Troubleshooting of direction control circuit
Troubleshooting of direction control circuit
The direction control circuit is a circuit that controls the starting, stopping and reversing of the actuator. This kind of circuit includes two basic circuits: commutation and locking. The function of the reversing circuit is to change the movement direction of the actuator, which can generally be realized by various reversing valves. In the closed volume high-speed circuit, the bidirectional variable pump can also be used to realize the reversing process. The function of the locking circuit is to stop the actuator at the specified position and prevent drift or movement due to external influence.
1、 Basic principles of fault analysis of directional control circuit
Among the control valves of hydraulic system, directional valves account for a considerable proportion in quantity. The working principle of the directional valve is relatively simple. It uses the change of the relative position between the valve core and the valve body to realize the connection or disconnection of the oil circuit, so as to start, stop (including locking) or reverse the actuator. The main faults of the direction control circuit and their causes are as follows.
1. The reversing valve does not reverse
(1) The electromagnet has insufficient suction and cannot push the valve core.
(2) The residual magnetism of the DC electromagnet is large, so that the valve element does not reset.
(3) The axis of the centering spring is skewed, so that the valve core is stuck in the valve.
(4) The valve core is roughened and stuck in the valve body.
(5) The oil is seriously polluted, blocking the sliding clearance, causing the valve element to be stuck.
(6) Due to the poor machining accuracy of the valve core and valve body, the radial clamping force is generated, which makes the valve core stuck.
2. The check valve leaks seriously or does not work
(1) The cone valve and valve seat are not tightly sealed.
(2) The cone valve or valve seat is roughened or there is dirt on the annular sealing surface.
(3) The valve core is stuck, and the cone valve cannot be closed when the oil flow reverses.
(4) The valve element cannot be reset because the spring is missing or skewed.
2、 Fault analysis and elimination of reversing circuit
1. The hydraulic control check valve loses control over the lowering of the plunger cylinder
In the circuit shown in Figure 24 (a), the electromagnetic directional valve is O-shaped, the hydraulic cylinder is a large plunger cylinder, and the lowering stop of the plunger cylinder is controlled by the hydraulic control check valve. When the reversing valve is in the middle position, the hydraulic control check valve should be closed, and the lowering of the hydraulic cylinder should be stopped immediately. But in fact, the hydraulic cylinder can't stop immediately, and it needs to drop a distance before it finally stops. This phenomenon that the stop position cannot be accurately controlled will not only cause the equipment to lose its working performance, but also cause various accidents.
Check all components of the circuit, the sealing cone of the hydraulic control check valve is not damaged, and the one-way seal is good. However, during the lowering process of the plunger cylinder, when the directional valve switches to the middle position, it takes a certain time for the hydraulic control check valve to close. As shown in Figure 24 (b), change the middle position of the reversing valve to Y-type. When the reversing valve is in the middle position, the control oil circuit is connected, its pressure immediately drops to zero, the hydraulic control check valve immediately closes, and the plunger cylinder quickly stops falling.
2. Mutual interference of hydraulic cylinder movement
In the circuit shown in Figure 25 (a), the hydraulic pump is a quantitative pump. Cylinder 1 is a plunger cylinder and cylinder 2 is a piston cylinder. The hydraulic control check valve controls the lower position of the plunger cylinder. The movement of the two cylinders is controlled by two electrohydraulic directional valves respectively.
The fault of this circuit is that when the plunger cylinder 1 is in the upper position and the hydraulic cylinder 2 starts to act, the plunger cylinder automatically drops.
In the circuit, when the electro-hydraulic directional valve controls the action of hydraulic cylinder 2, the outlet pressure of the hydraulic pump increases with the external load. Since the control oil circuit of the hydraulic control check valve is connected with the main oil circuit, at this time, the hydraulic control check valve is opened and the plunger of cylinder 1 drops. Due to the white weight of the plunger and its external load, the oil pressure discharged from the plunger cylinder is greater than the working pressure of cylinder 2, so the flow entering cylinder 2 is the sum of the output flow of the pump and the flow discharged from cylinder L, forming that the movement speed of cylinder 2 is higher than the set value.
In the circuit shown in Figure 25 (a), the hydraulic pump is a quantitative pump. Cylinder 1 is a plunger cylinder and cylinder 2 is a piston cylinder. The hydraulic control check valve controls the lower position of the plunger cylinder. The movement of the two cylinders is controlled by two electrohydraulic directional valves respectively.
The fault of this circuit is that when the plunger cylinder 1 is in the upper position and the hydraulic cylinder 2 starts to act, the plunger cylinder automatically drops.
