Szacowany czas czytania: 11 minutes
Interference Between Pressure Valves
1. Double Pump Hydraulic System
In the hydraulic system shown in the figure, pompy hydrauliczne 1 and 2 supply pressure oil to the hydraulic cylinders 7 and 8 respectively. The reversing valves 5 and 6 are all three-position four-way Y-type electromagnetic reversing valves.
There is a problem: when the pompa hydrauliczna is started and the system starts to run, the pressure of the relief valves 3 and 4 are unstable, vibrates make noise.
Tests show that when only one relief valve works, its set pressure is stable, and there is no obvious vibration and noise. The above fault occurs when the two relief valves work at the same time.
It can be seen from the system hydrauliczny that the two relief valves have no common connection except for a common oil return line. The fault was caused by this common oil return line. From the structural performance of the relief valve, it can be seen that the control oil passage of the relief valve is an internal leakage, that is, after the pressure oil in front of the relief valve enters the valve, it flows into the control chamber through the damping hole. When the pressure rises, it acts on the valve. When the hydraulic pressure above overcomes the pressure regulating spring, after opening the cone valve port to reduce pressure, the oil flows through the valve body orifice and flows into the oil return chamber of the relief valve, where it merges with the oil overflowing from the main valve port and flows together through the oil return line. Return to the oil tank, so in the oil return line of the relief valve, the flow state of the oil directly affects the set pressure of the relief 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 also increases, and shocks and fluctuations occur, resulting in unstable setting pressure of the relief valve, which is easy to arouse. vibration and noise.
Install the oil return lines of the two relief valves back to the oil tank respectively to avoid mutual interference. If due to some factors, it must be merged back to the fuel tank, the combined fuel return pipe should be thickened, and the two relief valves should be changed to external leakage type, the oil that will pass through the valve port of the poppet valve and the main valve will return The cavity is separated, and it becomes a leakage type relief valve when it is connected back to the fuel tank alone.
2. Lifting Platform Hydraulic System
In the circuit shown in the figure, each circuit acts independently, the specifications of the corresponding hydraulic components of the two circuits are the same, and the pipe diameter is the same.
There is a problem: when the two hydraulic pumps start working at the same time, the pressure fluctuations adjusted by the relief valves 3 and 4 are large, and vibration and noise occur.
The test shows that when one pump starts single-cylinder operation, the pressure adjusted by the relief valve is stable, and there is no obvious vibration and noise, but when the two pumps are started at the same time, that is, the two relief valves work at the same time, the above fault occurs.
It can be seen from the figure that the two relief valves share one oil return pipe, and there is no other connection. 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 backpressure 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 total resistance loss along the oil return pipeline increases by 1 time; in the turbulent flow state, it increases by 3 times, that is, the backpressure of the overflow valve oil return port increases by 1 or 3 times.
It can be seen from the structure and working principle of the relief valve that they control the oil to enter the control chamber through the damping hole on the main spool. When the pressure rises to overcome the pressure regulating spring force of the pilot valve, the pressure oil opens the valve port of the pilot valve, and the oil After the pressure is reduced through the valve port, it flows into the oil return cavity of the relief valve through the leakage channel in the valve body and merges with the oil overflowing from the main valve port, and flows back to the oil tank through the oil return pipeline. Therefore, the flow state of the oil flow in the oil return line of the relief valve directly affects the adjustment pressure of the relief valve. When the two pumps are working at the same time, the two relief valves share the same oil return line, and the interaction of the two oil flows easily causes pressure fluctuations. At the same time, the backpressure of the oil return port of the relief valve changes significantly. Under the action of interference, the oil pressure in the control cavity of the relief valve also changes, which will inevitably lead to the instability of the pressure adjusted by the relief valve, accompanied by vibration and noise.
To eliminate the above faults, the diameter of the oil return main pipe of the two relief valves can be enlarged, and the two relief valves can be replaced with external leakage types. The leakage pipes of the two valves flow back to the oil tank separately or configure the two overflow valves 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. Specifications, installed on the oil circuit of the output port of pump 1 and pump 2 respectively, used for constant pressure relief. The set pressure of the relief valve is 14MPa. When it starts to run, the system emits a whistle-like whistle.
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 worked, the noise disappeared, and when the pumps on both sides worked at the same time, there was a whistling sound. It can be seen that the reason for the noise is that the two overflow 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 very easy to arouse vibration and make noise. Once the pressure oil at the inlet and outlet of the relief valve and the control port fluctuates, a hydraulic shock will occur, and the main spool, cone valve, and their interacting springs in the relief valve will vibrate. fluid pressure shocks and fluctuations. Therefore, the more stable the oil flow associated with 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 dual pumps merges after passing through the one-way valve, resulting in fluid shock and fluctuation, causing the one-way valve to oscillate, resulting in the instability of the pressure oil at the outlet of the hydraulic pump. And because the pressure oil output by the pump is inherently pulsating, the pressure oil output by the pump will fluctuate strongly and cause the relief valve to vibrate. And because the natural frequencies of the two relief valves are the same, it causes the relief valve to resonate and emit abnormal noise.
Leakage of Relief Valve Control Oil Circuit
In the circuit shown in the figure, because the equipment requires continuous operation and is not allowed to stop for repair, the system has two sets of oil supply systems. When one oil supply system fails, another oil supply system can be started immediately to make the equipment run normally, and then the failed oil supply system can be repaired.
The performance specifications of the components of the oil supply system to which pump 1 and pump 2 belong are identical. The first-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.
However, when the system to which pump 2 belongs stops supplying oil, and only pump 1 is running, the system pressure cannot rise. Even when the electro-hydraulic reversing 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 12MPa, and the design requirement should reach 14MPa. When the pressure regulating knobs of the relief 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 pump and other components were tested respectively, and no quality problems were found, and the indicators met the performance requirements. There is no problem with the components, but the pressure cannot go up after being combined into a system, so the mutual influence of the combination of system components should be analyzed.
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, and at the same time flows into the spring cavity at the upper end of the main spool through the damping hole, and then enters the remote control port of the relief valve 3 and the external oil pipe. The main spool of the relief valve 4. The spring cavity at the upper end flows down through the damping hole through the lower cavity of the main spool, and the oil inlet of the relief valve 4 reversely flows into the oil outlet pipe of the stopped pump 2. There will be two situations: a. The one-way valve 6 is not tightly closed; b. The pressure oil in the oil outlet pipe of pump 2 will make pump 2 jog in the reverse direction like a hydraulic motor or flow into the oil tank through the gap of pump 2 . Therefore, the remote control port of relief valve 3 leaks hydraulic oil into the oil tank, and the failure of the above-mentioned pressure will inevitably occur.
Since a throttling device is provided on the control oil circuit, the oil on the remote control oil circuit of the relief valve 3 flows back to the oil tank under a certain throttling resistance, so the pressure is not completely absent. For this reason, overflow valve 3 overflows below the required pressure.
The improved circuit is shown in the figure. Check valves 11 and 12 are set in the circuit, and the oil outlet pipe entering pump 2 is cut off, thus eliminating the above-mentioned faults.
The Outlet Closure Problem of The Hydraulic Pump
The figure shows a pressure regulating circuit, which can switch the system pressure between the two pressures set by the relief valve 1 and relief valve 2. When the reversing valve 3 is in the left position, the system pressure is controlled by the relief valve 1. It is set by the relief valve 2 when it is in the right position, and the system is unloaded when it is in the neutral position. A hose burst accident occurred after the system was used for some time. After analysis, it is found that the cause of the accident is the unreasonable system design. During the pressure switching process, 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 the repeated pressure shocks make the hydraulic pressure soft.