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16 Cards in this Set
- Front
- Back
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Air brakes are really three different braking systems:
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- The service brake: applies andreleases the brakes when you use thebrake pedal during normal driving
-The parking brake: applies andreleases the parking brakes when you usethe parking brake control -The emergency brake: uses partsof the service and parking brake systemsto stop the vehicle in a brake system |
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Air Compressor
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- pumps air into the air storage tanks (reservoirs)
- connected to the engine through gears or a v-belt - may be air cooled or may be cooled by the engine cooling system - May have its own oil supply or be lubricated by engine oil (if it has its own oil supply check the oil before driving)
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Air Compressor Governor
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-controls when the air compressor willpump air into the air storage tanks
- When air tankpressure rises to the "cut-out" level (around 125pounds per-square-inch or "psi"), the governor stops the compressor from pumping air - When thetank pressure falls to the "cut-in" pressure (around100 psi), the governor allows the compressor to start pumping again |
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Air Storage Tanks
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- used to hold compressed air - number and size of air tanks varies among vehicles - will hold enough air to allow the brakes to be used several times, even if the compressor stops working |
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Air Tank Drains
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- water and oil are bad for the air break system (eg. water can freeze and cause break failure) - Be sure that you drain the air tanks completely - there are two types of drain valves: *Manually operated: turn a quarterturn or by pulling a cable. You must drain the tanks yourself at the end of each day of driving *Automatic: the water and oil are automatically expelled. These tanks maybe equipped for manual draining as well |
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Alcohol Evaporator
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- puts alcohol into the air system
- helps to reduce the risk of ice in air brake valves and other parts during cold weather |
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Safety Valve
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- is installed in the first tank the air compressor pumps air to
- protects the tank and the rest of the system from too much pressure - is usually set to open at 150 psi - If the safety valve releases air, something is wrong. Have the fault fixed by a mechanic. |
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The Brake Pedal (also called the foot valve or treadlevalve)
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- Pushing the pedal down harder applies more air pressure.
- Letting up on the brake pedal reduces the air pressure and releases the brakes. -Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced.It must be made up by the air compressor. - Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes won't work. |
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Foundation Brakes
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- Foundation brakes are used at each wheel.
- The most common type is the s-cam drum brake. - Wedge brakes and disc brakes are less common than s-cam brakes |
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Brake Drums, Shoes, and Linings
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Brake drums are located on each end of the vehicle's axles. The wheels are bolted to the drums. The braking mechanism is inside the drum. To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle (and creates heat). The heat a drum can take without damage depends on how hard and how long the brakes are used. Too much heat can make the brakes stop working.
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S-cam Brakes
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When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft. This turns the s-cam (so called because it is shaped like the letter "S"). The s-cam forces the brake shoes away from one another and presses them against the inside of the brake drum.When you release the brake pedal, the s-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again.
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Wedge Brakes
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In this type of brake, the brakechamber push rod pushes a wedge directly betweenthe ends of two brake shoes. This shoves themapart and against the inside of the brake drum.Wedge brakes may have a single brake chamber,or two brake chambers, pushing wedges in at bothends of the brake shoes. Wedge type brakes maybe self-adjusting or may require manual adjustment
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Disc Brakes
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In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like s-cam brakes. But instead of the scam,a "power screw" is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper,similar to a large c-clamp.
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Supply Pressure Gauges
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- All vehicles with air brakes have a pressure gauge connected to the air tank.
- If the vehicle has a dual air brake system, there will be a gauge for each half of the system. (Or a single gauge with two needles.) -These gauges tell you how much pressure is in the air tanks. |
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Application Pressure Gauge (This gauge is not on all vehicles.)
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- shows how much air pressure you areapplying to the brakes
- Increasing application pressure to holdthe same speed means the brakes are fading - Youshould slow down and use a lower gear -The needfor increased pressure can also be caused bybrakes out of adjustment, air leaks, or mechanicalproblems |
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Low Air Pressure Warning
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- required on vehicles with air brakes
- warning signal you can see must come on before the air pressure in the tanks falls below 60 psi. (Or one half the compressor governor cutout pressure on older vehicles.) - can be red light, buzzer, or wig wag (this device drops a mechanical arm into your view when the pressure in the system drops below 60 psi.) - on large buses it is common for the low pressure warning devices to signal at 80-85 psi. |