In the circuit, when the electro-hydraulic directional valve controls the action of hydraulic cylinder 2, the outlet pressure of the hydraulic pump increases with the external load. Since the control oil circuit of the hydraulic control check valve is connected with the main oil circuit, at this time, the hydraulic control check valve is opened and the plunger of cylinder 1 drops. Due to the white weight of the plunger and its external load, the oil pressure discharged from the plunger cylinder is greater than the working pressure of cylinder 2, so the flow entering cylinder 2 is the sum of the output flow of the pump and the flow discharged from cylinder L, forming that the movement speed of cylinder 2 is higher than the set value.
As shown in Figure 25 (b), the return port of the pilot solenoid directional valve controlling the plunger cylinder is directly led to the oil tank. When cylinder 2 moves, the control oil circuit of the hydraulic control check valve has no pressure, and the plunger of plunger cylinder 1 will not slide down.
3. Commutation failure
In the circuit shown in Figure 26 (a), the pressure oil output by the quantitative pump is transmitted to three hydraulic cylinders by three three position four-way directional valves respectively, and sometimes the reversing of the electromagnetic directional valve is ineffective.
After testing, all parts of the electromagnetic directional valve work normally, and the regulating pressure of the overflow valve is lower than the allowable working pressure of the electromagnetic directional valve. Sometimes two or three hydraulic cylinders act at the same time, sometimes only one acts. The hydraulic pump is a quantitative pump, and the output flow of the pump can meet the simultaneous action of three cylinders, so the flow is relatively large. When only one cylinder acts at a certain time, the flow through the solenoid valve greatly exceeds the allowable capacity value. At this time, the force of the solenoid valve pushing the slide valve exceeds the allowable reversing force. The electromagnet cannot push the slide valve to reverse, resulting in reversing failure. At the same time, too much flow into a hydraulic cylinder is also easy to cause the cylinder speed to lose control. Therefore, as shown in Figure 26 (b), a throttle valve is installed in front of the reversing valve to control the flow into the hydraulic cylinder. At this time, it is equivalent to the oil inlet throttle speed regulation circuit. If only one cylinder works, part of the output flow of the pump is adjusted and controlled by the throttle valve to control the speed of the hydraulic cylinder, and part is overflowed back to the oil tank by the overflow valve. In this way, the flow through the solenoid valve is controlled, and the fault of commutation failure caused by excessive flow is eliminated.
4. Impact occurs before the fast retreat action
In the system shown in Figure 27 (a), the hydraulic pump is a quantitative pump, and the median function of the three position four-way directional valve is Y-type. The throttle valve is on the oil inlet road of the hydraulic cylinder to throttle and regulate the speed of oil inlet. The overflow valve has the function of constant pressure overflow. The two position two-way valve is connected when the hydraulic cylinder is fast forward and fast backward.
System fault is; When the hydraulic cylinder starts to complete the fast back action, it first rushes forward in the working direction, and then completes the fast back action. This will affect the machining accuracy, and may damage the workpiece and tool in serious cases.
In the system shown in Figure 27 (a), the hydraulic pump is a quantitative pump, and the median function of the three position four-way directional valve is Y-type. The throttle valve is on the oil inlet road of the hydraulic cylinder to throttle and regulate the speed of oil inlet. The overflow valve has the function of constant pressure overflow. The two position two-way valve is connected when the hydraulic cylinder is fast forward and fast backward.
System fault is; When the hydraulic cylinder starts to complete the fast back action, it first rushes forward in the working direction, and then completes the fast back action. This will affect the machining accuracy, and may damage the workpiece and tool in serious cases.
In modular machine tools and automatic line hydraulic systems, the hydraulic cylinder is generally required to realize the action cycle of fast forward → work forward → fast backward. When the action speed is converted, it is required to be stable without impact. The reason why the above faults occur in the system is that when the hydraulic system performs the fast reverse action, the three position four-way electromagnetic directional valve and the two position two-way directional valve must be reversed at the same time. Due to the lag of the reversing time of the three position four-way directional valve, at the moment when the two position two-way directional valve is connected, some pressure oil enters the working chamber of the hydraulic cylinder, causing the hydraulic cylinder to rush forward. After the reversing of the three position four-way reversing valve is completed, all the pressure oil enters the rod chamber of the hydraulic cylinder, and the oil without rod chamber returns to the oil tank through the two position two-way valve.
Therefore, when designing the hydraulic system, the phenomenon that the three position reversing valve lags behind the two position reversing valve should be considered.
The method to eliminate the above faults is to connect a check valve in parallel on the two position two-way directional valve and throttle valve, as shown in Figure 27 (b). When the hydraulic cylinder retreats quickly, the oil in the rodless chamber returns to the oil tank through the one-way valve, and the two position two-way valve is still closed, so as to avoid the fault of hydraulic cylinder forward flushing.
