Diagnosis and elimination of common faults in hydraulic system

Common fault diagnosis methods

Hydraulic equipment is a combination of mechanical, hydraulic, electrical and other devices, so the failures that occur are also diverse. A certain failure phenomenon may be caused by many factors. Therefore, analyzing hydraulic failures must be able to understand the schematic diagram of the hydraulic system, have a general understanding of the role of each component in the schematic diagram, and then analyze and judge according to the failure phenomenon. The causes of failures caused by many factors need to be analyzed one by one, and the main contradictions can be grasped to better solve and eliminate. It is difficult for the outside world to understand the flow of working fluid in components and pipelines in a hydraulic system, so it brings more difficulties to analysis and diagnosis, so people are required to have a strong ability to analyze and judge faults. Find out the cause and location of the fault in the complex relationships between machinery, hydraulics, and electricity, and eliminate them in a timely and accurate manner.

Simple fault diagnosis method

The simple fault diagnosis method is the most common method currently used. It relies on the maintenance personnel based on personal experience, using simple instruments to objectively use the methods of asking, watching, listening, touching and smelling to understand the system’s working conditions based on the failure of the hydraulic system. , Perform analysis, diagnosis, and determine the cause and location of the fault. The specific methods are as follows:

1) Ask the equipment operator to understand the operating status of the equipment. It includes: whether the hydraulic system is working normally; whether the hydraulic pump is abnormal; the time and result of checking the cleanliness of the hydraulic oil; the cleaning and replacement of the filter element; whether the hydraulic components have been adjusted before the failure; whether the sealing components have been replaced; What abnormal phenomena have occurred in the front and rear hydraulic systems; what failures have occurred in the system in the past and how to eliminate them, etc., need to be understood one by one.

2) Look at the actual working conditions of the hydraulic system and observe whether there are problems with the system pressure, speed, oil, leakage, vibration, etc.

3) Listen to the sound of the hydraulic system, such as: impact sound; pump noise and abnormal sound; judge whether the hydraulic system is working normally.

4) Touch temperature rise, vibration, crawling and tightness of joints to determine whether the working state of moving parts is normal.

In short, the simple diagnosis method is only a simple qualitative analysis, which has a wide range of practicality for rapid judgment and troubleshooting.

Analysis method of hydraulic system schematic diagram

Analyze the faults of the hydraulic transmission system according to the schematic diagram of the hydraulic system, find out the location and cause of the fault, and propose the method of troubleshooting. The hydraulic system diagram analysis method is currently the most common method used by engineering and technical personnel. It requires people to have a certain foundation of hydraulic knowledge and understand the hydraulic system diagram to master the name, function, principle and structure of the components represented by each graphic symbol And performance should also have a certain understanding. With this foundation, it is easy to compare and analyze the action cycle table and judge the fault. Therefore, earnestly learning the basics of hydraulics and mastering the hydraulic schematic diagram is the most powerful assistant for fault diagnosis and troubleshooting, and it is also the basis for other fault analysis methods. Must be mastered carefully.

Other analytical methods

When a hydraulic system fails, it is often impossible to find out the location and root cause of the failure immediately. In order to avoid blindness, people must carry out logical analysis based on the principle of the hydraulic system or use causal analysis to eliminate them one by one, and finally find the location of the failure. It uses logical analysis to find out the fault. In order to facilitate the application, the fault diagnosis expert has designed a logic flow chart or other diagrams to make a logical judgment on the fault, which provides convenience for the fault diagnosis.

Elimination methods of system noise and vibration (see Table 10)

Table 10 Elimination methods of system noise and vibration

Failure phenomenon and cause	Elimination method	Failure phenomenon and cause	Elimination method
1. The noise and vibration in the pump cause resonance in the pipeline and tank	1. Connect the inlet and outlet of the pump with hoses 2. Do not install the pump on the fuel tank, install the motor and pump separately on the base, separate from the fuel tank 3. Increase hydraulic pump and reduce motor revolution 4. Insert anti-vibration materials under the base of the pump and the fuel tank 5. Choose a low-noise pump and use a vertical motor to immerse the hydraulic pump in the oil	4. Noise caused by intense oil flow in the pipeline	1. Thicken the pipeline to control the flow rate within the allowable range 2. Use less elbows and more use elbows with small curvature 3. Adopt hose 4. Right-angle elbows or tees are not used where the oil flow is turbulent 5. Use muffler, accumulator, etc.
2. System resonance caused by valve spring	1. Change the installation position of the spring 2. Change the stiffness of the spring 3. Change the relief valve to an external oil drain form 4. Using remote control relief valve 5. Completely exhaust the air in the circuit 6. Change the length, thickness, material, thickness, etc. of the pipe 7. Increase the pipe clamp to prevent the pipe from vibrating 8. Install a throttle valve on a certain part of the pipeline	5. The fuel tank has resonance	1. Thicken the box board 2. Add ribs on the side plate and bottom plate 3. Change the shape or position of the end of the return pipe
2. System resonance caused by valve spring		6. Impact noise generated by valve reversal	1. Reduce the control pressure of the electro-hydraulic valve reversing 2. Add a throttle valve to the control line or return line 3. Select components with pilot unloading function 4. The electrical control method is adopted to prevent more than two valves from being reversed simultaneously
3. Vibration caused by air entering the hydraulic cylinder	1. Exhaust air well 2. You can apply molybdenum disulfide grease to the piston and sealing gasket of the hydraulic cylinder	7. Pipeline vibration and noise caused by poor operation of overflow valve, unloading valve, hydraulic control check valve, balance valve, etc.	1. Install a throttle valve where appropriate 2. Change the form of leakage 3. Transform the circuit 4. Add pipe clamps

5.3 Elimination method of abnormal system pressure (see Table 11)

Table 11 Elimination method of abnormal system pressure

Failure phenomenon and cause		Elimination method
Under pressure	Overflow valve bypass valve damaged	Repair or replace
	Pressure reducing valve setting value is too low	reset
	Incorrect design of integrated channel block	Redesign
	Damaged pressure reducing valve	Repair or replace
	The pump, motor or cylinder is damaged and has large internal leakage	Repair or replace
Unstable pressure	Air mixed with oil	Plugging, refueling, exhaust
	Relief valve wear, poor spring rigidity	Repair or replace
	Oil contamination, blocking valve orifice	Clean, change oil
	Failure of accumulator or inflation valve	Repair or replace
	Pump, motor or cylinder wear	Repair or replace
Too much stress	Incorrect setting of pressure reducing valve, relief valve or unloading valve	reset
	Variable mechanism not working	Repair or replace
	Blocked or damaged relief valve, overflow valve or unloading valve	Clean or replace

5.4 Elimination methods for abnormal system operation (see Table 12)

Table 12 Elimination methods for abnormal system operation

Failure phenomenon and cause		Elimination method
The system pressure is normal, the actuator has no action	The solenoid in the solenoid valve is faulty	Exclude or replace
	Limit or sequence device (mechanical, electric or hydraulic) does not work or is adjusted incorrectly	Adjust, repair or replace
	Mechanical failure	exclude
	No command signal	Find, repair
	The amplifier does not work or is adjusted incorrectly	Adjust, repair or replace
	Valve does not work	Adjust, repair or replace
	Damaged cylinder or motor	Repair or replace
The actuator moves too slowly	The pump output flow is insufficient or the system leaks too much	Check, repair or replace
	Oil viscosity is too high or too low	Check, adjust or replace
	The control pressure of the valve is not enough or the orifice in the valve is blocked	Cleaning and adjustment
	External load is too large	Check and adjust
	The amplifier is malfunctioning or not adjusted correctly	Adjustment repair or replacement
	Spool jam	Clean, filter or change oil
	Cylinder or motor is severely worn	Repair or replace
Irregular movement	Abnormal pressure	See section 5.3 Elimination
	Air mixed with oil	Refuel, exhaust
	Unstable command signal	Find, repair
	The amplifier is malfunctioning or not adjusted correctly	Adjust, repair or replace
	Sensor feedback failure	Repair or replace
	Spool jam	Cleaning, filtering oil
	Worn or damaged cylinder or motor	Repair or replace

5.5 Elimination method of system hydraulic shock (see Table 13)

Table 13 Elimination method of system hydraulic shock

Phenomenon and cause		Elimination method
Shock when commutation	Turn off and on instantaneously when changing direction, causing hydraulic shock when kinetic energy or potential energy is converted	1. Extend the commutation time 2. Design a cushioned spool 3. Enlarge the pipe diameter and shorten the pipeline
The hydraulic cylinder is suddenly shocked by the hydraulic pressure generated by the brake during movement	When the hydraulic cylinder moves, it has a lot of momentum and inertia, and it is suddenly braked, causing a large increase in pressure and hydraulic shock.	1. The hydraulic cylinder inlet and outlet ports are set separately, a small safety valve with fast response and high sensitivity 2. Try to reduce the working pressure of the system when satisfying the driving force, or increase the system back pressure appropriately 3. Install a bladder accumulator near the hydraulic cylinder
The hydraulic shock generated when the hydraulic cylinder reaches the end	The momentum and inertia generated during the movement of the hydraulic cylinder collide with the cylinder body, causing the impact	1. Set up buffer devices at both ends of the hydraulic cylinder 2. Small relief valves with quick response and high sensitivity are installed at the inlet and outlet ports of the hydraulic cylinder respectively 3. Set the stroke (switch) valve

