I'm relatively new to hydraulics, and I'm trying to understand this schematic. I've got the majority of the system figured out, but I cant for the life of me understand what the symbols at positions 18/20/23 represent.

The unit actively controls a constant pressure in the two cylinders attached using an electronic pressure switch. Its the control unit of a hydraulic tensioning system in a ski lift. My feeling is that 18/20 may have something to do with electrical pressure cells?

enter image description here


  • $\begingroup$ I can't see "33" - did you mean 23? $\endgroup$ – Jonathan R Swift Dec 13 '17 at 1:16
  • $\begingroup$ I did! My mistake, thanks for catching that. $\endgroup$ – Brad Morris Dec 13 '17 at 1:18
  • $\begingroup$ No problem. 23 looks like a capacitor, which would back up your theory that some other parts on this diagram are electrical, as well as hydraulic, but, I don't have any direct experience with Hydraulic diagrams, so you should likely wait for a more experienced responder! $\endgroup$ – Jonathan R Swift Dec 13 '17 at 1:26
  • $\begingroup$ There's an interesting set of three videos on YouTube on reading these diagrams, starting here: youtube.com/watch?v=OPlDYvq15nc. The symbol at 23 is not a capacitor, rather something normally open, but I've only ever seen these in a "composite" symbol like 7, so still need someone more expert weigh in :) $\endgroup$ – scanny Dec 13 '17 at 3:32
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    $\begingroup$ 23 looks to be a pressure equaliser balancing the pressure between the two lines while isolating them. $\endgroup$ – Solar Mike Dec 13 '17 at 4:52

Symbol 23 is just a plugged cavity. The two incoming hydraulic lines are both blocked. No flow is exchanged between them. The designer's intent for the cavity is future use. Various cartridge valves can be inserted into the cavity for either pressure control, flow control, directional control, or other purposes. You can search "DCP-2 cavity plug" or see this link for a typical example: https://www.doering.com/pdf/DCP-2.pdf


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