5. There is no pressure in the control oil circuit
The method to eliminate the above faults is to connect a check valve in parallel on the two position two-way directional valve and throttle valve, as shown in Figure 27 (b). When the hydraulic cylinder retreats quickly, the oil in the rodless chamber returns to the oil tank through the one-way valve, and the two position two-way valve is still closed, so as to avoid the fault of hydraulic cylinder forward flushing.
5. There is no pressure in the control oil circuit
In the system shown in Figure 28, the hydraulic pump 1 is a quantitative pump, the overflow valve 2 is used for overflow, the hydraulic directional valve 3 is M-type, externally controlled and externally returned oil, and the hydraulic cylinder 4 pushes the load in one direction.
The system fault phenomenon is: when the solenoid valve in the electrohydraulic valve is reversed, the hydraulic reversing valve does not act, and the hydraulic system is detected. When the system does not work, the hydraulic pump output pressure oil directly returns to the oil tank through the middle position of the hydraulic valve in the electrohydraulic valve, and there is no back pressure in the return oil circuit. Check that the slide valve core of the hydraulic valve moves normally without clamping.
The system fault phenomenon is: when the solenoid valve in the electrohydraulic valve is reversed, the hydraulic reversing valve does not act, and the hydraulic system is detected. When the system does not work, the hydraulic pump output pressure oil directly returns to the oil tank through the middle position of the hydraulic valve in the electrohydraulic valve, and there is no back pressure in the return oil circuit. Check that the slide valve core of the hydraulic valve moves normally without clamping.
Because the electrohydraulic valve is externally controlled and returns oil, in the control oil circuit of the medium and low pressure electrohydraulic valve, the oil must generally have a pressure of 0.2 ~ 0.3MPa for the control oil circuit to operate the hydraulic valve.
When starting the system, the pump output oil is directly returned to the oil tank through the M-type hydraulic valve, so the control oil circuit of the electrohydraulic directional valve has no pressure. When the solenoid valve in the electrohydraulic valve is reversed, the control oil cannot push the hydraulic valve to reverse, so the hydraulic valve in the electrohydraulic valve does not act.
When starting the system, the pump output oil is directly returned to the oil tank through the M-type hydraulic valve, so the control oil circuit of the electrohydraulic directional valve has no pressure. When the solenoid valve in the electrohydraulic valve is reversed, the control oil cannot push the hydraulic valve to reverse, so the hydraulic valve in the electrohydraulic valve does not act.
Such failure of the system is caused by poor design. The method to eliminate this fault is: install a check valve on the oil outlet circuit of the pump, and at this time, the control pipeline of the electrohydraulic valve is connected between the pump and the check valve; Or install a back pressure valve in the oil return circuit of the whole system (the direct acting overflow valve can be used as the back pressure valve to make the back pressure adjustable) to ensure that there is still a certain pressure in the oil circuit when the system is unloaded.
For the high-pressure system, the control pressure of the control oil circuit of the electrohydraulic valve should be increased accordingly. For example, for the 21MPa hydraulic system, the control pressure should be higher than 0.35Mpa; For 32Mpa hydraulic system, the control pressure should be higher than 1MPa.
For the high-pressure system, the control pressure of the control oil circuit of the electrohydraulic valve should be increased accordingly. For example, for the 21MPa hydraulic system, the control pressure should be higher than 0.35Mpa; For 32Mpa hydraulic system, the control pressure should be higher than 1MPa.
It should also be noted here that in systems with back pressure, the electro-hydraulic valve must use external oil return instead of internal oil return.
6. The start / stop position of hydraulic cylinder is inaccurate
In the system shown in Figure 29, the middle position function of the three position four-way solenoid directional valve is O-shaped. When the pressure oil enters the rodless chamber of the hydraulic cylinder, the oil in the rodless chamber is controlled by the throttle valve (return oil throttle speed regulation), two position two-way solenoid valve (rapid descent), hydraulic control check valve and sequence valve (used as balance valve) to return to the oil tank, so as to meet the requirements of different working conditions. The three position four-way solenoid directional valve is reversed, and the hydraulic oil enters the rod chamber of the hydraulic cylinder through the hydraulic control one-way valve to realize the return movement of the hydraulic cylinder. The stroke of the hydraulic cylinder is controlled by the stroke switch.
