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Originally Posted by Sankar Can someone tell me why heavy vehicles use air-brakes instead of hydraulic braking systems found in cars and light commercial vehicles? What are the benefits of having air brakes? |
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Originally Posted by pjbiju From what I know, trucks, trains and other large vehicles use air brakes.
Air pressure is used to release the brakes and hence until the required pressure is built up, the brakes would remain engaged. This is the opposite of hydraulic brakes where hydraulic pressure is used to engage the brakes. |
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Originally Posted by BaCkSeAtDrIVeR I am afraid that this is not entirely accurate.
But my 1978 model AL had air brakes, and it was pressure actuated, NOT vacuum brakes. A small leak anywhere in the circuit and brakes would malfunction. |
Shankar, a cheeky answer if you like
Air is everywhere. Hydraulic fluid isn't.
But really not cheeky
The heavier forms of transport - Trains, buses and tractor-trailers - use air-brake systems so they don’t have to rely on the hydraulic fluid (which may not be there) in car braking systems, which can run out in the event of a leak.
The first air barking systems introduced were -
direct air-brake systems, which used an air compressor to feed air through a brake pipe into air tanks on each car. When the engineer applied these brakes, the pipe filled with air and squeezed the brakes. (These were the systems that the
Backseatdriver has stated was existing in our heavy vehicles until 1980s)
Actually the
vacuum based system was introduced in 1869, an engineer named
George Westinghouse realized the importance of safety in the relatively new railroad industry and invented the first triple-valve air-brake system for railcar use.
Westinghouse’s system worked the opposite way of a direct air-brake system. The triple-valve system performed three functions, thus its name (This is what the system
pjbigu has stated as in use in today’s heavy vehicles)
Charging: The system must be pressurized with air before the brakes will release. At rest, the brakes remain engaged. Once the system reaches its operating pressure, the brakes are freed and ready to use.
Applying: As the brakes are applied, air pressure decreases. As the amount of air decreases, the valve allows air back into the reservoir tanks, while the brakes move to the applied position.
Releasing: Once the brakes are applied and the air escapes after braking, the increased pressure releases the brakes.
Now when/how the change came about
In US of A in the 1970's truckers were dying whenever they lost air pressure heading down steep mountain grades. The Department of Transportation and the Federal Bureau of Motor Vehicles Safety Division changed the requirements for air brake systems on all motor vehicles.
The Direct Air Brake Systems would not work if there was a loss of air. Truckers would have a runaway truck on their hands. As per the federal intervention in the late '70s or early '80s the engineering of these brake systems was reversed (functionally almost similar to the Westinghouse System)
Now when air brakes are applied, there is a valve that receives the applied air pressure. When the valve begins to open, it releases stored air pressure that is being held inside a chamber. That chamber has a heavy spring inside of it that begins to apply brake pressure in the absence of air pressure. Now when a trucker loses air pressure on a mountain grade, the spring will apply fully to lock up the brake shoes against the brake drum, preventing (hopefully) a runaway truck/bus.
This was further refined with dual tanks and dual circuits, ABS etc.
Shankar - The air brake is not there (commonly) in cars due to the practicality and requirement of it. Cars have relatively smaller mass and even in case of a complete brake failure can be controlled with the engine braking and use of gear box, hand brake etc. The mass of a cra will not give it sufficient momentum to acheive the the proportions of a runaway truck/bus Also we really have not heard of a runaway car. The worst runaway bus I can remember in Kerala was the Elappara one in the mid 1980s).
Now with many cars having independent circuits for front & back or diogannaly the barking system has enough redundancy built in (I hope so). Hence no air brakes in cars
Hope you would find this to be useful
Raj – the buses look lovely and all the best with the venture and wish you a very long and fruitful ownership of them.
Best Regards & Drive/Ride Safe
Ram