There's a couple of ST-14 scooptrams working around the clock at the nearby mine. They employ these large 4+3 section proportional DCVs (Parker K220/L90 combination) for steering, brake, and bucket control, and our friends from Epiroc are kind enough to be occasionally sending these bad boys to us for an overhaul, which almost always boils down to "strip-it, clean-it, seal-it and test-it" routine. A rather tedious procedure due to the number of small parts one has to clean.
So, as I was working on one of these the other day, I realized that this valve is another perfect example of how even such a dull "strip-it'n'clean-it" intervention can become an educational experience with but a tiny extra step.
Now, I don't fancy cleaning parts, like at all! Cleaning is boring. I even mentioned cleaning as one of the downsides of this profession. But if you dismiss the "dirt removing element", you'll see that there's a much more interesting part in which you get direct hands-on contact with the component internals, which in its turn allows you to study and evaluate their design in a way that is impossible to attain through technical catalogs. So, instead of looking at it as a dull and tiresome drill, consider it as exclusive first-hand access to protected information that is only available to a very limited number of the original designers.
Let me give you one concrete example with this DCV. Take a look at this very interesting spool, lodged in the inlet section:
If you only care about removing, cleaning, and then putting it back in without much thought, - the valve will work, and the client will be happy, but you learned nothing. This makes you a cleaner, not more not less.
However, if you take one little extra step, and ask a simple question "what does it do?", and then do your best to find an answer - your horizon expands tenfold!
What do we see here? We see a spool, that has no bias spring, and if we follow the oil galleries in the manifold, we'll see that it connects to the lines marked at PX, PL, and LS. Further "gallery investigation" shows that the PX is actually the pump pressure port, and the PL is coming from the ball-type shuttle valve, which makes it a good candidate for being the load sensing line, coming from the directional spools, and the LS is, well, the LS - i.e. the port that we'll be connecting to the control valve of our pump. Very interesting!
Lucky for us, the internet is an endless well of technical information, and the Parker catalogs are easy to find. By the way, here's a tip for you - whenever you fish the internet for technical pdfs on hydraulic stuff, do look for different year editions and compare them. They are often complementary, and the latest catalog can even have worse drawings or missing cutaway views. Also - don't forget about spare parts lists, that can often provide additional details. The spool has the following schematic:
Furthermore, the lads from Parker even provide the name and explain the function. The spool is called the LS copy spool, its function is to copy the load pressure, supplied by the PL gallery, into the LS outlet. This allows for a much higher flow available from the LS port, without affecting the load signal, and it removes the "micro-sinking" of the load at the beginning of the lifting phase, when the directional control spool connects the low-pressure signal gallery to the work-port, pressurized by the load. (Maybe a far-fetched scenario, given that most modern systems would use a load-holding valve of some kind, but still - a feature worth mentioning).
The fact that the load sensing line can supply high flow allows engineers to use pump controls with significant drainage in the regulator (good for temperature control), or higher than normal LS line flow requirement (for example - torque liming controls that relieve the LS line).
Even though there are schematics in the catalog, I personally, like inspecting the galleries and draw "scribbles of my own", trying to make sense of a spool's function. The spool is basically a pressure-reducing valve, that uses load pressure as a spring. As you can see, the large hole in the center is used to supply the oil from the pump port to the LS line, and the small hole in the bottom vents the LS line when the load pressure gallery unloads.
So, we asked a simple question "what does it do?" when we saw a "strange" spool, and then we spend an extra half an hour to find our answer, and now:
What can I say? I guess I will never stop promoting self-education. Seems like the best way to learn hydraulics, in my humble opinion...