The Application of Hydraulic Closed Circuit in the Walking System of Construction Machinery

With the rapid development of infrastructure construction such as transportation and energy in our country, the demand and inventory of large and medium-sized construction machinery have been growing continuously and rapidly in recent years. Among the numerous types of construction machinery, especially imported equipment, the use of hydraulic transmission and full hydraulic drive is very common, such as tunnel boring machines, shield machines, large-ton beam transporters, cranes, lifting trolleys, as well as pavers, excavators, bulldozers, etc. Hydraulic closed circuits have low energy consumption, compact structure and are easy to achieve stepless speed regulation, and have been widely applied in the traveling systems of construction machinery. However, compared with open circuits, the design, installation, commissioning and maintenance of closed circuits all have higher difficulties and technical requirements. Below, we will analyze the technical requirements of the closed circuit and the characteristics of installation, commissioning and maintenance in combination with the hydraulic drive system of the 100t flatbed transport vehicle we drive.

The longer one is the schematic diagram of the hydraulic drive system of a 100t flatbed transport vehicle. The system consists of four parts: proportional variable pump, proportional variable motor, flush valve and fuel tank, and radiator assembly.

First, based on the load size, adjust the current of the proportional valve 17 to set the displacement of the motor 16. Generally, it can be set to three gears: no-load, heavy-load, and climbing, corresponding to 1/4 displacement, half displacement, and full displacement of the motor respectively. Then, by adjusting the current of proportional valve 1, the displacement of the main pump 3 can be changed. Since the engine speed is basically constant (1800r/min), the system flow can be directly altered, thereby achieving the purpose of changing the vehicle speed. The oil replenishment pump 4 supplies low-pressure oil to the main circuit through the filter 2 and the check valve 9 (or 12), and simultaneously supplies oil to the control circuit. The excess oil in the main circuit overflows at low pressure through the flushing valve and flows back to the oil tank via radiator 14. When instantaneous overload occurs, high-pressure oil overflows through the relief valve 7 (or 11) to the oil replenishment circuit. When severe overloading occurs, high-pressure oil overflows through the relief valve 7 (or 11) to the oil replenishment circuit. When severe overloading occurs (such as wheel jamming), the sequence valve 6 (or 10) opens, and the high-pressure oil rapidly reduces the displacement of the main pump 3. At the same time, due to the pressure drop of the throttle valve 8 (or 13), the relief valve 7 (or 11) opens, thereby rapidly reducing the pressure in the main circuit.

Requirements for the traveling system of the 100t flatbed transport vehicle (1) Main performance parameters: Load capacity: 1000kN; Heavy-load speed: 3km/h; No-load speed: 12km/h; Maximum climbing Angle: 6 degrees. (2) Power matching must properly handle the matching relationship among engine speed, vehicle speed, and system working pressure. When the engine speed is basically constant (1800r/min), the vehicle speed depends on the displacement ratio of the pump to the motor. In this system, the displacement of the pump can be adjusted by using a foot-operated potentiometer. By monitoring the working pressure of the system, the displacement of the motor can be corrected. When the pressure is too high, the displacement of the motor can be increased and the vehicle speed reduced. Conversely, when the pressure is too low, reduce the displacement of the motor and increase the vehicle speed. It is also possible to keep the vehicle speed constant by reducing the engine speed, but this can reduce power consumption. (3) Solving the differential problem: When a vehicle turns, the drive wheel motors at different turning radii require different flow rates; otherwise, synchronous turning cannot be achieved. This system connects 8 drive motors in parallel and randomly distributes flow during operation, completely solving the problem of differential speed. (4) Solving the problem of differential force: The problem of differential force is also an issue of adhesion. When the adhesion is unevenly distributed, wheels subjected to insufficient force may skid, while those subjected to excessive force may burst due to overload. In this system, first, the suspension hydraulic cylinders (responsible for adjusting the wheel height) are divided into three groups to achieve three-point support, ensuring that the vehicle body does not experience instability. Second, each group of suspended hydraulic cylinders is connected in parallel to achieve uniform load-bearing within the group. Thirdly, the rotational speed of each drive motor is monitored through a speed sensor. When a motor's rotational speed is detected to be too high (slipping), the displacement of that motor is immediately set to zero, thereby turning it into a free wheel (driven wheel) to achieve the purpose of anti-slip.

When selecting hydraulic components and setting parameters, the first step is to choose the appropriate system working pressure and motor displacement based on the load size. For instance, a high-speed motor with a wheel reducer (which has a wide speed regulation range and a relatively low price) can be selected. If a single motor is insufficient to meet the driving force requirements, multiple groups of motors can be selected for parallel drive. In the 100t flatbed transport vehicle, we used 8 high-speed motors with a displacement of 28.1cm3/r (including wheel-side reducers with a reduction ratio of 38.6:1) for parallel drive. Secondly, based on the vehicle speed requirements, select the displacement of the main pump 3. A multi-pump parallel oil supply scheme can also be adopted. In this example, we have used two pumps in parallel for oil supply. The displacement of the make-up oil pump 4 can be selected at 10% to 30% of that of the main pump 3. Its main function is to compensate for the leakage in the main circuit (between the variable pump and the variable motor). Choosing an overly large oil replenishment pump or an excessively high replenishment pressure will both cause the oil temperature to rise. In this example, the displacement of the make-up oil pump is 20cm ³ /r. The function of the flush valve 15 is to overflow the excess oil in the main circuit and to serve the purpose of circulating and renewing the oil medium in the main circuit. For general mobile machinery, in order to reduce its own weight or due to spatial limitations, the volume of its hydraulic oil tank is relatively small, and usually a radiator needs to be added to the return oil line. There are a total of 5 pressure regulating valves (3 relief valves and 2 sequence valves) in the pump unit. Valve 5 is designed for oil replenishment pressure, and valve 7 is set for the working pressure of the main circuit. Valve 6 is the main circuit unloading valve. When overloaded, it can quickly open to return the main pump's displacement to zero, playing a role in rapid unloading. Its pressure is 0.5 to 1MPa higher than that of valve 7. The functions of valves 10 and 11 are respectively the same as those of valves 6 and 7. To ensure the normal operation of the flushing valve, its overflow pressure should be 0.2 to 0.5MPa higher than that of valve 5. In this example, the pressure of each relief valve is 2.4MPa for the temple valve 5 respectively. Valve 6=10 is 32.5MPa.

In a closed loop, since there is no throttling loss, the main reasons for the system temperature rise are high-pressure overflow and leakage. Therefore, the cooling power of the cooler is calculated based on the power consumption of oil replenishment. This example adopts a 2kW air-cooled cooler.

When designing a closed-loop hydraulic system, it is advisable to use pipe connections as much as possible and avoid using hydraulic integrated blocks, as the working pressure in a closed loop is generally higher.