Estimated reading time: 12 minutes
Point 1: Interference Between Pressure Valves
1. Double Pump Hydraulic System
In the hydraulic system shown in the figure, hydraulic pumps 1 and 2 supply pressure oil to hydraulic cylinders 7 and 8, respectively, and reversing valves 5 and 6 are three-position four-way Y-type electromagnetic reversing valves.
There is a problem: when the hydraulic pump is started and the system starts to run, the pressure of the overflow valves 3 and 4 are unstable, and vibrates, and makes noise. The test shows that when only one relief valve is working, its set pressure is stable, and there is no obvious vibration and noise. When the two relief valves work at the same time, the above failure occurs.
It can be seen from the hydraulic system that the two relief valves have no other connection except for a common return line. The fault is caused by this common return line. It can be seen from the structural performance of the overflow valve that the control oil passage of the overflow valve is internally drained, that is, after the pressure oil in front of the overflow valve enters the valve, it flows into the control cavity through the orifice. When the pressure rises, it acts on the valve.
When the hydraulic pressure overcomes the pressure regulating spring, after opening the cone valve port to reduce the pressure, the oil flows through the valve body orifice into the oil return cavity of the overflow valve, merges with the oil overflowing from the main valve port, and flows back to the oil tank together through the oil return pipeline. Therefore, in the oil return line of the overflow valve, the flow state of the oil directly affects the set pressure of the overflow valve.
Fluid fluctuations such as pressure shock and backpressure directly act on the poppet valve of the pilot valve, so the pressure in the control chamber increases and shocks and fluctuations occur, resulting in unstable relief valve setting pressure and easy to arouse Vibration and noise.
Install the oil return lines of the two relief valves to the oil tank respectively to avoid mutual interference. If due to some factors, it is necessary to merge back to the tank, thicken the return pipe after the merge, and change the two relief valves to an external leakage type, that is, the oil that passes through the poppet valve port and the main valve return oil The cavity is separated and connected back to the fuel tank alone to become an external leakage relief valve.
2. Lifting Platform Hydraulic System
As shown in the figure, each circuit operates independently, the specifications of the corresponding hydraulic components of the two circuits are the same, and the pipeline diameter is the same.
There is a problem: when the two hydraulic pumps are started at the same time, the pressure adjusted by the overflow valve 3 and 4 fluctuates greatly, and vibration and noise occur.
The test shows that when one pump starts single-cylinder operation, the pressure adjusted by the overflow valve is stable, and there is no obvious vibration and noise. When the two pumps are started at the same time, that is, the two overflow valves work at the same time, the above-mentioned failure occurs.
It can be seen from the figure that the two relief valves share a return pipe, and there is no other connection. Obviously, the fault lies in this shared pipe. If the main oil return pipe is still designed according to the diameter of the separate circuit, it will inevitably increase the back pressure of the oil return port of the relief valve when the dual pumps supply oil at the same time. It can be seen that when the two pumps are working at the same time, in the laminar flow state, the resistance loss of the total oil return pipeline along the way increases by 1 time; in the turbulent flow state, it increases by 3 times, that is, the back pressure at the oil return port of the overflow valve increases by 1 or 3 times.
According to the structure and working principle of the relief valve, they control the oil to enter the control cavity through the orifice on the main spool. When the pressure rises and overcomes the force of the pressure regulating spring of the pilot valve, the pressure oil opens the valve port of the pilot valve. After the liquid flows through the valve port and reduces pressure, it flows into the oil return cavity of the overflow valve through the drain channel in the valve body. Therefore, the flow state of the oil flow in the oil return line of the overflow valve directly affects the adjustment pressure of the overflow valve. When the two pumps are working at the same time, the two overflow valves share the same oil return pipeline. The interaction of the two oil flows can easily cause pressure fluctuations. At the same time, the backpressure of the oil return port of the overflow valve changes significantly. Under the action of interference, the pressure of the oil in the overflow valve control chamber will also change, which will inevitably lead to unstable pressure adjusted by the overflow valve, accompanied by vibration and noise.
To eliminate the above-mentioned faults, the diameter of the oil return manifold of the two relief valves can be increased, and the two relief valves can be replaced with external leakage type, that is, the oil flow through the pilot valve port will be set separately. The leaking pipe of the leakage pipe flows back to the oil tank separately, or the two overflow valves are equipped with their oil return pipes to avoid mutual interference.
3. Multi-relief Valve Resonance Problem
In the hydraulic system shown in the figure, pump 1 and pump 2 are quantitative pumps of the same specification and supply hydraulic oil to the system at the same time. The neutral function of the three-position four-way reversing valve 7 is Y-type, and the overflow valves 3 and 4 are also the same. Specifications respectively installed on the output port oil circuit of pump 1 and pump 2 for constant pressure overflow. The set pressure of the overflow valve is 14MPa, and when starting to run, the system emits a whistling sound. After debugging, it was found that the noise came from the overflow valve, and it was found that when only one side of the pump and the overflow valve were working, the noise disappeared, and when the pumps on both sides were working at the same time, they made a whistling sound. It can be seen that the cause of the noise is that the two relief valves resonate under the action of the fluid.
