การออกแบบวงจรไฮดรอลิกของเครื่องอัดไฮดรอลิก

เวลาอ่านโดยประมาณ: 18 นาที

In modern machinery, The hydraulic drive systems used are made up of many basic hydraulic circuits. The so-called basic circuit is composed of the relevant hydraulic components, used to complete a specific function of the typical oil circuit. Master their role principle, composition, and characteristics, according to the machine’s performance, requirements, and working conditions, the correct and reasonable choice of these circuits, to form the complete hydraulic system.

Pressure control circuit design

Pressure control circuits are hydraulic circuits that use pressure control valves to control the overall or partial pressure of the system. Pressure valve control pressure circuit can be used to achieve pressure stabilization, pressure reduction, pressure boosting, and multi-stage pressure control to meet the force and torque requirements of the actuating element in the requirements. Standard components of pressure valves are relief valves, pressure reducing valves, sequence valves, as well as and check valves in parallel with the combination of one-way pressure reducing valves and one-way sequence valves.

 Pressure regulating circuits

Question 1: Selection of pressure regulation method

Firstly, Pressure limiting circuits

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

Secondly, Pressure remote control circuit

As shown in Figure 1-2, 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, the well passes through the four-way valve, and the pressure is set at 10MPa for the main relief valve. The valve is excited, the well through the four-way solenoid valve to change the main valve and remote control with relief valve a or b path, it can make the main circuit pressure conversion to 7 MPa or 5 MPa. The capacity of each valve, in addition to the main valve, other than the small flow valve.

Thirdly, Two-stage pressure regulating hydraulic circuit

In Figure 1-3, when the cylinder piston rises, falls and the piston remains in the highest position, the oil circuit pressure is P2 =5MPa (left register high-pressure pump unloading). However, when the piston reaches the bottom, the load increases and the pressure relay comes into play, manipulating the solenoid three-way valve so that P1 = 10MPa, high-pressure oil enters the circuit.

Fourthly, Pressure regulating circuit for compound pumps

In the design, the capacity of the pump must be compatible with the requirements of the work and break less useless heat generated at low drive speeds. Figure 1-4 The circuit is electrically controlled and can work at various flow rates and oil pressures as required to maintain maximum circuit efficiency, with the advantages offered by pressure make-up variable pumps. The electro-hydraulic reversing valve in the circuit is operated from the remote control port of the relief valve, preventing shocks caused by switching between the main reversing valves.

Question 2: Pressure parameter adjustmenNS

Firstly, Improper setting pressure of the relief valve

The set pressure of the relief valve is not appropriate, resulting in the hydraulic cylinder not moving at the required speed. Figure 1-5 circuit requires a flat movement when lifting. Total, speed adjustment range is large, the piston can stop in any position, but in operation, adjust the hoist rise speed, in a very rough with the speed does not change, only in the throttle valve opening to very small, rise speed change, can not reach the proper performance requirements. This is the reason why the pressure of the relief valve is adjusted high. The pressure setting of the relief valve should be the hydraulic pump working pressure is exactly equal to the hydraulic cylinder load pressure and pump all flow through the throttle valve when the required pressure drop.

Secondly, Improper pressure setting parameters

Improper pressure setting parameters lead to high oil temperature in the constant pressure pump supply system. As shown in Figure 1-6 in the hydraulic circuit of the constant pressure pump, due to improper pressure setting parameters, resulting in high oil temperature during system operation. The reason for the above problem is that the system pressure Pr set by the pressure valve 1 is lower than the pressure Pt set by the regulating spring of valve 2 so that the constant pressure pump always works under the maximum displacement, the excess flow to pressure Pr overflow back to the tank, and all transformed into heat so that the system temperature rises, therefore, the valve 1 for the use of safety valves, the pressure will be adjusted to the highest pressure than the system required 0.5 ~ 1MPa. The above problem can be solved.

Thirdly, Pressure parameter adjustment failure example

In Figure 1-7 quantitative pump pressure control hydraulic circuit, the hydraulic pump is a quantitative pump, three-way four-way reversing valve in position can be Y-type. Therefore, when the hydraulic cylinder stops running, the system does not unload, the hydraulic pump outputs pressure oil all by the relief valve overflow back to the tank. The relief valve in the system is a YF type pilot-operated relief valve, the structure of this relief valve is a three-stage concentric type.

ปัญหา: The system in the reversing valve in the middle position, adjust the pressure of the relief valve found, when the pressure value is 10MPa below, the relief valve normal work; when the pressure is adjusted to a higher than 10MPa any pressure value, the system issued a screeching sound like a whistle, at this time, you can see the pressure gauge pointer violent vibration. After testing, the noise was found to come from the relief valve.