5.6 Elimination method of system oil temperature is too high (see Table 14)

Table 14 Elimination method of system oil temperature is too high

Failure phenomenon and cause	Elimination method
1. Set pressure is too high	Adjust the pressure appropriately
2. The components of the unloading circuit such as overflow valve, unloading valve, pressure relay, etc. work poorly	Correct the abnormal working condition of each component
3. The component setting value of the unloading circuit is inappropriate, and the pressure relief time is short	Re-adjust, extend pressure relief time
4. The valve has large leakage loss and short unloading time	Repair large leakage valves, consider not using large-size valves
5. Do not overflow by the overflow valve at high pressure and small flow, low pressure and large flow	Change circuit, use unloading valve, variable pump
6. Due to low viscosity or pump failure, the internal leakage of the pump is increased and the temperature of the pump casing rises	Oil change, repair, replace hydraulic pump
7. Insufficient oil in the tank	Refuel, increase the fuel tank
8. Unreasonable fuel tank structure	Improve the structure to make the temperature rise evenly around the fuel tank
9. Insufficient or faulty accumulator capacity	Replace the large accumulator, repair the accumulator
10. A cooler needs to be installed, the capacity of the cooler is insufficient, the cooler is faulty, the water inlet valve is not working properly, the water volume is insufficient, and the oil temperature automatic adjustment device is faulty	Install the cooler, enlarge the cooler, repair the malfunction of the cooler, repair the valve, increase the water volume, repair the temperature control device
11. Excessive throttling of the remote control port of the relief valve, high residual pressure of unloading	Make appropriate adjustments
12. The resistance of the pipeline is large	Use proper pipe diameter
13. Affected by nearby heat sources, large radiant heat	Adopt heat-insulating material reflector or change the layout; install ventilation and cooling devices, etc., choose suitable working oil

