The difference between open hydraulic system and closed hydraulic system

Open system 

(1) Open hydraulic system means that the hydraulic oil pump sucks oil from the oil tank, and supplies oil to the hydraulic cylinder (or hydraulic motor) through the reversing valve to drive the working mechanism. The oil return of the hydraulic cylinder (or hydraulic motor) is then returned through the reversing valve. tank. Install a relief valve at the pump outlet. This system structure is relatively simple.

(2) Since the oil after the system is working is returned to the oil tank, it can play the role of heat dissipation of the oil tank and precipitation of impurities. However, because the oil is often in contact with air, it is easy for air to penetrate into the system, resulting in the need to set up a back pressure valve on the road, which will cause additional energy loss and increase the oil temperature. 

(3) In the open system, the hydraulic oil pump used is a fixed pump or a one-way variable pump. Taking into account the self-priming ability of the pump and avoiding the phenomenon of suction, the working speed of the hydraulic pump with poor self-priming ability is usually set Limit it to 75% of the rated speed, or add an auxiliary pump for filling.

(4) The reversal of the working mechanism is cast by means of a reversing valve. When the reversing valve changes direction, in addition to the hydraulic shock, the inertia of the moving parts will be converted into heat energy, which will increase the temperature of the hydraulic oil.

(5) Due to the simple structure of the open system, it is still used by most cranes.

 Closed system

(1) In a closed system, the oil inlet pipe of the hydraulic pump is directly connected with the oil return pipe of the actuator, and the working fluid is in a closed loop in the pipeline of the system.

(2) The structure of the closed system is relatively compact, with less chance of contact with air, and air is not easy to penetrate into the system, so the smoothness of the transmission is good.

(3) The speed change and reversal of the working mechanism are realized by adjusting the variable mechanism of the pump or motor, which avoids the hydraulic shock and energy loss in the reversing process of the open system. 

(4) The closed system is more complicated than the open system. Since the oil after the closed system does not return to the oil tank, the heat dissipation and filtering conditions of the oil are worse than the open system.

(5) In order to compensate for the leakage in the system, a small-capacity refill pump is usually required for refilling and cooling. Therefore, this system is actually a semi-closed system.

(6) Under normal circumstances, if the actuator in the closed system adopts a double-acting single-piston rod hydraulic cylinder, due to the unequal flow rate of the large and small cavities, the power utilization rate will decrease during the working process. Therefore, the actuator in the closed system is generally a hydraulic motor.

(7) The closed hydraulic drive system has continuous oil leakage during work (continuous high-pressure oil internal leakage is an inherent product of component design). In order to compensate for these leakages and costs, and to maintain the closed system working normally, it is necessary to provide the closed system Make up the oil in time.

(8) A small displacement charge pump is attached to the main pump of the closed system. Since the displacement and pressure of the charge pump are relatively small compared to the main pump, the additional power loss is usually only 1% of the total power of the transmission. 2% can be ignored.

Post time: May-27-2021
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