Open hydraulic system:
Open hydraulic system is the hydraulic oil pump sucks the hydraulic oil and generates the pressure from the oil tank through hydraulic valves to the hydraulic cylinder (or hydraulic motor) to drive the work of oil supply, hydraulic cylinders (or hydraulic motor) of the return valve back to the oil tank. Install relief valve at pump outlet port. This kind of system structure is relatively simple. Due to the oil return oil tank, the system can play the role of heat radiation and precipitation of impurities. But due to oil often in contact with air, so that the air easily penetrate the system, leading to the road to set back pressure valve, as this will cause the additional energy loss, the oil temperature rise.
In the open hydraulic system, a hydraulic pump for quantitative pump or one-way variable pump, taking into consideration the pump self-priming capability and avoid the suction phenomenon, hydraulic pump on self-priming ability is poor, usually the speed limit in less than 75% of the rated speed, or adding an auxiliary pump into the perfusion. The change of the working mechanism is by the aid of the reversing valve. In addition to the hydraulic shock, the inertia energy of the moving parts can be changed into heat energy, and the temperature of the hydraulic oil is increased. However, due to the simple structure of the open hydraulic system, it is still used by most of the crane.
Closed hydraulic system:
In the closed hydraulic system, the hydraulic pump’s inlet pipe is directly connected with the return pipe of the executing component, and the working fluid is closed and circulating in the pipeline of the system. Closed hydraulic system is more compact structure, less access to air contact less, air is not easy to penetrate the system, so the stability of the transmission is good. The changing speed of the working mechanism and the change of the reversing by adjusting the variable mechanism of the pump or motor can avoid the hydraulic impact and the energy loss in the process of the hydraulic system. But the closed hydraulic system is more complicated than the open type hydraulic system, because the closed hydraulic system works, the oil is not back to the tank, the heat dissipation of the oil and the filter conditions are more open than the hydraulic system. In order to compensate for the leakage in the system, a small volume of fluid infusion pump is usually required to fill up and heat the heat, so the system is actually a half-closed hydraulic system.
Under normal circumstances, the closed hydraulic system actuator if the double acting single piston rod hydraulic cylinder, ranging from the size of the cavity flow, in the working process, the power utilization rate of decline. Therefore, the implementation of the hydraulic system of the hydraulic system in general.
Hydraulic drive system of engineering machinery, open type hydraulic system and the closed hydraulic system, domestic small tonnage truck crane usually take open hydraulic system with control valve, fulfilling mechanism, activity in the opposite direction and braking request. Large tonnage crane most of the closed hydraulic system, closed hydraulic system adopts bidirectional variable displacement hydraulic pump, through pump variable transformation of hydraulic oil in the main oil flow rate and direction, to fulfill the speed changing and steering. This control method can fully embodies the hydraulic transmission strengths.
In the heavy machinery plant, the large tonnage crane hydraulic work device, usually adopts the angle type axial piston variable displacement pump and the quantitative motor closed type hydraulic system. The flow rate of the variable displacement piston pump is proportional to the driving speed and displacement, and can be continuously variable. Closed loop type variable displacement pump in the oil inlet and motor oil inlet connected, motor out of the oil inlet of the pump and the oil inlet connected, consisting of a closed hydraulic circuit, without reversing valve, by adjusting the variable pump swashplate angle to change direction of the pump flow and pressure oil, so as to change the motor speed and direction of the rotation. Variable pump flow with the ramp angle change can be changed from zero to the maximum value. When the inclined plate is placed in the middle position, the liquid flow direction can be changed smoothly, the micro motion is good, and the work is stable.
Closed type hydraulic drive system at work will continue to have oil leakage of fluid (continuous high pressure oil leak is design elements from the natural products), in order to make up the leakage and cost, keep closed type hydraulic system to work normally, it is necessary to give the closed system in a timely manner to compensate oil.
