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The VCCM equation ( force makes it go ) vs V=Q/A ( flow makes it go )

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  • The VCCM equation ( force makes it go ) vs V=Q/A ( flow makes it go )

    On our forum I was posting about the VCCM equation and the formula. One of the forum members came up with an interesting example.
    To keep thing simple I chose the following system parameters
    Cylinder 100mm diameter bore / 56mm rod.
    System Pressure 70 bar
    Valve - same as your example above.
    Opposing force 500 N
    The flow constant for the valve is:

    \[

    {K}_{vpl}=\frac {100lpm }{ \sqrt {35bar} }
    \]

    I give up. It doesn't work when I cut and paste using the Daum equation editor
    Kvpl=100lpm/sqrt(35bar)


    The VCCM equation is:
    \[
    v_{ss}=K_{vpl}\sqrt{\frac{P_{s}\cdot A_{pe}-F_{l}}{A_{pe}^{3}\cdot(1+\frac{\rho_{v}^{2}}{\rho_ {c}^{3}})}}
    \]
    The user calculated correctly that \[{v}_{ss}=259.7 \frac{mm}{s}\]
    however this doesn't make sense because this is faster than the the flow makes it go equation that doesn't even take into account opposing force.
    \[v=Q/A=212.2\frac{mm}{s}\]
    How can this be?
    Last edited by Peter Nachtwey; 02-11-2016, 08:21 PM.

  • #2
    We are doomed. After more 18 years on hydraulic forums people still don't get it. Who is teaching the hydraulic people? Aren't there any hydraulic engineers around? This is really pitiful.
    This problem on scratches the surface for servo hydraulic controls.

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    • #3
      But Peter, pressure is only resistance to flow! Don't you know pressure STARTS at the resistance? :P

      Comment


      • #4
        But Peter, pressure is only resistance to flow! Don't you know pressure STARTS at the resistance? :P
        Josh, remember that sarcasm is not understood by everyone unless they were part of the endless discussions we had at the old H&P FFluid Power Forum... ;-)

        Flow or the "liquid push rod" inside the tubing is just part of the motion that is started by the pump and the prime mover. The fluid in the tubing is just transferring the motion forward the same way a drive-chain would do. It takes force from the pump to move the mass of oil as well as the mass of load resting on the piston rod. But of course, we can not have the motion on the piston induced by the pump if we do not have fluid flow in the tubing between the two. But that do not mean that the flow in the tubing is what's creating the motion of the piston.
        I want to repeat my favorite quote or "akkamaan law"...
        "Flow do not create motion. Flow is motion"

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