Hydraulic circuits problem analysis: in the three-stage coaxial high-pressure relief valve, the main spool, and valve body, valve cover there are two sliding fits, if the valve body and valve cover assembly after the concentricity of the bore beyond the design requirements, the main spool can not move flexibly, but stick to a side of the bore to do the abnormal movement. When the pressure is adjusted to a certain value, it is bound to provoke the main spool vibration. This vibration is not the main spool in the work movement of the conventional vibration, but the main spool is stuck in a certain position (at this time because the main spool while bearing the hydraulic clamping force) and provoked by high-frequency vibration. This high-frequency vibration will cause strong vibration of the spring, especially the pressure-regulating bullet, and noise resonance.

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

สารละลาย

The manufacturing accuracy of YF type relief valve is relatively high. The concentricity of the inner and outer circular surface of the connection part between the break-in cover and the valve body and the concentricity of the outer circular surface of the three shoulders of the main spool should be within the specified range. Besides, the damping hole on the main spool has a limited damping effect when the main spool is vibrating. When the viscosity of the working oil is low or the temperature is too high, the damping effect will be reduced accordingly, therefore, the choice of suitable viscosity of the oil and control system temperature rise is too high is also conducive to vibration and noise reduction.

Fourthly, Pressure parameter adjustment failure problem

• Pressure can not be adjusted up. The main reason is that the pressure regulating spring of the relief valve is too soft, installed wrong or omitted; pilot type relief valve main valve damping hole blockage, slide valve in the lower end of the oil pressure, to overcome the liquid pressure of the upper cavity and the main valve spring force, so that the main valve moved up, the regulating spring lost the control of the main valve, so the main valve in the lower pressure to open the spillway overflow; spool and valve seat is not closed, the leakage is serious; spool is a burr or other dirt jammed in the open position.
• The pressure is too high and cannot be adjusted down. The main reason is that the spool is burr or dirt stuck in the closed position, the main valve can not open; installation, the valve inlet and outlet wrong connection, no pressure oil to push the spool to move, so the spool can not open; pilot valve before the damping hole blockage, resulting in the main valve can not be opened.
• Pressure oscillation is large. The main reason is that there is 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. Damping role is weak; resonance; spool in the valve body movement is not flexible. For the above problems, can be in the circuit design, component selection, component parameters, and system adjustment, pipeline installation, hydraulic oil use, etc. The above-mentioned problems can be dealt with in terms of circuit design, component selection, component parameters and system adjustment, piping installation, hydraulic oil use, and other targeted improvements.

Question 3: Outlet closure of a hydraulic pump

Figure 1-8 (a) shows a regulating circuit that switches the system pressure between the two pressures regulated by relief valve 1 and relief valve 2. The system pressure is regulated by relief valve 1 when reversing valve 3 is in the left position, by relief valve 2 in the right position, and unloads the system in the neutral position. The system. After a period of use a hose burst accident occurred. The accident was analyzed and found to be due to an unreasonable system design. Reversing valve 3 in the process of doing pressure switching must go through a short process of complete closure of the valve, in this process, due to the pump output oil no way to make the system pressure suddenly rise, repeated pressure shock to make the hydraulic hose fatigue burst.

สารละลาย

One solution is shown in Figure 1-8 (b).

This example illustrates: even a very short output closure, but also to the hydraulic system will cause a lot of pressure shock, if the system does not have a hose, a long time, will inevitably lead to damage to the hydraulic pump.

Question 4: Interference between pressure valves

Firstly, Two-pump hydraulic system

In the hydraulic system shown in Figure 1-9, hydraulic pumps 1 and 2 supply pressure oil to hydraulic cylinders 7 and 8 respectively, and reversing valves 5 and 6 are both three-way four-way Y-type solenoid reversing valves.

ปัญหา: When the hydraulic pump is started and the system begins to run, pressure in relief valves 3 and 4 is unstable and vibrates, making noise.

The test showed that when only one relief valve was working, its regulated pressure was stable and there was no obvious vibration and noise. When two relief valves work at the same time, the above faults occur.

From the hydraulic system, it can be seen that the two relief valves have no other connection than a common return line. Obviously, the fault is caused by this common return line. From the structure of the relief valve performance can be seen, the relief valve control oil channel for the internal drain, that is, the relief valve before the pressure oil into the valve, through the damping hole flow into the control capacity cavity when the pressure rises, the action on the valve.

