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     This is an Italian closed loop unit from HP Hydraulic (Bondioli i Pavezi group), equipped with automotive control, reference M6PV72-72. Here in Portugal, these pumps are commonly called HP pumps (mainly because of the letters H and P die-cast on the casing). Despite being small, these pumps carry lots of adjustment screws, and have universal fame of "being hard to adjust", so I have been wanting to write a back-engineering article on one of these for a long time...

    This particular pump has a "bonus" control option - pressure compensator, which is mounted on top of the automotive control, and conditions the variable motor displacement by supplying controlled servo pressure - a back-engineering opportunity that I can't afford to go to waste!

    Here you can see how the complete pump looks like. As far as the basic design is concerned there's no news - you get a classic rotatry group, servo-cylinder, swashplate, system check-valve/relief assembly, and gerotor type charge pump. The unit is also equipped with by-pass option (for towing and/or smart sabotage purposes).

   The control module of this pump is the part that deserves a more detailed look. For specifications and schematics you can download HP pumps catalogue here, or, if you don't want to  browse through multiple pages, you can see the pump's diagram here.

   Like any mechanical automotive control, this control assures the automotive function by modulating pump servo-pressure in relation to the input shaft speed, and like most automotive controls it defines control pressure by means of an orifice pressure drop, with the orifice placed in the path of the charge pump flow. The peculiarity of this control lies in the fact that not all charge pressure flow passes through the control, unlike in other common designs (Rexroth A4VG DA control, for example).

   If you look at the schematics, you will see that the charge pump outlet is connected directly to the loop check valves (optionally through a filter) in parallel to the control part, which means that the oil flow through the control equals charge pump flow minus loop leakage.

   Let us consider first the control part, to see how exactly the servo-pressure is conditioned.

   The oil from the charge pump passes the minimum charge pressure spool, which  makes sure the charge pressure doesn't drop below a certain level at low speeds. Than the oil is directed to the control spool, which is nothing else but a variable restrictor, and then the oil is dumped to the casing. The restriction creates the necessary servo-pressure, which is then directed to the servo-cylinder via directional solenoid valve. As the flow through the restriction increases, the pressure drop and consequently the servo-pressure also increases, assuring the automotive (shaft speed related) control function.

     The size of the strangulation section can vary with pressure. The control spool is being pushed on by the small rod, connected to the loop sides via two small check valves mounted inside the control body. This feature can be adjusted so that the restriction will increase at a certain high pressure level, thus limiting the transmission torque consumption (in the catalogue this adjustment point is called "power" adjustment). It  is also possible to leave the strangulation fixed by locking the power adjustment screw all way in.

    The oil flow through the control spool equals charge flow minus system leakage flow(loop flushing included, that is why an orifice type flushing valve should be used in the motor) , which means that the control pressure will actually decrease with the increase of the leakage factor (pressure rise), which also works as a torque limiting system.

     Now let us have a look at the pressure compensator, which is mounted on top of the automotive control module, and serves to supply controlled servo-pressure to the variable displacement motor. A small orifice connects internally the compensator outlet with the pump charge pressure gallery. The outlet is connected to the variable displacement motor. When the system pressure hits the compensator adjustment level,  the line is vented to tank, causing the motor servo-pressure to drop. As the servo-pressure drops, the motor displacement increases, which allows the transmission to follow the current speed/torque demand.

    The control is also equipped with a thermal valve to assure euqal operation with cold oil. The valve is based on the wax-motor principle. When the transmission cold starts, there's am additional oil path from the charge pressure side to the pump casing through these small orifices, compensating for the increased viscosity of the cold oil. As the wax capsule heats up, the plunger extends, dislocating the spool to the left and closing the orifices.

    In this article I already described one example of how a wax capsule can be used in cooler by-pass systems, but as I am often asked questions about these gizmos, I created a small page showing how a wax capsule works - here.

   Another adjustment point you can find on the pump is the maximum charge pressure relief valve, which serves as security relief valve when the variable restrictor is closed and/or during cold starts.

    This unit also allows adjustment of the valve plate pressure carry over angle (seen here)- which is yet another way to control the transmission torque demand. The matter of the valve plate angle is a topic for a separate article which I am planning to publish soon, so I won't go in much detail about this adjustment here.

   When you understand how this control works, its adjustment becomes a relatively easy task. The settings, of course, will depend on particular characteristics of a vehicle. I, personally, prefer the following pattern:

a) close the restrictor and adjust the maximum charge pressure relief valve
b) open the restrictor and adjust the minimum charge pressure level
c) adjust the starting point
d) block the power screw and check how the transmission responds, then unscrew it to the desired level (if necessary)
e) explain the operator that the machine can not go any faster than it already does. (more often than not I'm asked to tweak the transmission to drive "just a tiny bit faster"...)

    If you ask me what I think about these pumps, I will tell you that I have "mixed feelings". These controls are relatively simple, but there's something about this design that just doesn't "feel right" (please forgive the term and my subjective approach). For example, the trapped high pressure behind the check valves, which apparently doesn't get trapped due to the existing clearances and low volume of the pressurized chamber, the power control, which can function as an active pressure limiter. The changing charge pressure level. All this is absolutely functional, yet "strange". Practice shows that these pumps are reliable medium pressure closed loop solutions, and the overall build quality is very high. Would I prefer one of these to a Rexroth pump - no! To a Linde pump - yes!
HP closed loop pump
Rotary group
Relief valve/check valve
Gerotor charge pump
Maximum charge pressure relief valve
End cover
Control ports
Control assembly
Automotive control
Automotive control
Minimum charge pressure valve
Control spool
Thermal valve
Thermal valve orifices
High pressure acts on this rod
Pressure compensator control
Pressure compensator control
Measuring ports
HP pump hydraulic diagram