Common faults and troubleshooting measures for pressure relays

- Sep 27, 2019-

A pressure relay is a hydraulic electrical conversion element that utilizes the pressure of a liquid to open and close an electrical contact. When the system pressure reaches the set value of the pressure relay, an electrical signal is sent to cause electrical components (such as electromagnets, motors, time relays, electromagnetic clutches, etc.) to operate, so that the oil circuit is relieved and reversed, and the actuators are sequentially operated. Or turn off the motor to stop the system, for safety protection. How to solve the failure of the pressure relay?


First, no signal output


Analysis of the cause: deformation of the oil inlet pipe, oil leakage of the pipe joint, deformation or loss of elasticity of the rubber film, the valve core is stuck, the spring is permanently deformed or the pressure is too high, the contact screw, the lever are not properly adjusted, and the micro switch is damaged.


Key issue: pressure signal is not converted into electrical signal


Remedy: Replace the pipe, tighten the pipe joint, replace the film piece, clean, match the valve core, replace the spring, adjust the reasonable, adjust the position of the lever reasonably, and replace the micro switch.


Second, the sensitivity is poor


Analysis of the cause: the friction force is too large when the valve core moves, the assembly mechanism is poorly assembled, the moving parts are out of order, and the contact stroke of the micro switch is too long.


Key issue: slow signal conversion


Remedy: The assembly and adjustment should be reasonable, the valve core and other actions should be flexible, and the position of the lever should be adjusted reasonably.


Third, easy to send false signals


Analysis of the cause: The inlet orifice is too large, the system stamping pressure is too large, and the electrical system is not properly designed.


Key question: signal conversion that should not be there


Measures are excluded: reduce the damping hole appropriately, add a damper tube on the control line to reduce the pressure shock, and the electrical system design should consider the necessary interlocking.


Fourth, can not afford air flow or a small adjustment range


Analysis of the cause: the valve fitting clearance is too large, there is a serious internal leakage, the one-way valve in the one-way throttle valve is poorly sealed or the spring is deformed, the spool is stuck when the flow valve is opened, and the flow valve is throttled at the small opening. The mouth is clogged.


Key issue: Excessive liquid through the flow valve.


Remedy: Restore the valve body or replace the valve core, grind the check valve seat, replace the spring, disassemble the cleaning and repair, wash, clean and filter the oil.


5. The speed of the actuator is unstable, sometimes it is slow or beating.


Analysis of the causes: periodic changes in the blockage of the throttle, immediate blockage, periodic changes in leakage, changes in load, changes in oil temperature, failure of various types of compensation devices (load, temperature), do not stabilize.


The key issue: the flow through the valve is unstable.


Measures to be excluded: Strictly filter oil or replace new oil. For systems with large load changes and high speed stability requirements, speed control valves should be used to control temperature rise. After the oil temperature rises and stabilizes, adjust the throttle again. Valve opening to repair the pressure reducing valve or temperature compensation device in the speed control valve.



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Common faults and troubleshooting measures for pressure relays

The fault of the pressure relay is mainly caused by false alarms and no signal. Most of the correct use and adjustment can avoid such faults.


1. False or non-signal faults generated by the pressure relay itself


Take the membrane pressure relay as an example for analysis.


(1) Analysis of the cause.


1 The rubber diaphragm is broken. The membrane type pressure relay works by using the oil pressure to rise, causing the film to bulge upward to push the plunger. When the film breaks, the pressure oil acts directly on the plunger, and oil will leak out from the fitting gap between the plunger and the middle body hole, causing obvious changes and instability of the action value and return interval, thus causing Mistaken action.


2 The micro switch is insensitive and has poor resettability. The spring force in the micro switch is not enough. When the contact is pressed, it will not bounce, or the dust will stick to the contact, causing the micro switch signal to be abnormal and sending an action signal.


3 The molybdenum disulfide grease coated on the outer circumference of the plunger is washed away, causing the plunger to move inflexibly and malfunction.


4 Because the plunger is not well matched with the middle body (or frame), or the burr is not cleaned, the plunger is stuck and the pressure relay does not operate.


5 The micro switch is not positioned firmly or not. The micro-switch of DP-63 type pressure relay is originally pressed by a single screw, so that it does not move forward, and the back of a small screw top micro switch does not move backward. Later, after the top wire was added before and after the micro switch, there was improvement, but the rigidity was still insufficient. Therefore, during the wiring and disconnection, the force applied by the screwdriver to the micro switch and the force of the twisted wire when repairing the cover may cause the micro switch to be misaligned, causing the action value to change, that is, changing the original adjusted action. Pressure, and false signalling.


6 pairs of differential pressure relays (see Figure 78), because the micro-switch part and the drain chamber are separated by a rubber membrane. When the oil inlet chamber and the drain chamber are reversed, the pressure oil will break through the rubber diaphragm. The micro switch can damage the micro switch and cause malfunction or no action. In addition, since the pressure regulating spring chamber and the oil drain chamber communicate with each other, there is no sealing device at the adjusting screw, so there is an external leakage phenomenon at the adjusting screw, so the oil drain port L and the oil inlet chamber P cannot be reversed (mainly for the tube type) ), and the drain port must be returned to the tank separately.


