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Hydraulic Press Circuit

In modern machinery, the applied hydraulic transmission system is composed of some basic circuits. The so-called basic circuit is a typical oil circuit composed of related hydraulic components to complete specific functions. By mastering their working principles, composition, and characteristics, these circuits can be selected correctly and reasonably according to the machine’s working performance, requirements, and working conditions to form the complete hydraulic system required. Hydraulic Press Circuit

Pressure control loop design

The pressure control circuit is a circuit that uses a pressure control valve to control the entire or partial pressure of the system. Pressure circuit controlled by the pressure valve can be used to realize the control of voltage stabilization, decompression, boosting, and multi-stage pressure regulation to meet the requirements of the actuators in terms of force and torque. The pressure valves of standard components include overflow valves, pressure reducing valves, sequence valves, and one-way pressure reducing valves, and one-way sequence valves that are combined in parallel with one-way valves.

A pressure regulating circuit is a system that controls operating pressure so that it does not exceed a certain – pre-tuned value, or so that the working mechanism has different pressures during the various stages of movement. Hydraulic Press Circuit

1. Regulating method selection

• Pressure limiting circuit

A relief valve is best used to limit the maximum pressure in a hydraulic circuit. Figure 1 shows a common circuit for pressure processing machinery. The low-pressure relief valve1 is used to maintain the piston from falling by its self-weight when the piston of the cylinder rises (not working) to the end. This saves power consumption and avoids the heating of oil from the relief valve. Hydraulic Press Circuit

As shown in Figure 2, when the solenoid three-way valve is demagnetized, the circuit pressure is set at 10MPa for the main relief valve; when the solenoid three-way valve is excited and the path between the main valve and the remote relief valve a or b is changed through the four-way solenoid valve, the pressure of the main circuit can be converted to 7MPa or 5MPa. The capacity of each valve, except for the main valve, all other small flow valves are used. Hydraulic Press Circuit

• Secondary pressure regulating circuit

In Figure 3, when the cylinder piston rises, falls and the piston remains in the highest position, the oil circuit pressure is p2=5MPa (the left high-pressure pump unloads). However, when the piston reaches the bottom, the load increases, and the pressure relay acts to manipulate the solenoid three-way valve so that p1=10MPa and high-pressure oil enter the circuit. Hydraulic Press Circuit

• Pressure regulating circuit for compound pumps

In the design, the capacity of the pump must be adapted to the requirements of work and reduce the generation of unwanted heat at low-speed drives. The Figure 4 circuit is electrically controlled and can operate at various flow rates and oil pressures as required to maintain maximum circuit efficiency, with the advantages offered by a pressure compensated variable pumps. The operating oil circuit of the electro-hydraulic reversing valve in the circuit is led from the remote port of the relief valve, preventing shocks caused by the switching of the main reversing valve. Hydraulic Press Circuit

2. Pressure parameter adjustment

The set pressure of the relief valve is not appropriate, resulting in the hydraulic cylinder movement speed does not meet the requirements. Figure 5 circuit requires smooth movement when lifting, a large range of speed adjustment and the piston can stop at any position. But in operation, when adjusting the rising speed of the lift, the speed does not change in a wide range, only when the throttle valve opening is adjusted to a very small, the rising speed will change, not up to the proper performance requirements.

This is the reason why the pressure of the relief valve is adjusted high. The pressure of the relief valve should be set to the hydraulic pump working pressure is exactly equal to the hydraulic cylinder load pressure and pump all the flow through the throttle valve and the required pressure drop.

• Improper pressure tuning parameters Hydraulic Press Circuit

Improper pressure setting parameters lead to high oil temperature in the constant pressure pump supply system. In the hydraulic circuit of the constant pressure pump shown in Figure 6, improper pressure setting parameters lead to high oil temperature during system operation. The reason for the above problem is due to the pressure valve 1 set system pressure Pr, lower than the valve 2 regulating spring set pressure Pt so that the constant pressure pump always work under the maximum displacement, excess flow to pressure Pr,

overflow back to the tank, and all transformed into heat so that the system temperature rises, so the valve 1 as a safety valve, the pressure is adjusted to the highest pressure than the system required 0.5 ~ 1MPa, the above problem can be solved. Hydraulic Press Circuit

