Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question on positive displacement piston pumps

  1. Jun 4, 2014 #1
    This is a very simple question but I am struggling to grasp the answer. My question is in regards to a positive displacement piston pump.


    Say you are using this machine to pump hydraulic oil from one reservoir to another. How can this work? The reason I am confused is that I understand oil as being incompressible, so my question is how can the oil travel along the rotational path as the piston chamber reduces in volume. Essentially, how is the fluid compressing during operation?

  2. jcsd
  3. Jun 4, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper

    You are right that the oil is (almost) imcompressible. The oil flows into the piston chamber as it increases in volume and out again as it decreases. The inlet and outlet "holes" in the picture you posted are actually semicircular slots and the piston moves along them as it rotates.

    This might help.
  4. Jun 4, 2014 #3
    Thank you for the reply. That is my question though - if the fluid is incompressible, then how can it travel along the length of these slots if the volume of each piston chamber decreases along the path of travel?
  5. Jun 4, 2014 #4
    Oh nevermind, I just had the obvious a-ha moment. It's just moving fluid, not compressing it. I see the slots now. Thanks
  6. Jun 4, 2014 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    That's what 'positive displacement' means: the fluid is shunted along by the motion of the pistons as the wobble plate rotates. It also works for screw pumps, too.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook