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What advantage of oil in hydraulic brake over air?

  1. Oct 20, 2008 #1
    I know that in a hydraulic system, oil is used rather than water because water can easily vaporised under extreme use. What the is the advantage/s of using oil over air in a hydraulic system?
    Last edited: Oct 20, 2008
  2. jcsd
  3. Oct 20, 2008 #2
    Air, and any other gas, can be compressed, while oil and other liquids can be considered nearly incompressible.
  4. Oct 20, 2008 #3


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    Just a Follow-Up

    So why should that matter? One of the main reasons is response time. When a pneumatic system is activated, the air rushes into a cylinder. But, the air hitting the stopper in the cylinder may not have enough pressure to overcome the load and start the cylinder moving right away. As a small amount of time passes, air inside the cylinder, and the tubes leading to the cylinder, compresses to build pressure sufficient to overcome loading. With an incompressable fluid, response time is much quicker.

    Another advantage is that air leaks a lot more easily than oil, and when oil leaks, you know about it right away, so you can take action. Any leak is, of course, a loss of energy.
    Last edited: Oct 20, 2008
  5. Oct 20, 2008 #4


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    Another reason not to use a compressible fluid like air is thermal. If you compressed air in a cylinder it would heat up, as that heat leaks away the pressure would drop.
    You would have to have the entire system very well insulated.

    You can use air for systems which only use a small amout of pressure, it's advantage is that you can quickly release the force by just dumping the compressed air back into the enviroment - this is frowned upon when using hydraulic fluid!
  6. Oct 20, 2008 #5
    One more thing is in air system, pressure exists even in idle state. In oil, there's not.
  7. Oct 20, 2008 #6
    Good comments. Oil is also convenient as it lubricates seals although that's not as important as the reasons cited....water is bad because it corrodes components...
  8. Apr 16, 2009 #7
    I agree with advantages stated above. I have a pneumatic cylinder that I want to use with hydraulic fluid to eliminate compression issues. If I stay within the load limits of the cylindar, ie not exceed the 150psi, would it work? would it last?
  9. Apr 16, 2009 #8


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    Does this mean oil as opposed to air, as it seems that everyone is assuming? I just want to clarify that, since an air-over-oil hybrid system is used a fair bit (Citroen suspension systems, for example).

    edit: I see that OP has edited his question to be specific.
    Last edited: Apr 17, 2009
  10. Apr 16, 2009 #9
    Are you comparing Air brakes like on a truck or train with hydraulic oil brakes on a car?
    Like CaptainPanic said air would not work in place of hydraulic oil in a car because the air would just compress and not apply pressure to the pads or shoes. If you are talking about air brakes on a truck or train then the air is used to keep the brake from locking up.
    On a truck when you release air pressure by pushing on the brake pedal the brakes start to grab from spring pressure. The air pressure keep the brakes released and this is a safety factor because you can’t move the truck without air pressure. But maybe I misunderstood your question.
  11. Apr 17, 2009 #10

    Ranger Mike

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    Whoa there..ok,bad pun...imo, air is used because it is cheaper..a 60 foot trailer rig will take a whole lot of brake fluid and be a real bear to bleed...i think the trucks have an air compressor constantly running to maintain an air reservoir. they realize air will bleed thru the system so hence the continuous replacement..i may be full of it on this point not being a heavy truck guy..

    brake fluid is special mix with DOT rating and boiling point..our race brake fluid has 399 degree F boiling point wet and 617 degrees F dry..all brake fluid will aquire moisture over time and this means steam in the master cylinder and brake lines which, when you lean on the whoa pedal over time, will cause the pedal to go mushy because the moisture becomes steam and reduces volume, thus soft pedal. one more thing..racers don't use silicone brake fluid cause it can squeeze past the brake cylinders.
  12. Apr 17, 2009 #11
    My apologies, after reading again this morning I clearly see we are talking about braking systems. I built a machine tool using air on a punch press. It works, it just has delay and a surge after load is overcome that seems to offend my boss. Please forgive the off topic inquiry. Your thoughts are very welcome, however.
  13. Apr 17, 2009 #12


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    The OP didn't actually specify brakes; that came up in the answers. As far as I know, the only potential problem with using oil is the possible degradation of the seals. Check for compatibility. The best bet, though, is to use purpose-built equipment.
  14. Jul 30, 2009 #13
    could u please tell me if replacing a hydraullic brake in place of a shoe brake for all locomotives will be advantageous in all respects?
  15. Jul 30, 2009 #14
    no idea, sorry.
  16. Aug 5, 2009 #15
    Just to clarify the truck/train issue and answer oopadi's question, I believe vibjwb is right. George Westinghouse invented the air brake for trains, mostly for safety reasons. As I understand it, the same basic system is still used for train cars and most heavy vehicles. The system is based on a "triple valve" principle. Air pressure is used to unlock the brakes against a spring system. When the brake pedal is applied, air is released, lowering the pressure, and the brake begins to work. When the pedal is released, the air in the reservoir allows the brake to release again. Vehicles using this system cannot move unless there is air pressure. So, if there is a sudden loss of air pressure, the vehicle stops. In hydraulic systems, if you lose oil, you lose your brakes. Somewhat of a problem for a train!
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