Differences between Open And Closed Systems And Their Advantages And Disadvantages

Open system:

Open system means that the hydraulic pump from the tank suction, through the reversing valve to the hydraulic cylinder (or hydraulic motor) oil supply to drive the working mechanism, hydraulic cylinder (or hydraulic motor) of the return oil and then through the reversing valve back to the tank. Relief valve is installed at the outlet of the pump. This system structure is relatively simple. As the system works out of the oil back to the tank, so you can play the role of the tank cooling, precipitation of impurities. But because the oil is often in contact with the air, so that the air is easy to penetrate the system, resulting in the need to set the back pressure valve on the road, which will cause additional energy loss, so that the oil temperature rises.

In the open system, the use of hydraulic pumps for quantitative pumps or unidirectional variable pumps, taking into account the pump's self-priming ability and avoid the phenomenon of suction air, the self-priming ability of poor hydraulic pumps, usually limited to 75% of the rated speed of its operating speed, or the addition of an auxiliary pump for filling. The direction of the working mechanism is cast with the help of a reversing valve. Reversing valve reversal, in addition to producing hydraulic shock, the inertial energy of the moving parts will be converted into heat, and the temperature of the hydraulic oil. However, due to the simple structure of the open system, it is still used by most cranes.

Closed system:

In the closed system, the hydraulic pump inlet pipe is directly connected to the return pipe of the actuator, the working fluid in the system piping for closed circulation. Closed system structure is more compact, not the mouth of the air contact opportunity is less, the air is not easy to penetrate the system, so the smoothness of the transmission is good. The working mechanism of the variable speed and direction by adjusting the pump or motor variable institutions to achieve, to avoid the hydraulic shock and energy loss in the open system direction process. However, the closed system is more complex than the open system, because the closed system does not return to the tank after the work of the oil, the oil cooling and filtration conditions are worse than the open system. In order to compensate for the leakage in the system, usually need a small capacity make-up pump for oil and heat dissipation, so this system is actually a semi-closed system.

In general, the closed system in the executive element if double-acting single piston rod hydraulic cylinder, due to the size of the chamber flow is not equal, in the process of work, it will make the power utilization rate decline. Therefore, the actuating element in the closed system is generally a hydraulic motor.

Construction machinery hydraulic transmission system, open system and closed system, domestic small tonnage truck cranes usually take the open system with reversing valve control, to achieve the fulfillment of the positive and negative direction of the activities and braking requirements. Medium and large tonnage cranes mostly use closed system, closed system adopts bidirectional variable hydraulic pump, through the pump variables to change the flow and direction of the hydraulic oil in the main oil circuit to achieve the fulfillment of the agency's variable speed and direction change, this control method, you can adequately reflect the advantages of hydraulic transmission.

Heavy machinery plant medium and large tonnage crane hydraulic working device, usually take the swash plate axial piston variable pump and quantitative motor composed of closed system. Swash plate variable piston pump flow is proportional to the drive speed and displacement, and can be infinitely variable. In the closed circuit, the oil outlet of the variable pump is connected to the oil inlet of the motor, and the oil outlet of the motor is connected to the oil inlet of the pump, which forms a closed hydraulic oil circuit without a reversing valve, and the angle of the swash plate of the variable pump can be adjusted to change the flow rate of the pump and the direction of the pressure oil, which will change the rotational speed and rotational direction of the motor. Variable pump flow with the swashplate swing angle change can be added from zero to the maximum value. When the swash plate is swung over the center position, the liquid flow direction can be changed stably, so the micro-movement is good and the work is stable.

