The complete hydraulic system in the excavator includes 5 parts, namely power components (hydraulic pumps), actuating components (cylinders, oil motors), control components (valves), auxiliary components (, oil tanks, filters) radiator, etc.) and hydraulic oil. To analyze and eliminate hydraulic system faults, we must start from the above five aspects for detection, analysis and judgment. The common faults and diagnostic troubleshooting methods listed by the author are as follows:
1. The whole machine does not move:
When this kind of overall fault occurs, we must start with the components that may cause the overall fault. to judge.
(1) Whether the hydraulic oil is sufficient. Check oil level and quality.
(2) Whether the hydraulic pump is working normally. Excavators generally have two or more main pumps that supply oil to the system. If it is not a mechanical failure that prevents the main shaft of the oil pump from rotating, generally speaking, the probability of several pumps failing at the same time is very low. If necessary, you can install an oil pressure gauge with a suitable range at the pressure measuring port of each oil pump to measure the oil supply pressure to determine whether the oil pump is faulty.
(3) The safety locking valve is faulty. It is a simple mechanical switch located in the cab, which controls the on and off between the low-pressure oil circuit and three sets of PPC valves (proportional pressure control valves, namely left and right joysticks and walking push-pull rods) in the cab. When the valve is stuck or clogged, oil cannot be supplied to the PPC valve to push the main control valve to act, and the entire vehicle will be inactive. Clean and repair the safety locking valve.
(4) Self-pressure and decompression failure (if there is a pilot pump, there is no such valve). It is a pressure regulating valve located on the main control valve body. Since the oil pressure output by the oil pump is sometimes high and sometimes low, it is not conducive to controlling the oil pressure. The self-pressure reducing valve turns this unstable oil pressure into a constant pressure oil flow and outputs it to the PPC valve and solenoid valve of the entire vehicle to complete the control function. If the self-pressure reducing valve is clogged or stuck, so that the oil from the main pump cannot pass through the self-pressuring reducing valve but is safely discharged from the main relief valve, the control oil flow cannot enter the PPC valve and solenoid valve, and cannot be pushed. When the main control valve operates, the entire vehicle will inevitably become inactive. Clean and repair the self-pressure reducing valve.
(5) Main relief valve failure. If there is no problem with the safety locking valve and the self-pressure reducing valve, check the main relief valve at this time. It is a safety valve located on the main control valve body. Its function is to limit the maximum pressure of the entire system. If the maximum pressure is exceeded, the pressure will be automatically relieved to protect the entire system from damage. If the spring of the valve is broken or the set pressure is too low, the pressure of the entire system will be too low, the pressure oil of the main pump will not be able to push the actuator to work, and all the main relief valve will leak, resulting in the phenomenon of no movement of the entire vehicle. Judgment method: Start the engine, connect the safety locking valve, and control the handle to rotate or dig the machine. Since the faulty machine will not move, touch the main relief valve body with your hand. If the valve rises steadily and quickly and is hot to the touch, it can be concluded that the main relief valve is faulty. Test, repair and adjust the main relief valve.
2. The whole machine moves slowly and weakly:
The reason for this failure phenomenon is insufficient flow and pressure of hydraulic oil supplied to each actuator. Excluding the long-term use of the machine, poor sealing of various parts, increased wear, and decreased pressure and displacement of the oil pump. The possible reasons are as follows:
(1) Faulty merging and diverting valve. It is located on the main control valve body, and its function is to control the pressure oil of multiple pumps to be in a split or merge state according to work needs. Taking the PC400-5 machine as an example, the two main pumps at the front and rear of the machine are divided only when traveling, supplying the work of the right and left traveling motors respectively; in other states, the two pumps work together. Therefore, when the converging and diverting valves are stuck and are always in a diverting state, under working conditions other than walking, only the front pump works and the rear pump is unloaded, resulting in slow movement of the entire machine. If the machine travels normally but the excavation and rotation movements are slow, it can be determined that the converging and diverting valve is faulty, and the valve should be repaired at this time.
(2) The set pressure of the main relief valve is too low. Since the set pressure is lower than the standard value, the system is prone to overflow during operation. For a hydraulic system with multiple main relief valves, as long as any one of them has the above failure, it will cause the maximum pressure of the entire system to decrease and the system will be unable to work. For this purpose, the set pressure of the main relief valve needs to be measured. When measuring pressure, refer to the instruction manual to select an oil pressure gauge with a suitable range and install it at the corresponding pressure measuring port.
