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    Komatsu's hydraulic excavators brought into the world the OLSS hydraulic system, which stands for Open center Load Sensing System. What intrigues me the most, is the fact that it is practically impossible to find any usable information about this hydraulic system, nor about Komatsu pumps. Recently I got a couple of Komatsu pumps from a very old third generation Komatsu excavator to overhaul. The  reason I am calling these HPV pumps a "third generation" is simple, the excavators carry a dash three at the end of the model reference, like PC 200-3, making perfect sense to call it series or generation, but I have no "original" Komatsu data to back it up. Not often, but still a couple of times a year one of these pops up, and I keep seeing that the control modules, used on these pumps, puzzle many mechanics due to the intimidating amount of adjusting screws they carry along, as well as due to the lack of information about open center load sensing systems. This yellow couple gave me a perfect chance to try out once again my back-engineering skills, as well as to shed some light on this OLSS subject. The two pumps were - one with simple displacement control, and the other - with a more complex OLSS control.

    I understand that this information will have no practical value today, as these pumps and excavators (I mean third generation, not the open center load sensing) have become obsolete in most parts of the world. But I am considering the OLSS system to be a milestone in the history of industrial oil hydraulics, and so IH is contributing to make sure that these pumps don't die out without a decent tomb stone to be recalled from...

    The best way to understand how a third generation OLSS Komatsu pump works, would be to start with this base HPV-something model (code D7F50026). This is not an OLSS pump, but the OLSS control is built on this basic module, so the best way to start would be from here.

  The pump you see on the pictures is a simple basic module, and has a very simple  positive proportional feedback displacement control.

    Positive - starts at minimum displacement, when the pilot pressure increases, so does the displacement.
    Proportional - the displacement changes infinitely in relation to the pilot pressure
    Feedback - the control has a mechanical position feedback link connected to the swashplate

    The control spools are arranged in this manner (note, please, that this is something I made to demonstrate the general layout, therefore the drawing is out of proportion), and the schematics is here. As you can see, the first pump has one pilot port, while the second one has two. The control is basically the same, only the second pump can be controlled by an external delta P signal, while the first one relates the signal pressure to the pump casing (tank) pressure. I haven't seen the actual machine, so I, unfortunately,  have no idea about what exactly is the purpose of this pump, but it is just perfect to understand how the much more complex OLSS control with torque limiter and pressure limiting function works, because it is literally mounted "on top" of  such basic module.

    Now - the adjustments. The maximum and minimum displacement (mechanical stops) is adjusted by the spacers under the servo-cylinder caps. As you can see from the schematics, by turning the pilot piston stopping screw in (which also is a pilot pressure reading port) you can hydraulically position the servo-piston above the mechanical minimum displacement limiter, although to go there on this pump you would need a couple of full turns of the adjusting screw. Then you have the standard displacement control threshold adjustment. And finally, you have two adjusting screws on both sides of the servo-pressure spool. In fact, these aren't exactly adjusting screws, but rather are positioning screws, because their purpose is to lock the spool-spring-sleeve assembly in place (drawing). To turn one in you'd need to turn the other one out. As a rule, the position of these screws is factory set, and there is no need to adjust anything here, unless, of course, someone had already tampered with it. If so, remember that these two screws are to be locked against each other. I assume the correct position of the control sleeve/spool assembly would be somewhere around the point where the spring chamber of the servo-piston gets slightly pressurized, but shifting its position slightly doesn't change the control's function - except from altering the threshold setting it functions just the same. From what I've seen on these pumps, the sleeve is normally  locked somewhere around middle position of the casing cavity.

    This particular pump had 40 cu.cm. minimum displacement, and 120 cu.cm. maximum. Both controls were adjusted to start at 10 bars and end at 18 bars. Turning the adjusting screw one turn in increased the control threshold setting by two bars.

    I opened these controls some months ago, and now am restoring the control layout from memory, so some inaccuracies might be present in the drawing, however the most important thing here is to understand the function of this simple control - proportional positive - before going to a more complex OLSS control, which will be described in the next OLSS article in a couple of weeks.

    The above can be resumed to:

    -Displacement of the pump is controlled by a positive proportional control.
    -The pump starts at minimum displacement.
    -The control has a threshold setting adjustment.
    -Servo pressure must be supplied from an external source for this control to work.
    -There is an option of positioning the swashplate above the minimum mechanic displacement setting, which most probably  isn't used.
     -You can change the position of the servo-pressure spool, but most probably it is already in the correct position (close to middle).
     -The latter two serve only to give you an idea of what to adjust in case of "severe fiddling" situations.
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