Insane Hydraulics
Bold and audacious blog about fluid power
Let us troubleshoot a hydraulic circuit, shall we? I will even provide a hydraulic diagram to make this super easy:
Beautiful and simple, right? Those of you who work with core-drilling rigs will instantly recognize this double pressure reducing-relieving valve arrangement, universally used for providing the necessary feed and hold pressures.
Oh, wait, would you look at that?! There's even a closed-center directional valve and two pressure gauges mounted on the instrument panel, so if we want to take the reducing valves "for a test drive", all we need to do is center the DCV, turn them knobs and check them gauges, right? Yes, you are right, so let us do just that now and see what happens.
So, you turn the feed pressure knob while looking at the respective pressure gauge and clearly see that the needle faithfully follows your input. Turn it to the right - the pressure goes up, turn it to the left - the pressure goes down. Marvelous!
Then you do the holding pressure valve in the same way, and... wait a minute... that doesn't seem right at all! The pressure response is so sluggish. You turn the knob to the right, the pressure does go up, but boy, is it slow to pick up. You make your input, and then it is one... two... three... four... five... finally!
A five-second delay is a big delay for a control valve. Something is definitely fishy. To see how it fares the other way, you dial it down, and... hmm... now the response is actually pretty decent.
Strange, isn't it? Surely, there is something wrong with the valve or the oil supply, like an obstructed orifice or something. So, what would your next steps be? Replace the valve? Inspect the manifold for orifices?
I can tell you what my next step in such scenarios would be. By "such scenarios," I mean diagnosing a machine equipped with an instrument panel with analog pressure gauges and seeing that a pressure reading has an abnormal response (especially when it is abnormally slow). My next step would alwyas be to double-check the pressure reading with a good digital pressure gauge.
You may say that since the response of the gauge I just described was fast when the valve was dialed out, surely its function and connection must have been good. It can't be slow and sluggish when the pressure goes up and then fast when the pressure goes down, can it? The thing is... it actually can! Here, let me show you. This video shows a control panel of a drilling rig with exactly the arrangement I just showed you, and I have a wireless pressure sensor connected in parallel to the analog gauge showing the holding pressure (the bottom-right gauge on the panel):
See what I mean? So, the instrument panel gauge rule number one goes like this: "Do not trust a pressure gauge on an instrument panel when the pressure is acting out without double-checking it with another instrument."
If the bad reading checks out, you are all good to continue using the convenient panel. If it does not, however, it is not a bad reading, but a bad pressure gauge trying to throw you off track!
Over-crimped capillaries, over-tightened gauge dampers, contaminated orifices (and in some gauges they can be really tiny), and gauges purposefully built for slower response - all of these can make you chase an issue that does not exist. This is why you should always (but always!) double-check a suspicious panel reading with a gauge that you trust.
Unfortunately, I didn't have time to investigate "all the way" with the rig in the video, but I did find that one of the ends of the capillary hose connecting the gauge was over-crimped, although even when I replaced it with a "normal" hose, the gauge was still a bit slow. When I removed the connector fitting, I saw that the gauge had a damper, but it wasn't removable, so I left it as it was.
In any case, I hope that my point is super clear: panel gauges are great, but they can fail for a number of reasons, and you should not trust them blindly after you see a reading that feels "off" or sluggish.