My usual readers already know that I had a very unpleasant experience with UFI FPB pressure filters equipped with reverse flow by-pass and my general opinion on the use of pressure filters in closed loops. Today's post is an add-on of sorts that showcases the points I mentioned earlier. I like presenting concrete examples whenever I can because pictures are worth a thousand words (or so they say).
So, take the following system (real-life example, obviously simplified):
This is a closed loop drive, in which a pump drives a fixed displacement motor. The designer of this HPU decided to use two UFI pressure filters with a reverse flow by-pass function on both legs of the loop. In theory, this is perfect, because if the by-pass valves worked the way they're supposed to, you'd get all the benefits of pressure filtration, and none of the fuss with external reverse flow by-passing. Just slap in an inline pressure filter and you're done! Cheap and effective, right? Wrong! These reverse flow by-passes suck!
This particular system works in one direction only, and most likely this fact was "obscured!" from the designer (at least I hope it was - otherwise this filter placement was a mistake). Now, check out how the filter elements look after several weeks of operation:
The filter element that "ballooned out" and blew off the end cap is the one that was supposed to be by-passing, only it was by-passing "not entirely". Truth be told, in this particular system, despite the nasty look, this does not break anything (at least till the by-pass valve itself breaks, but let's hope it doesn't), so if you ever run into such a predicament, but can't engineer it out, just replace the element with one with larger micron rating, and advise the client to ditch this filter altogether, or maybe turn in the right way, so it actually filters something instead of shamelessly bypassing part of the flow through god knows what.
In any case, my point is, once again - if you ever find yourself in need of a pressure filter with a reverse flow bypass function, be very very careful about the solution that you choose.
Now - a few words about closed loop pressure-side filtration. I know that a lot of people consider that pressure filters in closed sloop transmissions are unnecessary. And I kind of agree -in the sense that a closed loop can work absolutely fine with charge pressure filtration only. But you should always (and I mean always) consider the conditions that the transmission that you are designing (and building) will be working in. And by conditions I mean both the environment and the maintenance. This is very important!
Think about what will happen when a loop hose bursts a year down the line. Who will replace it and how? Where is it routed? Even if the routing looks clean when you commission the drive, how will look a year from now? Will it be buried under heaps of something that is not "hydraulics-friendly"? I have an example for you:
Mining conditions are ruff on hydraulics. I know this first hand, believe me. Take a look at the low-side pressure filter from the HPU I just discussed:
You can see that it caught a lot of millimeter-sized rocks. How does rock enter a hydraulic system? When you replace a hose, of course! Hadn't this filter been there, these rocks would've ground the pump to death in a snap. And, despite my dirty hands, I assure you that these bits were in the filter cap and embedded into the element when I opened it!
Something like this should never happen, but unfortunately, it does. That's the industrial hydraulic paradox for you.
For some goddamn reason, mechanical is always regarded as something inferior when compared to electrical. You need to call a certified electrician to replace a 20-bucks worth of a button or disconnect a wire, god forbid you do it yourself! But the first hand available, experienced or not, will be tasked to replace a hydraulic hose without any questions simply because it's dirty work that requires hand tools. Even on a million-dollar system.
A lot of people still think I am crazy when they see me clean hoses with projectiles and "waste money" on proper metallic caps when I do the routing or replace hoses.