Hydraulic loaders often work in wet, dust, mud, low temperature or high temperature, as well as strong light radiation and other environments, requiring its hydraulic system to work reliably for a long time. If the hydraulic system leaks, it should be repaired in time.
1. Type of leakage
There are mainly two kinds of leakage of the loader hydraulic system. One is the sealing leakage of the fixed part (i.e. the static joint surface, such as the joint between the cylinder head and the cylinder); The second is the leakage of the sliding part (that is, the dynamic joint surface, such as the hydraulic cylinder piston and the inner wall of the cylinder, the piston rod and the cylinder head guide sleeve) seal, which can also be divided into internal leakage and external leakage. Internal leakage is mainly generated in the hydraulic valve, hydraulic pump (hydraulic motor) and hydraulic cylinder internal oil flow from the high pressure chamber to the low pressure chamber; The external leakage is mainly generated on the outside of the hydraulic pipeline, hydraulic valve, hydraulic cylinder and hydraulic pump (hydraulic motor) of the hydraulic system, that is, the leakage to the outside of the parts. Specific performance for pipe joints, seals, component joint surface, shell and system itself caused by the oil leakage.
2. Cause of leakage
Leakage of hydraulic system is usually produced after a period of use. From the surface phenomenon, most of the seals failure, damage
Broken, extruded, or strained sealing surface, etc. The main reasons are: oil pollution, improper sealing surface roughness, unqualified sealing groove, loose pipe joint, gap increase of fitting parts, high oil temperature, sealing ring deterioration or poor assembly.
(1) The leakage of the pipe joint is related to the processing accuracy, fastening strength and whether the burr is removed and other factors. The main performance is that the type of pipe joint is inconsistent with the conditions of use; The structure design of pipe joint is not reasonable; The processing quality of the pipe joint is poor, and the sealing effect is not possible. The pressure pulsation causes the pipe joint to loosen, and the bolt is not tightened timely after the creep looseness; The connector tightening torque is too large or insufficient.
(2) The leakage caused by the seal is related to the damage or failure of the seal. The main performance is the seal material or structure type and use conditions do not match; Failure of seals, insufficient compression, aging, damage, unqualified geometric accuracy, poor processing quality, informal products; Seal hardness, pressure grade, deformation rate and strength range of indicators are not required; Improper installation of seals, surface wear or hardening, and expiration of life but not timely replacement.
(3) Leakage caused by the component joint surface is related to design, processing and installation. The main performance is that the design of the seal does not meet the requirements of the specification, the size of the sealing groove is unreasonable, the sealing precision is low, the clearance is out of whack; The error of sealing surface roughness and flatness is too large, and the machining quality is poor. Improper selection of sealing structure, resulting in deformation, so that the joint surface can not fully contact; The assembly is not careful, and the joint surface has sand or plastic deformation due to damage.
(4) Shell leakage mainly occurs on the defects of castings and welding parts, and gradually expands under the action of pressure pulsation or shock vibration of the hydraulic system.
(5) The main reason for the leakage of the system itself is that the system assembly is rough, the lack of vibration reduction and vibration isolation measures; System overpressure use; Failure to timely check and deal with the system as required; The service life of wearing parts is expired but not replaced in time.
3. Prevention and control of leakage
(1) Prevent oil pollution
The suction port of the hydraulic pump should be installed with a coarse filter, and the suction port should be a certain distance from the bottom of the tank; High pressure fine filter should be installed at the oil outlet, and the filtering effect should meet the working requirements of the system, to prevent the blockage of dirt caused by hydraulic system failure; A filter screen should be installed on the separator of the hydraulic tank to remove the impurities not filtered by the return oil filter. Metal protective ring should be installed on the hydraulic cylinder to prevent dirt from being brought into the cylinder, and to prevent mud water and light radiation from eroding the hydraulic cylinder and causing leakage; Check and clean up the iron filings and impurities inside the hydraulic components before installation; Check the hydraulic oil regularly. Once the oil deteriorates, foams, precipitates, oil and water separation and other phenomena are found, the system should be cleaned and the oil changed immediately. The new oil should be settled before being added to the tank, and can be added after being filtered. If necessary, the middle tank can be set to precipitate and filter the new oil to ensure the cleanliness of the oil.
(2) the roughness of the sealing surface should be appropriate
Leakage will occur when the roughness of the relative moving pair surface of the hydraulic system is too high or when axial scratches appear. Roughness is too low, when reaching the mirror, the lip of the sealing ring will scrape the oil film, so that the oil film is difficult to form, sealing edge produces high temperature, aggravate wear, so the roughness of the sealing surface can not be too high or too low. The sliding surface in contact with the sealing ring must have a low roughness. The roughness of the surface of the hydraulic cylinder, slide valve and other dynamic seals should be between Ra0.2 ~ 0.4 ratio m, so as to ensure that the oil film on the sliding surface is not destroyed during movement. When the axial scratch appears on the rod of hydraulic cylinder and slide valve, the light can be polished by metallographic sandpaper, and the heavy should be repaired by electroplating.
