1. Select hydraulic circuit with high transmission efficiency and appropriate speed regulation mode
The throttling speed regulation system of constant displacement pump, which is widely used at present, has a low efficiency (<0.385), because the efficiency of constant displacement pump and oil cylinder is about 85% and 95% respectively, and the loss of directional valve and pipeline is about 5%. Therefore, even without flow control, there is a 25% power loss. With throttling speed regulation, at least half of the waste is generated. In addition, there are leakage and other pressure loss and volume loss, which will be converted into heat energy to cause the temperature rise of hydraulic oil. Therefore, the constant displacement pump plus throttling speed regulation system can only be used for small flow system. In order to improve efficiency and reduce temperature rise, efficient energy-saving circuits shall be adopted. The above table shows the comparison of power loss of several circuits. In addition, the efficiency of the hydraulic system depends on the load. In the same circuit, the efficiency of the circuit is high when the ratio of load flow QL to pump Z large flow Qm is large. For example, manual servo variable, pressure control variable, pressure compensation variable, flow compensation variable, speed sensing power limit variable, torque limiter power limit variable and other forms can be used to achieve the matching of load flow QL and pump flow.
2 For the commonly used throttling speed regulating circuit of constant displacement pump, the overflow loss shall be reduced as much as possible
2.1 Unloading circuit
When the working parts of machinery stop working for a short time, generally let the hydraulic pump in the hydraulic system run with no load (that is, let all the oil output from the pump flow back to the oil tank under zero pressure or very low pressure), instead of frequently opening and closing the motor. This can save power consumption, reduce the heating of the hydraulic system, and extend the service life of the pump and motor. Generally, hydraulic systems with power greater than 3kw are equipped with unloading circuits. Several typical unloading circuits are described below.
2.1.1 Unloading circuit with three position valve
The hydraulic pump can be unloaded by using a three position reversing valve with a neutral unloading function. This method is simple and reliable. The middle unloading function is M, H, K type. Figure 1 shows the unloading circuit with M-type middle position directional valve. This method is relatively simple. The pump unloads when the valve is in the middle position. It is applicable to the hydraulic system with low pressure and small flow; It is used in high pressure and large flow system to provide certain control oil pressure when the pump is unloaded [(2~3) × 105Pa], a check valve (or back pressure valve) can be added at the outlet of the pump (or on the oil return circuit). However, this will increase the unloading pressure of the pump accordingly.
2.1.2 Unloading circuit with two position two-way valve
The unloading circuit of two position two-way valve is adopted, and the position shown in the figure is the unloading state of the pump. The specification of the two position two-way valve of this unloading circuit must be consistent with the rated flow of the pump. Therefore, this unloading method is not suitable for the occasion of large flow, and hydraulic impact will occur when reversing. It is usually used for hydraulic systems with the rated flow of the pump less than 63L/min.
2.1.3 Unloading circuit with pilot operated roll over valve
Connect a small two position two-way solenoid valve 2 to the remote control port of the pilot relief valve 1. The unloading pressure depends on the strength of the main valve spring of the relief valve, generally (2~4) × 105Pa。 Since the valve 2 only needs to control the oil in the oil circuit through the pilot relief valve 1, a smaller valve can be selected for remote control. This type of unloading circuit is suitable for hydraulic systems with large flow.
There are many unloading circuits, such as the unloading circuit of the external control sequence valve commonly used in the dual pump oil supply system; Unloading circuit of pressure compensated variable displacement pump; The pressure maintaining and unloading circuit that the system still needs to maintain pressure when unloading the hydraulic pump; Two way cartridge valve unloading circuit suitable for large flow system; Unloading circuit composed of "accumulator+pressure relay+electromagnetic overflow valve", etc.
2.2 Double pump and double pressure oil supply circuit
Fast motion circuit with dual pump oil supply. Hydraulic pump 1 is a high-pressure small flow pump, its flow should be slightly greater than the flow required by Z large working speed, and its working pressure is set by relief valve 5. Pump 2 is a low-pressure large flow pump (the flow of the two pumps can also be equal). The sum of its flow and the flow of pump 1 should be equal to the flow required by the rapid movement of the hydraulic system. Its working pressure should be lower than the set pressure of hydraulic control sequence valve 3.
This fast loop has the advantages of reasonable power utilization and high efficiency. The disadvantage is that the loop is more complex and the cost is higher.