Closed hydraulic drive system in the work of continuous oil leakage (continuous high-pressure oil leakage is an inherent product of component design), in order to make up for these leaks and depletion, to maintain the normal operation of the closed system, it is necessary to make up for the closed system in a timely manner to make up for the oil. Closed system on the main pump shaft attached to a small-displacement oil pump, because the oil pump displacement and pressure relative to the main pump are very small, so its additional power loss is usually only 1% to 2% of the total power of the transmission device, can be ignored. In the closed system hydraulic work device is equipped with make-up oil relief valve and make-up oil check valve, make-up oil relief valve to limit the maximum make-up oil pressure, make-up oil check valve based on the two sides of the pipeline hydraulic oil pressure of high and low, select the make-up oil direction to the main oil circuit low pressure side of the oil to compensate for the loss of volume due to pumps, motors leakage of the flow; the main pump is equipped with two high-pressure relief valves on both sides of the swash plate fast swing pressure peak and maximum Pressure is maintained by the high-pressure relief valve to prevent overloading of the pump and motor; the hydraulic device is also equipped with a pressure cut-off valve, the pressure cut-off valve is equivalent to a pressure regulation, when the pressure reaches the set pressure, the displacement of the pump will be adjusted back to zero condition. In addition, there is a filter at the outlet of the refill pump, the hydraulic system working medium filtration, progressing the cleanliness of the hydraulic oil.

The closed system has the following advantages:

(1) the current closed system variable pumps are integrated structure, replenishment pumps and replenishment, overflow, control and other functions of the valve group integrated in the hydraulic pump, so that the pipeline connection becomes simple, not only to reduce the installation space, but also reduces the leakage caused by the pipeline connection and pipeline vibration, improve the reliability of the system, simplify the operation process.

(2) oil replenishment system not only in the main pump displacement changes to ensure that the response of the positive displacement transmission, improve the frequency of the system's action, but also increase the main pump inlet pressure, to prevent the production of large flow rate betting on air erosion, can effectively increase the speed of the pump and to prevent the pump from sucking air to improve the life of the work; replenish the system is equipped with a filter to improve the reliability of the transmission device and the application of the life of the pump; in addition, replenishment of the oil pump can also be convenient for some of the Low-voltage help organizations to supply power.

(3) Since only a small amount of oil is drawn from the oil tank, the loss of the oil tank is reduced.

Characteristics of open hydraulic systems:

(1) Generally, double or triple pumps are used to supply oil, and the pilot oil is supplied by a separate pilot pump. Some hydraulic actuators require large power need to merge the oil supply, merge there are two ways: ① Combined oil supply in valve. Generally there are two pumps combined flow supply a valve stem, in the general valve stem control oil supply to the required combined flow of hydraulic actuators. The merge of the valve stem of the aperture design needs to take into account the virtual circulation area of the multi-pump oil supply. ② Valve outside the combined flow. Double pumps through their respective valve stems, through the two valves flooding linkage operation, in the valve stem outside the combined flow of oil supply to the required combined flow of hydraulic actuators. Although the manipulation structure is relatively complex, large volume, but due to the flow through the valve stem of the hunger is a single pump flow, the valve stem aperture is relatively small, and it is possible to generalize with other valve stems.

(2) multi-way valves are often divided into blocks and pumps for oil supply, each valve group according to the actual need to utilize the straight through the oil supply channel and parallel oil supply channel two kinds of oil channels. The former can realize the priority oil supply, both upstream valve stem action, the pressure oil is supplied to the valve stem manipulation of the hydraulic components, while the downstream valve stem manipulation of the hydraulic components can not act. The latter can realize parallel oil supply.

(3) in order to meet a variety of operating conditions and composite action requirements, generally use a simple on-off type two-position two-pain valve and cartridge valves, the oil from one oil line directly to another oil line, and often use check valves to prevent the oil back to constitute a one-way channel. Manipulation of the cut-off valve has the following three ways: ① the use of pilot manipulation oil linkage manipulation, pilot manipulation oil in the control of the manipulation of the valve stem movement at the same time, linkage manipulation of the cut-off valve. ② The use of manipulation of the valve to add an oil channel as the control of the valve oil channel, so that in the manipulation of the valve at the same time, but also manipulate the opening and closing of the valve.

Another disadvantage of the open oil circuit is that when a pump is used for simultaneous action of multiple actuators, the hydraulic oil flows first to the lightly loaded actuator, resulting in difficulty in the action of the highly loaded actuator, and therefore, the lightly loaded actuator control valve stem needs to be throttled.