6. The start / stop position of hydraulic cylinder is inaccurate
In the system shown in Figure 29, the middle position function of the three position four-way solenoid directional valve is O-shaped. When the pressure oil enters the rodless chamber of the hydraulic cylinder, the oil in the rodless chamber is controlled by the throttle valve (return oil throttle speed regulation), two position two-way solenoid valve (rapid descent), hydraulic control check valve and sequence valve (used as balance valve) to return to the oil tank, so as to meet the requirements of different working conditions. The three position four-way solenoid directional valve is reversed, and the hydraulic oil enters the rod chamber of the hydraulic cylinder through the hydraulic control one-way valve to realize the return movement of the hydraulic cylinder. The stroke of the hydraulic cylinder is controlled by the stroke switch.
The fault phenomenon of the system is that when the reversing valve is in the middle position, the hydraulic cylinder cannot stop moving immediately, but deviates from the specified position for a short distance.
In the system, because the change-over valve adopts O-shape, when the change-over valve is in the middle position, the pressure in the oil inlet pipe of the hydraulic cylinder is still very high. Often open the hydraulic control check valve to make the piston of the hydraulic cylinder drop a short distance and deviate from the contact switch. When the next letter is sent, it cannot act correctly. This kind of fault is called micro action fault in hydraulic system. Although it will not directly cause major accidents, it may cause secondary faults when cooperating with other machinery, so it must be eliminated.
In the system, because the change-over valve adopts O-shape, when the change-over valve is in the middle position, the pressure in the oil inlet pipe of the hydraulic cylinder is still very high. Often open the hydraulic control check valve to make the piston of the hydraulic cylinder drop a short distance and deviate from the contact switch. When the next letter is sent, it cannot act correctly. This kind of fault is called micro action fault in hydraulic system. Although it will not directly cause major accidents, it may cause secondary faults when cooperating with other machinery, so it must be eliminated.
The troubleshooting method is: change the middle position function of the three position four-way reversing valve from O-shape to Y-shape. When the reversing valve is in the middle position, the oil inlet pipe of the hydraulic cylinder and the oil tank are connected, and the hydraulic control check valve remains locked, so as to avoid the piston sliding.
7. The pressure does not go up after reversing
In the circuit shown in Figure 30 (a), three pumps supply oil to the system, among which pump 1 is a high-pressure small flow pump, and pump 2 and pump 3 are low-pressure large flow pumps. The electro-hydraulic directional valve is a larger M-type valve. Overflow valve 7 is used as the safety valve of pump 1 in this circuit. Overflow valve 8 and two position two-way valve 9 make pump 2 and pump 3 unload and overflow. In the circuit, when 1ya is powered on, the pressure oil output by the hydraulic pump enters from port P of the electro-hydraulic directional valve and outputs from port a into the load working chamber of the hydraulic cylinder, and the pressure cannot rise to the set load working pressure.
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In the circuit shown in Figure 30 (a), the internal working principle of the reversing valve 12 when reversing the pressure oil (i.e. P → a or P → b) is shown in Figure 30 (d). When 1ya is powered on, pressure oil p is connected with valve port a, and B is connected with oil return port T. therefore, B and T are low-pressure chambers, while P and a and control chamber K belong to high-pressure chambers. Therefore, there are three annular gaps S1, S2 and S3 in the matching part of the valve core and the inner hole of the valve body, so that the high-pressure oil leaks to the oil return port. Especially at s, the length of the annular cover of some valves is designed to be short, and the leakage of pressure oil will increase. Due to the serious leakage, the pressure cannot go up.
In the circuit shown in Figure 30 (a), the internal working principle of the reversing valve 12 when reversing the pressure oil (i.e. P → a or P → b) is shown in Figure 30 (d). When 1ya is powered on, pressure oil p is connected with valve port a, and B is connected with oil return port T. therefore, B and T are low-pressure chambers, while P and a and control chamber K belong to high-pressure chambers. Therefore, there are three annular gaps S1, S2 and S3 in the matching part of the valve core and the inner hole of the valve body, so that the high-pressure oil leaks to the oil return port. Especially at s, the length of the annular cover of some valves is designed to be short, and the leakage of pressure oil will increase. Due to the serious leakage, the pressure cannot go up.
As shown in Figure 30 (b), exchange the two chambers of the hydraulic cylinder with ports a and B of the electrohydraulic directional valve, that is, let port B connect to the load working chamber of the cylinder and port a connect to the return working chamber of the cylinder. In this way, when 2ya is powered on, pressure oil p enters the load working chamber of the cylinder from port B. At this time, the flow condition of oil in the directional valve is shown in Figure 30 (d) left position of valve element. It can be seen that only the annular gap at S1 'leaks high-pressure oil. At this time, the control oil of the electrohydraulic directional valve comes from the main oil circuit, so there is no leakage of high-pressure oil to low-pressure oil in the S2 'shaped clearance.
