Re: Questions about AIR BRAKES ???

Posted by RodInNS on January 12, 2006 at 14:27:20 from (142.177.102.222):

In Reply to: Questions about AIR BRAKES ??? posted by Way Up Here on January 11, 2006 at 18:47:13:

Air brakes are used on heavy trucks and equipment due to the ease of applying large amounts of breaking power with minimum driver effort, along with the ease of coupling multiple units together. Air lines are easy and reliable to couple; hydraulic lines are not so easy of reliable. The fail safe aspect of the system is also of primary concern. As has been stated in several other posts, if the truck starts losing air pressure, the maxi (parking) brakes will start to apply below 60 PSI. Short of having a quick release valve fail with the maxi's off, or the adjustment being run out too far, air brakes are fail safe. They will always stop you in the event of no air.
The system itself consists of a compressor, quite often an air dryer, then a primary resevoir, and then often a secondary resevoir, with check valves between the tanks. From there the service brakes are controlled by a treadle valve, with the front wheels getting air supply directly from the treadle valve, and each axle behind having it's own relay valve, which is controlled by the treadle valve. The relay valves are tied into the air tanks by large lines, to provide quick supply of air close to the brake chambers. The service brakes are applied by air pressure, which can range up to 150 PSI on some trucks, depending upon how high the govenors are turned up on the compressor.
The parking brakes are applied by spring pressure. As stated above, the system needs about 60 PSI to fully release the parking brakes. The maxi brakes are also set up with a quick release valve near the rear axles to speed the release of air form the chambers, rather than exhaust the air out through the control valve in the dash.
The brake chambers themselves are shown in another post, with the single chambers being a straight service brake, and the double chamber being a piggyback, with the service and parking brake acting on the same linkage. The brake chambers themselves act on a slack adjuster, which turns an S-cam, which acts on hte rollers of the brake shoes. The shoe rollers ride the "S", and cause the shoe to expand against the drum. The slack adjuster is used to compensate for wear in the shoes, while keeping the the chamber rod within it's operating limits. That's about the extent of it, summed up in one page.... from a 30 page book.....
Air brakes are generally more reliable than hydraulic brakes, with the exception of systems that are bad to freeze without the air dryer. Gotta have that up here in the great white north.... Hope I didn't create more confusion...

Rod

Replies:

• Re: Questions about AIR BRAKES ??? old 21:19:04 01/12/06 (4)
  • Re: Questions about AIR BRAKES ??? RodInNS 13:56:43 01/14/06 (3)
    • Re: Questions about AIR BRAKES ??? old 14:06:49 01/14/06 (2)
      • Re: Questions about AIR BRAKES ??? RodInNS 09:25:02 01/15/06 (1)
        • Re: Questions about AIR BRAKES ??? old 09:45:20 01/15/06 (0)

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