According to the working principle of the overflow valve, the overflow valve works under the interaction of hydraulic pressure and spring force, so it is easy to arouse vibration and make noise. Once the pressure oil at the inlet and outlet of the overflow valve and the control port fluctuates, hydraulic shock will occur. The main spool, poppet valve, and their interacting bullets in the overflow valve will vibrate, the degree of vibration, and its state It varies with fluid pressure shocks and fluctuations. Therefore, the more stable the oil flow related to the relief valve, the more stable the relief valve can work, and vice versa. In the above-mentioned system, the pressure oil output by the double pumps merges after passing through the one-way valve, causing fluid shock and fluctuation, causing the one-way valve to oscillate, resulting in unstable pressure oil at the outlet of the hydraulic pump. Since the pressure oil output by the pump is originally pulsating, the pressure oil output by the pump will fluctuate strongly and arouse the vibration of the relief valve. And because the natural frequencies of the two overflow valves are the same, it causes the overflow valve to resonate and emit abnormal noises.
Method of exclusion：
- Replace the overflow valves 3 and 4 with a large-capacity overflow valve and place them at the junction of the two pumps so that although the overflow valve will vibrate, it is not too strong because the resonance conditions are eliminated.
- Stagger the set pressure values of the two relief valves by about 1 MPa to avoid resonance. At this time, if the working pressure of the hydraulic cylinder is between 13MPa and 14MPa, the setting value of the relief valve should be increased respectively, so that the minimum setting pressure meets the working requirements of the hydraulic cylinder, and the pressure difference of 1MPa should still be maintained.
- Change the above circuit to the form of the figure, that is, connect the remote control ports of the two overflow valves to a remote pressure regulating valve 11. The adjustment pressure of the system is determined by the pressure regulating valve and has no direct relationship with the pilot valve of the overflow valve, But it is necessary to ensure that the set pressure value of the pilot valve pressure regulating spring must be higher than the maximum regulating pressure of the pressure regulating valve. Because the adjustment pressure range of the remote pressure regulating valve must be lower than the regulating pressure of the pilot valve of the relief valve to work effectively, otherwise the remote pressure regulating valve will not work.
Point 2: The Leakage Problem of The Relief Valve Control Oil Circuit
In the circuit shown in the figure, because the equipment requires a continuous operation, it is not allowed to shut down for repairs, so the system has two sets of oil supply systems. When a certain fuel supply system fails, another fuel supply system can be started immediately to make the equipment operate normally, and then the malfunctioning fuel supply system can be repaired.
The performance specifications of each component of the oil supply system belonging to pump 1 and pump 2 are the same. The–stage pressure is set by the relief valves 3 and 4 and the second-stage pressure is set by the remote pressure regulating valve 9.
But when the system of pump 2 stops supplying oil and only pump 1 is running, the system pressure cannot rise. Even when the electro-hydraulic directional valve is placed in the neutral position, the output oil circuit of pump 1 cannot rise to the required pressure value.
After debugging, it is found that the maximum pressure of pump 1 can only reach 12 MPa during operation, and the design requirement should reach 14 MPa. When the pressure regulating knobs of the overflow valve 3 and the remote pressure regulating valve 9 are all tightened, the pressure still cannot rise. When the oil temperature is 40°C, the pressure rises to 12MPa; when the oil temperature rises to 55°C, the pressure can only rise to 10MPa. The pumps and other components were tested separately, and no quality problems were found, and all the indexes met the performance requirements. There is no problem with the components, and the pressure after being combined into a system. If you can’t go up, you should analyze the mutual influence of the system component combination.
When pump 1 is working, the pressure oil enters the lower end of the main spool from the oil inlet of the relief valve 3, flows into the upper spring cavity of the main spool through the orifice, and then enters through the remote control port of the relief valve 3 and the external oil pipe. The bomb chamber at the upper end of the main spool of the overflow valve 4 flows downwards through the lower cavity of the main spool through the orifice, and the oil inlet of the overflow valve 4 flows backward into the oil outlet pipe of the stopped pump 2. There will be two situations: a. Make the one-way valve 6 not tightly closed; b. The pressure oil in the outlet pipe of pump 2 will make pump 2 move in the opposite direction like a hydraulic motor or flow into the oil tank through the gap of pump 2. As a result, the remote control port of the relief valve 3 leaks hydraulic oil into the oil tank, and the above-mentioned pressure failure will inevitably occur.
Since the control oil circuit is provided with a throttle device, the oil on the remote control oil circuit of the overflow valve 3 flows back to the oil tank under a certain throttle resistance, so the pressure is not completely absent. For this reason, overflow valve 3 overflows when the pressure is lower than the required pressure.
The improved circuit is shown in the figure). One-way valves 11 and 12 are set in the circuit, and the oil outlet pipe into pump 2 is cut off, eliminating the above-mentioned faults.
Point 3: The Outlet of The Hydraulic Pump is Closed
The picture shows a pressure regulating circuit, which can switch the system pressure between the two pressures set by the overflow valve 1 and the overflow valve 2. When the reversing valve 3 is in the left position, the system pressure is controlled by the overflow valve 1. Set, set by overflow valve 2 in the right position, and unload the system in the middle position. A hose burst accident occurred after the system was used for some time. The analysis found that the cause of the accident was the unreasonable system design. In the process of pressure switching, the reversing valve 3 must go through a short process in which the valve port is completely closed. During this process, because the output oil of the pump has no way to go, the system pressure suddenly rises, and repeated pressure shocks make the hydraulic pressure soft. The tube bursts due to fatigue.
One of the solutions is shown in the figure.
This example illustrates that even a very short output closure will cause a great pressure shock to the hydraulic system. If there is no hose in the system, the hydraulic pump will inevitably be damaged over time.