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Can you get something for nothing?

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  • Can you get something for nothing?

    As someone who as designed and upgraded plenty of hydraulic systems, I have noticed how often a machine user will want a system upgrade to increase either pressure or flow, while changing nothing else. For example, sometimes a customer will want "more power," so they install a larger pump on a machine. The error is two-fold, of course. They assume a bigger pump can lift a larger load (it will only make it faster). They also assume they can install any size of pump with their current prime mover. Bigger pump (or higher pressure) means more power is required, which means larger prime mover. You can't get something for nothing, and the laws of thermodynamics win every time.

    Does anyone else experience these difficulties?
    Last edited by JoshCosford; 12-29-2015, 06:16 PM. Reason: Oh yeah ... first ever FPW post!

  • #2
    This is the most common misunderstanding in the first opening hour of a class of "fluid power students". And I have thrown out the question many times:
    -How do we make this crane lift a heavier load?
    9 out of 10 spontaneous answers will be
    -put in a larger pump
    Last edited by akkamaan; 12-30-2015, 12:40 AM. Reason: Oh yeah ... first ever FPW reply!


    • #3
      Per, it's great to see you here already!

      Yes, it's counter-intuitive to put a smaller pump on when you want to lift a heavier load, isn't it?


      • #4
        Yes it is for sure counter-intuitive...But you can't blame people for getting it wrong when they only "think" they know how it works. Of course some people can be pretty sure they "know2 too....
        But if we continue on this a little more...
        If we add a larger motor (cylinder) you can get to your goal..... if the prime mover power can keep up.
        To relate to my first comment in my previous post, my students always got it quickly when I asked them what the muscle of the logging crane arm was...
        -The cylinder, they responded all like a choir...
        And after that, the discussion to the pre-assumed healthy-track and we can start to talk about the power, ie force and velocity on the motor.
        But when we get into the discussion about the definition of power, force and velocity of the cylinder, and how that is put together with flow and pressure it get little more interesting.
        Most students will start claming the we need flow to create motion on the cylinder......
        Newton, Newton....where are you?????


        • #5
          No free lunches ... entropy of any isolated system not in thermal equilibrium almost always increases.