6 common faults and treatment of hydraulic parts

6.1 Common faults and treatments of hydraulic pumps (table 15)

Table 15 Common faults and treatment of hydraulic pump

Failure phenomenon	Cause Analysis		Elimination method
(1) The pump does not deliver oil	1. The pump does not turn	(1) The motor shaft is not rotating 1) No power supply 2) Electrical circuit and component failure	Check electrical and troubleshoot
		(2) Motor thermal trip 1) The pressure of the overflow valve is too high, and the pump will be stuffy after overload 2) The valve core of the relief valve is stuck and the center oil hole of the valve core is blocked or the damping hole of the relief valve is blocked, causing overpressure and no overflow 3) The pump outlet check valve is installed reversely or the valve core is stuck and the pump is stuffy 4) Motor failure	1) Adjust the pressure value of the overflow valve 2) The maintenance valve is stuffy 3) Check the check valve 4) Repair or replace the motor
		(3) There is no connecting key on the pump shaft or motor shaft 1) Broken 2) Missing installation	1) Change the key 2) Repair key
		(4) The sliding pair inside the pump is stuck 1) The fit clearance is too small 2) The accuracy of parts is poor, the assembly quality is poor, the deviation of gear and shaft coaxiality is too large; the plunger head is stuck; the blade verticality is poor; the rotor swing is too large, the rotor groove is wound or the blade is broken after force And stuck 3) The oil is too dirty 4) Too high oil temperature causes thermal deformation of parts 5) The suction cavity of the pump gets into the dirt and gets stuck	1) Disassemble for overhaul, choose clearance as required 2) Replace parts and re-assemble to make the fit clearance meet the requirements 3) Check the oil quality, filter or replace the oil 4) Check the cooling effect of the cooler, check the amount of oil in the oil tank and add oil to the oil level line 5) Disassemble for cleaning and install the suction filter on the suction port
	2. The pump reverses	The electric motor turns incorrectly 1) Wrong electrical wiring 2) Wrong direction arrow on the pump body	1) Correct the electrical wiring 2) Correct the upward rotation arrow on the pump body
	3. The pump shaft can still rotate	Internal fracture of pump shaft 1) Poor shaft quality 2) The sliding pair in the pump is stuck	1) Check the reason and replace with a new shaft 2) See (1) 1 (4) for the treatment in this table
	4. The pump does not suck oil	(1) The oil level in the fuel tank is too low (2) The suction filter is blocked (3) The valve on the pump suction pipe is not open (4) The pump or suction pipe is not tightly sealed (5) The pump suction height exceeds the standard and the suction pipe is slender and has too many elbows (6) The filtration precision of the oil suction filter is too high, or the oil passage area is too small (7) The viscosity of the oil is too high (8) The vane of the vane pump is not extended or stuck (9) The variable mechanism of the vane pump does not work well, making the eccentricity zero (10) The variable mechanism of the plunger pump fails, such as poor machining accuracy, poor assembly, too small matching clearance, too much internal friction resistance of the pump, and the servo piston, variable piston and spring mandrel are stuck, leading to individual oil passages of the variable mechanism Blockage, dirty oil, high oil temperature, thermal deformation of parts, etc. (11) There is no seal between the cylinder block of the plunger pump and the oil distribution plate (such as the center spring of the plunger pump is broken) (12) There is no seal between the oil distribution plate of the vane pump and the pump body	(1) Refuel to the oil level line (2) Clean or replace the filter element (3) Check and open the valve (4) Check and tighten the joints, tighten the pump cover screws, apply grease to the joints of the pump cover and the joints, or fill the oil suction port of the pump first (5) Lower the oil suction height, replace the pipe and reduce the elbow (6) Select the appropriate filtering accuracy and increase the oil filter specification (7) Check the viscosity of the oil, change the appropriate oil, and check the effect of the heater in winter (8) Disassemble and clean, reasonably select gaps, check oil quality, filter or replace oil (9) Replace or adjust variable mechanism (10) Disassemble and inspect, repair or replace parts, and reasonably select the gap; filter or replace the oil; check the effect of the cooler; check the oil level in the oil tank and add it to the oil level line (11) Replace the spring (12) Disassemble, clean and reassemble
(2) The pump is noisy	1. Severe suction phenomenon	(1) The oil suction filter is partially blocked and the oil suction resistance is large (2) The suction pipe is closer to the oil surface (3) The suction position is too high or the oil tank level is too low (4) The pump and suction pipe are not tightly sealed (5) The viscosity of the oil is too high (6) The speed of the pump is too high (improper use) (7) The passage area of the suction filter is too small (8) The auxiliary pump of the non-self-priming pump has insufficient oil supply or malfunction (9) The air filter on the fuel tank is blocked (10) Failure of pump shaft oil seal	(1) Clean or replace the filter (2) Properly lengthen and adjust the length or position of the suction pipe (3) Reduce the installation height of the pump or increase the liquid level (4) Check the sealing of the joint and the joint surface, and tighten it (5) Check the oil quality and select the oil viscosity as required (6) Control below the maximum speed (7) Replace the filter with a large oil passage area (8) Repair or replace auxiliary pump (9) Clean or replace the air filter (10) Replace
	2. Inhale bubbles	(1) A certain amount of air is dissolved in the oil, and bubbles are generated during the working process (2) The oil return vortex strongly generates foam (3) There is air in the pipeline or in the pump casing (4) The depth of the suction pipe immersed in the oil surface is not enough	(1) Add a baffle in the fuel tank, pass the return oil through the baffle to defoam and then suck it in, add a defoamer to the oil (2) The suction pipe and the return pipe should be separated by a certain distance, and the return pipe port should be inserted below the oil level (3) Carry out no-load operation to remove air (4) Lengthen the oil suction pipe and fill the oil tank to raise the liquid level
	3. Poor operation of hydraulic pump	(1) The bearings in the pump are severely worn or damaged (2) The internal parts of the pump are damaged or worn 1) The inner surface of the stator ring is severely worn 2) Low gear accuracy and large swing difference	(1) Disassemble, clean and replace 1) Replace the stator ring 2) Research and distribution repair or replacement
	4. Pump structural factors	(1) Severe trapped oil produces larger flow pulsation and pressure pulsation 1) Poor design of unloading groove 2) Poor processing accuracy (2) The variable mechanism of the variable pump works poorly (the clearance is too small, the machining accuracy is poor, the oil is too dirty, etc.) (3) The pressure distribution valve of the two-stage vane pump is not working properly. (The clearance is too small, the machining accuracy is poor, the oil is too dirty, etc.)	1) Improve design and increase unloading capacity 2) Improve machining accuracy (2) Disassemble, clean, repair, reassemble to meet performance requirements, filter or replace oil (3) Disassemble, clean, repair, reassemble to meet performance requirements, filter or replace oil
	5. Poor pump installation	(1) Poor coaxiality between pump shaft and motor shaft (2) Poor coupling installation, poor coaxiality and looseness	(1) Reinstallation meets the technical requirements, and the coaxiality should generally be within 0.1mm (2) Re-install to meet the technical requirements, and tighten the coupling with jack screws
(3) Insufficient pump oil	1. Low volumetric efficiency	(1) The sliding parts inside the pump are severely worn 1) The end face of the oil distribution plate of the vane pump is severely worn 2) The gear end face and the measuring plate are severely worn 3) The gear pump has serious wear and tear due to bearing damage 4) The plunger of the plunger pump and the cylinder bore are severely worn 5) The oil distribution plate of the plunger pump and the end face of the cylinder body are seriously worn	(1) Disassemble for cleaning, repair and replacement 1) Grind the end face of the oil distribution pan 2) Grinding repairmanship or replacement 3) Replace bearing and repair 4) Replace the plunger and grind it to the required clearance, clean it and reassemble it 5) Grind both ends to meet the requirements, clean and reassemble
		(2) Poor assembly of the pump 1) The gap between stator and rotor, plunger and cylinder, gear and pump body, gear and side plate is too large 2) The screw tightening torque on the cover of vane pump and gear pump is uneven or loose 3) Reverse installation of blades and rotors	1) Re-assembly, matching clearance according to technical requirements 2) Re-tighten the screws and achieve uniform force 3) Correct the direction and reassemble
		(3) The viscosity of the oil is too low (such as using the wrong oil or the oil temperature is too high)	(3) Replace the oil, check the reason for the high oil temperature, and propose cooling measures
	2. The pump has suction	See (2) 1 and 2 of this table.	See (2) 1 and 2 of this table.
	3. The internal mechanism of the pump is not working well	See (2) 4 of this table.	See (2) 4 of this table.
	4. Insufficient fuel supply	The auxiliary pump of the non-self-priming pump has insufficient oil supply or malfunction	Repair or replace auxiliary pump
(4) Insufficient pressure or low pressure rise	1. Serious oil spill	See (3) 1 of this table.	See (3) 1 of this table.
	2. The driving mechanism power is too small	(1) Motor output power is too small 1) Unreasonable design 2) The motor is faulty (2) The output power of the mechanical drive mechanism is too small	1) Calculate the motor power and replace it if it is insufficient 2) Check the motor and troubleshoot (2) Calculate the driving power and replace the driving mechanism
	3. The pump displacement is selected too large or the pressure is adjusted too high	Causes insufficient power of the drive mechanism or motor	Recalculate matching pressure, flow and power to make them reasonable
(5) Unstable pressure and unstable flow	1. The pump has suction	See (2) 1 and 2 of this table.	See (2) 1 and 2 of this table.
	2. The oil is too dirty	Individual blades are difficult to jam or extend in the rotor slot	Filter or change oil
	3. Poor assembly of the pump	(1) The clearance of individual blades in the rotor slot is too large, causing high pressure oil to flow to the low pressure cavity (2) The clearance of individual blades in the rotor slot is too small, which makes it difficult to jam or extend (3) The clearance between the individual plunger and the cylinder bore is too large, causing a large amount of oil leakage	(1) Disassemble and clean, repair or replace the blades, and reasonably select the clearance (2) Modification to make the blade movement flexible (3) Make the gap meet the requirements after repairing
	4. Pump structural factors	See (2) 4 of this table.	See (2) 4 of this table.
	5. Fluctuations in fuel supply	The auxiliary pump of the non-self-priming pump is faulty	Repair or replace auxiliary pump
(6) Abnormal fever	1. Poor assembly	(1) Improper clearance selection (for example, the matching clearance between the plunger and the cylinder, the blade and the rotor slot, the stator and the rotor, the gear and the measuring plate is too small, causing the sliding parts to overheat and burn) (2) The assembly quality is poor, the coaxiality of the transmission part does not meet the technical requirements, and there is another phenomenon in operation (3) The bearing quality is poor, or it is broken during assembly, or it is not cleaned during installation, resulting in uneasy operation (4) The lubricating oil drain through the bearing is not unblocked 1) The screw plug of the oil return port is not opened (the pipe is not connected) 2) The oil passage was not cleaned during installation, and dirt was blocked 3) There are too many elbows or flattening of the return pipe during installation	(1) Disassemble and clean, measure the gap, and re-grind to reach the specified gap (2) Disassemble, clean and reassemble to meet technical requirements (3) Disassemble for inspection, replace bearings, and reassemble 1) Install the return pipe 2) Cleaning the pipeline 3) Replace the tube and reduce the tube head
	2. Poor oil quality	(1) The viscosity-temperature characteristics of the oil are poor, and the viscosity changes greatly (2) The oil contains a lot of water and causes poor lubrication (3) Serious oil pollution	(1) Select hydraulic oil according to regulations (2) Replace qualified oil to clean the inside of the oil tank (3) Replace the oil
	3. Pipeline failure	(1) The drain pipe is flattened or blocked (2) The diameter of the oil drain pipe is too small to meet the oil drain requirements (3) The oil suction pipe has a small diameter and a large oil suction resistance	(1) Clean and replace (2) Change the design and replace the pipe (3) Increase the pipe diameter, reduce elbows, and reduce oil absorption resistance
	4. Affected by external conditions	High external heat source and poor heat dissipation conditions	Remove external influences and add heat insulation measures
	5. Large internal leakage, low volumetric efficiency and heat generation	See (3) 1 of this table.	See (3) 1 of this table.
(7) Oil leakage of shaft seal	1. Poor installation	(1) The seal lip is installed reversely (2) The frame spring falls off 1) The chamfer of the shaft is improper, the sealing lip is turned over and the spring falls off 2) Carelessly when installing the shaft, the spring falls off (3) Foreign matter stuck to the sealing lip (4) The sealing lip is strained when passing through the spline shaft (5) The oil seal is inclined 1) The inner diameter of the groove is too small 2) The chamfer of the groove is too small (6) The oil seal is seriously deformed during assembly (7) The sealing lip is rolled up 1) Shaft chamfer is too small 2) Shaft chamfer is too rough	(1) Disassemble and reinstall, do not damage the lip during disassembly, and replace if deformed or damaged. 1) Reprocess according to the requirements of the processing drawings 2) Reinstall (3) Remove, clean and reassemble (4) Reinstall after replacement 1) Check the size of the groove and reprocess it as required 2) Reprocessing according to regulations (6) Check the groove size and chamfer (7) Check the chamfer size and roughness of the shaft, use emery cloth to polish the chamfer, and apply grease to the chamfer of the shaft during assembly
	2. Poor machining of shaft and groove	(1) Shaft processing error 1) The shaft journal is not suitable, causing the oil seal lip to wear and heat 2) The shaft chamfer is not up to the requirements, which causes the oil seal lip to be strained and the spring falls off 3) There are turning or grinding marks on the surface of the journal 4) The rough surface of the journal makes the oil seal lip wear faster (2) Groove processing error 1) The groove size is too small to make the oil seal oblique 2) The groove size is too large and the oil leaks from the outer circumference 3) There are scratches or other defects on the groove surface, and oil leaks from the outer periphery	1) Check the size and change the shaft. The tolerance of the oil seal is usually h8 2) Re-machine the chamfer of the shaft 3) Re-grind to eliminate grinding marks 4) Reprocessing to meet the drawing requirements (2) Replace the pump cover and repair the groove to meet the matching requirements
	3. The oil seal itself is defective	The oil seal is not of good quality, is not resistant to oil, or has poor compatibility with hydraulic oil, and causes oil leakage due to deterioration, aging, and failure	Replace the appropriate oil seal rubber parts
	4. Volumetric efficiency is too low	See (3) 1 of this table.	See (3) 1 of this table.
	5. The drain hole is blocked	After the oil drain hole is blocked, the oil drain pressure increases, causing the seal lip to deform too much, the contact surface increases, and the friction produces thermal aging, which makes the oil seal ineffective and causes oil leakage	Clean the oil hole and replace the oil seal
	6. The diameter of the external drain pipe is too thin or the pipe is too long	Difficulty in draining oil, increased drain pressure	Properly increase the pipe diameter or shorten the length of the drain pipe
	7. The drain pipe is not connected	The drain pipe is not opened or the drain pipe is not connected	Open the screw plug and connect the drain pipe