When the pressure rises and the hydraulic pressure on the valve overcomes the regulating spring, open the cone valve port to lower the pressure, the oil flows into the return cavity of the relief valve through the valve body orifice, and the oil overflowing from the main valve port converges and flows back to the tank through the return pipeline – the same flow, so in the return pipeline of the relief valve, the flow state of the oil directly affects the regulating pressure of the relief valve.

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

สารละลาย

Return the two relief valves to the oil 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 back to the oil chamber is separated, the separate connection back to the tank on the external leakage type relief valve.

Secondly, Lift table hydraulic system

As shown in Figure 1-10 circuit, each circuit acts separately, the two circuits correspond to the same specifications of hydraulic components, the same pipeline diameter.

ปัญหา: when two hydraulic pumps start working at the same time, relief valves 3 and 4 to adjust the pressure fluctuations, and vibration and noise.

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

As can be seen from Figure 1-10, the two relief valves share a common return pipe, and there is no other connection. Obviously, the fault lies in this common pipe. If the total return pipe is still designed according to the diameter of the separate circuit, it is inevitable that the backpressure of the return port of the relief valve will increase when the two pumps are supplied with oil at the same time.

The backpressure at the return port of the relief valve will increase when the two pumps are supplied simultaneously. 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 back pressure increased by 1 or 3 times.

From the relief valve structure and working principle can be seen, they control the oil through the main spool on the damping hole into the control cavity, when the pressure rises to overcome the pilot valve regulating spring force, the pressure oil opens the pilot valve port, the oil flow through the valve port pressure reduction, through the valve body drain passage.

The oil will flow into the oil return cavity of the relief valve, and the oil overflowing from the main valve port will merge and flow back to the oil tank through the oil return pipe. The same flow back to the tank. 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. 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, in the two causes of mutual interference, the relief valve control capacity cavity in the oil pressure also changes, will lead to unstable pressure adjustment of the relief valve, and accompanied by vibration and noise.

In order to eliminate the above-mentioned failure, the two relief valves can be increased the diameter of the total return pipe, and the two relief valves are replaced by external leakage type, that is, the flow of oil through the pilot valve orifice by another leaky pipe flow back to the tank, or the two relief valves will be equipped with their own return pipe, to avoid the two relief valves can be fitted with their own return lines to avoid mutual interference

Thirdly, Multiple relief valve resonance problem

In the hydraulic system shown in Figure 1-11 (a), pump 1 and pump 2 is the same specification of the quantitative pump, while supplying hydraulic oil to the system, three-way four-way reversing valve 7 in position can be Y type, relief valve 3 and 4 is also the same specification, respectively installed in the pump 1 and pump 2 output port oil.

The relief valves 3 and 4 are of the same size and are installed in the output ports of pump 1 and pump 2 respectively, for constant pressure relief. The pressure setting of the relief valve is 14MPa, and when the system is started, the system makes a whistling sound like a siren. The system whistles when starting operation.

After commissioning, the noise came from the relief valve and found that when only one side of the pump and relief valve work, the noise disappeared, and when both sides of the pump work at the same time, a whistling sound. It can be seen that the reason for the noise is that the two relief valves resonate under the action of the fluid According to the working principle of the relief valve, it can be seen that the relief valve is working under the mutual action of liquid pressure and spring force, so it is very easy to provoke vibration and noise. Once the pressure oil at the outlet and control port of the relief valve fluctuates, i.e. hydraulic shock occurs, the main spool of the relief valve, the cone valve, and its interacting spring will vibrate, and the degree of vibration and its state will vary with the fluid pressure shock and fluctuating conditions. Therefore, the more stable the oil flow associated with the relief valve, the more stable the relief valve will be able to work, and vice versa.

In the above system, the double pump outputs pressure oil through the check valve after the combined flow, fluid shock, and fluctuations occur, causing the check valve oscillation, which leads to the hydraulic pump outlet pressure oil instability. 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.

Exclusion methods

• Relief valves 3 and 4 will be replaced by a large-capacity relief valve, placed at the double pump merge, so that although the relief valve will also vibrate, but not too strong because the resonance of the conditions excluded.
• The two relief valves will be set at a pressure of about 1MPa staggered, but also 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 Figure 1-11(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, there is no direct relationship with the pilot valve of the relief valve, but to ensure that the set pressure value of the pilot valve regulating spring must be higher than the maximum adjustment pressure of the regulator.

• This is because the adjustment pressure range of the remote regulator must be lower than the adjustment pressure of the relief valve’s pilot valve in order for the remote regulator to work effectively, otherwise, the remote regulator will not work.