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Basic question about hydraulic pumps

  1. Jan 11, 2012 #1
    Hi everyone,

    I have been messing around studying Hydraulics on my own, but I can't get round something. The following things are things I concluded after reading books, sites, ...

    - The pump itself doesn't generate pressure, but when there is a resistance in the circuit, there is pressure being built up (for example: a hydraulic piston which is loaded, will provide a pressure p = F/A)

    - A hydraulic pump pumps up fluid just under atmospheric pressure, and it pushes it out just above atmospheric pressure.

    Now my question is:

    If the cilinder is loaded, and causes a pressure of lets say 50 bars (using bars here because i'm from the EU),

    I don't understand how the pump (the pump can withstand >>> 50 bar) will lift the piston (if the output pressure is just slightly higher than the atmospheric pressure, and the pump doesn't supply pressure itself).

    I hope this makes sence...

    Maybe someone can give a example of a simple circuit and some example values or something...


  2. jcsd
  3. Jan 12, 2012 #2
  4. Jan 12, 2012 #3
    That only confirms what I read and understand...

    Still didn't answer my question: when there is a load which causes a pressure of 50 bar, the pump will still generate no pressure and will merely cause flow?
  5. Jan 13, 2012 #4
    Uhm, no.

    Let's try this analogy: Open your mouth wide and blow. How much pressure is generated against your cheeks? Virtually none. Next, close your lips as if you're going to whistle -- now blow. You feel pressure, correct? Same basic principle.
  6. Jan 13, 2012 #5
    the pump sucks in fluid (say from the rear) at a tiny pressure because the tank it is sucking from is not pressurized (only atmospheric really: 1.01 bar), if there is no load i.e no pressure build up on the front half, the pump is just creating flow really, which is not much use in hydraulics.

    When the circuit needs pressure, for example cylinders doing work, the pump has to convert the flow with next to no pressure into substantial pressure. Which is done by the pump throwing in more fluid passing through the front half of the pump into the circuit.

    This is done by the pump MOTOR which will deliver enough torque to spin the pump at working pressure. the torque produced by the motor will always be in equilibrium or more than the pressure in the system. if the torque from the pump motor is more than the pressure behind the relief valve, it will cause the relief to crack off and send the un-needed to tank.
  7. Jan 17, 2012 #6
    So basically, when more load is put on for example a cilinder, the motor driving the pump will create enough torque for the pump to overcome the working pressure?

    Does this mean the output pressure of the pump will always stay a bit higher than the atmospheric pressure (relative to the load that is put on), to create flow?
  8. Jan 17, 2012 #7


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    I doubt this has been an acceptable reply any other time on this board. LMAO
    YeeHaa, flow NEVER occurs unless the there is a pressure differential in the system. This is the case with any system.
  9. Jan 17, 2012 #8
    Yeah if for example an up stroking cylinder were stroking, and someone placed an extra weight on it, the pump motor would have to supply the extra torque to keep the pump spinning which keeps the cylinder stroking with extra load.

    The output pressure of the pump will be the pressure that is used to stroke the cylinder, industrially the pump pressure output generally is between 100 to 250 bar in my experience. Look on you tube for videos of hydraulic presses etc, you will hear the pump change pitch in sound when the load is placed on, I.e the pump working harder
  10. Jan 17, 2012 #9
    That is more than clear to me.

    This is exactly what is confusing me (see my original question). Everyone always says: a pump doesn't create pressure.
    Well you say now that the pump pressure output is 100-250 bar...

    I understand that increasing load will increase the pressure (basic math...) and I understand that this pressure will spread equally to the outtake of the pump and that it will use the motor to overcome this pressure.

    It's just the definition: pump doesn't create pressure, that has me confused...

    For example, I read here http://www.hydraulicspneumatics.com/200/FPE/Pumps/Article/True/6401/Pumps: [Broken]

    "A pump produces liquid movement or flow: it does not generate pressure. It produces the flow necessary for the development of pressure which is a function of resistance to fluid flow in the system. For example, the pressure of the fluid at the pump outlet is zero for a pump not connected to a system (load). Further, for a pump delivering into a system, the pressure will rise only to the level necessary to overcome the resistance of the load."

    Well, when reading this, it sounds like it DOES create pressure when there is a resistance present (like 1988ajk said).
    Last edited by a moderator: May 5, 2017
  11. Jan 17, 2012 #10
    but without the pump there would be no pressure in the case of a cylinder stroking. the two things go hand in hand, so you could say the book is half right. just remember this;

  12. Jan 22, 2012 #11
    There are basically two kinds of pumps
    1.Positive displacement pumps eg.Piston pump
    Here a fixed amount of fluid is compressed,thereby increasing the pressure of the fluid and pushing it into the hydraulic system.
    Is capable of generating higher fluid pressure
    2.Non positive displacement pump
    Here the output of pump varies as the pressure in the system builds
    This may be an axial flow pump where the blades of the pump impart energy to fluid molecules and thus causing flow.

    When the output pressure is greater than the pressure that can be generated by the pump there will be back flow or leakage.
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