Insane Hydraulics

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A Cylinder That is Both Pneumatic and Hydraulic at the Same Time

Don't you just love the assistance calls when you have no information about the malfunction, no detailed description of what is going on, or even what type of machinery you're going to deal with? (Except, of course, the client's address, the confirmation of the fact that "something is not operational", and the request for the problem to be solved yesterday). And you grab your gear and go there without even being sure that the problem is hydraulic? I call these calls "Blind Calls". Despite being discouraging, they often turn out to be quite educational.

So, I got a call about a series of pneumatic cylinders that had been re-sealed and the client was claiming that three of them did not work. That's it. I had no idea what type of a cylinder it was, whether it was big or small, what kind of function it performed, or what type of machine it was.

When I got to the client's, I saw that I would be working on a pretty complex tomato transplanter, and the "jack in question" was this L-shaped contraption here:

The malfunctioning pneumatic cylinder
The malfunctioning pneumatic cylinder

You can see that there are two pneumatic lines connected to it, and there's a small solenoid valve on the side. I manually tested the (pneumatic) DCV connected to the cylinder - and it was OK. When it was to lift the frame, it would pressurize the hose connected to the horizontal section (the left end on the picture above), and when it was to lower - it would pressurize the rod end of the vertical cylinder.

The machine was composed of three identical sections, that were planting tomatoes in three rows. None of the three cylinders was moving. When a cylinder would "try" to lift the frame, which was pretty light (around 20 kg), one could feel that it was making some force, as the frame seemed to lighten up a little, but it was not enough to lift it. The air pressure was not an issue also, it was around 7 bar, the manufacturer-recommended level.

Let's ask some questions. Why the strange L-shaped format? Why is there a solenoid valve in the middle? Why such a complicated system just to lift and lower a light 20 kg structure?

The appearance wasn't giving me many clues. But what do you do, when you have no idea about a component and there's no documentation around? Back-engineer, of course! Well... first you ask the operator how the unknown component is supposed to perform, and then you back-engineer!

According to the operator the jack was supposed to lift the frame holding several plant-filled cassettes and then move down in precise 30-mm steps, allowing the row of feeding needles to push out a dozen of plants from the cassette one batch at a time. The positioning had to be very accurate, there was even a position transducer on the frame. At that point, I already had the first clues, and immediately started looking for confirmations of my theory.

The first clues are - controlled descent and precise positioning. Two things that are very hard (if not impossible) to achieve with pneumatic cylinders. Air is good stuff, but it's way too compressible, which is why pneumatic jacks work "full stroke" and are not suited to position anything mid-stroke.

On the contrary, controlled descent and precise positioning can easily be achieved with hydraulics because oil is almost incompressible. So a bell was ringing in my head saying that somewhere in this system oil was missing.

Fortunately, the cylinders were very easy to remove and open:

Diassembling the cylinder
Diassembling the cylinder
Diassembling the cylinder
Diassembling the cylinder

And here's the diagram I came up with for this actuator:

The schematic of the hydraulic-pneumatic cylinder

After having seen all the pieces and visualizing the schematics, I had no doubts that the mid-section was to be filled with oil, so I put the jacks back together, pushed the loose piston back, and filled the assembly with ATF fluid (the only hydraulic fluid available on site). The machine worked perfectly. When the loose piston chamber received air pressure it would force the oil through the two check valves to the vertical section and raise the structure. Then, the only way to lower the frame was to open the by-pass valve manually, or to energize the solenoid valve, allowing the oil to flow back to the loose piston chamber through the orifice (approx. 1.5 mm in diameter). When the frame was lowering, the air was injected in the rod cam of the vertical cylinder to "help" the descend, then the machine's controlling system did an excellent job of lowering the cassette-holding structure exactly 30 mm at a time.

I confess that this type of jack assembly was new to me at the time. I did find the idea very elegant and worthy of description. The combined use of pneumatics and hydraulics allowed the designer to benefit from the advantages of both. Cleanliness and ease of connection of air, yet precision and control of oil. Gentlemen, I take my hat off!

Apparently, the machine's owner wasn't aware of the existence of oil-filled pneumatic jacks either. Unfortunately in this case the lack of information caused almost three weeks of downtime.

Another good reason to have a detailed manual for your machinery on hand at all times!