• Example of failure of pressure parameter adjustment

In the quantitative pump pressure control circuit shown in Figure 7, the hydraulic pump is quantitative, and the three-way four-way reversing valve neutral function is Y type. Therefore, when the hydraulic cylinder stops running, the system does not unload, hydraulic pump outputs pressure oil all the pressure oil output from the hydraulic pump is overflowed back to the tank by the relief valve. The relief valve in the system is YF type pilot-operated. The structure of the valve is a three-stage concentric type. Hydraulic Press Circuit

. Problems: When the reversing valve in the system is placed in the neutral position and pressure of the relief valve is adjusted, it is found that the relief valve works normally when pressure value is below 10MPa; when pressure is adjusted to any pressure value higher than 10MPa, the system emits a screeching sound like a blowing flute, at which time the pressure gage pointer can be seen to vibrate violently. After testing, the noise was found to have come from the relief valve. Hydraulic Press Circuit

. Analysis: In the three-stage coaxial high-pressure relief valve, the main spool has two sliding spools with the valve body and cover. If the co axiality of the bore after the assembly of the valve body and cover exceeds the design requirements, the main spool cannot move flexibly, but stick to one side of the bore to do the abnormal movement.

When pressure is adjusted to a certain value, it will inevitably provoke the main spool to vibrate. This vibration is not the regular vibration of the main spool in working movement, but the high-frequency vibration caused by the main spool stuck in a certain position (at this time because the main spool is also subjected to hydraulic clamping force). This high-frequency vibration will certainly cause strong vibration of the spring, especially the regulating spring, and noise resonance.

In addition, since the high-pressure oil does not overflow through the normal overflow port, but overflows back to the tank through the stuck overflow port and the internal drainage channel, this high-pressure oil flow will emit a high-frequency fluid noise. This vibration and noise are excited under the specific operating conditions of the system, which is the reason why it does not squeal at pressures below 10 MPa. Hydraulic Press Circuit

. Solutions: The manufacturing accuracy of the YF type relief valve is relatively high. The coaxiality of the inner and outer circular surface of the connecting part of the valve cover and the valve body, and the coaxiality of the outer circular surface of the three shoulders of the main spool should be within the specified range.

In addition, the damping hole on the main spool has a damping effect when the main spool vibrates. When the working fluid viscosity is low or the temperature is too high, the damping effect will be reduced accordingly, therefore, the selection of the appropriate viscosity of the fluid and the control system temperature rise too high is also conducive to vibration and noise reduction. Hydraulic Press Circuit

• Pressure parameter adjustment failure problem
• The pressure can’t be adjusted up. The main reason is that the regulating spring of the relief valve is too soft, wrongly installed, or omitted; the main valve damping hole of the pilot-operated relief valve is blocked, the slide valve is under the action of oil pressure at the lower end to overcome the liquid pressure of the upper chamber and the spring force of the main valve, so that the main valve moves up, the regulating spring loses its controlling effect on the main valve, so the main valve opens the relief port overflow at a lower pressure; the spool and the valve seat are not closed tightly, and the leakage is serious; the spool is stuck in the open position by burrs or other dirt. Hydraulic Press Circuit
• The pressure is too high to adjust down.The main reason is that the spool is stuck in the closed position by burrs or dirt, and the main valve cannot be open; when installed, the inlet and outlet of the valve are connected wrongly, and there is no pressure oil to push the spool to move, so the spool cannot be opened; the damping hole in front of the pilot valve is blocked, resulting in the main valve cannot be opened. Hydraulic Press Circuit
• Large pressure oscillation. The main reasons are air mixed in the oil; poor contact between the spool and the valve seat; the diameter of the damping hole is too large and the damping effect is weak; resonance is generated; the spool moves inflexibly in the valve body. Hydraulic Press Circuit

For the above problems, the circuit design, component selection, component parameters, and system adjustment, piping installation, hydraulic oil use, etc. Can be targeted to improve the processing. Hydraulic Press Circuit

