The loader repairman takes you to understand the hydraulic system principle of the front-end working device of the loader, taking the common hydraulic circuit of the boom cylinder as an example. The front-end working device of the loader is mainly a bucket, and the normal operation of the bucket requires the cooperation of the bucket cylinder, arm cylinder, and boom cylinder to complete various compound actions. One of the more problems present on these cylinders is the problem of pressure relief. The performance in the operation may be the boom drop, arm drop and bucket active loosening.
Inlet circuit: The hydraulic oil goes from the large pump to the main control valve, and then enters the cylinder after passing through the pipeline safety valve and sticking to the valve.
Oil return circuit: The hydraulic oil on the other side of the cylinder returns to the hydraulic oil tank through the main control valve to the hydraulic oil radiator.
Safety relief valve
Safety relief valve is also commonly known as pipeline safety valve on the loader. It is a type of valve that protects the left and right in the hydraulic circuit of the loader. When the pressure in the hydraulic circuit is too high to exceed the specified value, the spool in the safety relief valve is opened, and a part of the hydraulic oil in the hydraulic circuit is discharged into the oil return pipe, so that the pressure of the hydraulic circuit drops, avoiding the excessive pressure. Something happened.
Stick valve
Taking the boom cylinder stick valve as an example, the boom stick valve is generally installed between the boom cylinder and the boom control valve, and its structure is composed of an oil suction safety valve, a pilot valve and a pilot valve spring, a main control spool valve, and a main control The spool valve is composed of springs. When the operating lever of the boom cylinder is in the neutral position and there is no action, insist that the valve can maintain the hydraulic pressure, and the boom will not fall and retract due to its own weight.
The effect of the arm sticking valve is equivalent to the hydraulic safety lock, because the sticking valve passes a large flow during normal operation, and the cartridge valve structure is generally used.
Check valve
One-way valve is also called safety charge valve in the hydraulic circuit of the loader boom. Its effect is to avoid the abnormal high pressure and vacuum caused by the inertia of the hydraulic actuator. During the operation process of the loader, the hydraulic cylinder returns under a sudden commutation or stop condition. Due to inertia and other factors, abnormal high pressure occurs in one cavity, and suction will occur in another cavity. At this time, the safety charge valve can be The abnormal high pressure is released, and the suction cavity is filled with oil to avoid harm to the hydraulic system.
Some check valves on the main control valve of the loader
The effect of one-way valve has some other effects besides filling the charge valve:
Insist on pressure: The slide valve directional valve has a gap leakage phenomenon, which can only maintain pressure for a short time. When there is a pressure-retaining requirement, a hydraulic control check valve can be added to the oil line, and the tightness of the cone valve is used to keep the oil line pressure-maintained for a long time.
"Bearing" of the hydraulic cylinder: In the hydraulic cylinder, due to the leakage of the spool valve and the tube, under the weight of the loader working device, the piston and the piston rod may slide down. Connect the hydraulic control one-way valve to the oil path of the lower chamber of the hydraulic cylinder to avoid the sliding of the movable parts such as the piston and slider of the hydraulic cylinder.
Realize the locking of the hydraulic cylinder: when the directional valve is in the neutral position, the two hydraulic control check valves are closed, which can tightly seal the oil in the two chambers of the hydraulic cylinder. At this time, the piston cannot move due to external force effects.
Large flow oil drain: The useful working area of the two chambers of the hydraulic cylinder differs greatly. When the piston retracts, the oil displacement of the right chamber of the hydraulic cylinder suddenly increases. At this time, if a small flow spool valve is used, a saving effect will occur, limiting the retreat speed of the piston; if a hydraulic control check valve is added, the piston in the hydraulic cylinder When retreating, the control pressure oil will open the hydraulic control check valve, and the right cavity oil can be smoothly discharged.
Understand the hydraulic circuit and the main hydraulic components of the front-end working device of the loader. Now let's analyze the reasons for the loader's arm loss.
At that time, the control valves used by the hydraulic loader were all spool valves. In order to make the spool in the spool valve control valve switch normally, there must be a certain gap between the spool and the valve body, and if there is a gap, there must be leakage .
As shown in the figure below, the A1 and B1 ports are separated from the rodless cavity and the rod cavity of the connecting hydraulic cylinder. When the loader is operating or parked, the hydraulic pressure on one side of A1 or B1 will increase under the effect of its own weight (yellow area), and a pressure difference will occur between this increased pressure and the hydraulic tank channel connected to the blue area. Under the effect of this pressure difference, the hydraulic oil will move and flow from the yellow area to the blue area through the gap between the valve core and the valve body. Because of the above-mentioned oil movement, the oil cylinder will be displaced in the direction of its own weight, that is, the "oil cylinder sinks".