Common faults and troubleshooting measures for pressure relays

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7 The manufacturing accuracy of the pressure relay is not good. For example, the lever is not strong in the middle body groove, the clearance between the spring seat and the upper body is too small, the plunger size is wrong, and the main and auxiliary adjustment screws of the pressure relay are loosely adjusted. The function of the micro switch can not be pressed, and the pressure relay is disabled.


(2) Troubleshooting.


1 The new film should be replaced in time.


2 The micro switch should be repaired or replaced.


3 Re-apply aluminum disulfide grease on the outer circumference of the plunger to ensure the flexibility of the plunger movement.


4 Check that the clearance between the valve plug (plunger) and the middle body hole is 0.007~0.015mm, the burr should be cleaned, cleaned first, and then molybdenum disulfide (black) is lubricated on the surface. Users often think that molybdenum disulfide is washed off as dirt, which is wrong.


5 Check that the micro-switch is not in a good position and is tight, and repair it according to the situation.


6 Carefully check the differential pressure relay. If the rubber diaphragm is damaged, it should be replaced in time.


7 Check and replace the high precision pressure relay.


For the erroneous or non-signaling of the plunger type pressure relay, refer to the above-mentioned film type, in addition to the following points.


1 The plunger moves inflexibly and is stuck with dirt or burrs.


2 pairs of PF type pressure relays, such as the oil drain chamber is not directly connected to the fuel tank, but share a pipeline with the system return oil, the drain port may have a back pressure is too high and the wrong action signal.


2. The pressure relay malfunctions due to the loop


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As shown in Figure 79, Figure 79 (a) uses the inlet throttle speed regulation, and Figure 79 (b) uses the oil return throttle speed regulation. In the two circuits, if the position of the pressure relay is not correct, the malfunction of the pressure relay may occur due to the circuit. In Fig. 79 (a), if the pressure relay VI is installed in the figure a, when the reversing valve 3 is suddenly switched, a hydraulic shock will occur to cause the pressure relay to malfunction; if it is installed in the figure S, then S The pressure in the oil pool is constant (zero) and no pressure change can be generated to send a signal to the pressure relay. Only when the pressure relay 5 is installed behind the one-way throttle valve 4 and close to the hydraulic cylinder inlet position, the pressure shock of the reversing valve 3 is absorbed by the one-way throttle valve 4, and no malfunction occurs, and the pressure P1 during operation is The change has created conditions for the pressure relay 5 to send a letter.


Common faults and troubleshooting measures for pressure relays


For the oil return throttle mode in Fig. 79 (b), it is only correct to install the pressure relay 5 at the C in the figure, and the position of the figure is incorrect. Because at this time, P1 is basically equal to PP (PP is the pressure relief valve outlet pressure), there is no pressure change, only the pressure P2 at C is changed, and the pressure can change the pressure relay. Since the back pressure at C is low, a low pressure pressure relay should be used.


3. Mis-sending action signals or no signal generated by other reasons


(1) Analysis of the cause.


1 Due to the failure of the pump or other valves (such as relief valve, pressure reducing valve, etc.), the system pressure cannot be established, or there is a large pressure deviation phenomenon, so that the pressure relay does not send an action signal or a false alarm signal.


2 The accident occurs when the hydraulic cylinder is stuck in the middle, causing the pressure relay to make a conversion in advance (false signal).


3Because the pressure relay of the pressure relay is blocked, in particular, it should not share a pipeline with the return pipe of components such as large flow valves, otherwise the pressure relay will send a signal by mistake or not signal.


4 The pressure in the oil circuit is often fluctuating. When the fluctuation value is too large and exceeds a certain range, that is, wider than the on-off regulation interval value and the return interval value of the pressure relay, the pressure relay may falsely signal.


5 Due to the serious internal leakage of the hydraulic cylinder, the pressure chamber and the oil return chamber are in series, the working chamber pressure is not up, and the pressure in the return chamber is increased, so it is possible to make the pressure relay (in the inlet) of the oil inlet throttle mode. If the signal is not sent, the pressure relay of the oil return mode will be falsely signaled; in addition, the leakage will cause the pressure relay to malfunction or not signal.


6 When the hydraulic system is started or the speed is changed, the pressure shock is large, and the pressure relay is malfunctioned.


7 The pressure relay spring is broken.


(2) Troubleshooting


1 should eliminate the pump valve failure.


2 It is more reliable to use the stroke valve or the stroke switch as the signal conversion component than the pressure relay. However, some of the loops are not suitable for the control because of the need of the control. In order to prevent the false signal from being sent out at the end point, the electrical stroke can be added at the end point. The switch makes the signal from the pressure relay at the end point valid.


3 Check the drain line of the pressure relay, and make sure it is unobstructed.


4 The return interval of the pressure relay should be appropriately widened.


5 Check the cause of the leakage of the hydraulic cylinder and take measures in a targeted manner.


6 The general impact pressure is unavoidable. A fixed damping method can be installed in the pipeline close to the inlet of the pressure relay, or the transmitting circuit of the pressure relay can be opened in the impact pressure.


7 Replace the new spring.