According to NFPA 1901 : standard for Automotive Fire Apparatus ( Section 12.3.1 ), air-actuated braking systems must include the following :
- An automatic moisture ejector
- An air dryer
- A pressure protection valve to prevent all air-operated accessories from drawing air from the air brake system when the air systems pressure drops below 80 psi
- A quick build-up section in the air reservoir system arranged so that if the apparatus has a completely discharged air system, it can move within 60 seconds of startup
The standard explains : “ The immediate buildup arrangement shall provide enough air imperativeness so that the apparatus has no brake drag and is able to stop under the mean engage conditions following the 60-second buildup meter. For a chassis that can not be equipped with a quick buildup atmosphere bracken system, an onboard automatic electric compressor or a fire station compressed air shoreline hookup shall be permitted in arrange to maintain full operating vent pressure while the fomite is not running. ”
All air brake systems use air atmospheric pressure to apply the brakes when you step on the bicycle. The breeze is pressurized by the air compressor on the truck ‘s engine and then stored in a series of pressure tanks on the truck. An tune governor on the compressor regulates the press. Most, if not all systems, cultivate on 120 pounds per square inch of pressure.
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Most systems have an air dry, which removes moisture from the air to inhibit corrosion and keep the system from freezing in the winter. The air dry has a cartridge that should be maintained according to the manufacturer ‘s recommendations .
The supercharge air passes through the vent dry and into the tanks. From the tanks, it goes to two unlike systems .
air brakes: two-component system
Air brakes for heavy trucks, including fire apparatus, are a two-component system. The first component releases the parking brake and holds some reserve air travel to stop you if there is a escape. The parking brake on most trucks has springs in the air chambers on the rear axle or axles that apply the park brakes .
Pushing the parking brake knob in supplies air travel to those tune chambers and pushes against the springs to release the park brakes. If the pressure in the air organization pulls below 60 pounds per square inch, the parking brake knob will pop out and set the parking brakes. This is a safety feature so the truck will stop, because if the pressure falls besides gloomy, the service brakes will not work .
The early system part stops the vehicle when you push on the brake pedal. When you push on the brake pedal point it goes through the bicycle valve, which modulates the airflow to the brake chambers. The brake chambers in turn advertise on the abate adjusters, which keep the brakes in adaptation .
There are different types of slump adjusters : manual and automatic. manual slack adjusters are precisely as they sound. You must manually adjust them to keep the brakes in adaptation. many truck mechanics say that this is the prefer style .
The automatic pistol slack adjusters automatically keep the brakes in alteration. These work well if you keep them greased. The problem is that most people don ’ thyroxine keep and they seize up. If you have automatics, the best grease to use is white lithium grease .
The slack adjusters turn a diaphysis that turns the S-cams, which push the brake shoes out into the drums. The S-cam shafts have bushings that should be checked every time you change the shoes. They besides require grease .
air brakes: Replacement parts
There are such things as air disk brakes, but they are rare. Most brake shoes have an index built into the end of the pads that tell when they need to be replaced .
When you replace the bracken shoes get hardware kits that include the springs, pins and bushings for the brakes, and replace the drums. This is a good estimate because the drums will have a wear ridge and could besides have heat cracks.
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here are some steps to take before heading out on the road :
- Make sure the minimum operating pressure for a vehicle air-brake system is no less than 100 psi for a truck.
- Check that it takes no longer than 2 minutes for air pressure to rise from 85 psi to 100 psi at 600 to 900 rpm. This is called the air pressure buildup rate.
- Confirm that the correct cut-out governor pressure for the air compressor is between 120 psi and 135 psi. Cut-in pressure is 20 psi to 25 psi below cut-out pressure.
Winter weather practices
watch for urine in the air-brake arrangement, a by-product of the condense publicize. Air-brake lines do n’t like water, specially in cold climates where ice can block air from reaching the brake mechanism and cause the rack to lock .
To prevent this problem, many advanced systems have automatic pistol drain valves installed in each air cooler. Check these valves according to the manufacturer ‘s recommendations on a regular basis to ensure they are functioning by rights, particularly during extreme cold conditions .
The air travel dry is worth its slant in gold. In cold climates, water can freeze in the valves and cause dangerous problems with the mathematical process of your air-brake system .
Air dry care is imperative mood. If you have n’t done so ahead winter arrived, now is the arrant time to have your air dry filter serviced according to the manufacturer ‘s recommendations .
I posted a wonder on the Emergency Vehicle Technician TechTalk forum asking if air brake system anti-freeze was necessary or if the vent dryer was sufficient. I received several replies from EVTs and they were all reproducible in their message : Take commodity care of the breeze dry and the air dry will take care of you .
Stopping distances on slippery surfaces are from three to 12 times loner than on dry roads. Under ideal conditions, the safe following distance dominion is 1 second for every 10 feet of vehicle length, so adjust your distance according to the existing conditions .
A recommend way to stop safely without Antilock Brake System brakes is to use a rapid light pump of the brakes. By pumping the brakes, you can maintain steering control. Apply the brakes for an clamant and release them. Repeat this military action until you come to a complete diaphragm .
ABS automatically pumps the brakes if the vehicle ‘s wheels begin to lock up. This allows the vehicle operator to maintain effective steering dominance and reduces the hazard of skidding. The bracken pedal will pulsate, but this is normal.
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With tune brakes, be careful to avoid reducing the air pressure to a low grade. The air pressure required to lock wheels on frost can be a short as 10 psi, so a great deal of pumping can be done with a gentle touch on the brake pedal point .
winter driving with air brakes can best be summarized as follows : Keep the system dry and keep the pressure up .
This article, primitively published in 2014, has been updated .