Spool valve control valve
In order to reduce the inconvenience caused by the "sinking of the cylinder", combined with the operating characteristics of the loader, most loader manufacturers have added the determination valve shown in the figure below to the control valve. Its working principle can be briefly described as the installation of a poppet-sealed poppet between the cylinder and the control valve spool. After confirming the effect of the valve poppet, the previous gap seal was changed to a conical seal, which greatly reduced the leakage of hydraulic oil, and then basically prevented the drift of the hydraulic cylinder.
However, considering the operating characteristics of hydraulic loaders and other factors, all loader manufacturers only install a determination valve in the direction of the arm chamber of the arm (stick) cylinder and the rod chamber of the boom (boom) cylinder. The effect of these two determination valves separately determines the oil in the rod arm with a rod cavity and the oil in the boom cylinder without a rod cavity, and then effectively reduces the displacement of the cylinder under the outstretched condition and the boom lift condition 3. Sinking (the picture above shows the working principle diagram of the determination valve of the arm cylinder).
In both the stowed condition of the forearm and the lifting of the vehicle body through the boom, the slight displacement of the cylinder caused by the gap between the control valve spool and the valve body does not affect the normal operation of the loader, so all loader The manufacturer has no device to determine the valve in these two directions, nor does it conduct special special inspection of the amount of cylinder sinking in this direction when the equipment is shipped from the factory. And the amount of sinking in this direction will tend to increase with the addition of loader operating time and control valve wear.
In addition to the large amount of sinking of the cylinder caused by the gap between the control valve spool and valve body, the sinking amount of the cylinder will also increase when the oil seal of the cylinder is damaged or the safety valve is damaged. Taking the forearm as an example, when the oil seal of the forearm cylinder is damaged, it will show that the amount of sinking when the arm is extended is significantly too large, but the amount of sinking when the arm is retracted will not increase significantly at this time. Big. Therefore, when the amount of sinking when the above-mentioned arm is retracted is greater than the amount of sinking when it is extended, do not attempt to disassemble the arm cylinder. If there is a situation where the amount of sinking of the forearm is collapsed very quickly, it is possible to determine the faulty location by exchanging the safety valve on the side of the arm cylinder with and without a rod cavity.
Cylinder oil seal damaged
We know that the rod cavity and rodless cavity of the loader cylinder are strictly sealed in theory. The hydraulic oil that blocks the rod cavity and rodless cavity of the cylinder through multiple oil seals on the piston of the cylinder moves at a high temperature and pressure for a long time. The oil seal is prone to deformation, aging, wear and abrasion. Such a situation will cause the cylinder to leak, causing the hydraulic oil with the rod cavity to enter the rodless cavity or the hydraulic oil without the rod cavity to enter the rod cavity.
Valve leakage
Valves on the hydraulic circuit, including stick valves, overflow valves, check valves, safety valves, etc., because displacement occurs during operation, internal leakage is likely to occur after wear, resulting in the phenomenon of the loader dropping the arm.
Cylinder structure
However, no matter how good the quality of the oil seal, there will be a certain amount of leakage, as long as it can be used normally within a reasonable range, it is called the amount of drift on the loader arm cylinder. It is within the normal range for the natural sinking amount of a typical loader arm to sink 1-3 cm within five minutes.
General measures to eliminate problems
We have mentioned that there are many reasons for the loader arm to fall, so how to accurately determine the root cause of a loader, we can refer to the following methods.
First of all, you can first check the safety valve with rod cavity and rodless cavity in the adjustment cylinder. After the adjustment, investigate whether the fault disappears. If the arm failure disappears at this time, it means that the safety valve in the cylinder without rod cavity is severely leaking and needs repair or replacement. .
If the safety valve is still missing, you can check the hydraulic oil return filter element to investigate whether there are rubber fragments or iron debris on the filter element. If there are rubber fragments on the oil return filter element, it may be that the oil seal is worn seriously and needs to be replaced. After the oil cylinder is sealed.
If the arm failure still occurs afterwards, you need to check the sticking valve of the hydraulic circuit. You need to disassemble it to see if it is severely worn, timely grinding and correcting or replacing the sticking valve.
After checking and replacing the sticking valve, if the loader arm drop still exists, it is necessary to disassemble the main control valve, check all the spools on the hydraulic circuit, and correct or replace it in time.
to sum up:
The failure of the arm of the loader is a common problem. Most of the loaders show the phenomenon of arm loss because the oil seal is worn and the cylinder shows internal leakage. In the process of viewing, the simple fault point can be viewed first. The spool of the main control valve Wear usually appears after the loader has been used for a long time, and repairing the main control valve is a relatively complicated process. It is recommended to check it at the end.