3. Problems in the secondary regulation circuit

• Pressure shock problem

In the secondary regulating circuit shown in Figure 8 (a), when 1DT is not energized, the system pressure is regulated by relief valve 2; when 1DT is energized, the system pressure is regulated by relief valve 3, the pressure switching of this circuit is realized by valve 4, when the pressure is switched from p1 to p2 (p1>p2) because there is no pressure in the oil circuit between valve 4 and valve 3 before switching, so when valve 4 is switched (1DT is energized), the instantaneous pressure at the remote port of relief valve 2 drops from p1 to almost zero and then rises to p2, the system naturally produces a large pressure shock. Hydraulic Press Circuit

Exclusion method: As shown in Figure 8 (b), valve 4 is connected to the outlet of valve 3, i.e., the position of valve 4 is interchanged with valve 3. Since the remote control port from valve 2 to valve 4 is often filled with pressure oil, the system pressure drops from p1 to p2 when valve 4 is switched, which will not produce excessive pressure shock. Hydraulic Press Circuit

• The problem of long boost time when regulating pressure Hydraulic Press Circuit

In the secondary regulating circuit shown in Figure 9, when the remote control pipeline is long and the system changes from the state of unloading (valve 3 is in the middle position) to the state of boosting (valve 3 is in the left or right position), the boosting time is long because the remote control pipe is connected to the oil pool and the pressure oil has to fill the remote control pipeline first before boosting.

Note the question: Try to shorten the remote control pipeline, and add a back pressure valve (or check valve) at the remote control pipeline return 5, so that there is certain pressure so that boost time can be shortened. In the remote regulating circuit, the minimum regulating value of the relief valve is increased, and at the same time, the fault of hysteresis in action is generated.

The reason for this failure is that the piping between the main relief valve and the remote pilot relief valve is too long (for example, more than 10m) and the pressure loss in the remote control pipe is too large. Therefore, the remote control pipeline should not exceed 5m in general. Hydraulic Press Circuit

4. Interference between pressure valves

• Double pump hydraulic system

In the hydraulic system shown in Figures 10, hydraulic pumps 1 and 2 supply pressure oil to hydraulic cylinders 7 and 8, respectively, and reversing valves 5 and 6 are three-way, four-way, Y-type solenoid-operated reversing valves. Hydraulic Press Circuit

. Problem: When the hydraulic pump is started and the system begins to run, the pressure of relief valves 3 and 4 is unstable, and vibration occurs and noise is emitted. Hydraulic Press Circuit

The test shows that when only one relief valve works, its regulation pressure is stable, and there is no obvious vibration and noise. When two relief valves work at the same time, the above-mentioned fault occurs.

From the hydraulic system, it can be seen that the two relief valves are not connected other than a common return line. The fault is caused by this common return line. From the relief valve structure performance can be seen, the relief valve control oil channel for the internal drain. That is, the pressure oil before the relief valve into the valve, through the damping hole into the control volume cavity, when the pressure rises, the hydraulic pressure acting on the valve to overcome the regulating spring, open the cone valve port pressure reduction,

the oil flow through the valve body orifice into the relief valve return cavity, and the main valve port overflow oil confluence through the return line together back to the tank, so in the relief valve return, Therefore, in the oil return line of the relief valve, the flow state of the oil directly affects the adjustment pressure of the relief valve. Hydraulic Press Circuit

Pressure shock, back pressure, and other fluid fluctuations directly on the pilot valve cone valve, so the pressure in the control cavity also increased, and shock and fluctuations, resulting in unstable pressure adjustment relief valve, easy to provoke vibration and noise. Hydraulic Press Circuit

. Rozwiązanie: The oil return lines of the two relief valves should be placed back to the tank separately to avoid mutual interference. If due to some factors, must be combined back to the tank, should be combined after the return pipe thicker, and the two relief valves are changed to external leakage type, that is, after the cone valve mouth of the oil and the main valve return cavity is separated, a separate connection back to the tank becomes external leakage type relief valve. Hydraulic Press Circuit

• Lift table hydraulic system

As shown in Figure 11 circuit, each circuit acts separately, the two circuits correspond to the same specifications of hydraulic components and the same piping diameter. Hydraulic Press Circuit