6.2 Common faults and treatments of hydraulic motors (see Table 16)

Table 16 Common faults and treatments of hydraulic motors

Failure phenomenon	Cause Analysis		Elimination method
(1) Low speed and low torque	1. Insufficient oil supply of hydraulic pump	1) The motor speed is not enough 2) The filter screen of the suction filter is blocked 3) Insufficient fuel in the fuel tank or too small diameter of the suction pipe causes difficulty in suction 4) The seal is not tight, does not leak, and air invades inside 5) The viscosity of the oil is too large 6) The axial and radial clearance of the hydraulic pump is too large and the internal leakage increases	1) Find out the reason and make adjustments 2) Clean or replace the filter element 3) Add enough oil and increase the pipe diameter appropriately to make the oil suction smooth 4) Tighten the relevant joints to prevent leakage or air intrusion 5) Choose oil with low viscosity 6) Properly repair the hydraulic pump
	2. The output pressure of the hydraulic pump is insufficient	1) The efficiency of the hydraulic pump is too low 2) The pressure of the overflow valve is insufficient or malfunctions 3) Tubing resistance is too large (the pipeline is too long or too thin) 4) The viscosity of the oil is small, and the internal leakage is large	1) Check the failure of the hydraulic pump and eliminate it 2) Check the failure of the overflow valve, and re-adjust the pressure after troubleshooting 3) Replace the pipe with a larger diameter or minimize the length 4) Check the sealing condition of the internal leakage part, replace the oil or seal
	3. Leakage of hydraulic motor	1) The joint surface of the hydraulic motor is not tightened or the seal is not good, and there is leakage 2) The internal parts of the hydraulic motor are worn and leaking seriously	1) Tighten the joint surface to check the sealing condition or replace the sealing ring 2) Check the damaged parts and grind or replace parts
	4. Failure	The support spring of the oil distribution plate is fatigued and loses its function	Check and replace the support spring
(2) Leakage	1. Internal leakage	1) Serious wear of oil distribution plate 2) The axial clearance is too large 3) The end face of the oil distribution plate and the cylinder body is worn, and the axial clearance is too large 4) Spring fatigue 5) Serious wear of plunger and cylinder	1) Check and repair the contact surface of the oil distribution pan 2) Check and adjust the axial clearance to the specified range 3) Grind the cylinder body and the end face of the oil distribution plate 4) Replace the spring 5) Grind the cylinder bore and refit the plunger
(2) Leakage	2. External leakage	1) Oil end seal, wear 2) The sealing ring at the cover plate is damaged 3) There is dirt on the joint surface or the bolts are not tightened 4) The pipe joint is not tightly sealed	1) Replace the sealing ring and find out the cause of wear 2) Replace the sealing ring 3) Check, remove and tighten bolts 4) Tighten the pipe joint
(3) Noise		1) The seal is not tight and air invades inside 2) The hydraulic oil is contaminated and air bubbles are mixed in 3) The coupling is not concentric 4) Hydraulic oil viscosity is too large 5) The radial dimension of the hydraulic motor is severely worn 6) The blades are worn 7) Poor contact between the blade and the stator, there is a collision phenomenon 8) Stator wear	1) Check the seals of relevant parts and tighten all connections 2) Replace clean hydraulic oil 3) Correcting concentricity 4) Replace oil with lower viscosity 5) Grind the cylinder hole. Refit the plunger 6) Repair or replace as much as possible 7) Perform trimming 8) Repair or replace. If the spring is too hard to cause wear and tear, you should replace the spring with less rigidity

6.3 Common faults and treatments of hydraulic cylinders (see Table 7)

Table 7 Common faults and treatments of hydraulic cylinders

Failure phenomenon	Cause Analysis		Elimination method
(1) The piston rod cannot move	1. Insufficient pressure	(1) The oil does not enter the hydraulic cylinder 1) The reversing valve is not reversing 2) The system does not supply oil (2) Although there is oil, there is no pressure 1) The system is faulty, mainly pump or overflow valve fault 2) The internal leakage is serious, the piston and the piston rod are loosened, and the seal is seriously damaged (3) The pressure does not reach the specified value 1) The seal is aging or invalid, the lip of the sealing ring is installed reversely or is damaged 2) The piston ring is damaged 3) The set pressure of the system is too low 4) The pressure regulating valve is faulty 5) When the flow through the adjusting valve is too small and the leakage in the hydraulic cylinder increases, the flow is insufficient, resulting in insufficient pressure	1) Check the reason why the reversing valve is not reversing and eliminate it 2) Check the causes of failures of hydraulic pumps and main hydraulic valves and eliminate them 1) Check the cause of the pump or overflow valve failure and eliminate it 2) Tighten the piston and piston rod and replace the seal 1) Replace the seal and install it correctly 2) Replace the piston rod 3) Readjust the pressure until it reaches the required value 4) Check the cause and eliminate 5) The flow rate of the regulating valve must be greater than the leakage in the hydraulic cylinder
(1) The piston rod cannot move	2. The pressure has reached the requirement but still does not move	(1) Problems with hydraulic cylinder structure 1) The end face of the piston is closely attached to the end face of the cylinder, and the working area is insufficient, so it cannot be started 2) The one-way valve circuit on the cylinder with buffer device is blocked by the piston (2) The piston rod moves “don’t be strong” 1) The clearance between cylinder barrel and piston, guide sleeve and piston rod is too small 2) The matching clearance between the piston rod and the cloth bakelite guide sleeve is too small 3) Poor assembly of the hydraulic cylinder (such as poor coaxiality between the piston rod, piston and cylinder head, poor parallelism between the hydraulic cylinder and the worktable) (3) The cause of the hydraulic circuit is mainly that the oil in the back pressure chamber of the hydraulic cylinder is not in communication with the oil tank, the throttle of the speed control valve on the oil return line is adjusted too small or the reversing valve connected to the return oil does not act.	1) An oil groove should be added to the end face to make the working fluid flow into the working end face of the piston quickly 2) The oil inlet and outlet positions of the cylinder barrel should be staggered with the end face of the piston 1) Check the fit clearance and match it to the specified value 2) Check the fit clearance and repair the guide sleeve hole to meet the required fit clearance 3) Re-assembly and installation, unqualified parts should be replaced Check the cause and eliminate
(2) The speed is not up to the specified value	1. Serious internal leakage	(1) The seal is severely damaged (2) The viscosity of the oil is too low (3) The oil temperature is too high	(1) Replace the seal (2) Replace hydraulic oil with suitable viscosity (3) Check the cause and eliminate
	2. Excessive external load	(1) Design error, selection pressure is too low (2) Process and use errors caused the external load to be larger than the predetermined value	(1) Replace components after accounting and increase working pressure (2) Use according to the specified value of the equipment
	3. “Don’t be jealous” when the piston moves	(1) The accuracy is poor, and the taper and roundness of the cylinder bore are out of tolerance (2) Poor assembly quality 1) Coaxiality difference between piston, piston rod and cylinder head 2) Poor parallelism between hydraulic cylinder and worktable 3) The clearance between the piston rod and the guide sleeve is too small	Check the size of the parts and replace the parts that cannot be repaired 1) Reassemble as required 2) Reassemble as required 3) Check the fit clearance and repair the guide sleeve hole to meet the required fit clearance
	4. Dirt enters the sliding part	(1) The oil is too dirty (2) Damaged dust ring (3) Not cleaned or brought in dirt during assembly	(1) Filter or replace oil (2) Replace the dust ring (3) Disassemble and clean, pay attention to cleaning when assembling
	5. The speed of the piston drops sharply during the end stroke	(1) The orifice of the buffer control valve is adjusted too small. When entering the buffer stroke, the piston may stop or the speed drops sharply (2) The diameter of the orifice in the fixed buffer device is too small (3) The gap between the fixed buffer throttle ring on the cylinder head and the buffer plunger is too small	(1) The opening degree of the buffer throttle valve should be adjusted appropriately, and can play a buffer role (2) Increase the diameter of the orifice appropriately (3) Increase the gap appropriately
	6. The piston moves halfway and finds that the speed slows down or stops	(1) The inner diameter of the cylinder has poor machining accuracy and rough surface, which increases the internal leakage (2) The cylinder wall expands, when the piston passes through the enlarged part, the internal leakage increases	(1) Repair or replace the cylinder (2) Replace the cylinder
(3) Crawling of hydraulic cylinder	1. The movement of the piston rod of the hydraulic cylinder is “not strong”	See (2) 3 of this table.	See (2) 3 of this table.
(3) Crawling of hydraulic cylinder	2. Air enters the cylinder	(1) New hydraulic cylinders, repaired hydraulic cylinders or cylinders whose equipment has been stopped for a long time, there is air in the cylinder or the exhaust gas in the hydraulic cylinder pipeline is not exhausted (2) Negative pressure is formed inside the cylinder, sucking in air from outside (3) The volume of the pipeline from the cylinder to the reversing valve is much larger than the internal volume of the hydraulic cylinder. When the hydraulic cylinder is working, the oil in this section of the pipeline has not been drained, so it is difficult to drain the air. (4) The pump sucks air (see hydraulic pump failure) (5) Air is mixed into the oil (see hydraulic pump failure)	(1) No-load large-stroke reciprocating movement until the air is exhausted (2) Seal the joint surface and joint with grease first, if the suction condition improves, tighten the fastening screw and joint (3) An exhaust valve can be added at the height of the pipeline near the hydraulic cylinder. Unscrew the exhaust valve, the piston moves several times in the full stroke, and then close the exhaust valve after exhausting the air See troubleshooting measures for hydraulic pump failures (5) Refer to the elimination of hydraulic pump failure
(4) Buffer device failure	1. Excessive cushioning	(1) The throttle opening of the buffer control valve is too small (2) Cushioning plunger “don’t be tough” (such as the gap between the plunger head and the buffer ring is too small, the piston is inclined or eccentric) (3) There is dirt between the plunger head and the buffer ring (4) The gap between the plunger head of the fixed buffer device and the bush is too small	(1) Adjust the orifice to a suitable position and tighten it (2) Disassemble and clean properly to increase the gap, and the unqualified parts should be replaced (3) Repair, deburr and clean (4) Increase the gap appropriately
	2. Cushioning failure	(1) The buffer control valve is fully open (2) Inertial energy is too large (3) The buffer control valve cannot be adjusted (4) The check valve is fully open or the seat of the check valve is not tightly closed (5) The seal on the piston is damaged. When the pressure in the buffer chamber increases, the working fluid flows back from this chamber to the working pressure side, so the piston does not decelerate (6) There are scars on the inner surface of the plunger head or bushing (7) The buffer ring inlaid on the cylinder head falls off (8) The length and angle of the taper of the buffer plunger are inappropriate	(1) Adjust to a suitable position and tighten (2) Appropriate buffer mechanism should be designed (3) Repair or replace (4) Check the size, replace the cone valve core or steel ball, replace the spring, and repair with research (5) Replace the seal (6) Repair or replace (7) Replace with a new buffer ring (8) Amendment
	3. “Crawl” appears in the buffer stroke	(1) Poor processing, such as the cylinder head, the verticality of the piston end face is not up to the requirements, the gap between the piston and the cylinder barrel is uneven over the entire length, and the cylinder head and the cylinder barrel are not concentric: the cylinder bore has a large deviation from the center line of the cylinder head, and the piston The verticality of the end face of the nut does not meet the requirements, causing the deflection of the piston rod. (2) Poor assembly, such as the eccentric or inclined hole of the buffer plunger and the buffer ring	(1) Carefully check every part, and unqualified parts are not allowed to be used (2) Re-assembly to ensure quality
(5) There is external leakage	1. Poor assembly	(1) When assembling the hydraulic cylinder, the end cover is biased, and the piston rod is not concentric with the cylinder tube, which makes it difficult to extend the piston rod and accelerates the wear of the seal (2) The poor parallelism between the hydraulic cylinder and the guide rail surface of the worktable makes it difficult for the piston to extend and accelerates the wear of the seal (3) Seal installation errors, such as the seal is scratched or cut, the sealing lip is installed reversely, the lip is broken or the shaft chamfer size is incorrect, the seal is installed incorrectly or is missing (4) The sealing gland is not installed 1) The gland installation has deviation 2) Uneven force of fastening screws 3) The fastening screw is too long, so that the gland cannot be compressed	(1) Disassemble, inspect and reassemble (2) Disassemble for inspection, reinstall, and replace the seal (3) Replace and reinstall the seal 1) Reinstall 2) Re-install and tighten the screws to make the force evenly 3) Choose the screw length reasonably according to the depth of the screw hole
	2. The quality of the seal	(1) The storage period is too long, and the seals will naturally age and fail (2) Poor storage, deformation or damage (3) Poor rubber performance, not oil-resistant or poor compatibility between rubber and oil (4) The product quality is poor, the size is wrong, and the tolerance does not meet the requirements	replace
	3. Poor machining quality of piston rod and groove	(1) The surface of the piston rod is rough, and the chamfer on the head of the piston rod does not meet the requirements or is not chamfered (2) The groove size and accuracy do not meet the requirements 1) There are errors in the design drawings 2) The groove size processing does not meet the standard 3) Poor groove accuracy and many burrs	(1) The surface roughness should be Ra0.2μm, and chamfered as required (2) 1) Design grooves according to relevant standards 2) Check the size and correct it to the required size 3) Correction and deburring
	4. The viscosity of the oil is too low	(1) Use the wrong oil (2) There are other brands of oil in the oil	Change the appropriate oil
	5. The oil temperature is too high	(1) The oil inlet resistance of the hydraulic cylinder is too large (2) The ambient temperature is too high (3) The pump or cooler is faulty	(1) Check whether the oil inlet is unblocked (2) Take heat insulation measures (3) Check the cause and eliminate
	6. High frequency vibration	(1) The fastening screws are loose (2) The pipe joint is loose (3) The installation position has moved	(1) The screws should be tightened regularly (2) The joints should be tightened regularly (3) The mounting screws should be tightened regularly
	7. Piston rod strain	(1) Aging and failure of the dust-proof ring to invade sand and chips and other dirt (2) The fit between the guide sleeve and the piston rod is too tight, causing overheating of the movable surface, causing the chromium layer on the surface of the piston rod to fall off and strain	(1) Clean and replace the dust ring, repair the strain on the surface of the piston rod (2) Check and clean, use a scraper to repair the inner diameter of the guide sleeve to reach the matching clearance

