Insane Hydraulics
Bold and audacious blog about fluid power
I've used SUN hydraulics cartridge valves for years, and my experience with this brand has always been highly positive. Solid quality, a vast selection of time-proven solutions, and a decent price - what else would one want from a hydraulic valve brand, right? These guys have done a lot to shape this industry, and they are constantly inventing new stuff too. Take, for example, the Energen™ (EVEC) - a T-16A cartridge valve with a freaking three-phase generator built into it for recovering energy - who on earth would have thought of something like that?! I guess what I want to say is that I really like SUN's stuff, but I am a simple man - I see something "peculiar" - I point it out - even when it comes from a favorite of mine. In my defense - at least I rant in writing - so everyone has a chance to examine my point and disagree or prove me wrong.
So, a brand new SUN cartridge valve failed on a rig of one of our clients, and their tech brought it over to see what was going on. It was the PPDB8DN reducing valve, piloted by an RBAEKWN direct-acting relief valve. The PPDB8 is a version of the standard (and very dear to me) PPDB valve that uses an external pilot valve - but their internal construction is virtually identical. The valve, apparently, got stuck in the open position. Here's the diagram from the SUN's data sheet that shows the valve's function, and the (horrendously low-res) cutaway view:
Now, before we go any further I feel I must say one thing to the folks at SUN Hydraulics: "Guys, please, I am begging you, please increase the resolution of the hydraulic symbols and the cutaway views in your otherwise beautiful data sheets! Just look at this hideous pixelated garbage - I am sure you can do better than that! Having access to clean and detailed cutaway views of cartridge valves is very important for us - in-the-filed techs - because we need to know how they operate. And even if we can't grasp all of the engineering wonders you did to your cartridges to make them damn near perfect, we are the ones who will be unclogging those orifices when they clog - so we need to know exactly where they live!"
I can even show you how this should be done. Let us have a look at "similarly-sized competition" and compare the PPDB to, say, Rexroth equivalent of a pressure-reducing and relieving, pilot-operated spool-type valve - the VRPX-10A. Open the PDF files ( PPDB8DN vs VRPX-10A), zoom in on the symbols and the cutaway views and compare them - which one, in your honest opinion, looks better?:
I don't think I need to say anything else about it...
Anyhow, when I hooked the valve to the test bench - it was a "pipeline" all right - same pressure at the inlet and the outlet, even with the pilot dialed out:
Now let us think about it - how can a pilot-operated pressure-reducing valve get "stuck open"?
The first thing that comes to mind is - it can be literally stuck open due to contamination. But that was not the case - the modulating spool was moving freely.
The second thing that comes to mind is, of course, the contamination, namely - the clogged orifice in the pilot section. But this was the first thing that I checked, and I confirmed that the pilot part was 100% OK.
So, what else could be happening here? If you look at the hydraulic diagram of the valve, you can see that in order for the spool to stay open "hydraulically" - you would need to remove the orifice in the spool:
But is it even possible? Once again - a clear cutaway view would be such a helper here! You can immediately tell from the Rexroth's drawing that something like that would be impossible to happen with the VRPX-10A because the spool orifice is clearly a part of the spool itself, but the blob of pixels in the SUN's cutaway, while suggesting that something is definitely there, does not give you much detail now, does it? And the orifices just don't fall out of brand-new spool valves anyway, do they?
As it turns out... they actually do (brand-new cartridge on the left, and the one with a missing orifice on the right):
Yes, Sir, they surely do! As soon as I removed the failing cartridge and placed it next to a new one - the problem became apparent - while the new cartridge valve had "something bronze-ish and pointy" inside of the spool, the malfunctioning one had nothing - just a wide open hole, and, this was, obliviously, why the spool would not move from the default open position - no orifice = no pressure drop to shift it!
Naturally, I could simply replace the damaged valve with a new one and be done with it - but I really wanted to know what kind of an orifice it was. The (very sad) cutaway view did not show much, and no matter how hard I looked inside the spool - all I could see was that there was something made out of bronze there, but I couldn't see any orifices at all - and I really like to know where the orifices in my cartridge valves are located!
Luckily - these valves are very easy to disassemble - all you need is a good eye to spot the end of the wire circlip as you rotate the nose and then a pointy tool to pry it out:
While Rexroth beat SUN with their crisp drawings, any tech will prefer the circlip design of the SUN's cartridge to the threaded (and glued) design of the Rexroth valve because removing the thread-locking agent is... well... how do I put it mildly?.. a bit harder than it may seem...
I disassembled both the faulty and the brand-new valves, but even when I removed the spool of the new valve I couldn't see the orifice (I already suspected that it would be somewhere on the side of the bronze insert, but it wasn't visible through any of the holes:
One thing was clear so far - the "orifice thing" was made out of bronze, and it was pressed into the spool from the front end and riveted (flared out) from the back - so to remove it, I would have to break the riveting. I would not do this to a brand-new spool but I am glad I keep scrap around (just in case), so I found an old PPDB and hammered the orifice out with a 3 mm pin punch:
Finally! I will sleep better now because my curiosity is totally satisfied. But how can a riveted part jump out of a brand-new spool? I am convinced it was a manufacturing defect - insufficient riveting. Look at what I found inside of the spool:
See how extremely thin the leftover brim is? When I punched the other orifice out from the other spool, the "ring" was much thicker:
The SUN's assembly line can fail, too, apparently.
So, the big question is - what do I think about this orifice design? Frankly - I don't like it! Because it is hidden and un-cleanable! Maybe on some spools, the orifice aligns so that you can see and clean it through one of the holes in the spool - but it wasn't the case with the two spools that I had here (actually, there were three spools, but we punched through the first orifice when we attempted to remove it with a smaller punch - so I am not showing it here).
Now that I know for sure that it is an insert with an orifice on the side of it, I can look at the SUN's cutaway view and kind of imagine it's there...
Still - I am urging SUN's designers to improve the low-resolution issue and maybe think of a different design choice for the orifice. I really like the Rextroth approach - a hole drilled in a steel part - this one is definitely not going anywhere!
P.S.: An update from 01/09/2024: A reader sent me an e-mail pointing out the existence of basically the same valve but with a drilled orifice! I wasn't aware of that! The part number would be PPDF8, and it is called "Pilot-operated, pressure reducing/relieving main stage with drilled piston orifice and integral T-8A control cavity."