Problems: When two hydraulic pumps start working at the same time, the pressure adjusted by relief valves 3 and 4 fluctuates greatly, and vibration and noise occur. Hydraulic Press Circuit

The test shows that when a pump starts a single-cylinder operation, the relief valve adjusts pressure stable, with no obvious vibration and noise. while when two pumps start at the same time, that is, when the two relief valves work at the same time, the above-mentioned failure occurs. Hydraulic Press Circuit

You can see Figure 11, the two relief valves share a common return line and no other connections. The fault lies in this common pipe. If the total return pipe is still designed according to the diameter of the separate circuit, when the two pumps supply oil at the same time, it will certainly increase the backpressure of the return port of the relief valve. Hydraulic Press Circuit

It can be seen that when the two pumps work at the same time, the laminar flow state, the total return line along the resistance loss increased by 1 time. Turbulent flow state, increased by 3 times, that is, the relief valve return port back pressure increased by 1 or 3 times. Hydraulic Press Circuit

From the relief valve structure and working principle, they control the oil to enter the control volume cavity 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 pilot valve port,

and the oil flows through the valve port to lower the pressure and then flows into the return cavity of the relief valve through the drainage channel in the valve body and the oil overflowing from the main valve port converges and flows back to the oil tank together through the return pipeline. Therefore, the flow state of the oil flow in the return line of the relief valve directly affects the adjustment pressure of the relief valve. Hydraulic Press Circuit

When the two pumps work at the same time, the two relief valves share the same return line, the interaction of the two streams of oil flow, it is easy to produce pressure fluctuations, while the relief valve returns port backpressure changes significantly, under the mutual interference of these two factors, the pressure of the oil in the control cavity of the relief valve also changes, which will inevitably lead to unstable pressure adjustment of the relief valve, and accompanied by vibration and noise. Hydraulic Press Circuit

To eliminate the above-mentioned problems, you can increase the diameter of the return pipe of the two relief valves, and replace the two relief valves with external leakage type, that is, the oil flowing through the pilot valve orifice will flow back to the tank separately from the other leakage pipe, or configure the two relief valves with their return pipes to avoid mutual interference. Hydraulic Press Circuit

• Multiple relief valve resonance problems Hydraulic Press Circuit

In Figure 12 (a), pump 1 and 2 are the same specification quantitative pump, while supplying hydraulic oil to the system, three-way four-way reversing valve 7 neutral functions for Y type, relief valve 3 and 4 are same specification , respectively, installed in the output port of pump 1 and pump 2 oil road, for fixed pressure relief with. The relief valve pressure is set to 14MPa, the system siren-like whistling sound when starting operation.

After debugging found that the noise from the relief valve, and found that when only one side of the pump and relief valve work, the noise disappears, both sides of the pump work at the same time, a whistling sound. It can be seen that the reason for the noise is the resonance of the two relief valves under the action of the fluid. Hydraulic Press Circuit

According to the operating principle of the relief valve, the relief valve is working under the interaction of liquid pressure and spring force, so it is very easy to Stimulates vibration and makes noise. Once the pressure oil at the inlet and outlet of the relief valve and the control port fluctuates, that is, the hydraulic shock, the main spool,

cone valve and its interaction with the spring in the relief valve should vibrate, and the degree of vibration and its state changes with the fluid pressure shock and fluctuation conditions. Therefore, the more stable the oil flow associated with the relief valve, the more stable the relief valve can work, and vice versa, it cannot work stably. Hydraulic Press Circuit

In the above system, the pressure oil output from the double pump merges after the check valve, fluid shock and fluctuations occur, causing the check valve to oscillate, which leads to unstable pressure oil at the outlet of the hydraulic pump. And because the pump output pressure oil is originally pulsating, so the pump output pressure oil will fluctuate strongly, and provoke the relief valve vibration. And because the inherent frequency of the two relief valves is the same, it causes the relief valve resonance and emits abnormal noise. Hydraulic Press Circuit

Exclusion method: The exclusion method will relieve valve 3 and 4 with a large capacity relief valve instead, placed in the double pump merge, so that although the relief valve will also vibrate, but not too strong because of the exclusion of the resonance conditions. Hydraulic Press Circuit

. Stagger the setting pressure of the two relief valves by about 1MPa also can 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 will increase respectively, so that the minimum setting pressure meets the working requirements of the hydraulic cylinder, and the pressure difference value of 1MPa should still be maintained.