The engine should be turned at full throttle and the hydraulic oil temperature should be between 45°C and 55°C. Extend and retract the bucket cylinder, arm cylinder and boom cylinder respectively to the end to overflow the system and measure the pressure value. If 4 or more of these 6 values ??are basically equal and lower than the standard value, it can be concluded that the fault lies in the main relief valve. In order to further clarify which main relief valve is faulty, you can measure the relief pressure of the left and right traveling pumps respectively (at this time, the front and rear main pumps are working separately). Still taking the PC400-5 machine as an example: In the above test state, a strong rod-shaped object is stuck between the driving wheel and the crawler track, so that the machine cannot move and the system overflows. The value measured by the pressure measuring port of the rear pump is the left traveling overflow pressure, and the value measured by the pressure measuring port of the front pump is the right traveling overflow pressure. Then compare it with the standard value to determine the faulty valve and perform debugging at the same time.
(3) The set pressure of the self-pressure reducing valve is too low (if there is a pilot pump, measure the pump output pressure and flow rate). If the output oil pressure of the self-pressure reducing valve is too low, the opening of the main control spool through the PPC valve will be smaller than normal, and the force of the main pump pressure oil passing through the main control spool will be reduced, causing the actuator action to change. slow. Therefore, it is necessary to measure the pressure of the self-pressure reducing valve. When measuring, refer to the instructions, select an oil pressure gauge with a suitable range and install it at the corresponding pressure measuring port; the working device should be lowered to the ground, the temperature of the hydraulic oil should be between 45°C and 55°C, the engine should be turned off, and the filler cap of the hydraulic oil tank should be slowly loosened. , release the pressure in the fuel tank; then put the safety locking valve in the oil circuit disconnection position, start the engine and measure the circuit oil pressure under no-load condition; if it is lower than the standard value, adjust it at the same time, turn the adjustment screw clockwise to increase the pressure , until the standard value, and then tighten the nut.
(4) The unloading valve and the LS (load sensing system) bypass valve are faulty. The unloading valve is located on the main control valve. Its function is that when the control handle is in the neutral position, the hydraulic oil of the main pump does not have to overflow from the main relief valve at high pressure due to no action of the actuator, but returns through low-pressure unloading of the unloading valve. The oil tank prevents the oil pump from outputting high-pressure oil under no-load conditions, thus avoiding unnecessary waste. During normal operation, the PLS (load sensing pressure) pressure is always slightly less than the main pump pressure P0, plus the elastic force of the spring at the bottom of the valve core, it maintains balance with P0, and the unloading valve is closed; when the handle is neutral, the PLS pressure is 0, At this time, the main pump pressure P0 opens the valve core, and the main pump pressure oil returns to the tank to unload. A common fault is that the PLS pressure is low, resulting in insufficient back pressure of the unloading valve, causing the unloading valve to be in a half-open or fully-open state during normal operation of the machine. The main pump pressure oil is partially or completely unloaded, causing the system to be slow and weak. The common cause of low PLS pressure is a failure of the LS bypass valve. The function of the LS bypass valve is to increase the flexibility of operation and prevent sharp changes in the main pump flow rate caused by sudden changes in PLS oil pressure. With the LS bypass valve, when the PLS oil pressure changes sharply, part of the PLS oil is slightly leaked by the throttle hole on the LS bypass valve and flows into the tank, thereby reducing the PLS pressure. There is an O-type sealing ring on the LS bypass valve core to ensure that the PLS oil pressure does not leak into the tank when the pressure is stable. If the O-ring is damaged, the PLS oil will leak into the tank and the pressure will decrease, causing the unloading valve to have insufficient back pressure and be in a half-open or fully-open state, causing the system to move slowly and weakly. Inspect the unloading valve and LS bypass valve. A simple way to judge the quality of the unloading valve is: when the car is hot, the engine rotates at high speed, the control handle reaches the full stroke, and the machine is moved and placed in the overflow state. Since the unloading pressure of the unloading valve is lower than that of the main relief valve, as long as you touch several unloading valves with your hands, the one with the fastest temperature rise and hotness will be the faulty valve. Of course, if the unloading valve is not hot but one of the main relief valves is heating up and is hot, it means that the fault is not in the unloading valve, but is caused by the low relief pressure of the main relief valve. At this time, the repair should be performed. valve.
