Semi-closed hydraulic circuits

Have you ever heard about the semi-closed hydraulic circuit?

The idea is to use open loop pumps in closed loop circuit. In the latest eighties, Mannesmann Rexroth (yes, that time Mannesmann yet!) even offered pumps A4VSH, especially designed for semi-closed circuits.

Bosch Rexroth A4VSH pump

Bosch Rexroth A4VSH pump, from catalog RE92110/01.89

The main difference between closed loop pumps and semi-closed loop pumps was in addition block mounted to the ports A and B with check valves which let to supply suction flow from the tank, while mainstream comes from the motor to pump’s suction port:

Since that time Rexroth still builds A4VSH, but there are a limited number of sizes and options available and its no longer a mainstream published option.

But to make a semi-closed loop you do not need some special pump, it’s really easy to turn most standard open loop pump into a semi-closed loop circuit. Of course, you have to have some knowledge about the application and actuators before trying to apply it. If the pump does not have an integrated boost pump and check valves these are commonly added via an external gear pump and external check valves.

Here is the simplest example of the semi-closed circuit schematic where a regular open loop pump with LS control is involved to run two motors with independent proportional control of their speeds:

Semi-closed hydraulic circuit

Semi-closed hydraulic circuit

Notes to the image:
– 0 psi check valve prevents cavitation at the suction.
– 40 psi check valve removes extra flow from the charge pump and keeps pumps suction port under the constant pressure.
– Proportional valves, of course, can be combined in the multi-section valve block.

With a semi-close circuit, you get all advantages that both closed loops and open loops circuits offer:
– smaller tank size;
– smaller cooler size;
– smaller suction hoses;
– less hydraulic oil;
– lower weight;
– the cost of open loop pumps is much lower than the cost of closed loop pump (for the same displacement);
– the flexibility of using the more simple pump controls (what open loop pumps have), and load sensing concept.

This concept is not for every application. For example, it doesn’t work well if you are using inefficient components like geroller or gerotor motors. If the internal case leakages (volumetric efficiencies) are present and pronounced on the pump/motor or actuators, the higher these are or become the less attractive this semi-closed loop option becomes.

Thanks to Grant Green, Bosch Rexroth Canada consultant for providing this interesting info.

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