Construction machinery hydraulic transmission system, an open system and closed system, the domestic small tonnage truck crane usually adopts the open valve control system with open, to achieve the implementing agency positive and negative direction of movement and braking requirements. Most of the large tonnage cranes adopt a closed system (as shown in the following figure). The closed system uses a bidirectional variable hydraulic pump to change the flow and direction of the hydraulic oil in the main oil passage through the variable of the pump to realize the speed change and commutation of the actuator. This control method can fully reflect the advantages of hydraulic transmission. Xuzhou Heavy Machinery Factory, large-tonnage crane hydraulic work device, usually a swash plate type axial piston variable displacement pump and a closed-loop system composed of quantitative motor. Swash plate variable displacement piston pump flow and drive speed and displacement is proportional to, and can be infinitely variable. In the closed circuit, the outlet of the variable displacement pump is connected to the inlet of the motor. The outlet of the motor is connected to the inlet of the pump to form a closed hydraulic circuit. No need to change the valve, Point of view to change the pump flow and pressure oil direction, thereby changing the motor speed and direction of rotation. Variable pump flow with swashplate angle can be increased from zero to maximum. When the swashplate is centered, the direction of fluid flow can be smoothly changed. Therefore, it has good micro-mobility and stable operation. Closed hydraulic drive system in the continuous oil leakage (continuous high-pressure oil leakage is the inherent product design), in order to supplement these leaks and consumption, to maintain the normal operation of closed-system, closed-end systems must be replenished oil liquid. The closed system main pump is provided with a small displacement charge pump through the through shaft. Since the charge pump displacement and pressure are relatively small relative to the main pump, its additional power loss is usually only 1% to 2% of the total power of the transmission. Can be negligible. In the closed-system hydraulic work device is equipped with a charge relief valve and a charge check valve, charge relief valve to limit the maximum charge pressure, charge check valve according to both sides of the pipeline hydraulic oil pressure level, select Fill the direction of the oil to the main oil side of the low-pressure side of the oil to compensate for the pump, motor volume loss by the leakage of the main pump on both sides with two high-pressure relief valve, swash plate when the peak pressure swing and The maximum pressure is protected by a high-pressure relief valve to prevent overloading of the pump and motor. The hydraulic unit also has a pressure cut-off valve that acts as a pressure regulator. When the set pressure is reached, the pump displacement is recalled To zero state. In addition, the charge pump outlet also has a filter to filter the hydraulic system working medium, improve the cleanliness of the hydraulic oil. Closed-loop system has the following advantages: (1) At present, the closed-system variable-displacement pumps are all of an integrated structure. The functional valve groups such as the charge pump, the charge pump, the overflow and the control are integrated in the hydraulic pump to make the connection of the pipeline simple. Not only does it reduce installation space, it also reduces leakage and piping vibrations caused by pipe connections, improves system reliability and simplifies operation. (2) The charge system can not only ensure the response of the positive displacement pump when the displacement of the main pump is changed, but also increase the operating frequency of the system and also increase the pressure at the inlet of the main pump to prevent cavitation when large flow rate is generated. Effectively improve the pump speed and pump to prevent emptying and improve working life; equipped with a filter in the charge system to improve the reliability and service life of the transmission; In addition, the charge pump can be convenient for some low-voltage auxiliary mechanism to provide power. (3) As only a small amount of oil sucked from the tank, reducing the loss of the tank.
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