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Air brake over hydraulic brakes

  1. Jun 8, 2012 #1
    Air brake over hydraulic brakes!!!

    Why air brakes are used in long vehicles like trains, instead of hydraulic brakes?
     
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  3. Jun 8, 2012 #2

    sophiecentaur

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    Re: Air brake over hydraulic brakes!!!

    Air brakes are naturally 'on' on trains, at least they always used to be. The brakes are applied with springs and held off with 'vacuum pressure'. This means that the system is fail safe. Lose power and the brakes are applied. The emergency stop always used to open a valve on the vacuum line and all brakes on the train were applied.
     
  4. Jun 8, 2012 #3
    Re: Air brake over hydraulic brakes!!!

    When connecting brake lines between many trailers or cars, air works much better than hydraulic. But there are many different types of air brakes. In some cases the air actuates a hydraulic brake system. The most common is a canister that has two actuators in it. One is air applied, spring released. The other is spring applied, air released. The canister is supplied with two lines. One is the service brake, the other the parking or emergency brake. Diesel systems use an air compressor system, but most gasoline powered vehicles us a vacuum system.
     
  5. Jun 8, 2012 #4

    sophiecentaur

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    Re: Air brake over hydraulic brakes!!!

    In what way does air work better? Could it be a practical reason, to do with the relative ease of making non-permanent pneumatic connections? Also, a minor air leak doesn't involve the same dramatic loss of fluid that a hydraulic system would suffer.
     
  6. Jun 8, 2012 #5
    Re: Air brake over hydraulic brakes!!!

    The title on this these is confusing because a system that uses air to actuated a hydraulic brake system is called a "Air over hydraulic brake system."
     
  7. Jun 8, 2012 #6
    Re: Air brake over hydraulic brakes!!!

    Nobody worries about small air leaks, but hydraulic leaks bother everyone. Air fittings are cheaper. You don't have to bleed an air system. If I had more time, this list could get to be very long.
     
  8. Jun 8, 2012 #7

    sophiecentaur

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    Re: Air brake over hydraulic brakes!!!

    So why do they use hydraulic in motor cars? They really can be such a pain as a car gets old - sticky discs, sudden failure, sponginess. Like you said, the list is endless.

    Having said that, I never saw a large crane that used pneumatics. But I guess the pressures needed are so massive in that application.
     
  9. Jun 8, 2012 #8
    Re: Air brake over hydraulic brakes!!!

    A large crane is going to have a hydraulic system driving many things anyway. So it costs nothing extra to power the load brakes with hydraulics. But all the mobile cranes that I've worked on up to 500 tons have air brakes for the road brakes. Some are air over hydraulic.

    Hydraulic brakes on an automobile are easy, simple, and cheap. Best of all, no QD's. They work well with a vacuum assist because you can pull a vacuum off of a gasoline but not a diesel engine. For larger systems, air pressure works better than vacuum, so you add a compressor and tank to the system.
     
  10. Jun 8, 2012 #9

    Danger

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    Re: Air brake over hydraulic brakes!!!

    I always figured that the reason is the incompressibility of fluids. It seems to me that air would be too slow to react in a panic stop situation.
     
  11. Jun 9, 2012 #10
    Re: Air brake over hydraulic brakes!!!

    So why do they use hydraulic in motor cars? They really can be such a pain as a car gets old - sticky discs, sudden failure, sponginess. Like you said, the list is endless.

    Because you need a vacuum pump for air brakes to work and that would be too expensive on a car.
     
  12. Jun 10, 2012 #11
    Re: Air brake over hydraulic brakes!!!

    Actually, Danger does bring up a good point. The hydraulic system on modern automobiles has a quicker response time than an air system on a truck. I suspect there is a problem with scaling the truck's air system down to automobile size that makes the response time difference an even bigger problem. I see a good allegory here when considering that all small gas turbines are centripetal because the larger axial turbines don't scale down well.
     
  13. Jun 10, 2012 #12

    NascentOxygen

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    Re: Air brake over hydraulic brakes!!!

    Air brake systems seem to be a lot noisier than the silent hydraulic. Frighten the horses.

    Sometimes see a sign for heavy-vehicle drivers, "Please refrain from using air brakes through town."
     
  14. Jun 10, 2012 #13

    sophiecentaur

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    Re: Air brake over hydraulic brakes!!!

    Frighten cyclists! REEESULT!
     
  15. Jun 11, 2012 #14
    Re: Air brake over hydraulic brakes!!!

    Whoever posted that sign shows a great misunderstanding of the braking mechanisms used by trucks. Air brakes make very little noise. But they are often supplemented by a Jake brake, which screams very loudly. They are intended only for descending mountains, but I've had many truck drivers who love to use them everywhere because they think the noise sounds so cool.

    Since my trucks were only used on the flat ground of Florida and because Jake brakes cost extra I always bought trucks without them. But a few Jake brakes found their way into the fleet anyway. Those were the trucks the drivers preferred the most because they liked making noise. They would be coming up to a stop light at 45 mph and switch the Jake on. I could hear them from a mile away and I knew who was about to show up in the yard.
     
    Last edited: Jun 11, 2012
  16. Jun 11, 2012 #15

    NascentOxygen

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    Re: Air brake over hydraulic brakes!!!

    The sound I associate with air brakes is that of compressed air being released, usually in short half second bursts. You hear it when a driver parks his rig as he shuts down the engine; I assumed that to be a parking air brake being applied.

    When I was a kid we lived in a tiny village alongside an interstate highway. At night, long distance transport drivers would often park off the highway opposite our house for a few hours sleep. At any time of the night my nightmares would be dispersed by the trumpeting of elephants as the big engine was heaved into life by its compressed air starter! http://img839.imageshack.us/img839/4549/80c0b098.gif [Broken]
     
    Last edited by a moderator: May 6, 2017
  17. Jun 12, 2012 #16
    Re: Air brake over hydraulic brakes!!!

    I find that interesting. I was in the heavy equipment and trucking business for 25 years, and I've never seen an air starter. I've heard of them and I've read about them. If a customer wanted me to design one, I could offer him several different design approaches. But I've never seen one even though I've worked with equipment dating as far back as 1953. I bet some of the designs really screamed, though.
     
    Last edited: Jun 12, 2012
  18. Jun 12, 2012 #17

    NascentOxygen

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    Re: Air brake over hydraulic brakes!!!

    They may have still been petrol engines then, in the 60s. I think the idea was to feed compressed air into a couple of the cylinders, and its squeals of protest would soon break into a throaty roar, leaving my dreams in tatters. https://www.physicsforums.com/images/icons/icon6.gif [Broken]
     
    Last edited by a moderator: May 6, 2017
  19. Jun 12, 2012 #18

    Danger

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    Re: Air brake over hydraulic brakes!!!

    How could truckers be asked to not use their air brakes when it's the only kind they have? (A Jake doesn't really count, since it's more of a deceleration device and speed governor.)
    The signs in my town ask them to not use "engine retarder brakes", which is another term for Jakes.
    Pkruse, I'm confused by your reference to them "screaming". The ones that I've heard made a horrendously loud clattering noise, similar to normal diesel operation but vastly amplified.
     
  20. Jun 12, 2012 #19
    Re: Air brake over hydraulic brakes!!!

    Danger: I think we are using different words to describe the same noise. Sounds like an engine at high rpm and under a heavy load.
     
  21. Jun 12, 2012 #20

    Danger

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    Re: Air brake over hydraulic brakes!!!

    Agreed. "Scream" always implies to me something at a high frequency, like a turbine or a whistle, but that isn't necessarily correct.
     
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