A Komatsu PC200-6 excavator works normally when it is cold. As the temperature of the hydraulic oil rises, its action gradually slows down. The performance is that the whole machine is weak and moves slowly after the hot car. In order to accurately find the cause of the failure, we first start with the working principle of the hydraulic system of the Komatsu excavator, and investigate one by one the various factors that may cause the work weakness after the hot car.
1 Analysis of the principle of main pump output flow control
The Komatsu PC200-6 hydraulic excavator is a dual-pump dual-circuit variable hydraulic system. One pump supplies oil for the left travel, boom and bucket, and the other pump supplies oil for the right travel, swing and stick. The main pump assembly (double pumps) is composed of the main pump, PC valve and LS valve. The two pumps merge through the manifold and enter the main control valve, so that certain working devices can merge.
The hydraulic system of PC200-6 excavator adopts the closed center load sensing system CLS9. Figure 1 is a control principle diagram of the output flow of the main pump in the closed center load sensing system.
From Figure 1, we can see that the LS valve is a valve that senses load and controls flow. The LS valve is based on the main pump pressure p. The output pressure of the operating valve p% Pis is determined by the opening of the operating valve. The pressure difference △pis=prPis controls the output flow of the main pump, and △ps is called the pressure difference of the LS valve.
The relationship between the pressure difference △pus of the LS valve and the output flow Q of the main pump is shown in Figure 2.
When the pressure difference △pis of the LS valve is lower than the set pressure of the LS valve, the set pressure is 2.2MPa), the swing angle of the main pump variable mechanism changes in an increasing direction; when the pressure difference △pis of the LS valve is greater than that of the LS valve When the set pressure is high, the swing angle of the variable mechanism of the main pump changes in a decreasing direction. The size of △pis depends on the stroke of the operating valve core.
The working process of the main pump is as follows:
(1) When the main operating valve is in the neutral position, the LS valve A end acts on the main pump pressure and the ppB end is the operating valve outlet pressure, that is, the LS pressure pus. When the main operating valve is in the neutral position, pus is (same as the drain pipe. The pressure difference of the LS valve becomes larger, and the LS valve moves to the right, so that the large cavity of the servo piston communicates with the oil passage of the main pump. The swing angle of the variable mechanism moves in the minimum direction, and the flow of the main pump becomes smaller.
(2) When the opening area of the main operating valve becomes larger, the pressure difference of the LS valve becomes smaller, the LS pressure pus and the spring force form a combined force, which makes the LS slide valve move to the left, so that the large cavity of the servo piston is connected to the drain and return oil. . Due to the difference in the area of the servo piston, the servo piston is pushed to the right to make the swing angle of the variable mechanism move in the direction of increasing flow.
(3) As the opening area of the main operating valve becomes smaller, the pis pressure becomes smaller, △pms increases, and the LS valve moves to the right. When △pus increases to 2.2MPa, the LS spool valve is in the middle position, and the pressure in the large cavity of the servo piston The LS valve throttles half of the main pump pressure, and the servo piston stops at this position, making the swing angle of the variable mechanism unchanged. At this time, the relationship between the pressure area at both ends of the servo piston is: Ao:A1=1:2 where: Ao——pressure area at the small end of the servo piston; A——pressure area at the large end of the servo piston. The pressure ratio added to both ends during balance is: Pr:Pex-2:1 where: pp——main pump pressure; Px——servo piston large end pressure.
(4) When △ps further increases and is greater than 2.2MPa, that is, the combined force of pis and spring is less than the main pump pressure pp, and the LS valve moves to the right to increase the input pressure of the servo piston cavity and push the servo piston to the right On the side, the swing angle of the variable mechanism moves in the direction where the flow decreases.
2 Fault diagnosis and troubleshooting
Generally, the viscosity of hydraulic oil will drop to about one-sixth every time the temperature of hydraulic oil rises by 10°C. With the gradual decrease of hydraulic oil viscosity, the internal leakage of the hydraulic system increases. According to the normal operation of the system when the car is cold, the preliminary judgment may be that the internal leakage exceeds the allowable amount due to the wear of some parts of the system or the stuck valve core, which causes the pressure of the system to drop after the hot car, and the work is weak. Based on the above analysis of the cause of the fault, the following steps can be used to diagnose and eliminate the fault.
