One of Fluid Power World’s resident technical experts, Carl Dyke, has related a number of field-service issues he’s come across over the years. In a presentation at the 2017 Fluid Power Technical Conference in Milwaukee, he enlightened us on one mobile application that had the machine owners scratching their heads.
This one cropped up last year and concerned a container stacker. It’s one of those mobile units that runs around at a shipping terminal, looks a little like a fork lift, but also has a boom that can stack shipping containers four units high and two or three rows deep. The report was that one of the two cylinders on either side of the boom was continuously drifting and creeping downward, giving them a bit of a skewing or a racking. It wouldn’t stay locked.
The operators of the terminal said, “This block attached to the blind end of the cylinder, we’re convinced it’s cracking internally. We have to constantly order a new one from the manufacturer and they want $4,000 each. Maybe you can help us figure out what’s going on in there.”
Well, this valve manifold is a solid block of steel, not aluminum. It contains several screw-in type cartridge valves and, according to the schematic, it does more than just load holding. There’s load locking, there are regenerative functions and one or two other functions all going on inside this valve block mounted on the cylinder’s blind end. Yet the mechanic was convinced, “Something is wrong with the way they make the steel on this block and it must be cracked inside, because we’ve had to order three or four in the last year.”
Where do we go from here? The machines in question have two boom cylinders separated by a bit of distance between them. So if one of the two cylinders starts to creep down a bit, you get uneven racking or skewing. It’s a very unsettling circumstance, because they want to lift that boom, hold it for a while and not see anything move—especially if it’s loaded.
What is in that block, that’s about twice the size of a shoebox? We put one in the back of our pickup truck and took it back to the shop. We like to rip things apart and get inside, and see if we can find out what was going on.
Right off the bat there are two, pilot-operated valves. Then there’s a solenoid valve to operate the hydraulic pilot-operated valve in two locations. There’s one check valve and one work port relief for the blind end of those boom cylinders. It didn’t take too long to get it apart and get the main load-holding valve out. This is the valve that stays locked if you don’t intend for the cylinder to raise or lower. There’s no directional valving in this bank—the directional valves are mounted elsewhere on the machine.
We removed one cartridge and looked into the back of the threaded cavity, where we could see there was a place for our hex key. And we ran our hex key in there, which squished a little piece of contaminant further in. When we pulled the key out, we noticed that the contaminant was sort of bundled up in there, so we pulled it loose again down at the little “tail” sticking out. And we thought, “Well that’s interesting.”
So was the steel block cracked inside, allowing fluid to pass from one place to another, when maximum relief pressure was 3,200 psi? No. There was nothing wrong with the steel.
But we patiently took it apart and documented what’s there and try to understand what the circuit is all about. Among other things, there was a pressure-drop controlling orifice for the pilot line. It prevents bleeding out an excessive amount of fluid from the blind end and making the pilot functions of this valve work correctly. But that was jammed full of that little sliver of contaminant. As soon as we took out the main load-locking cartridge and looked in the back, we could see that there was a steel sliver sitting right there. Not in the poppet anymore, just sticking to the hydraulic fluid that was there. And more loose bits of polymer material sitting on the poppet.
Right away that’s a clue that likely, they are just dealing with a common issue: contamination has come to rest on poppets.
Valve designers work very hard to design these cartridges so that just at the moment before the poppet closes you’re at the highest possible velocity, to flush away any contaminants that might be close by and to resist trapping of contaminants.
But of course, valve manufacturers don’t intend these things to work with a slurry of a fluid, so you can’t guarantee you’re not going to trap something. That’s basically what was at work here. We went back to the client and said, “It looks like some of these little slivers of metal have an arc shape to them. The bits of seal that are in there are probably off of the pistons in the cylinders. And since this is a load-holding valve right against the blind end of the cylinder, the question is: How old are those container stackers? How old are the boom cylinders?”
I believe the number was twelve years old, and the number of cycles up and down was estimated to be pretty high. And we asked, “Well, what’s the expected service life of those cylinders for the kind of work you’ve done?” And that number wasn’t known. So I said maybe it’s time to go back to your mobile equipment manufacturer and talk to them about it. Because without further investigation on our part, it would appear the contaminants that you’re dealing with are coming from the cylinder that’s sitting there and that is the root cause here, not this block of steel. It’s not cracked inside.
We always talk about the invisible particulates, reading reports about things that are five and ten microns in size that are spoiling reliable functioning of valves. But here, I didn’t even have to get out my microscope. With just a look we could see big particles. This is the visible side of things that wreck your mobile valves. The cylinder, to the best of our knowledge in this case, was simply scrubbing away and creating wear particles—metal to metal and metal to seal material—and causing these issues.
So some of these mysteries aren’t too terribly hard to start to solve. Or at least it was easy enough to find out where the leakage path was. The terminal port managers didn’t seem particularly interested in getting into a cylinder study, but that is certainly what we recommended they do, because our suspicion was they were well past the end of expected service life for those units.