6.4 Common faults and treatment of pressure valve

6.4.1 Common faults and treatments of relief valves (see Table 18)

Table 18 Common faults and treatment of overflow valve

Failure phenomenon	Cause Analysis		Elimination method
(1) Can’t adjust the pressure	1. Main valve failure	(1) The main spool orifice is blocked (the main spool is not cleaned during assembly, and the oil is too dirty) (2) The main spool is stuck in the open position (such as low accuracy of parts, poor assembly quality, and dirty oil) (3) The return spring of the main spool is broken or bent, so that the main spool cannot be reset	(1) Clean the orifice to make it unblocked; filter or replace the oil (2) Disassemble, repair, and reassemble; the tightening force of the bonnet fastening screws must be even; filter or replace the oil (3) Replace the spring
	2. Pilot valve failure	(1) The pressure regulating spring is broken (2) The pressure regulating spring is not installed (3) Cone valve or steel ball is not installed (4) The poppet valve is damaged	(1) Replace the spring (2) Reinstall (3) Reinstall (4) Replace
	3. The remote cavity port solenoid valve is faulty or the remote control port is directly connected to the fuel tank without plugging.	(1) The solenoid valve is not energized (normally open) (2) The slide valve is stuck (3) The solenoid coil is burnt or the iron core is stuck (4) Electrical circuit failure	(1) Check that the electrical circuit is connected to the power supply (2) Maintenance and replacement (3) Replace (4) Overhaul
	4. Wrong installation	Incorrect installation of oil inlet and outlet	correct
	5. Hydraulic pump failure	(1) The gap between sliding pairs is too large (such as gear pumps, plunger pumps) (2) Most of the blades of the vane pump are stuck in the rotor slot (3) The blade and rotor are installed in reverse	(1) Repair the gap to an appropriate value (2) Clean and repair the gap to an appropriate value (3) Correct the direction
(2) The pressure is not high	1. Main valve failure (if the main valve is a poppet valve)	(1) Poor sealing of the main spool cone 1) The main spool cone is worn or out of round 2) Worn or out-of-round valve seat cone 3) Dirt sticking to the cone surface 4) The conical surface of the main valve core is not concentric with the conical surface of the valve seat 5) The main valve core is stuck in work, and the valve core cannot be tightly combined with the valve seat (2) There is leakage at the main valve gland (such as damaged gasket, poor assembly, loose gland screws, etc.)	1) Replacement and research 2) Replacement and research 3) Clean and equip research 4) Repair to make it well combined 5) Repair to make it well combined (2) Disassemble and repair, replace the gasket, reassemble, and ensure the screw tightening force is even
(2) The pressure is not high	2. Pilot valve failure	(1) The pressure regulating spring is bent, or is too weak, or the length is too short (2) Poor sealing at the junction of the poppet valve and the valve seat (for example, the poppet valve and the valve seat are worn, the contact surface of the poppet valve is not round, the contact surface is too wide to enter the dirt or be stuck by glue)	(1) Replace the spring (2) Maintenance, replacement and cleaning to make it meet the requirements
(3) Sudden increase in pressure	1. Main valve failure	The main spool is not sensitive to work, and it is suddenly stuck in the closed state (such as low precision of parts processing, poor assembly quality, dirty oil, etc.)	Overhaul, replace parts, filter or replace oil
(3) Sudden increase in pressure	2. Pilot valve failure	(1) The connecting surface of the pilot valve spool and valve seat suddenly sticks and cannot be separated (2) The bending of the pressure regulating spring causes jamming	(1) Clean and repair or replace the oil (2) Replace the spring
(4) The pressure drops suddenly	1. Main valve failure	(1) The main spool orifice is suddenly blocked (2) The main spool is not sensitive to work and suddenly gets stuck in the closed state (such as low parts processing accuracy, poor assembly quality, dirty oil, etc.) (3) The gasket at the main valve cover is suddenly damaged	(1) Clean, filter or replace oil (2) Repair and replace parts, filter or replace oil (3) Replace the seal
	2. Pilot valve failure	(1) The pilot valve spool suddenly ruptured (2) The pressure regulating spring suddenly breaks	(1) Replace the spool (2) Replace the spring
	3. The remote cavity port solenoid valve failure	The electromagnet is suddenly cut off, unloading the relief valve	Check and eliminate electrical faults
(5) Pressure fluctuation (unstable)	1. Main valve failure	(1) The action of the main spool is not flexible, and sometimes it is stuck (2) The main spool orifice is sometimes blocked and sometimes open (3) Poor contact between the cone surface of the main valve core and the cone surface of the valve seat, and uneven wear (4) The damping aperture is too large, resulting in poor damping	(1) Overhaul and replace parts, the tightening force of the gland screws should be even (2) Disassemble and clean, check the oil quality, and replace the oil (3) Repair or replace parts (4) Reduce the damping aperture appropriately
(5) Pressure fluctuation (unstable)	2. Pilot valve failure	(1) The pressure regulating spring is bent (2) Poor contact between the poppet valve and the poppet valve seat and uneven wear (3) The pressure adjustment screw changes due to the loosening of the lock nut	(1) Replace the spring (2) Repair or replace parts (3) The lock nut should be tightened after pressure regulation
(6) Vibration and noise	1. Main valve failure	The radial force of the main spool is unbalanced during operation, resulting in unstable performance 1) The geometric accuracy of the valve body and the main valve core is poor, and the edges have burrs 2) There is dirt adhering to the valve body, which makes the matching gap increase or uneven	1) Check the accuracy of the parts, replace the parts that do not meet the requirements, and remove the edge burrs 2) Repair and replace parts
	2. Pilot valve failure	(1) Poor contact between the cone valve and the valve seat, poor roundness of the circumferential surface, and large roughness values, resulting in unbalanced force of the pressure regulating spring, which intensifies the cone valve oscillation and produces screams (2) The axis line of the pressure regulating spring is not perpendicular to the end face, so the needle valve will tilt, causing uneven contact (3) The pressure regulating spring is biased to one side on the positioning rod (4) Valve seat is biased during assembly (5) Lateral bending of pressure regulating spring	(1) Control the roundness error of the oil sealing surface within 0.005～0.01mm (2) Improve the accuracy of the poppet valve, and the roughness should reach Ra0.4μm (3) Replace the spring (4) Improve assembly quality (5) Replace the spring
	3. The system has air	The pump sucks in air or there is air in the system	Remove air
	4. Improper use of valve	Pass flow exceeds allowable value	Use within the rated flow range
	5. Poor oil return	The oil return line resistance is too high or the return oil filter is blocked or the return line is close to the bottom of the tank	Properly increase the pipe diameter and reduce the elbow. The return pipe port should be more than twice the pipe diameter from the bottom of the fuel tank. Replace the filter element.
	6. Improper selection of remote control port diameter	The pipe diameter between the remote control port of the relief valve and the solenoid valve should not be too large, which may cause vibration	Generally 6mm is more suitable

