Don't you just love the assistance calls when you have absolutely no
information about the problem, no detailed description of what is going
on or what type of machinery you're going to deal with? Except, of
course, for its location and the "urgency factor", meaning that that
the problem must be solved "yesterday"? So you grab all your gear and
off you go without even being sure that the problem is hydraulic? I
call them "Blind Calls". Despite being rather depressing due to the
"blind" factor, they sometimes turn out to be quite educational.
The information I got this time was that, some time
before, a series of pneumatic jacks had been repaired (new seals) and
the client was claiming that three of them did not work. That's all. I
had no idea what type of a jack it was, whether it was big or small,
what kind of function it performed, what type of machine it was.
Nothing, nada, zero.
When I finally came to destination point, the
machine turned out to be a pretty complex tomato planting machine I had
never seen before, and the jack in question was this.
On the picture you can see that it had two air lines connected to it,
and a small solenoid valve on one side. The air directional valve worked OK.
In manual mode, when it was to lift the structure, it would pressurize
the left-end hose, and when it was to lower the structure, it would
pressurize the rod side of the vertical section. The machine was
composed of three identical separate sections, that were planting
tomatoes in three rows. None of the three jacks was moving. When it was
to lift the structure, which was not so heavy, around 20 kg maybe, one
could feel that the jack was making some force, as the structure 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 bars, the manufacturer
recommended level.
First series of questions: why such a strange
L-shaped format? Why solenoid valve in the middle? Why such a
complicated system just to lift and lower a light 20 kg structure? What
the hell am I doing here? (just kidding). Did I close the garage door
this morning? (kidding again).
It was clearly an unknown component to me and its
appearance didn't give me many clues (my superman x-ray vision gave up
on me that morning). So, what do you do, when you have no idea about a
piece and no documentation is available? Back-engineer, of course! No,
first you ask the operator how the unknown component was 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 was
supposed to move down in precise 30mm 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, in fact there was a
position transducer present on the frame. At this point I already had
the first clue, and immediately started looking for confirmations of my
theory.
The first clue is - controlled descend and precise
positioning. Two things which are very hard to achieve with pneumatics.
Air is good stuff, but it's way too compressible. When you have a
pneumatic jack moving and you shut the directional valve it won't stop
immediately. When you alter load of a pneumatic jack, with a piston in
the middle position, the rod will move, compressing the air. That's
why most pneumatic jacks work "full stroke" and are not suited to
position anything in the mid stroke.
On the contrary, controlled descend 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 jacks were very easy to remove and to open (pic.3). The schematics of the gismo is on the pic.7 (PDF). On the diagram you can see that the solenoid valve passage is strangulated. You can see the seat/orifice on pic.5, sorry, forgot to take a close up. Pic. 4 shows the solenoid valve. On pic. 5
you can see the external by-pass connection tube, which is used to
manually lower the structure when needed. Note that these are not
normal pneumatic quick fittings, but rather threaded hydraulic, another
point in favor of "missing oil theory". Pic. 6 shows the check valve.
After having seen all the pieces and visualizing the
schematics, I had no doubts that the mid section of the assembly 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. Afterwards, the only way to lower the structure 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 structure was lowering, the air
was being injected in the rod cam of the vertical cylinder to "help"
the descend, then the machines controlling system did an excellent job
of lowering the cassette-holding structure exactly 30 mm at a
time.
You might ask why the hell, as long as the
system air pressure was OK, the structure didn't lift before filling
the oil? Well, it's easy. By moving the loose piston, we are
compressing the air in the mid section. It is obvious that the pressure
inside can't go higher than the system air pressure, because both sides
of the loose piston are equal, and it is obvious, that to make the
structure lift, a sufficient pressure must be created in the mid
section, BUT, as the mid section is "filled" with air under the atmospheric pressure, it must be compressed
to reach the necessary pressure level. Let's say 5 bars is sufficient
to move the structure up. In this case, the air in the mid section has
to be compressed to a volume five times as small, before reaching 5
bars. And it is this compressed air volume, that will define the travel
of the vertical jack. Now if you take into account the air in between
the two jacks, you'll see that maximum possible pressure for this
arrangement is limited by the relation of volumes of the loose piston
cylinder and the mid section. As soon as the loose piston hits its
mechanical stop, you may pressurize it to a billion bars, and the
pressure inside the mid section will remain the same (provided it was
filled with air under the atmospheric pressure). In fact, if the volume
in between the two pistons is big enough, it is possible for the loose
cylinder to hit its mechanical stop BEFORE the lifting pressure level
is reached.
I confess that this type of jack assembly was news
to me, due to the fact that I work very little with pneumatic systems,
but I did find the idea very elegant and worthy of description.
Combined use of pneumatics and hydraulics allowed 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 oil filled pneumatic jacks were news to
the machine's owner the same way it was to me. Unfortunately in this
case the lack of information caused almost three weeks of downtime.
Just another good reason to get a detailed manual for your machinery!
