Created on:2021-09-16 10:26

Construction machinery_ Excavator hydraulic system failure

Construction machinery_ Excavator hydraulic system failure

(1) Working principle of hydraulic system

The basic working principle of the hydraulic system of pc200-7 excavator is shown in Figure a.


Provide the whole system with pressure oil with certain pressure. Part of the pressure oil enters the pilot oil circuit after passing through the pressure reducing valve, and the other part enters each working device as required after passing through the main control valve. The pump controller analyzes the data returned by each sensor, and then adjusts the pressure and displacement of the main pump through pc-epc solenoid valve, PC valve, LS valve, servo piston and other control components to match the load of the working device with the power of the engine.

(2) Fault analysis of control circuit

① Fault of self reducing valve the self reducing valve uses the output oil flow of the main pump and reduces it as the control pressure to act on the solenoid valve and PPC valve, which is similar to the function of hydraulic pilot pump to provide control oil pressure.

Fault phenomenon: the working device of the whole machine is slow.

Inspection results: the PPC pressure is 20kgf / cm2, which is less than the rated pressure of 33kgf / cm2.

Fault analysis: after inspection, the engine speed is normal and the hydraulic oil circuit pressure is normal; Then check the self pressure reducing valve and find that the cone valve is stuck, dirt in the hydraulic oil is clamped between the cone valve and the shell, resulting in a gap, and the pressure of PPC valve cannot reach the set pressure. No matter how the operating handle acts, the control oil pressure output from the PPC valve does not change much, so the movement of the valve core of the main control valve is small, and the flow to the working device is also small. As a result, the speed of the working device is low.

Troubleshooting: after cleaning, install the self pressure reducing valve, check that the pressure of PPC valve returns to normal and the fault disappears.

② Accumulator failure is a device for storing and controlling oil circuit pressure. The accumulator is generally installed between the main pump and the PPC valve. Its function is to keep the pressure of the control oil circuit stable and to put down the working device when the engine stops, so as to ensure the safety of the machine.

Fault phenomenon: after the engine is shut down, the control lever moves to the "down" position of the working device, and the working device does not act.

Inspection results: check the accumulator and find nitrogen leakage.

Fault analysis: the gas in the accumulator leaks. After the engine is started, because the oil pressure in chamber B is compressed, but the gas in chamber a is not compressed, the oil entering chamber B cannot be used as control pressure oil to push the main control valve. Therefore, the control valve and the working device do not act.

Troubleshooting: replace the accumulator and the fault disappears.

③ PPC valve fault PPC valve is a proportional pressure control valve, which is installed under each operating handle in the cab. It can output the corresponding control oil pressure according to the stroke of the driver's operating handle, so that the main control valve core has the corresponding movement, so as to control the speed of the working device. The common faults of PPC valve are failure of a certain action of the operating handle, slow action of a working device, etc.

Fault phenomenon; Cannot rotate to the left.

Inspection result: the dirty hydraulic oil causes the PPC valve spool to be stuck when turning to the left.

Fault analysis: the filter element of the hydraulic oil filter was not replaced on time, resulting in the blockage of the filter element, the opening of the bypass valve of the oil filter, the dirty hydraulic oil entered the PPC circuit without filtration, resulting in the jamming of the slide valve in the PPC valve, and the control oil pressure could not flow to the main control valve through the PPC valve turning left, resulting in the failure of turning left.

Troubleshooting: clean the PPC valve, replace the filter element and hydraulic oil of the hydraulic oil filter, and clean the relevant oil circuit.

④ Ls valve fault the main function of LS valve is to sense the stroke state of the driver's control lever and give corresponding signals to the main pump to adjust the appropriate flow. The movement of the control lever changes the movement of the valve core inside the main control valve, and the movement of the main control valve generates pls pressure (the pressure represents the movement of the valve core). The PLS pressure is fed back to the LS valve of the main pump, and then the displacement of the main pump is changed through the LS valve according to the movement of the control lever.

Fault phenomenon: no matter how the control lever changes, the speeds of various working devices remain unchanged. Inspection result: the small hole in the slow return valve core in the PLS feedback circuit is blocked.