6.4.2 Common faults and treatment of pressure reducing valve (see Table 19)

Table 19 Common faults and treatment of pressure reducing valve

Failure phenomenon	Cause Analysis		Elimination method
(1) No secondary pressure	1. Main valve failure	The main valve core is stuck in the fully closed position (such as parts with low precision); the main valve spring is broken and bent; the orifice is blocked	Repair, replace parts and springs, filter or replace oil
(1) No secondary pressure	2. No oil source	No oil is supplied to the pressure reducing valve	Check the oil circuit to eliminate the fault
(2) Cannot reduce stress	1. Misuse	Drain port is blocked 1) The screw plug is not unscrewed 2) The drain pipe is slender, with many elbows and too much resistance 3) The drain pipe is connected to the main oil return pipe, and the oil return back pressure is too high 4) The oil drain channel is blocked or blocked	1) Unscrew the screw plug 2) Replace the pipe that meets the requirements 3) The oil drain pipe must be separated from the oil return pipe and flow back to the oil tank separately 4) Clean the drain channel
	2. Main valve failure	The main spool is stuck in the fully open position (such as low precision parts, dirty oil, etc.)	Repair, replace parts, check oil quality, and replace oil
	3. Poppet valve failure	The pressure regulating spring is too hard, bent and stuck	Replace the spring
(3) Unstable secondary pressure	Main valve failure	(1) The geometric accuracy of the main spool and the valve body is poor, and it is not sensitive when working (2) The main valve spring is too weak, deformed or jams the main valve core, making it difficult to move the valve core (3) When the damping hole is blocked, it is open	(1) Overhaul to make it flexible (2) Replace the spring (3) Clean the damping holes
(4) The secondary pressure is not high	1. External leakage	(1) Oil leakage on the joint surface of the top cover, the reasons are such as: the seal is aging and invalid, the screw is loose or the tightening torque is uneven (2) There is oil leakage at each plug	(1) Replace the seals, tighten the screws, and ensure even torque (2) Tighten and eliminate leakage
(4) The secondary pressure is not high	2. Poppet valve failure	(1) Poor contact between poppet valve and valve seat (2) The pressure regulating spring is too weak	(1) Repair or replace (2) Replace

6.4.3 Common faults and treatment of sequence valve (see Table 20)

Table 20 Common faults and treatment of sequence valve

Failure phenomenon	Cause Analysis	Elimination method
(1) The oil is always discharged and does not function as a sequence valve	(1) The valve core is stuck in the open position (such as poor geometric accuracy, too small gap; spring bent or broken; oil is too dirty) (2) The one-way valve is stuck in the open position (such as poor geometric accuracy, too small gap; spring bent or broken; oil is too dirty) (3) Poor sealing of one-way valve (such as poor geometric accuracy) (4) The pressure regulating spring is broken (5) Missing installation of pressure regulating spring (6) No cone valve or steel ball	(1) Repair, make the fit clearance meet the requirements, and make the valve core move flexibly; check the oil quality, if it does not meet the requirements, filter or replace; replace the spring (2) Repair, make the fit clearance meet the requirements, and make the one-way valve core move flexibly; check the oil quality, if it does not meet the requirements, filter or replace; replace the spring (3) Repair to make the one-way valve seal well (4) Replace the spring (5) Reinstall the spring (6) Reinstall
(2) No oil is discharged at all, no sequence valve function	(1) The valve core is stuck in the closed position (such as poor geometric accuracy; bent spring; dirty oil) (2) The control oil flow is not smooth (for example, the damping hole is blocked, or the remote control pipeline is flattened and blocked) (3) The remote control pressure is insufficient, or the joint of the lower end cover is leaking seriously (4) The orifice on the oil path leading to the pressure regulating valve is blocked (5) The back pressure in the drain pipeline is too high to prevent the slide valve from moving (6) The adjusting spring is too hard, or the pressure is adjusted too high	(1) Repair, make the slide valve move flexibly, replace the spring; filter or replace the oil (2) Clean or replace pipes, filter or replace oil (3) Increase the control pressure, tighten the end cover screws and make them evenly stressed (4) Cleaning (5) The drain pipeline cannot be connected to the return pipeline, it should be connected back to the fuel tank separately (6) Replace the spring and adjust the pressure appropriately
(3) The set pressure value does not meet the requirements	(1) Improper adjustment of pressure regulating spring (2) The pressure regulating spring deforms laterally, and the maximum pressure cannot be adjusted up (3) The slide valve is stuck and difficult to move	(1) The pressure required for readjustment (2) Replace the spring (3) Check the fit clearance of the spool valve and repair it to make the spool valve move flexibly; filter or replace the oil
(4) Vibration and noise	(1) Oil return resistance (back pressure) is too high (2) The oil temperature is too high	(1) Reduce oil return resistance (2) Control the oil temperature within the specified range
(5) The one-way sequence valve cannot return oil in the reverse direction	The one-way valve is stuck and cannot be opened	Check the check valve

