reservoir on top of the diaphragm pushes the diaphragm down compressing the diaphragm release spring. The downward movement is transferred to the relay valve plunger, closing the air exhaust valve and opening the air inlet valve allowing compressed air to flow from the reservoir to the brake chamber locking the brakes.
The relay valve plunger will remain in the applied position as long as there is air pressure in the trailer reservoir or until the service line is reconnected to a truck. To release the trailer brakes without connecting the trailer to a truck, the drain cock in the bottom of the reservoir should be opened to allow all the compressed air in the system to escape. |