Fault analysis: the possible causes of such faults include the jamming of the internal mechanical parts of the swash plate, servo piston, PC valve and LS valve in the pump, and the jamming of the LS circuit fed back by the main control valve to the pump.

Troubleshooting: clean the slow return valve and LS oil pipe, replace the hydraulic oil filter element and hydraulic oil, and remove the fault after cleaning the hydraulic oil tank.

⑤ The change of fault external load of PC valve is reflected as the change of working pressure. PC valve senses the change of pressure by inputting the front and rear pump pressure of the pump, and automatically adjusts the displacement of corresponding pump according to the principle of best matching between pump horsepower and engine horsepower, so as to improve productivity. The common fault is that the engine stalls when the load is too large.

Fault phenomenon: the engine will automatically shut down when re excavating the overflow state.

Inspection results: it is found that the valve core of PC valve is stuck.

Fault analysis: the valve core of PC valve is stuck, resulting in that the large head pressure of servo piston does not change with the change of external load. When the machine is in overflow state, the pump flow is too large, resulting in engine flameout.

Troubleshooting: remove the PC valve, clean all internal parts and install, clean relevant oil circuits, and return to normal after installation.

(3) Fault analysis of working device

① Traveling device fault traveling device is composed of traveling motor and deceleration part. There are left and right final drives on the machine, which directly drive the crawler to make the machine move forward, backward and turn. The common faults include inability to walk on one side, deviation, etc.

Fault phenomenon: one side cannot walk.

Inspection results: dirt blocks the small oil circuit k that releases the brake, resulting in the failure to release the parking brake.

Fault analysis: during disassembly and assembly, when the hydraulic oil is too dirty, dirty things enter the oil circuit, blocking the small oil circuit for releasing the brake. When walking, the pressure oil cannot enter the oil circuit, so it cannot overcome the thrust of the spring to release the brake. As a result, the travel motor cannot rotate, so it cannot travel.

Troubleshooting: clean the oil circuit, pay attention to avoid the entry of dirt during disassembly and assembly in the future, and replace the hydraulic oil and filter element on time.

② Excavation device fault the excavation device is composed of two boom hydraulic cylinders, a jib hydraulic cylinder, a bucket hydraulic cylinder and a bucket. The common fault is that the hydraulic cylinder is unable to work.

Symptom: bucket digging is weak.

Inspection results: opening the bucket hydraulic cylinder for inspection is very time-consuming. The simple way to judge the internal leakage is to move the hydraulic cylinder to the right end, remove the end oil pipe and apply force, and observe the oil leakage at the separation. If there is no oil leakage, there is no internal leakage, otherwise there is internal leakage in the hydraulic cylinder.

Fault analysis: the bucket hydraulic cylinder leaks, resulting in weak excavation.

Troubleshooting: replace the seal in the bucket hydraulic cylinder and eliminate the fault.

③ The slewing device is mainly composed of slewing motor, planetary reduction mechanism and slewing gear ring. The slewing motor drives the upper vehicle body to rotate through the planetary gear reduction mechanism. Common faults include failure to rotate, rotation locking failure, etc.

a. Fault I

Fault phenomenon: unable to rotate.

Inspection result: the coil in the swing lock solenoid valve is damaged.

Fault analysis: when the rotary lock switch is turned to "off", because the rotary lock solenoid valve is damaged and the solenoid valve does not act, the pressure oil from the pressure reducing valve cannot reach port B of the rotary motor and the brake cannot be released, so the motor cannot rotate.

Troubleshooting: replace the swing lock solenoid valve.

b. Fault II

Fault phenomenon: the rotary lock cannot be locked, and the rotary lock switch is turned to "on".

Inspection result: the spool of the swing lock solenoid valve is stuck.

Fault analysis: the filter element of the hydraulic oil filter was not replaced as required, the hydraulic oil became dirty, and the dirty things stuck the valve core of the rotary locking solenoid valve when the rotary locking switch was turned to "off", and the pressure oil from the pressure reducing valve could always flow to port B of the rotary motor, resulting in the release of the brake and the failure of the rotary locking.

Troubleshooting: clean the rotary locking solenoid valve and replace the filter element and hydraulic oil of the hydraulic oil filter.

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