I "hotfixed" an old Walvoil DLS7 directional control valve the other day, and the client ordered a new one to keep on hand. We don't stock these tiny valves, so I had to order all the parts to match the required specs. Sectional DCVs are like LEGOs - if a catalog is any good (and Walvoil's catalogs are of decent quality, in my opinion) anyone can make a list of parts - so that's exactly what I did.
The parts arrived in nice plastic bags, and there weren't that many parts since it was but a simple one-section DCV. In all, I received:
As a "veteran" hydraulic tech, I knew exactly how this valve operated - I've seen hundreds of similar valves before. Here's the hydraulic diagram taken from Walvoil's catalog. The diagram shows a two-spool version, mine had only one (and you'll see why I am showing the catalog version of the valve's schematic in a minute):
A classic proportional load-sensing DCV with a single common compensator in the inlet section configured for a fixed displacement pump operation. Nothing I haven't seen before. "Half an hour's job tops!" - thought I to myself - took the parts out of their bags, installed the anti-shocks into the working section, stacked the "sandwich", tied the rods, and voilá - what a beauty!
Only... the beauty would not work. It did direct the oil correctly, but no matter what I did - the pressure in the work ports wouldn't go past 10 bar!
As I am writing these words, and I already "know what I know" - I would immediately suggest the most probable cause of the "malfunction" if somebody described to me exactly what I just described to you (i.e. bought all the right parts for a single-section DLS7 - assembled the valve from the said parts - saw that it doesn't work). So, can you?
First tip - there's nothing wrong with the valve (in the sense that no part is damaged or malfunctioning - the inlet section is OK, the compensator is OK, the relief valve is OK, the anti-shocks are OK). Any wild guesses yet? Obviously - I screwed something up during the assembly, but what exactly?
Let me give you one last tip - there's a hint about where the problem might be in the hydraulic diagram (if you look at it ve-e-ery carefully). That's exactly what prompted me about the location of the "bug" when I gave up and went to the office to check out the catalog - something I should have done before assembling the valve. You have 30 seconds to figure it out on your own because I am about to tell you!
So here's what I did wrong - I missed a small detail when I assembled the stack. I over-looked the fact that the shuttle ball in this DCV does not live in a safe and securely isolated "home" inside the section's body, but is loosely tossed between the sections (which is ingeniously represented in the schematic by the placement of the shuttle valve right on the line separating the sections), and when I took the working section out of the bag - the tiny 4-mm ball rolled out and got left behind! Without the shuttle the LS gallery was permanently connected to the return line in the outlet section - and thus, obviously, nothing worked!
I went to the bunch of empty plastic bags that were still lying on my workbench and, sure thing - there it was:
What can I say in my defense? Here's the exploded view of the valve from the section of the catalog you choose parts from. Do you see any loose balls here?
Neither do I. It's only on page 97 that they (finally) depict the loose ball between the sections (at least in an exploded view). There's also a cutaway view on page 87, and that's about it.
The exploded view makes it look like there's a plug (or something resembling an allen-keyed plug) in the place where the shuttle valve sits, and this is kind of misleading as well. Here's how the shuttle looks in real life:
I think that Walvoil folks should have done two things:
First - they should have placed a warning arrow in every exploded view of the valve in the catalog, drawing attention to the loose (and thus easily losable) ball, and second - they should have put the ball in a separate plastic bag with a marking that clearly indicates the presence of a tiny and important part (as opposed to simply tossing it in the same bag). I'm lucky I didn't have to fish for the ball inside our garbage bin!
These are lame excuses, and I know it. I should have been more careful. I've seen the loose inter-section shuttle ball designs before. And "fished" for them "runaway balls" as well. Many times. But I can promise you that this is not happening to this fellow ever again!
So, today's takeaway points are: