Before we talked to everyone about the "high temperature of the excavator engine", many excavator owners dialed the phone to consult about the failure of the "excavator's arm", so the small editor arranged some problems that were easy to appear in the front work device of the excavator for the owners to read the reference.
The front work device of the excavator is mainly the shovel, and the normal work of the bucket needs to cooperate with each other in the bucket oil cylinder, the bucket rod oil cylinder and the movable arm oil cylinder. There are a lot of problems on these cylinders, which may be the problem of pressure relief, the drop of the bucket and the bucket automatically in the work. A loose bucket.
Let's first understand the hydraulic system principle of the excavator front end working device, taking the most common hydraulic circuit of the boom oil cylinder as an example.
Oil inlet circuit: the hydraulic oil is from the large pump to the main control valve, and enters the cylinder after passing through the pipeline safety valve and the retaining valve.
Oil return circuit: the hydraulic oil on the other side of the cylinder is returned to the hydraulic tank through the main control valve to the hydraulic oil radiator.
Safety relief valve
Safety relief valve, also commonly known as pipe safety valve on excavator, is a kind of valve in the hydraulic circuit of excavator. When the pressure in the hydraulic circuit exceeds the specified value, the valve core in the safety overflow valve is opened, and a part of the hydraulic oil in the hydraulic circuit is discharged into the oil pipeline to reduce the pressure of the hydraulic circuit and prevent the pressure of the hydraulic circuit. The pressure is too high to cause an accident.
Retaining valve
Take the large arm cylinder retaining valve as an example, the large arm retaining valve is usually installed between the big arm cylinder and the large arm control valve. Its structure is composed of the oil suction safety valve, the pilot valve and the pilot valve spring, the main control slide valve and the main control slide valve spring. When the operating rod of the large arm cylinder is in the middle or no action, the retaining valve can maintain the hydraulic pressure. Force, the big arm will not fall and shrink because of the weight.
The role of the large arm retaining valve is equivalent to the hydraulic safety lock. Generally, the cartridge valve structure is adopted due to the large flow rate of the valve during normal operation.
Unidirectional valve
The function of the unidirectional valve is to prevent the abnormal high pressure and air suction caused by the inertia of the hydraulic actuator in the hydraulic circuit of the big arm of the excavator. In the working process, the hydraulic cylinder is in the case of sudden change or stop, and the abnormal high pressure is produced in a certain cavity because of the inertia and so on. The suction valve will release the abnormal high pressure, and fill the cavity with suction to prevent damage to the hydraulic system.
The one-way valve has some other functions besides acting as an oil filling valve.
Keep pressure: the valve type reversing valve has gap leakage and can only keep the pressure in a short time. When there is a pressure protection requirement, a hydraulic control one-way valve can be added to the oil road, using the tightness of the cone valve to close the oil road for a long time.
The "support" of the hydraulic cylinder: in the hydraulic cylinder, because of the leakage of the slide valve and tube, the piston and the piston rod may fall down under the gravity of the excavator's working device. The liquid controlled one-way valve is connected to the oil path of the lower chamber of the hydraulic cylinder, and the piston and slider and other active parts of the hydraulic cylinder can be prevented from falling.
Hydraulic cylinder locking: when the reversing valve is in the middle, two hydraulic control unidirectional valves are closed, and the oil of the two chamber of the hydraulic cylinder can be closed tightly, and the piston can not move because of the external force.
Large flow of oil discharge: the effective working area of the two chamber of the hydraulic cylinder is very different. When the piston is returned, the amount of oil in the right chamber of the hydraulic cylinder suddenly increases. At this time, if a small flow valve is used, the throttle effect will be produced to limit the back speed of the piston; if the hydraulic control one-way valve is added, the pressure oil will be controlled to open the one direction valve when the piston is back. The right chamber oil can be discharged smoothly.
We have learned the hydraulic circuit and main hydraulic components of the excavator's front end working device. Now let's analyze the cause of the excavator's arm dropping.
At present, the control valves used by hydraulic excavators are all slide valves. In order to make the valve core in the valve type control valve switch normally, there must be a certain gap between the valve core and the valve body, and there must be a leakage between the valve core and the valve.
As shown below, the A1 and B1 ports are connected to the rod cavity and the rod cavity of the hydraulic cylinder respectively. When the excavator works or stops, the hydraulic pressure of the side of the A1 or B1 will rise under the action of the weight (yellow area). The pressure and the hydraulic oil tank channel in the blue area will produce pressure difference, and the hydraulic oil will be under the action of the pressure difference. Flow through the gap between the valve core and the body from the yellow area to the blue area. As the fluid flows, the oil cylinder will shift to the direction of the weight, that is, the "cylinder sinking".