(1) First, we install pressure gauges on the main pump and pilot pressure measuring points. When the car is cold, the main pump pressure is 33MPa, the pilot pressure is 4MPa, the system pressure is normal, and the speed is normal. When the temperature rises to about 70°C, the main pressure drops to 23MPa and the pilot pressure is 4MPa. It can be seen that the pilot pressure is normal but the main pump pressure is abnormal. Under normal conditions, the pressure reducing valve can reduce and stabilize the output oil pressure of the main pump at about 3.7MPa, forming a pilot control oil pressure. If the conical valve core of the pressure reducing valve is worn out and the closing is not tight, which causes the output pressure of the pressure reducing valve to be low when the car is hot, and the spool of the main control valve moves less, causing the direction valve of the main control valve to change direction The opening is not in place enough). As a result, the flow to the working device is also too small, causing the whole machine to work weakly.
(2) The main relief valve, unloading valve spool and valve seat are strained or severely worn, causing serious leakage of the hydraulic system, and the hot car cannot build up the required pressure, resulting in slow and weak movements. We disassemble and inspect the main relief valve and unload
There is no obvious wear on the valve.
(3) The gap between the cylinder body of the hydraulic pump and the oil distribution plate or the cylinder body and the plunger is too large due to severe wear or strain, so that the oil film cannot be established between the two, resulting in the low volumetric efficiency of the pump when the car is hot , The output pressure is low, it will be reflected in the mechanical working device that the whole machine will be weak. Not long ago, the hydraulic pump of this car was overhauled due to the slow heating. The cylinder block, plunger, oil distribution plate, swash plate, etc. of the hydraulic pump were all replaced with new ones. There should be no problem with the hydraulic pump.
(4 After the hot car, the settlement of the hydraulic cylinder of the working device is within the normal range, indicating that the main control valve is normal.
(5) We install pressure gauges on the pressure measuring point of the oil outlet of the hydraulic main pump, the pressure measuring point of the large cavity of the servo piston and the LS oil passage respectively for testing. It is found that in the cold car state, when the stick overflows, the system pressure and speed are normal, the pressure p) output by the main pump is almost double the pressure (pe) of the servo piston cavity, the LS loop pressure pu) and the main pump The pressure p) is almost the same, which means that the flow control of the pump is normal, that is, the LS valve, PC valve, and PC-EPC solenoid valve work normally. As the temperature increases, the pressure difference between pp and pR gradually decreases, and the pressure difference between pp and pis gradually increases. When the hydraulic oil temperature rises to about 70°C, the main pump pressure pp drops to 23MPa. The pressure difference between pp and px is almost zero, and the pressure difference between pp and pis is greater. It can be seen that the pressure pex in the large cavity of the servo piston is too high, and the pressure pus is low, and both are abnormal.
In summary, when the stick overflows, the pressure in the large cavity of the servo piston is the same as the pressure of the main pump: the pressure difference △pis between the main pump output pressure pp acting on the left end of the LS valve and the pressure pis acting on the right end of the LS valve Greater than the set pressure of the LS valve 2.2MPa), move the LS valve to the right to connect the servo piston large cavity oil path with the main pump output oil path, causing the servo piston large cavity pressure to be the same as the pressure of the main pump. The area difference, so that the angle of the swash plate moves to the minimum, the flow of the main pump becomes the minimum, and the hot car shows slow action; theoretically, the pressure of the pus should be the same as the pressure of the main pump when the stick overflows, which also indicates that the pus pressure is low , The low pus pressure indirectly causes the unloading valve to be normally open, so that the unloading valve cannot build up high pressure, the main pressure of the system becomes lower, and the hot car appears to be weak. It can be seen that low pus pressure is the main cause of failure.
So why does the hot car pis pressure become lower?
According to the above analysis, there is internal leakage in the LS pipeline. We dismantled and inspected the LS bypass valve, LS shuttle valve, unloading valve, etc. and found no faults; finally, we removed the main control valve and disintegrated, and through the blowing and pressure test, it was found that the LS oil circuit was between the unloading valve and the oil return duct. Air leaks are severe from time to time. The unloading valve was disassembled and inspected again, and it was found that the gap between the input end of the unloading valve LS and the oil return passage was too large due to long-term wear and tear, which caused LS oil to leak to the oil return passage. As the temperature increased, the viscosity of the hydraulic oil decreased and LS leaked. The amount gradually increased, causing pus to gradually decrease. After replacing the unloading valve, the fault was eliminated immediately. There are many reasons for the inability of the excavator to work on the hot car. Although the fault is concentrated on the hydraulic pump and the main control valve, it is not necessarily the problem of the hydraulic pump and the main control valve itself, because there is a problem with any component of the entire system of the excavator Will affect the work of the hydraulic pump. When making specific judgments and analysis, according to the possible causes of the failure, the order should be eliminated in the order of first outside, then inside, easy first and difficult first, and finally find the source of the failure.
