Home>> Back-Engineer This!>> John Deere Radial Piston Pump>>
InsaneHydraulics - Sergiy Sydorenko 2009-2010 All Ridghts Reserved
Home
Introduction
The Simple Test
Back-Engineering?
The Most Basic Basics
Let's Talk Hydraulics
Back-Engineer This!
Battlefield
Kaboom
Library
News Archive
Bla-Blaimer
Contacts
Guestbook
HomeIntroductionThe Simple TestBack-Engineering?The Most Basic BasicsLet's Talk HydraulicsBack-Engineer This!BattlefieldKaboomLibraryNews ArchiveBla-BlaimerContactsGuestbook |
1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8

9

9

10

10

11

11

12

12

    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.

            An Add-On for this article from 10th May, 2010.