In order to reduce the inconveniences caused by the "oil cylinder sinking", and combined with the working characteristics of the excavator, most excavator manufacturers have installed a lock valve as shown below in the control valve. The working principle can be described simply as a lift head passing through the cone seal between the cylinder and the valve spool. The lock valve is moved by the lock valve. The function of the head changes the clearance seal to the cone seal, which greatly reduces the leakage of hydraulic oil and basically eliminates the drifting of the hydraulic cylinder.
But considering the working characteristics of the hydraulic excavator, all the excavator manufacturers only install the lock valve in the direction of the rod and the big arm (ARM) cylinder without the rod. The oil of the small arm cavity and the oil of the large arm cylinder without rod cavity are locked by the two locking valves. It effectively reduces the displacement and subsidence of the small arm under the extended condition and the lifting state of the upper arm (the working principle diagram of the upper arm cylinder locking valve) is shown above.
In the two states of the retracting state of the small arm and the lifting of the body through the large arm, the slight displacement of the cylinder caused by the clearance of the valve spool and the valve body does not affect the normal work of the excavator, so all the excavator manufacturers do not install the lock valve in these two directions, and the oil cylinder will not sink in this direction when the equipment is out of the factory. The amount of special measurement is made. And the sinking of this direction will increase with the increase of excavator's use time and the increase of wear of the control valve.
In addition to the heavy oil cylinder sinking caused by the valve spool and the valve body clearance mentioned earlier, the oil cylinder oil seal damage or the safety valve will also cause the increase of the cylinder sink. Take the small arm for example, when the oil seal of the small arm oil cylinder is damaged, the sinking of the small arm is obviously too large, and the small arm is in state at this time. The amount of sunk will not increase significantly. Therefore, do not try to check the small arm cylinder when the overarm subsidence is greater than that of the extended state. If there is a very fast sinking of the small arm, the fault can be judged by exchanging the rod cavity and the non rod side safety valve. Position.
Oil cylinder oil seal damage
We know that the rod cavity and the non rod cavity of the excavator cylinder are strictly sealed and sealed in theory, and the hydraulic oil of the cylinder rod cavity and the non rod cavity is isolated by the multiple oil seals on the piston of the cylinder. In the long period of high temperature and pressure movement, the oil seal is prone to Deformation Aging wear and wear. This will lead to the leakage in the cylinder. The hydraulic oil with rod cavity enters the rod less cavity or the hydraulic oil of the rod less cavity enters the rod cavity.
However, no matter how good the quality of the oil seal will have a certain amount of leakage, as long as it is in a reasonable range, it can be used normally, which is called the drift of the big arm cylinder of the excavator. The natural subsidence of the general excavator arm is 1-3 centimeters in five minutes.
Valve discharge
The valves in the hydraulic circuit include the retaining valve, the overflow valve, the one-way valve, the safety valve and so on. Because of the displacement during the work, it is easy to produce the internal leakage after the wear and tear, which causes the excavator to drop the arm.
General methods for troubleshooting
We have said before that there are many reasons why the excavator dropped the arm, so how to accurately find out the root of the fault of a excavator, we can refer to the following methods.
First of all, you can first adjust the oil cylinder with the rod cavity and the non rod cavity safety valve. After the adjustment, we can observe whether the failure phenomenon disappears. If the failure of the arm is disappearing at this time, it indicates that the safety valve of the cylinder without the rod has been discharged seriously, and it needs to be repaired or replaced.
If the control valve still appears to fall out of the arm phenomenon, you can check the hydraulic oil back oil filter core, observe whether there are rubber fragments or iron chips on the filter core. If there are rubber fragments on the filter core, it is very likely that the oil seal is badly worn, and the oil seal should be replaced.
Since then, if the arm failure still remains, check the retaining valve of the hydraulic circuit. It needs to be disassembled and checked for serious wear. Timely grinding and repairing or replacing the retaining valve.
After checking and replacing the retaining valve, if the excavator still has its arm dropping, the main control valve must be disassembled, and all the spool on the hydraulic circuit should be checked to repair or replace in time.
Summary:
The failure of the excavator's arm is a common fault. The phenomenon of most excavators falling out of the arm is due to the leakage of the oil seal wear cylinder. In the process of inspection, a simple fault point can be checked. The wear of the main valve core in the main control valve will appear after the excavator is used for a long time, and the maintenance of the main control valve is a ratio. The more complex process is recommended for final inspection.