(3) Reasonable design and processing of sealing groove
The design or processing of hydraulic cylinder seal groove is a prerequisite for reducing leakage and preventing premature damage of oil seal. If the static sealing groove size of the piston and piston rod is small, the sealing ring has no small room for activity in the groove, the bottom of the sealing ring will be damaged by the action of the reaction force and lead to oil leakage. The design of the sealing groove (mainly the structural shape, size, tolerance of the groove and the roughness of the sealing surface, etc.) should be carried out in strict accordance with the standard requirements.
To prevent oil leakage from the static seals, it is necessary to reasonably design the sealing groove size and tolerance of the static seals, so that the static seals after installation can be squeezed and deformed to plug the micro pits on the surface, and the internal stress of the seals can be raised to higher than the sealed pressure. When the stiffness of the parts or the bolt preload is not large enough, the mating surface will be separated under the action of oil pressure, resulting in too much clearance. With the movement of the mating surface, the static seal becomes a dynamic seal.
(4) Reduce shock and vibration
The impact of the hydraulic system is mainly produced in the process of variable pressure, variable speed and reversing. At this time, the liquid flowing in the pipeline forms a high pressure peak instantly due to the rapid reversing and the sudden closure of the valve port, which causes the leakage of the connectors, joints and flanges loose or the sealing ring squeezed into the clearance. To reduce leakage due to shock and vibration, the following measures can be taken:
① All pipes are fixed with damping supports to absorb shock and vibration energy.
② The use of damping reversing valve, slow switching valve, in the end of the hydraulic cylinder set buffer device (such as one-way throttle valve).
③ Use low impact valves or accumulators to reduce impact.
④ Arrange pressure control valves appropriately to protect all components of the system.
⑤ Try to reduce the number of pipe joints, and pipe joints as far as possible with welding connection.
⑥ The use of threaded direct head, tee joint and elbow instead of taper pipe threaded joint.
⑦ Try to use back oil block instead of each configuration.
⑧ For the use of the highest pressure, the installation of the use of bolt torsion and plug torsion, prevent joint surface and seals are damaged.
(5) Reduce the wear of dynamic seals
Most of the dynamic seals in the hydraulic system are precisely designed. If the dynamic seals are processed and qualified, installed correctly and used reasonably, no leakage can be guaranteed for a long time. From the design point of view, the following measures can be used to extend the life of the moving seals:
① Eliminate the radial load on the piston rod and driving shaft seals.
② Protect the piston rod with dust ring, protective cover and rubber sleeve to prevent dust and other impurities from entering.
③ Design and select the appropriate filtration device and easy to clean the tank, in order to prevent dust accumulation in the oil.
④ Keep the speed of the piston rod and shaft as low as possible.
(6) Reasonable design of the mounting plate
When the loader hydraulic system valve group or bottom plate is bolted on the mounting surface, in order to obtain a satisfactory initial seal and prevent the sealing parts from being extruded groove and worn, the mounting surface should be flat, the sealing surface should be finished, the surface roughness should be less than Ra0.8μm, the flatness error should be less than 0.01/100mm; There should be no radial scratches on the surface, and the preload of the connecting screws should be large enough to prevent the surface from separating.
(7) To correctly assemble the sealing ring
When assembling the sealing ring, oil should be applied to its surface. If you need to pass through the opening parts such as the keyway and thread on the shaft, use guiding tools instead of metal tools such as screwdrivers. Otherwise, the sealing ring will be scratched and oil will leak. For the directional sealing ring (such as V, Y and Yx sealing ring), the assembly should be lip against the pressure oil chamber, pay attention to protect the lip, to avoid the sharp edge of the parts, such as burr scratches. For the sealing surface of rotary contact (such as the end of the driving gear shaft of the hydraulic pump), the double sealing ring should be selected. Before installing the combined seals, the seals should be boiled in hydraulic oil to a certain temperature; Special guide sleeves and closing tools should be used during installation, and the manufacturer's installation instructions for the seals should be strictly followed.
(8) Control oil temperature to prevent deterioration of seals
An important cause of premature seal deterioration is high oil temperature. In most cases, when the oil temperature often exceeds 60℃, the oil viscosity decreases greatly, and the sealing ring expands, ages and fails, resulting in leakage of the hydraulic system. According to research, every 10℃ increase in oil temperature will halve the life of the seal, so the oil temperature should be controlled within 65℃. To this end, the oil outlet pipe and the return pipe inside the tank should be separated by a partition to reduce the distance between the oil tank and the actuator (cylinder or motor), and the pipe should use as few right Angle elbows as possible; In addition, attention should be paid to the compatibility of oil and sealing materials, and the type and material of hydraulic oil and sealing parts should be selected according to the instruction manual or relevant manual.
(9) Attach importance to the repair and assembly process
The repair process of leakage prevention and control should be strengthened. For example, the whole or part of valve stem, piston surface and cylinder inner wall can be thickened by electric brush plating and electrostatic spraying, and then processed to the required size by lathe. When installing threaded pipe joints, wrap teflon raw tape around the thread. The casting or welding parts should be inspected and tested before installation. The pressure of the test is equivalent to 150% ~ 200% of the highest working pressure. When the oil seal is loaded into the seat hole, special tools should be used to import it to prevent the position from skewing.
