Insane Hydraulics

Site theme image

Three (and a Half) Hydraulic Fittings That Can Fool a Technician

In today's post, I want to talk about three (and a half) fittings that can fool a hydraulic technician. I call them "unorthodox" fittings, because they look like normal off-the-shelf fittings on the outside, but carry concealed functionality on the inside, and can make troubleshooting a nightmare when a tech is not used to running into these "crafty inventions". You will see why I said "... and a half" in a minute.

Let me begin with a troubleshooting story.

A client called me the other day inquiring about the price of a two-axis hydraulic joystick. When I wondered what the joystick was for, he said it was for the excavator, that sat still for several years, and was going to be "revived" as a piece of auxiliary equipment. So, I asked about what led him to conclude that the joystick was faulty - the man said: "Oh, I tested it. The pilot pressure is going in, but no oil is coming out of any of the outlets. I even teed into the pilot pressure line directly at the joystick inlet - and can read stable 35 bar there, but even when I disconnected the lines - I saw no oil. It's fried. I need a new one!"

So, I told him that I would be happy to sell him a new hydraulic joystick, even more than one if he needed, but I was almost sure that his problem was being caused by an "unorthodox fitting".

"Unorthodox what?.."

"Never mind the "technical" term. Your joystick is made of four pressure-reducing valves. It's possible but very unlikely for all four to fail at once. I believe there's a tiny wire mesh filter hidden inside the joystick's P-line fitting, and most likely the mesh is completely clogged because when you started the machine that sat in the field for several years, you didn't check the oil tank for "sludge, muck, and goo", did you?"

"I'll call you back!"

and, in five minutes:

"Dude, you're right! The joystick's fine, the fitting has a mesh in it, and I can't even blow it through with our compressor! I yanked the mesh out, and she's moving now! Also... the oil in the tank does look like crap. I guess I should look into that..."

And this, my friends, is the first type of an "unorthodox fitting" in action for you. Here's another example:

Very often mobile equipment manufacturers place wire mesh filters inside fittings (especially the ones related to sensible pilot circuits) that feed stuff like joysticks or proportional valves. Their purpose is not to filter oil (because, obviously, their filtering and dirt holding capacity are nonexistent), but rather to increase the reliability of the equipment, preventing stray particles of precarious sizes from entering contamination-sensitive places. And like all mesh filters, they can easily clog when an unusually dirty oil is pushed through them.

Since they look like "normal fittings" - i.e. something one would never expect to be able to get clogged - they can easily mislead a tech in his "troubleshooting efforts". Remember - the keywords here are "pilot circuits" and "mobile equipment".

Now, let me show you a second type of "unorthodox fitting" - the hidden orifice. Here's an example:

Once again, you have a normal-looking fitting, that houses an orifice. In this example, it's an outrigger cylinder, that needs this orifice to control the motion. Two "bad things" can happen here. First - the orifice can get obstructed, and if you don't know it's there - you'll waste time troubleshooting other things. And second - if the fitting is old and rusty, you can replace it with a shiny new "normal" fitting (orifice-free) and then wonder why the hell the outrigger that was moving so smoothly is "acting out" now.

My tip is - you need to look at a hydraulic system and think about what makes sense. For example - when you see an outrigger that moves a heavy machine up and down, but apparently uses a simple piloted check valve to hold the load - i.e. not a flow-modulating valve - you should realize that there should be a restriction point somewhere, and if it's not apparent - most likely the orifice is hidden, and a fitting is as a good a place as any to hide an orifice!

Now for the third type (my favorite) - a shuttle valve that looks like a normal fitting:

You may say. "But.. there's clear marking that shows it's a shuttle, isn't there?". You're right, there is, but let it sit for a couple of weeks in a mine or near the sea, or even better - cover it with a fat coat of paint, and voila - it'll look like any other metric Tee next to it.

Like I said before - always look at what makes sense and what doesn't. When you look at a hydraulic system as a whole, places, where shuttle valves should logically exist, are not hard to identify. So, don't immediately assume its' a simple T-junction fitting when you see one.

And now - to the "three and a half" type.

Another way a system designer can give a simple hydraulic fitting additional functionality lies in placing "things" directly under it, turning it into a valve. For example - a check valve, or a uni-directional throttle valve.

I once assisted a case, when a grader steering stopped working, and first - the client paid for the steering valve repair, then when the steering continued to malfunction, he bought a new steering valve, and in the end, when a savvy tech arrived at the "crime scene" - he discovered that underneath the fittings in what looked like a simple path-through manifold, there were metallic washers with a calibrated orifice in the center and radial grooves machined on the other side placed under the fittings, which turned them into uni-directional throttle valves (to dampen the oil coming out of the steering cylinder). When a piece of plastic lodged in one of the orifices - it turned the throttle valve into a check valve - and the steering function broke!

The owner told me that he looked at the manifold countless times, and it would never occur to him that it could hold any additional functionality!

A note: I have such a plate somewhere in the shop - but I couldn't find it, which is why there's no picture here, I am sure I'll run into it sooner or later, and then post a picture of it here.

I called this type "three and a half" because the fittings are perfectly normal, it's the stuff that gets placed underneath them that makes them "unorthodox".

I, personally, would love to see all such "catch" cases be identified with stainless steel plates with clearly engraved messages like "Mesh filter fitting" or "Shuttle valve here", but unfortunately no one will ever do that, and un-prepared techs will continue to struggle with these invisible troubleshooting traps till they gain enough experience (or, hopefully, till they find and read this article).