Closed type of main pump of hydraulic system on the through shaft attached to a small displacement compensation pump, due to repair the pump displacement and pressure relative to the main pump are very small, so the additional power loss usually only 1 to 2% of the total power of the transmission device can suddenly regardless. Fill oil overflow valve and fill oil one-way valve is arranged in the closed hydraulic system of the working device, fill oil relief valve limits the maximum fill oil pressure, oil supply one-way valve on both sides of the pipe of the hydraulic oil pressure high, choose fill oil, fill oil to the low pressure side of the main oil pump and motor, so as to compensate the loss of volume the leakage flow; both sides of the main pump with two high pressure relief valve, the peak pressure and the maximum pressure of the swash plate fast swing by high pressure relief valve maintenance, prevent pump and motor overload; the hydraulic device is also provided with a pressure shut-off valve, pressure shut-off valve is equivalent to a pressure regulator, when reaching the set when the pressure of the oil displacement back to zero state. In addition, the filter is also provided at the outlet of the oil pump, the working medium of the hydraulic system is filtered, and the cleanliness of the hydraulic oil is improved.
Closed system has the following advantages:
(1) the closed hydraulic system of variable displacement pump are integrated structure, oil supply pump and fill oil, overflow, control efficacy valve group integrated in the hydraulic pump, the pipeline connection become simple, not only reduce the installation space, but also reduce the connection caused by the leakage and piping vibration in the pipeline, improve the reliability of the system and simplify the operation process.
(2) oil filling system can not only produce a change in the displacement of the main pump to ensure the transmission volume type response, progress system operating frequency, also can add the main pump inlet pressure, prevent the flow of intrapartum anger inhibition, can effectively improve the speed of the pump and to prevent the pump suction air, improve work life; fill oil system is provided with a filter, to improve the transmission reliability and service life; in addition, fill pump can also be convenient for some low help institutions to provide power.
(3) because only a small amount of oil is drawn from the tank, the loss of the fuel tank is reduced.
Features of open hydraulic system:
(1) generally use a double pump or three of the supply, the pilot oil provided by a separate pilot pump. Some of the hydraulic power required to perform the necessary power of the combined supply, the confluence of two ways:
a. The confluence of the valve. Generally a dual pump supply a stem, the valve in the general rod to control the required confluence of the hydraulic actuator. The design of the combined mode of the valve stem needs to consider the virtual flow area of the multi pump oil supply.
b. Outside the valve confluence. Double pump through the respective stem, through the two valve pan linkage control, in the valve stem outside the confluence of the confluence of the hydraulic actuator. Although the control structure is relatively complex, large, but because of the flow through the stem is a single pump flow, the stem diameter is relatively small, and there may be common with other stem.
(2) multi way valve is often divided into block and sub pump supply, according to the actual needs of each valve group can be used straight through the supply channel and parallel supply of two kinds of oil. The former can be realized as a priority supply, both upstream stem action, the pressure oil on the supply of the stem control of the hydraulic components, and the downstream stem control of the hydraulic components can not move. The latter can be realized and oil supply.
(3) in order to meet the requirements of various operating modes and composite action. In general, the simple pass off type two pain valve and cartridge valve, the oil from an oil directly lead to another circuit, and often use one-way valve oil to prevent the return, constitutes a one-way channel. There are 3 ways to control the on-off valve:
a. The pilot operated oil linkage control, the pilot control oil in the control of the control valve stem movement, linkage control valve.
b. The use of a control valve in the oil channel as a control valve through the oil way, so that the operation of the control valve at the same time, but also to control the opening and closing of the valve.
Another shortcoming of the open circuit is: when a pump for multiple actuators also action, due to hydraulic oil first to lightly loaded actuator flow, resulting in high load actuator movement difficult. Therefore, it is necessary to the light load actuator control valve for throttling.
Features of closed hydraulic system:
(1) the closed hydraulic system of variable displacement pump are integrated structure, oil supply pump and fill oil, overflow, control valve group integrated in the hydraulic pump, make the pipeline connection become simple, not only reduce the installation space, but also reduce the connected by pipe caused by leakage and piping vibration, improve the reliability of the system, which can simplify the operation process.
(2) fill oil system can not only guarantee the transmission volume type of response in the main pump displacement change and improve the frequency of system, but also increase the main pump inlet pressure, prevent the flow cavitation, can effectively improve the speed of the pump and to prevent the pump suction air, improve work life; fill oil system is provided with a filter, to improve the reliability of the transmission device and the service life; in addition, fill pump can be convenient for some low voltage auxiliary mechanism to provide power.
(3) because only a small amount of oil is drawn from the tank, the loss of the fuel tank is reduced.