6.5 Common faults and treatment of flow valve (see Table 21)

Table 21 Common faults and treatment of flow valve

Failure phenomenon	Cause Analysis		Elimination method
(1) Adjust the throttle valve handle without flow change	1. The pressure compensation valve does not operate	The pressure compensation spool is stuck in the closed position 1) Poor geometric accuracy of valve core and valve sleeve, and the gap is too small 2) The spring is laterally bent and deformed and the valve core is stuck 3) The spring is too weak	1) Check the accuracy, the repair gap meets the requirements, and the movement is flexible 2) Replace the spring 3) Replace the spring
	2. Throttle valve failure	(1) The oil is too dirty, blocking the orifice (2) The assembly position of the handle and the throttle valve core is inappropriate (3) The connection of the throttle valve spool is missing or no key is installed (4) The throttle valve core is stuck due to too small fitting clearance or deformation (5) The thread of the adjusting rod is blocked by dirt, causing poor adjustment	(1) Check oil quality and filter oil (2) Check the reason and reassemble (3) Replacement key or reinstall key (4) Cleaning, repairing gaps or replacing parts (5) Disassemble and clean
	3. The system does not supply oil	Reversing valve spool not reversing	Check the cause and eliminate
(2) The moving speed of the actuator is unstable (the flow is unstable)	1. Pressure compensation valve failure	(1) The pressure compensation valve core is not sensitive 1) The valve core is stuck 2) When the damping hole of the compensation valve is blocked, it will open 3) The spring is laterally bent or deformed, or the end face of the spring is not perpendicular to the spring axis (2) The spool of the pressure compensation valve is stuck in the fully open position 1) The damping orifice of the compensation valve is blocked 2) Poor geometric accuracy of valve core and valve sleeve, and the matching clearance is too small 3) The spring is laterally bent and deformed and the valve core is stuck	1) Repair and fit to achieve mobile flexibility 2) Clean the orifice, if the oil is too dirty, replace it 3) Replace the spring 1) Clean the orifice, if the oil is too dirty, replace it 2) Repair to achieve mobile flexibility 3) Replace the spring
	2. Throttle valve failure	(1) Dirt accumulates at the orifice, causing it to be blocked and open (2) The change of external load of simple throttle valve will cause flow change	(1) Disassemble and clean, check the oil quality, if the oil quality is unqualified, replace it (2) For systems with large changes in external load or requiring very stable movement speeds of actuators, speed control valves should be used instead
	3. Deterioration of oil quality	(1) The oil temperature is too high, causing changes in the flow through the orifice (2) The compensation rod of the flow control valve with temperature compensation has poor sensitivity and is damaged (3) The oil is too dirty, blocking the orifice or orifice	(1) Check the cause of temperature rise, reduce oil temperature, and control it within the required range (2) Use temperature-sensitive materials as compensation rods, and replace the damaged ones (3) Clean, check the oil quality, replace the unqualified ones
	4. Check valve failure	In the flow control valve with check valve, the sealing performance of check valve is not good	Grind check valve to improve sealing
	5. Pipeline vibration	(1) There is air in the system (2) The set position changes due to pipeline vibration	(1) The air should be exhausted (2) Lock with a locking device after adjustment
	6. Leak	Internal leakage and external leakage make the flow unstable and cause uneven working speed of the actuator	Eliminate leakage, or replace components

6.6 Common faults and treatment of directional valve

6.6.1 Common faults and treatments of electric (hydraulic, magnetic) directional valve (see Table 22)

Table 22 Common faults and treatment of electric (hydraulic, magnetic) directional valve

Failure phenomenon	Cause Analysis		Elimination method
(1) The main spool does not move	1. Electromagnet failure	(1) The solenoid coil is burned out (2) Insufficient driving force of electromagnet or magnetic leakage (3) The electrical circuit fails (4) No control signal is applied to the electromagnet (5) The electromagnet core is stuck	(1) Check the reason, repair or replace (2) Check the reason, repair or replace (3) Eliminate the fault (4) Add control signal after inspection (5) Check or replace
	2. First conduction solenoid valve failure	(1) The valve core and the valve body hole are stuck (such as poor geometric accuracy of the parts; the valve core and the valve hole are too tightly matched; the oil is too dirty) (2) The spring is bent sideways, causing the spool valve to jam	(1) Repair the fit clearance to meet the requirements, make the valve core move flexibly; filter or replace the oil (2) Replace the spring
	3. The main spool is stuck	(1) Poor geometric accuracy of valve core and valve body (2) The valve core and valve hole fit too tightly (3) There are burrs on the valve core surface	(1) Repair and research clearance meets the requirements (2) Repair and research clearance meets the requirements (3) Deburr and rinse
	4. Hydraulic control oil circuit failure	(1) No oil in control oil circuit 1) Control oil circuit solenoid valve is not reversing 2) The control oil circuit is blocked (2) Insufficient control oil pressure 1) Oil leakage at the valve end cover 2) The throttle valve on the side of the oil discharge chamber of the slide valve is adjusted too small or blocked	(1) 1) Check the cause and eliminate 2) Check and clean, and make the control oil circuit unblocked (2) 1) Tighten the end cover screws 2) Clean the throttle valve and adjust appropriately
	5. The oil has deteriorated or the oil temperature is too high	(1) The oil is too dirty and the valve core is stuck (2) The oil temperature is too high, which causes thermal deformation of the parts and causes the phenomenon of seizure (3) The oil temperature is too high, and gelatin is produced in the oil, which sticks to the valve core and becomes stuck (4) The oil viscosity is too high, making the valve core difficult to move and jam	(1) Filter or replace (2) Check the cause of excessive oil temperature and eliminate it (3) Clean and eliminate excessive oil temperature (4) Replace with suitable oil
	6. Poor installation	Valve body deformation 1) Uneven tightening torque of mounting screws 2) The pipes connected to the valve body are “don’t be tough”	1) Re-tighten the screws and make them evenly stressed 2) Reinstall
	7. The return spring does not meet the requirements	(1) The spring force is too large (2) The spring is deformed sideways, causing the valve core to jam (3) The spring cannot be reset	Replace with suitable spring
(2) Insufficient flow through the spool after reversing	Insufficient valve opening	(1) The push rod in the solenoid valve is too short (2) The geometric accuracy of the valve core and the valve body is poor, the gap is too small, and there is jamming when moving, so it is not in place (3) The spring is too weak and the thrust is insufficient, so that the spool stroke is not in place	(1) Replace the putter of suitable length (2) The matching research meets the requirements (3) Replace the appropriate spring
(3) The pressure drop is too large	Improper selection of valve parameters	The actual passing flow is greater than the rated flow	Should be used within the rated range
(4) The spool reversing speed of the hydraulic control reversing valve is difficult to adjust	Adjustable device failure	(1) Poor sealing of check valve (2) The processing accuracy of the throttle valve is poor and the minimum flow cannot be adjusted (3) Oil leakage at the valve cover of the oil discharge chamber (4) Poor adjustment performance of needle throttle	(1) Repair or replace (2) Repair or replace (3) Replace the seals and tighten the screws (4) Change to triangular groove throttle valve
(5) The electromagnet is overheated or the coil is burned out	1. Electromagnet failure	(1) Poor coil insulation (2) The iron core of the electromagnet is not suitable and cannot be attracted (3) The voltage is too low or unstable	(1) Replace (2) Replace (3) The voltage change value should be within 10% of the rated voltage
	2. Load changes	(1) Reversing pressure exceeds regulations (2) The commutation flow exceeds the regulation (3) The back pressure of the oil return port is too high	(1) Reduce pressure (2) Replace the electro-hydraulic directional valve with appropriate specifications (3) Adjust the back pressure to be within the specified value
	3. Poor assembly	Poor coaxiality between solenoid core and valve core axis	Reassemble to ensure good coaxiality
(6) The electromagnet has insufficient suction	Poor assembly	(1) The putter is too long (2) The contact surface of the electromagnet iron core is uneven or poor contact	(1) Grind the push rod to an appropriate length (2) Eliminate the fault and reassemble to meet the requirements
(7) Shock and vibration	1. Commutation shock	(1) The large-diameter electromagnetic directional valve has an impact due to the large size of the electromagnet and the fast pull-in speed (2) Hydraulic reversing valve, because the control flow is too large, and the spool moving speed is too fast, causing shock (3) The steel ball of the one-way valve in the one-way throttle valve is missing or broken, and it has no damping effect.	(1) When large-diameter directional valves are needed, electro-hydraulic directional valves should be selected first (2) Adjust the orifice of the throttle valve to slow down the movement speed of the spool (3) Overhaul the one-way throttle valve
(7) Shock and vibration	2. Vibration	The screw fixing the electromagnet is loose	Fasten the screws and add lock washers

6.6.2 Common faults and treatment of multi-way directional valve (see Table 23)

Table 23 Common faults and treatment of multiple directional valve

Failure phenomenon	Cause Analysis	Elimination method
(1) Pressure fluctuation and noise	The spring of the relief valve is bent or too soft Relief valve orifice blocked The one-way valve is not closed tightly Poor contact between poppet valve and valve seat	Replace the spring Clean, make the passage unblocked Repair or replace Adjust or replace
(2) The valve stem is not flexible	Damaged return spring and limit spring Damaged circlip for shaft The dust seal is too tight	Replace damaged spring Replace circlip Replace the dust seal
(3) Leakage	Poor contact between poppet valve and valve seat Double-head screws are not tightened	Adjust or replace Tighten as required

6.6.3 Common faults and treatment of hydraulic control check valve (see Table 24)

Table 24 Common faults and treatment of hydraulic control check valve

Failure phenomenon

Cause Analysis

Elimination method

(1) The opposite direction is not sealed and leaks

Check valve is not sealed

(1) The check valve is stuck in the fully open position

1) The valve core and valve hole fit too tightly

2) The spring is bent, deformed or too weak

(2) The contact between the conical surface of the check valve and the conical surface of the valve seat is uneven