As shown in Figure 30 (c), connect the control oil circuit of the electrohydraulic directional valve with the low-pressure oil circuit, so that the control oil circuit of the electrohydraulic directional valve is low-pressure, and the annular gap of s will not produce leakage from high pressure to low pressure, thus reducing the leakage of the system. However, at this time, it is necessary to change the electro-hydraulic directional valve from high-pressure control to low-pressure control, and ensure the basic pressure value in the low-pressure oil circuit.
From the above analysis, it can be seen that in the form shown in Figure 30 (b), the leakage in the electro-hydraulic directional valve is the least, which can be considered as a better scheme. By reducing the leakage, the working pressure of the system can rise to the design value.
From the above analysis, it can be seen that in the form shown in Figure 30 (b), the leakage in the electro-hydraulic directional valve is the least, which can be considered as a better scheme. By reducing the leakage, the working pressure of the system can rise to the design value.
8. Hydraulic shock during reversing
Figure 31 (a) shows the three position four-way electromagnetic reversing unloading circuit, and the intermediate function of the reversing valve is M-type. The system of this circuit is a high-pressure and large flow system. When the directional valve is switched, the system will have a large pressure impact.
In addition to M-type, H-type and K-type with unloading performance in the middle position of three position valve. Such a circuit is generally used for hydraulic systems with low pressure (pressure less than 2.5MPa) and small flow (flow less than 40l/min), which is a simple and effective unloading method.
Figure 31 (a) shows the three position four-way electromagnetic reversing unloading circuit, and the intermediate function of the reversing valve is M-type. The system of this circuit is a high-pressure and large flow system. When the directional valve is switched, the system will have a large pressure impact.
In addition to M-type, H-type and K-type with unloading performance in the middle position of three position valve. Such a circuit is generally used for hydraulic systems with low pressure (pressure less than 2.5MPa) and small flow (flow less than 40l/min), which is a simple and effective unloading method.
For the hydraulic system with high pressure and large flow, when the outlet pressure of the pump is switched from high pressure to almost zero pressure, or rapidly switched from zero pressure to high pressure, hydraulic shock will inevitably occur when the reversing valve is switched. At the same time, due to the rapid switching of the electromagnetic directional valve, there is no buffer time, which forces the hydraulic impact to intensify.
Replace the three position solenoid directional valve with an electro-hydraulic directional valve, as shown in Figure 31 (b). Because the reversing time of the hydraulic valve in the electro-hydraulic directional valve is adjustable, the reversing has a certain buffer time, which makes the pump outlet pressure rise or fall in a changing process, improves the reversing stability, and thus avoids obvious pressure impact. The function of the one-way valve in the circuit is to make the pump still have a certain pressure value (0.2 ~ 0.3MPa) during unloading, which is used for controlling the oil circuit.
Replace the three position solenoid directional valve with an electro-hydraulic directional valve, as shown in Figure 31 (b). Because the reversing time of the hydraulic valve in the electro-hydraulic directional valve is adjustable, the reversing has a certain buffer time, which makes the pump outlet pressure rise or fall in a changing process, improves the reversing stability, and thus avoids obvious pressure impact. The function of the one-way valve in the circuit is to make the pump still have a certain pressure value (0.2 ~ 0.3MPa) during unloading, which is used for controlling the oil circuit.
The above analysis is mainly applicable to the hydraulic system of machine tools, because hydraulic shock is not allowed in the hydraulic system of machine tools, and any small impact will affect the machining accuracy of parts. For the hydraulic system of construction machinery, it is generally a high-pressure and large flow system, and there are many M-type directional valves. Why won't there be hydraulic shock? This is because in the hydraulic system of construction machinery, the change-over valve is generally manual, and the buffer effect during the change-over of the change-over valve is realized by the operator. The valve port of the change-over valve is also a throttle port. When operating the handle, the operator should gradually open or close the valve port to avoid hydraulic shock.
The hydraulic pump shall be unloaded during the interval when the working mechanism of the hydraulic system stops working or pushes the load to run, or even if the hydraulic pump operates without load under almost zero pressure. This can reduce power consumption, reduce system heating, and extend the service life of the hydraulic pump. Generally, the hydraulic system with power greater than 3KW should have unloading function.
The hydraulic pump shall be unloaded during the interval when the working mechanism of the hydraulic system stops working or pushes the load to run, or even if the hydraulic pump operates without load under almost zero pressure. This can reduce power consumption, reduce system heating, and extend the service life of the hydraulic pump. Generally, the hydraulic system with power greater than 3KW should have unloading function.
