Insane Hydraulics

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Pressure Filter Reverse-Flow Valve Failure

This post is a reminder that even a well-established brand can deliver a mediocre product, which is why it is very important to evaluate the overall design, the build quality, and the materials used in the manufacturing process when you are evaluating a new component for your next hydraulic project. "New" in this context stands for "new to you", i.e. a component that you haven't used before, but would like to, because the catalog states that, in theory, it is exactly what your project needs.

I don't want to be specific, and I won't mention any brands or models, but I do want to share how I learned this formula "the hard way".

Several years ago I got involved in designing and building several closed-loop drives, relatively big (combined pump displacements in the range of 360 - 500 cc), and since I was a huge fan of pressure filters, and the client didn't mind investing in extra filtration, we decided to install pressure filters on all HPUs.

Naturally, using a pressure filter in a bi-directional closed-loop system is tricky, and our first units used two external check valves on each filter to address the "reverse-flow" problem. It worked quite well, but, of course, was bulky, and at a certain point the client approached me with a brochure showing these very nice looking reverse flow capable pressure filters he found online. Very interesting! The drawing explaining how the bypass system worked looked very compelling:

Even though it seemed to be "kind of weaker" than the heavy-duty external checks that we were using, the fact that we would get rid of the ugly bulkiness of the external valves and gain a lot of space convinced everyone to give the filters a try. And in any case - what were we risking, anyway? The loops were running 99 percent of the time in one direction, so even if the bypass was not as sturdy, it seemed to be the perfect choice for this particular application, and so we installed these babies in four systems, that worked problem-free for several years. Hurray?.. No, no hurray!

Now comes the revelation and the miracle... Yes, the miracle.

All of the filters were installed in a classic vertical fashion, with the jugs pointing down. So, one sunny morning, a person that was servicing the rig, removed the jugs and the elements for service, but before he put the new ones in, he leaned into the power pack to check something, placed his head under the filter head, and then looked up and... Wa-a-a-it a minute... This isn't right now, is it? There was something "clearly strange" going on in the filter head, exactly in the filter by-pass valve area. And when the valve was removed from the filter... Boom!:

The bypass valve is gone! Missing! Totally! This has to be a fluke. So, a pressure filter on a second rig was inspected, and...

Then the third one...

Then the fourth...

See any patterns yet?

Miraculously, no loop components were damaged. A true miracle, if you ask me! We never found the missing bits. I can only guess that since the pumps didn't explode, the heavy chunks fell into the filter elements and then were discarded. But this does make you think, doesn't it? The metal strip used for the rod support was very thin, and eventually broke - and remember - this loop reverses only occasionally. Had it been a 50/50 loop, the failure would have happened much sooner.

I did open another pressure filter, brand new, same make and model but a more recent series - and here's the current version of the bypass valve:

As you can see, the metal strip is much thicker now, although the quality of the assembly leaves much to be desired. The strip is welded crooked and is asymmetric. Very painful to look at, actually. When we tried to push open the by-pass valve, it got stuck open so bad that I had to smash the assembly against the metal bench top several times to "usher" it back home.

Not a very convincing design solution, in my humble opinion.

I understand (first hand) that putting even the simplest of products on the market is a very complicated endeavor, and the fact of life is - design and manufacturing mistakes will continue to happen. I learned to live with it - I made it a rule of mine (at least for the components I can "put my hands on and in") to evaluate how a component is made before I decide to use it. A good learning experience as well, by the way, so my advice is - do the same, use your own head, and if a "gut feeling" tells you that there's something "fishy" going on with a part - either find hard proof that somebody has used it problem-free for an extensive period o time, of don't use it at all.

At least do that with reverse-flow capable pressure filters. And should you ever consider using a pressure filter in a closed loop - check out my post on pressure filtration in closed-loop transmissions.