Care of Air Brakes Requires Understanding of System
With the increasing weight of fire apparatus (the gross vehicle weight of a pumper for example, has more than doubled in the last 30 years) the use of air brakes has become both common and mandatory for adequate braking When the gross vehicle weight is 25,000 pounds or more, air brakes should be considered, particularly for hilly areas. For vehicles with a gross weight of 30,000 pounds or more, air brakes are essential. See NFPA Standard No. 19, paragraph A-2312.
To effectively perform preventive maintenance on an air brake system, an understanding of the way the system works and the function of each of its components is essential. Figure 1 is a diagram of the basic, rapid buildup system as used on two-axle fire apparatus. As a basic system, it is relatively simple with a minimum of maintenance required. But, with the addition of accessory devices and controls, such as a front brake limiting valve, a brake locking valve, windshield wipers, air horns and spring brakes, to mention only a few, the need for preventive maintenance to preserve the original reliability is obvious.
An air brake system is precisely engineered to provide effective balance of braking with reliability. Brake chambers are selected to match the size of the brakes and the load to be carried on each axle. Tubing sizes are selected to provide a fast, equalized transmission of air pressure to all brakes. The air compressor is selected to provide a capacity adequate to meet emergency service needs. Each is a part of tire system engineering. So, the system will be effective only in direct proportion to the preventive maintenance it receives.
Rapid buildup system: Referring to Figure 1, the engine-driven air compressor provides the compressed air necessary to operate the braking system. On fire apparatus, a rapid buildup system quickly provides operating pressure in the system for normal service braking. In areas where responses average once a day or less the air pressure may decrease below the safe minimum of 60 psi.
With the rapid buildup system, only one reservoir is charged until the pressure reaches 65 psi. When this pressure is reached, the pressure protection valve opens to admit air to a second (usually larger) reservoir and any other reservoirs in the system. This rapid buildup reservoir may have a capacity of 830 cubic inches (7 x 24 inches) or it may be the same size as the main and other reservoirs, 1,200 cubic inches (8 x 26 inches). This system arrangement permits a quick buildup to over 60 psi in only 10 to 20 seconds with the engine running at a fast idle, 1,000 to 1,200 rpm. Thus no delay is experienced in response time on an alarm as the engine warmup and time for the crew to get in place on the vehicle involve more time than this.
The diagonally shaded lines on the diagram (Figure 1) indicate the air lines under pressure at all times. The double lines indicate the lines that have air pressure only when the brakes are being applied. This is controlled by the foot brake valve. A quick release valve is used in the air lines at each axle for both front and rear brakes. Its function is to quickly exhaust the air in the brake lines and brake chambers, permitting release of the brakes as the pressure is reduced or completely exhausted at the foot brake valve.
Pressure control: A governor operated by air pressure controls the air compressor discharge. The governor is connected to a separate line from the rapid buildup reservoir to the air compressor. When the compressor has charged all the reservoirs, the governor limits the compressor discharge pressure to 110 to 120 psi by unloading the compressor. In this unloading action the air pressure above the predetermined setting is used to hold the air compressor inlet valves off their seats. Thus, no work is done by the air compressor until the air pressure in the reservoirs drops to a cut-in pressure of 95 to 105 psi.
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There is a safety valve in the system to limit the pressure to 150 psi. Any pressure above this setting opens the valve to exhaust compressed air to the atmosphere.
Warning signals: The low-pressure warning signal unit is set to sound a warning buzzer and light a red signal lamp whenever the air pressure in the rapid buildup reservoir is below the normal safe operating pressure of 60 psi. The tolerance on this setting is plus or minus 6 psi. The air pressure gage may be used to confirm this setting.
The air brake system is designed to function effectively at pressures above 60 psi. Therefore, the vehicle should never be driven out of quarters if the low-pressure buzzer is sounding or the signal light indicates low pressure. The pressure must be built up sufficiently to cut off these warning signals.
Notice that there are check valves at one end of each reservoir, permitting air to flow in only one direction.
Moisture drains: Each reservoir has a drain cock so that moisture can be drained off. The air taken in by the air compressor contains a percentage of moisture or humidity, which is carried into the reservoirs as vapor. As the compressed air cools in the reservoirs, the vapor condenses. This condensation must be drained at regular intervals, nominally once a week. In the winter time, this maintenance is important to prevent the freezing of lines and valves. This is a simple operation, but it is important preventive maintenance.
In recent years, many fire departments have bought apparatus with diesel engines and air-operated brakes (airhydraulic or air). This combination requires some adjustments in maintenance procedures.
It is apparent that some fire departments have not fully understood the maintenance requirements for compressed air brake systems, as indicated by the increased number of reports of frozen brakes and air lines during this past winter.
When to purge reservoirs: In commercial service, the air reservoirs are usually purged daily to remove any condensed water. In the fire service, due to limited mileage and reduced frequency of operation, the quantity of water condensation in the air reservoirs is at a lower level. Therefore, it is recommended that the reservoirs be drained at least once a week. This is, however, only a general recommendation and does not apply to all fire apparatus. The busy company that responds two or more times daily should drain the reservoirs daily or on alternate days. For fire departments in the northern tier of states in the United States, and in Alaska and Canada, strict attention to this maintenance must be observed to avoid a freeze-up during the winter.
To simplify this maintenance, two optional types of reservoir drains are available—a semi-automatic remote control drain permitting the reservoir to be drained without getting under the apparatus and automatic drain valves.
An automatic drain valve for each reservoir (Bendix-Westinghouse No. DV-2) is recommended for emergency vehicle service, as the valve automatically performs this essential maintenance. The construction of this valve is shown in Figure 2.
Alcohol evaporator: In areas where atmospheric temperatures below 32°F are experienced, it is also necessary to protect the air lines from freezing of the moisture in the air. For this protection, an alcohol evaporator is required. This is a simple device, mounted usually on the rear wall of the engine compartment, and consists of a cast support unit with a polypropylene jar for methyl alcohol, a jar cover and a screen.
The installation can easily be made on apparatus now in service, as indicated by Figure 3 for apparatus where the air intake for the air compressor is through a filter mounted on the side of the compressor. Figure 4 is a schematic arrangement of the installation of an evaporator for a turbocharged diesel engine with the air intake for the air compressor supplied through the engine air cleaner.
The evaporator is connected to a spacer at the air inlet manifold of the air compressor, permitting some of the air taken into the air compressor to first pass through the liquid in the evaporator. This air picks up alcohol vapor and carries it into the braking system to prevent freezing.
Other than keeping a supply of pure methyl alcohol (188 proof) or BendixWestinghouse Air-Cuard in the evaporator jar, maintenance is required only once each year. This requires disassembly of the evaporator, cleaning of all parts and replacement of all gaskets.
Illustrations and technical data are through the courtesy of Bendix-Westinghouse Automotive Air Brake Co.
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