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Sheet N ° 22

Difference between

 open circuit and loop circuit

 Hydraulic training

Often the term "loop circuit" is used incorrectly by technicians.

In general, the open circuit will be used to perform linear and rotary movements (jacks & motors). This will sometimes be the only circuit of the machine.

The pressure of the open circuits may reach 350 bars .

The loop circuit will be reserved for rotary movements (motors). In very rare cases to linear movements. (Marine for example).

Also called hydrostatic circuit, it is mainly found on mobile machines (public works, agriculture, forestry, marine, etc.)

It is often used to move vehicles forward, or to rotate motors generating significant forces (winches, cable cranes, etc.)

The pressure of the loops circuits can reach 450bars.

1 - Open circuit

The architecture of the open circuit consists of one or more pumps (fixed or variable displacement) generally driven by an electric or thermal motor. The pumps suck the oil into a tank. The oil is directed towards the receivers (cylinders, motors, etc.) using distribution devices (directional valves, valves, etc.). The return oil from the receivers returns to the tank.

We find various pressure valves to protect pumps and receivers (pressure limiters, pressure reducers ...)

Various flow valves can be found to reduce the speed of the receivers (flow restrictors, flow regulators ...)

The oil is filtered and cooled using a heat exchanger installed on the return to the tank.

 

Circuit ouvert 1
 

In the diagram above, the pump (item 1) is driven by an electric motor (item 2), it sucks the oil into the tank (item 8).

The oil is directed to the cylinder (item 5) by the 6/3 directional valve (item 4).

The pressure relief valve (item 3) protects the pump, it is integrated in the directional valve.

The oil is filtered (item 6) and cooled by the exchanger (item 7) on the return to the tank.

2- Loop circuit

The architecture of the loop circuit consists of one or more pumps with two directions of flow generally driven by an electric or thermal motor. The pumps are filled with oil using a booster pump that sucks oil into a tank. The oil from the main pumps directly feeds the receivers (motor in most cases). The return oil from the receivers returns to the suction of the main pumps.

There are various pressure valves to protect pumps and receivers (HP valves, BP valves ...)

Unlike the open circuit, to reduce the speed of the receivers, it changes the flow of the main pumps by changing their displacement.

To filter and cool the oil, an exchange block is installed to create a leak to the tank. The booster pump compensates for the lack of oil and brings fresh oil to the circuit.

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Circuit ferme de base 1

In the diagram above, there is a pumps unit and a motor connected together by 2 flexible A & B. The main pump (item 1) and the booster pump (item 2) are coupled.

The BP valve (item 7) protects the booster pump, which draws oil into the tank (item 13). The poppet valves (items 5 & 6) allows feeding the circuit.

HP valves (4 & 3) protect the main pump.

The exchange spool (item 8) allows the circuit oil to be withdrawn to filter it (item 11) and cool it (item 12). The exchange valve (item 9) makes it possible to control the leak that returns to the tank.

To note: On the drain of the main pump is found the flow of the booster pump when the hydraulic motor (item 10) is off (stand-by).

On the motor drain, there is the flow of the booster pump when the hydraulic motor is running.

 

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