. Change the above circuit to the form of Figure 12 (b), that is, the remote control ports of the two relief valves are connected to a remote regulator 11, the adjustment pressure of the system is determined by the regulator and has no direct relationship with the pilot valve of the relief valve, but to ensure that the set pressure value of the pilot valve regulator spring must be higher than the maximum adjustment pressure of the regulator. Hydraulic Press Circuit

Because the adjustment pressure range of the remote regulator must be lower than the adjustment pressure of the pilot valve of the relief valve to work effectively, otherwise the remote regulator will not work.

5. Leakage problems in the relief valve control oil circuit

In the circuit shown in Figure 13(a), the system has two oil supply systems because the equipment requires continuous operation and does not allow downtime for repair. When one oil supply system fails, the other oil supply system can start immediately to make the equipment run normally and then repair the failed oil supply system. Hydraulic Press Circuit

The performance specifications of the components of the oil supply system to which pump 1 and pump 2 belong are identical. The primary pressure is regulated by relief valves 3 and 4, and the secondary pressure is regulated by remote regulator 9. Hydraulic Press Circuit

However, when the system belonging to pump 2 stops supplying oil and only pump 1 is running, the system pressure does not go up, even when the electro-hydraulic reversing valve is placed in the middle position, pump 1 output oil circuit still cannot rise to the required pressure value. Hydraulic Press Circuit

After debugging, it was found that the maximum pressure of pump 1 could only reach 12MPa when running, and the design requirement should reach 14MPa. when the pressure regulator knob of relief valve 3 and remote regulator 9 were tightened, the pressure still could not go up. When the oil temperature is 40℃, the pressure rises to 12MPa; when the oil temperature rises to 55℃, the pressure can only rise to 10MPa. separately tested the pump and other components, no quality problems were found. The indicators meet the performance requirements. Components no problem, combined into the system pressure does not go up, should analyze the system components combined with the mutual impact. Hydraulic Press Circuit

When pump 1 is working, pressure oil enters the lower end of the main spool from the inlet of relief valve 3. And flows into the spring chamber at the upper end of the main spool through the damping hole. Then enters the spring chamber at the upper end of the main spool of relief valve 4 through the remote control port of relief valve 3 and the external oil pipe, and flows downward through the lower chamber of the main spool through the damping hole, and flows backward from the inlet of relief valve 4 into the discharge pipe of pump 2, which is stopped.  Hydraulic Press Circuit

This will be two situations: a. the check valve 6 not close; b. pump 2 out of the pipe pressure oil will make the pump 2 like a hydraulic motor reverse micro-motion or through the gap of the pump 2 flow into the tank. So the remote control port of relief valve 3 leaks hydraulic oil into the tank, and the above-mentioned pressure failure will occur.

Since there is a throttling device set on the control oil line, the oil on the remote control oil line of relief valve 3 flows back to the tank under a certain condition of throttling resistance, so the pressure is not completely absent. For this reason, relief valve 3 then overflows at a pressure lower than the required pressure.

Figure 13 (b) shows the improved circuit with check valves 11 and 12, the outlet pipe into pump 2 is cut off, eliminating the above fault.

6. Hydraulic pump outlet closure problem

Figure 14(a) shows a pressure regulating circuit, which can switch the system pressure between the two pressures regulated by relief valve 1 and relief valve 2. When reversing valve 3 is in the left position, the system pressure is regulated by relief valve 1, and in the right position by relief valve 2, the system unload in the middle position.

System use for some time after the hose burst accident. After analysis: The unreasonable design of the system cause the accident. Reversing valve 3 in the process of doing pressure switching must go through a short process of valve mouth completely closed, in this process, because the pump output oil has no way to go, so that the system pressure suddenly rises, repeated pressure shock to make the hydraulic hose fatigue burst.

One of the solutions shows in Figure 14 (b). This example illustrates: even a very short output closure can cause a large pressure shock to the hydraulic system, and if there is no hose in the system, it will inevitably lead to damage to the hydraulic pump over time.

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