Insane Hydraulics
Bold and audacious blog about fluid power
I've been repairing hydraulic pumps and motors for many years, and I've been blogging about it, too, so it may create the impression that I should be able to instantly recognize a unit and its detailed function at first glance - but, alas, my friends, nothing could be further from the truth, because I forget things just as fast as I learn them. I suppose this is a norm for the age we live in, given the number of distractions and things we have to juggle every single day (or, at least, this is what I've been telling myself to stop feeling bad about it). Nevertheless, I do my best to "stay in shape" through silly and, arguably, unnecessary challenges that I invent for myself when I overhaul hydraulic stuff, and today I want to share one of them with you.
I used the word challenge, but I think the word exercise might be an even better fit for what I am about to describe. If you think about it, an exercise (as a workout) is something that you definitely don't have to do to be able to function, and is also something that feels like a total waste of time to a lot of people, but there's no arguing about the fact that regular exercise, however unintensive it might be, undoubtedly conditions you into a better form.
So, the exercise is called "Count the Plugs," and I will demonstrate it with a concrete example. This is my workbench from yesterday morning:
Not too much to look at, right? If you are an engineer, you will look at these pics and say: "Just another pump being put together." If you overhaul hydraulic pumps regularly (with your own hands), you will say: "Just another Rexroth A11VLO being put together," and after you zoom in, you'll be able to tell that the barrel and the plate had been lapped, and if you are a detail-freak like myself, you might say something like: "Dude, why didn't you replace the sealing nuts on the displacement limiters?" or, maybe: "Dude, where's the controller?" On a side note, I absolutely agree with this last question - a pump picture without a controller in it feels... uncomfortably incomplete. This one runs the DRS control (PC + LS), in case you want to know, and I did replace the sealing nuts after I took the picture, but this is not the subject of today's blog - these little guys are:
One, two, three, four, five - five plugs that came out of the end-plate, and the challenge that I like to impose on myself whenever I come across a hydraulic part "embellished" with plugs (they all are, aren't they?) is to tell, from memory, where exactly each of the plugged channels leads to and what pressure I would read during the unit's operation if I were to connect a pressure gauge to it.
Now, of course, you can get most of this information from a catalog, and I am all for consulting catalogs, but the challenge is to first investigate using all of the means at your disposal, including technical literature, and only after you're done with the investigation to come up to the part (which, in this case, is the end plate), point to each of the plugs one at a time, and tell "what's behind it" from memory.
Even if you forget 90% of what you learned the very next day, 10% will remain. This means that the exercise did its job and that your brain has encoded a small piece of useful hydraulic information into the permanent storage area. Here, I'll show you - let us do this one together. Let us start with these two plugs:
There's an "M" right next to the plug, and if you are used to working with German units, you will recognize that this is a diagnostic port ("M" for "Manometer"). Since there is, apparently, no number or letter next to it, like "M1" or "Ma," you might think that this is the main pressure port - i.e. - the pump outlet pressure, but this is actually not the case here. If you blow compressed air into this port, you will see that it leads directly to the larger servo-piston, and then if you consult the catalog (we'll be looking at the diagrams in a minute), you'll see that this port was supposed to be marked as "M1," and the pump outlet pressure port (to which we'll also get in a minute) is, indeed, the "M" port. Arguably, the missing "1" may be "lost" in the irregular surface of the end-plate casting, but this does not change the fact that you clearly see just the "M" and not much else.
See? We already learned something useful! We know now the physical location of the larger servo-piston diagnostic port, and we also know that it can be (pretty treacherously) marked with a simple "M"! Imagine somebody checking the pump diagram and then coming across this port and thinking: "Hey, there's an "M" next to this plug, so this is definitely the pump outlet pressure!" and then trying to interpret a servo-pressure reading as an outlet pressure. Or, in an alternative "remote diagnostics" scenario, getting a call from a client stating that he installed, just as you instructed him, a test fitting in the "M" port, and now the pressures are all screwed up for some reason...
What about the tiny plug? Compressed air coupled with a "flashlight inspection" will tell you that it connects the larger servo-piston to the controller and that there's a thread in this hole, which means that you can install an orifice in this channel. There were no orifices in this particular end-plate, but I am obsessed with verifying everything, so I took an M6 plug just to test if it threads into the hole, and it does:
OK, this part was easy - these two holes make the larger servo-cylinder line, and one of them is for reading the servo-pressure, while the other one is for installing an optional damping orifice. Now there are only three more plugs to go through:
Hmm... The top port (in this picture) also has a faint "M" marking to the left of it, and the bottom one has no markings, but it does have a cavity suggesting that something optional can be inserted there. Blowing compressed air into these holes seems to suggest that they are all interconnected. The only thing that is clear is that the small plug hides a channel that leads directly to the bias servo-piston, and that there's also an M6 thread in it for an optional orifice.
Now, at this point you could, of course, try to figure out the puzzle behind these connections for yourself, but there's no point in that because then it would fall under the category of "over-excessive back-engineering." We don't want that because we want our exercise to be quick. This would be the right moment to consult the official docs. Here's the diagram for the base A11VLO unit without a controller:
I must say I love how the engineer who made this drawing represented the removable damping orifices in the servo lines, clearly showing that they are placed in channels behind external plugs! This is what every hydraulic diagram should be like - an accurate and detailed representation of real life. No corners were cut here. Now, if you dig a bit further through the different controls available for this pump and compare them to the DRS control diagram, the function of the "cavity for something optional" becomes quite apparent:
It is a place that can house the shuttle valve for controls that require an additional external servo-pressure source for situations when the pump outlet can't supply one. If only they had kept the representation of the plugged G port in the DRS diagram! Going back to our exercise, we can state now that the small plug gives access to the damping orifice of the bias servo-piston, and both the "M" plug and the unmarked plug (a would-be G port for other controls) will read the outlet pressure.
That's it! Super fast and super-educational. I like to make notes when I do this exercise - here are mine for this particular pump:
I've overhauled many A11VOs before, but it's been a while since I worked on one, and this 10-minute exercise was a welcome memory refresher.
There's also a "bonus question" in this overhaul, and you can see it in the pictures above. A "classic" LS + PC control valve, like the one you'd find in, say, an A10VO, has three lines connecting to the control valve - the outlet pressure, the servo pressure, and the tank. This controller interface plate has four, and if you are as obsessed with details like I am, you might wonder: "Why the extra hole?" I hope that you can see the answer in the diagrams above. See what kind of questions bother me when I overhaul hydraulic components? Totally insane, I know!
So, the next time you work on a hydraulic pump or a motor, turn the plugs into mnemonic tools by answering the question "What's behind them?" from memory. You will be surprised how much you can learn!