An important Add-On for this article was added on 10th May, 2010.
Every once in a while one of these stops by. Quite a simple and
reliable pump, a hydraulic heart of many Deere's tractors, as I've
heard. Unfortunately, I haven't yet hat an opportunity to lay my hands
upon one of them tractors and its hydraulics (remember, I am mainly
pumps/motors guy, only occasionally a field service guy) BUT, I did get
its "heart", and have found it worthy of looking into.
Understand, please, that ALL the information I am
putting here (as well as on other pages of this site), is my own vision
and understanding of how things work. Rest assured, also, that I
believe completely in what I dare to express. There is absolutely no
disclosure of protected information, all there is, is a set of my own
logical assumptions based
on my previous experience and understanding of hydraulics. Take this
pump, for example, I have never laid my eyes upon neither any of John
Deer's manuals, nor the machine itself. In fact, I only call these John
Deere pumps because they are brought in as such. The company I
currently work with does not represent this brand and does not stock
spares. So the only thing I've seen so far is the pump itself.
Nevertheless, I have an opinion about it, an idea about its operation,
and an itch to back-engineer stuff which I find curious and
interesting. If you, by any chance, are a John Deere technician and
find that something I am putting here is not accurate, please feel free
to prove me wrong, I will appreciate it!
The pump, at first glance, looks like a classic radial piston pump. It has spring loaded radial pistons, an excentric shaft (pic4, pic11), suction check valves (pic5) and pressure check valves, seen on pic1.
This particular model had four pistons, but on the front end there are
eight cavities to mount check valves, so there is for sure an eight
piston model around. Further looking into it reveals
some differences that make this pump stand out among other radial
piston pumps.
A simple radial piston pump has an inlet, an outlet,
and is of a fixed displacement type. However extra digging reveals a
few "extra" parts, like the small filter (pic6, extremely shitty condition), a valve with an adjustment screw (pic7), and yet another small valve inside (pic8), no adjustment this time, and two orifices (pic8, pic9).
If you stare directly into this pumps body for about
an hour, you will see another key difference. A simple radial piston
pump has two separate volumes - suction volume and pressure volume, and
this one has three - suction, pressure and , as I called it, the
excentric chamber (which on a "normal" pump would have a connection to
the suction side).
Now is the time to put it all on paper. Making
diagrams works great for me, it helps me to grasp "the big
picture". So after an hour or so of "air gun practice" with
corresponding sprinkling of everything around me with, I must say,
rather smelly oil residues this pump was carrying inside, and taking
all kind of measurements I came up with this! Eureca!!! I found it!!!
When I drew the diagram I suddenly understood why
the mechanic who brought the pump called it a "closed loop". This is
definitely not a closed loop pump, but I am sure he was referring to
"closed center", which can only mean that this pump is working as a
constant pressure source with a closed center directional valve/valves.
Which, of course, also means that this is a variable displacement pump.
When it starts, it works just like a classic radial
piston pump, but when the outlet pressure reaches the setting of the
sequence valve, it opens and starts to pressurize the "excentric
chamber", no, let's call it the "control chamber", and what happens
here is the pistons actually stop retracting after a certain pressure
level inside the control chamber is reached (around 2-3 bar, depending
on the pistons return spring force), so that the excentric moves
around barely touching the pistons just enough to maintain the set
pressure level at the pump's outlet. An ideal constant pressure source.
No swashing plates, no complicated control valves and servo-pistons, no
proportional or load sensing signals. Just pistons, check valves, and a
poppet with a spring. A bullet-proof design, from my point of view.
Now what would be the purpose of the second
valve? I called it "fast responce" valve, and , must confess, I spent
plenty of time thinking about its purpose. My best theory is that it
improves the pumps response time to a sudden pressure drop at the
outlet, like when you open really fast the closed center valve. The
volume of oil inside the control chamber is bigger when the pump is at
"stand by" and all the pistons are in a "not retracted" position. When
the direction valve opens suddenly, the pressure at the outlet drops
abruptly and causes the "response" valve to connect the control chamber
with the suction side, thus allowing the pistons to retract faster by
expelling the excess oil from the control chamber to the suction side
through the additional passage.
Check out the wear of the body in the video (right click and save).
I call it "my pumping days are over" type of
wear. Next stop - scrap yard.
Such an excessive wear had an interesting effect.
When I tested the pump, I wasn't able to go over 120 bars, but not due
to the low efficiency, but due to the fact that excessive leakage to
the inside of the control chamber would cause the pressure to rise and
lift the pistons to the "stand by" position, just like the control
valve does. On the side of the pump there is a control chamber
measuring port, and when I opened it to the tank, the pump would go
beyond 250 and even more, of course with volumetric efficiency falling
beyond 60 per cent, which is way too low for any respectful radial
piston pump.
On my opinion, this is neat and simple variable
displacement pumping solution, especially suitable for constant
pressure source systems (which doesn't mean it can not work with open
center valves), with the drawbacks of excessive noise, typical for
pumps using check valves, BUT with the additional advantage of improved
"dirt tolerance" (no valve plate to bust). Unfortunately I can not tell
you if these pumps are proved to be reliable or "hourly-capable", but
the parts seem robust and I like the design idea, so were there a pump
election this one would gain a vote of mine for sure.