1) Poor coaxiality between valve core cone and valve seat

2) The outer diameter of the spool is not concentric with the cone

3) The outer diameter of the valve seat is not concentric with the cone

4) The oil is too dirty

(1)

1) Repair and make the spool move flexibly

2) Replace the spring

(2)

1) Repair or replace

2) Repair or replace

3) Repair or replace

4) Filter oil or replace

(2) Reverse cannot be opened

One-way valve cannot be opened

(1) Control pressure is too low

(2) The control pipeline joint has serious oil leakage or the pipeline is bent, which is flattened and the oil is not smooth

(3) The control spool is stuck (such as low processing accuracy and dirty oil)

(4) Oil leakage at the end cover of the control valve

(5) The one-way valve is stuck (such as the spring is bent; the processing precision of the one-way valve is low; the oil is too dirty)

(1) Increase the control pressure to make it reach the required value

(2) Tighten joints, eliminate oil leakage or replace pipes

(3) Cleaning and repairing make the valve core move flexibly

(4) Tighten the end cover screws and ensure that the tightening torque is even

(5) Clean, repair, make the valve core move flexibly; replace the spring; filter or replace the oil

6.6.4 Common faults and treatments of pressure relay (pressure switch) (see Table 25)

Table 25 Common faults and treatment of pressure relay (pressure switch)

Failure phenomenon	Cause Analysis	Elimination method
(1) No output signal	(1) The micro switch is damaged (2) Electrical circuit failure (3) The valve core is stuck or the orifice is blocked (4) The oil inlet pipe is bent and deformed, which makes the oil flow obstructed (5) The adjusting spring is too hard or the pressure is adjusted too high (6) The contacts connected to the micro switch are not adjusted properly (7) Poor assembly of spring and ejector rod, and jamming phenomenon	(1) Replace the micro switch (2) Check the cause and troubleshoot (3) Cleaning, repairing and meeting requirements (4) Replace the pipe to make the oil flow unblocked (5) Replace the appropriate spring or adjust the pressure value as required (6) Carefully adjust so that the contacts are in good contact (7) Reassemble to make the action sensitive
(Two) the sensitivity is too poor	(1) The friction force at the pin of the ejector pin is too large, or the friction force at the contact point between the steel ball and the plunger is too large (2) Poor assembly, inflexible action or “unkind” (3) The contact stroke of the micro switch is too long (4) Improper adjustment of adjusting screw, ejector rod, etc. (5) The steel ball is not round (6) Spool movement is not flexible (7) Improper installation, such as uneven and inclined installation	(1) Reassemble to make the action sensitive (2) Reassemble to make the action sensitive (3) Reasonably adjust the position (4) Reasonably adjust the position of screws and ejector pins (5) Replace the steel ball (6) Cleaning and repairing to achieve flexibility (7) Change to vertical or horizontal installation
(3) Send the signal too fast	(1) Large damping hole at the oil inlet (2) The diaphragm is broken (3) The system shock pressure is too large (4) The electrical system design is incorrect	(1) The damping hole should be appropriately reduced, or a damping tube (snake tube) should be added to the control pipeline (2) Replace the diaphragm (3) Add a damping tube to the control pipeline to reduce the impact pressure (4) Design electrical system according to process requirements

6.7 Key points of installation, debugging and troubleshooting of hydraulic control system

6.7.1 Installation and debugging of hydraulic control system

The difference between hydraulic control system and hydraulic transmission system is that the former requires the movement of its hydraulic actuator to track the changes of random control signals with high accuracy. Hydraulic control systems are mostly closed-loop control systems, so there is a need for system stability, response and accuracy. To this end, mechanical-hydraulic-electrical integrated electro-hydraulic servo valves, servo amplifiers, sensors, high-cleanliness oil sources and corresponding pipeline arrangements are required. The main points of installation and debugging of hydraulic control system are as follows:

1) The inner wall material or paint of the oil tank should not be a source of oil pollution. The oil tank material of the hydraulic control system should preferably be stainless steel.

2) Using high-precision filters, according to the requirements of the electro-hydraulic servo valve for filtration accuracy, generally 5-10μm.

3) After the oil tank and piping system go through general pickling and other treatment processes, inject low-viscosity hydraulic oil or turbine oil for no-load cycle flushing. Pay attention to the following points during cyclic flushing: a) The position where the servo valve is installed before flushing should be replaced by a short-circuit channel plate; b) The filter is blocked quickly during the flushing process, and should be checked and replaced in time; c) The oil sample is taken regularly during the flushing process. Use a pollution measuring instrument to carry out pollution measurement and record until the flushing is qualified; d) After the flushing is qualified, release all the cleaning oil, and inject qualified hydraulic oil into the oil tank through a precision filter.

4) In order to ensure that the hydraulic control system has a better purification function during operation, it is best to add a low-pressure self-circulating cleaning circuit.

5) The installation position of the electro-hydraulic servo valve is as close as possible to the hydraulic actuator, and there are as few hoses as possible between the servo valve and the actuator. These are all for improving the frequency response of the system.

6) Electro-hydraulic servo valve is a precision product integrating mechanical, hydraulic and electrical. You must have relevant basic knowledge before installation and commissioning, especially to read and understand product samples and instructions in detail. Pay attention to the following points: a) Whether the model of the installed servo valve meets the design requirements, whether the servo valve dynamic and static performance test data at the factory is complete; b) Whether the model and technical data of the servo amplifier meet the design requirements, it can be adjusted The parameters should match the servo valve used; c) Check the control coil connection mode of the electro-hydraulic servo valve, series, parallel or differential connection mode, which one meets the design requirements; d) Feedback sensor (such as displacement, force, Speed and other sensors) whether the model and connection mode meet the design requirements, pay special attention to the accuracy of the sensor, which directly affects the control accuracy of the system; e) Check whether the oil source pressure and stability meet the design requirements, if the system has an accumulator, Need to check the inflation pressure.

7) The hydraulic cylinder used in the hydraulic control system should be a low-friction hydraulic cylinder, and its minimum starting pressure should be measured before installation as a basis for checking the hydraulic cylinder in the future.

8) The gas should be carefully removed before the hydraulic control system is put into operation, otherwise it will have a greater impact on the stability and rigidity of the system.

9) System debugging should be carried out before the hydraulic control system is officially used, and can be carried out according to the following points: a) Zero adjustment, including the zero adjustment of the servo valve and the zero adjustment of the servo amplifier. In order to adjust the zero position of the system, sometimes an offset voltage is added. B) System static test, determine the static relationship between the controlled parameter and the command signal, and adjust the reasonable magnification. Generally, the larger the magnification, the smaller the static error and the higher the control accuracy, but it is easy to cause system instability; c) Systematic Dynamic testing, using dynamic testing equipment, usually needs to measure system stability, frequency response and error to determine whether it can meet the design requirements. The system dynamic and static test records can be used as the basis for future system operation evaluation.

10) After the hydraulic control system is put into operation, the following recorded data should be checked regularly: oil temperature, oil pressure, oil pollution degree; stable operation, zero deviation of the actuator, and signal tracking of the actuator.

6.7.2 Troubleshooting of the hydraulic control system (table 26)

Table 26 Troubleshooting of hydraulic control system

Failure phenomenon of hydraulic control system	Troubleshooting method
(1) The actuator does not operate after the control signal is input to the system	1) Check whether the oil pressure of the system is normal, and judge the working conditions of the hydraulic pump and overflow valve 2) Check whether the actuator is locked 3) Check whether the input and output electrical signals of the servo amplifier are normal, and judge its working condition. 4) Check whether the hydraulic output is normal when the electrical signal of the electro-hydraulic servo valve is input or changed, to judge whether the electro-hydraulic servo valve is normal. Servo valve failure should generally be handled by the manufacturer
(2) After the control signal is input into the system, the actuator moves to the end in a certain direction	1) Check whether the sensor is connected to the system 2) Check whether the output signal of the sensor and the servo amplifier are incorrectly connected as positive feedback 3) Check the possible internal feedback failure of the servo valve
(3) The zero position of the actuator is not accurate	1) Check whether the zero adjustment bias signal of the servo valve is adjusted properly 2) Check whether the servo valve zero adjustment is normal 3) Check whether the flutter signal of the servo valve is adjusted properly
(4) Vibration of the actuator	1) Check whether the magnification of the servo amplifier is adjusted too high 2) Check whether the output signal of the sensor is normal 3) Check whether the system oil pressure is too high
(5) The actuator cannot keep up with the change of the input signal	1) Check whether the magnification of the servo amplifier is adjusted too low 2) Check whether the system oil pressure is too low 3) Check that the clearance between the actuator and the motion mechanism is too large
(6) Crawling phenomenon of actuator	1) The gas in the oil circuit is not exhausted 2) The friction of moving parts is too large 3) The oil source pressure is not enough

Common fault diagnosis methods

Simple fault diagnosis method

Analysis method of hydraulic system schematic diagram

Other analytical methods

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