Partial faults:
1. Boom cylinder. Sometimes the hydraulic excavator will experience slow and weak expansion and contraction of the arm cylinder or bucket cylinder; slow and weak walking or rotation. The possible reasons are as follows:
(1) Seals of oil lines, pistons, etc. The parts failed, causing the hydraulic oil in the low-pressure chamber and the high-pressure chamber to leak and flow into each other. A simple judgment method is: loosen the seal of the low-pressure chamber to prevent the hydraulic oil in the chamber from falling out, and then enable the high-pressure chamber to operate. If hydraulic oil flows out of the low-pressure chamber at this time, it indicates that the cylinder piston seal is poorly sealed and the high-pressure chamber Oil has escaped into the low-pressure chamber and the seal needs to be replaced. There is also the possibility of hydraulic oil leakage. You can diagnose it by observing whether there is oil leakage when the cylinder and motor are working.
(2) The safety suction valve is faulty.
The safety suction valve is installed on each actuator (cylinder, motor). Its function is to leak the abnormal high pressure in the cylinder or motor to protect it from damage when the working device is impacted by a huge force from the outside (such as rockfall from a cliff). The safety suction valve is a two-way protection valve. When an instantaneous high pressure occurs in a certain chamber of the oil cylinder or motor, the high-pressure oil in the high-pressure chamber will open the relief valve in the safety suction valve to overflow. At the same time, the safety suction valve on the side of the negative pressure chamber will overflow. The valve opens the one-way valve to suck oil from the tank to supplement the negative pressure, which plays a role in oil circuit protection. There are two safety suction valves on the boom cylinder, stick cylinder, bucket cylinder, slewing and traveling working circuits respectively. Obviously, if the one-way valve on a certain circuit fails and becomes a two-way straight-through; or if the relief valve fails and the opening pressure is very low, the oil pressure in the oil chamber on that side cannot be established, but directly passes through the one-way valve. Or the relief valve partially or completely leaks into the fuel tank, causing the cylinder and motor to move slowly, weakly or even fail to work. This kind of failure is also accompanied by the phenomenon that the oil cylinder will slowly fall down (or called cylinder drop) after neutralization or shutdown. If the above two fault phenomena occur at the same time, the corresponding safety suction valve needs to be repaired. The safety suction valve on the faulty circuit can be replaced with a normal safety suction valve; if the fault disappears, the diagnosis can be made. (3) PPC valve failure. If there is a fault phenomenon in which individual movements are slow but other movements are normal, check the PPC valve after ruling out the possibility of internal leakage of the cylinder motor. The PPC valve is a proportional pressure control valve that outputs corresponding control oil pressure and flow according to the stroke of the driver's operating handle, so that the main control spool has a corresponding movement amount to control the speed of the work actuator. If a certain valve core is worn, stuck, or even clogged, the corresponding action will be slow or even non-action. Just switch the control of the long circuit to the PPC valve that is working normally. If the fault disappears, it is determined that the problem is the corresponding PPC valve fault, and it should be disassembled and repaired.
(4) Main control valve failure. If the possibilities (1), (2), and (3) are eliminated but the fault still exists, you need to check whether the valve core in the main control valve that controls the corresponding action is worn, stuck, or clogged. Each valve core controls the opening and closing of an oil circuit. If the valve core is worn, stuck, or clogged, the pressure and flow rate of the pressure oil supplied to the corresponding actuator will be reduced, resulting in weak, slow or even no action. Phenomenon.
2. Accumulator failure. The accumulator is a device that stores and controls oil pressure. Installed between the main pump and the PPC valve, its function is to maintain the stability of the control oil pressure, and when the engine is turned off, the working device can still be put down through it to ensure the safety of the entire machine. Its energy storage comes from the pressure oil of the self-pressure reducing valve compressing the nitrogen inside the accumulator bladder during normal operation. After the engine is shut down, if the safety locking valve is opened, the control handle and the main control spool can push the control oil circuit to open by compressed nitrogen, and the working device will fall down by its own weight. On the contrary, if the working device does not move, the accumulator is faulty and needs to be inspected and replaced. 3. The LS (load sensing system) selector valve is faulty. During compound action, if the boom lifts slowly and rotates faster, it usually means that the LS selector valve is faulty. This valve is installed on the circuit where the rotary PLS oil pressure comes out. Its main function is to control the coordination of the boom and rotary composite movements. Clean and inspect the LS selector valve seal.
Finally, when a fault occurs, you should first determine whether it is a complete machine fault or an individual action fault, and then determine whether it is a pressure fault or a speed fault. Starting from the components that may cause the fault, you can quickly diagnose and eliminate it. Fault.
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