The Brake System:
Air-Hydraulic Brake System Components
Air Reservoir
Two round steel tanks are used on each truck to hold a supply of compressed air. The tanks are large enough to provide enough air under pressure for several brake applications after the engine has stopped running. The reservoirs also provide a place to trap condensed oil and water vapors. The air, which is heated during compression, is cooled in the tanks causing any vapors in the air to condense. A drain cock is provided in the bottom of each reservoir to drain trapped condensation.
Safety Valve
A relief valve, known as the safety valve, is installed on one of the reservoirs. It is used to prevent air pressure in the system from building up above a safe operating pressure if the governor or unloader mechanism should fail. The valve is held closed by spring pressure and opened by air pressure. When the air pressure in the reservoir exceeds 150 PSI, the valve opens exhausting the excess pressure.
Warning Signal Buzzer
An electrically operated buzzer is located under the dash panel to warn the vehicle operator if the air pressure falls below a safe operating level. The buzzer sounds when an air-operated switch closes to connect an electrical circuit between the buzzer and the vehicle batteries. The switch is generally mounted under the dash panel near the buzzer and is connected to an air line from the air reservoir. Air pressure on a piston in the switch tends to open the switch contacts, while spring pressure tends to close the switch contacts. When the air pressure falls below 60 PSI, spring pressure closes the switch and the buzzer sounds.
Pressure Gage
An air pressure gage is located on the dash panel to show the amount of pressure in the air system. The gage is made to read pressures from 0 to 120 PSI.
Hand Control Valve
Some trucks have a hand control valve mounted on the steering column for individual control of the brakes on towed vehicles. Two air lines are connected to the valve: One is a supply line from the truck's air reservoir, and the other is a control line that is attached to the brakes of the trailer. A third threaded air opening in the valve is an air exhaust outlet which is left open.
The hand control valve assembly has an inlet and an exhaust valve mounted on one stem. The valve is normally held in the exhaust position by the intake and exhaust valve spring. One type of hand control valve has a movable exhaust tube that is pushed upward by an exhaust tube spring and downward by a cam plate spring. A hand lever on the control valve is used to rotate a cam changing the tension on the cam plate spring.
With the hand lever in the released position, the exhaust tube spring holds the exhaust tube up. Any air pressure in the trailer brake control line is exhausted through the exhaust tube and outlet on the control valve. Pulling the hand lever to the applied position increases the cam plate spring tension on top of the exhaust tube. This moves the exhaust tube down, first contacting the exhaust valve and blocking the air exhaust passage through the tube. Then it moves the air exhaust and inlet valve assembly down, pushing the air inlet valve off its seat. Air supply pressure flows past the open inlet valve into the control line to apply the trailer brakes. The control air pressure also pushes upward on the exhaust tube tending to lift it and close the inlet valve. When the pressures above and below the tube are equal, the inlet valve closes. By changing the position of the hand lever, the amount of spring tension pushing the exhaust tube down will change, and the driver can regulate the amount of control line pressure. |