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Tag Archive: open loop

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:

SANFAB Injector

What is a wonderful gadget I met today! Will try to use it in next applications. It used for oil recirculation, but it seems like you can make a close loop system with open loop pump, and get almost all benefits of the close loop:

  • reduced tank size
  • lower weight
  • higher pump work rpm

With benefits above this device is really welcome at the mobile applications where open loop pumps need to be involved. For example, at the project I’m working now, I got a situation where the open loop pump has to work at high 2500 rpm (2100 diesel engine + PTO ratio) and by some reasons open loop pump I need to use. And it isn’t easy to find not expensive open loop pump for so hight cont. work RPM.

If it is interesting here the link for the gadget: https://www.sunfab.com/products/accessories/injector/