Leakage and prevention measures of loader hydraulic system
Hydraulic loaders often work in wet, dust, mud, low or high temperatures, as well as strong light radiation and other environments, requiring their 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 two main types of leakage in the loader hydraulic system, one is the leakage of the seal of the fixed part (that is, the static joint surface, such as the joint of the cylinder head and the cylinder); The second is the leakage of the sliding part (that is, the dynamic joint surface, such as between the hydraulic cylinder piston and the inner wall of the cylinder, the piston rod and the cylinder head guide sleeve), which can also be divided into internal leakage and external leakage. Internal leakage mainly occurs in the hydraulic valve, hydraulic pump (hydraulic motor), and hydraulic cylinder internal oil flow from the high-pressure chamber to the low-pressure chamber; 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, to the outside of the parts. The specific performance is the oil leakage caused by pipe joints, seals, component joints, shells, and the system itself.
2. Cause of leakage
Leakage of the hydraulic system is generally generated after a period of use. From the surface phenomenon, most of the seals fail, damage
Broken, extruded, or strained sealing surfaces. The main reasons are oil pollution, improper sealing surface roughness, unqualified sealing groove, loose pipe joints, increased clearance of mating parts, high oil temperature, seal ring deterioration, or poor assembly.
(1) The leakage of the pipe joint is related to the processing accuracy of the connection, the tightening strength, and whether the burr is removed. The main performance is that the type of pipe joint selected is inconsistent with the conditions of use; The structural design of the pipe joint is not reasonable; The processing quality of the pipe joint is poor, and it cannot seal; The pressure pulsation causes the pipe joint to loosen, and the bolt is not tightened in time after creep loosening; 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 that the material or structural type of the seal does not match the conditions of use; Seal failure, insufficient compression, aging, damage, unqualified geometric accuracy, poor processing quality, informal products; The seal hardness, pressure grade, deformation rate, and strength range are not required; Improper installation of seals, surface wear or hardening, and the expiration of life but not a timely replacement.
(3) Leakage caused by the component bonding 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, and the matching gap is out of line; Sealing surface roughness and flatness error is too large, the processing 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) The leakage of the shell mainly occurs in the defects of castings and welds, 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, and the lack of vibration reduction and isolation measures; System overpressure use; Failure to check and deal with the system in a timely manner as required; The life of the wearing part expires but is not replaced in time.
3. Leakage prevention
(1) Prevent oil pollution
The oil suction port of the hydraulic pump should be installed with a coarse filter, and the oil suction port should be a certain distance from the bottom of the tank; The high pressure fine filter should be installed at the oil outlet, and the filtration effect should meet the working requirements of the system to prevent the blockage of dirt and cause the failure of the hydraulic system; A filter screen should be installed on the diaphragm of the hydraulic tank to remove the impurities not filtered by the return oil filter. Metal protective rings 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; Before the installation of hydraulic components, check and clean up the iron filings and impurities inside; Regularly check the hydraulic oil, once found oil deterioration, foam, sediment, oil and water separation phenomena should be cleaned immediately and oil change. The new oil should be settled before being added to the tank, and can be added after filtration. If necessary, the middle tank can be set up 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 surface roughness of the hydraulic system relative to the moving pair is too high or axial scratch occurs. When the roughness is too low, the lip of the sealing ring will scrape the oil film off when the mirror is reached, making the oil film difficult to form, and the sealing edge will produce high temperature, which will aggravate the 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, and the roughness of the surface of the hydraulic cylinder, slide valve and other dynamic seals should be between Ra0.2 ~ 0.4HCM to ensure that the oil film on the sliding surface is not destroyed during movement. When axial scratches appear on the rod of hydraulic cylinder and slide valve, the light can be polished with metallographic sandpaper, and the heavy should be repaired by electroplating.
(3) Reasonable design and processing of sealing grooves
The design or processing of the hydraulic cylinder seal groove is a prerequisite for reducing leakage and preventing premature damage to the oil seal. If the groove size of the static seal of the piston and piston rod is small, the sealing ring has no small room for movement in the groove, and the bottom of the sealing ring will be damaged by the action of the reaction force, resulting in oil leakage. The design of the sealing groove (mainly the structural shape, size, shape and position tolerance 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 seal, it is necessary to reasonably design the sealing groove size and tolerance of the static seal, so that the static seal can be squeezed and deformed after installation, and the internal stress of the seal can be increased to a higher pressure than the sealed. When the stiffness of the part or the pre-tightening force of the bolt is not large enough, the mating surface will be separated under the action of the oil pressure, resulting in a large gap, and the static seal will become a dynamic seal with the movement of the mating surface.
(4) Reduce shock and vibration
The impact of the hydraulic system is mainly generated 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, or the sealing ring is squeezed into the clearance damage. In order to reduce leakage due to shock and vibration, the following measures can be taken:
① All pipes are fixed with shock absorber brackets to absorb the energy of shock and vibration.
② Use a damped reversing valve, a slow switching valve, and a buffer device (such as a one-way throttle valve) at the end of the hydraulic cylinder.
③ Use low impact valves or accumulators to reduce impact.
④ Arrange pressure control valves appropriately to protect all components of the system.
⑤ Minimize the number of pipe joints used, and pipe joints should be welded as far as possible.
⑥ Use threaded direct heads, tee joints and elbows instead of taper pipe threaded joints.
⑦ Try to replace each configuration with a return oil block.
⑧ According to the highest pressure used, the bolt torque and plug torque used during installation are specified to prevent the joint surface and seals from being damaged.
(5) Reduce the wear of dynamic seals
Most of the dynamic seals in the hydraulic system are precisely designed, and if the dynamic seals are processed properly, installed correctly and used reasonably, no leakage can be guaranteed for a long time. From a design point of view, the following measures can be used to extend the life of the dynamic seal:
① Eliminate radial load on the piston rod and drive shaft seals.
② Protect the piston rod with a dust ring, protective cover and rubber sleeve to prevent dust and other impurities from entering.
(3) Design and select a suitable filtration device and a fuel tank for easy cleaning 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 valve group or bottom plate of the loader hydraulic system is bolted on the mounting surface, in order to obtain a satisfactory initial seal and prevent the seal from being extruded and worn, the mounting surface should be flat and straight, the sealing surface should be finished, the surface roughness should be less than Ra0.8μm, and 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 it is necessary to pass through the opening parts of the keyway and thread on the shaft, guide tools should be used. Do not use metal tools such as screwdrivers, otherwise, the sealing ring will be scratched and cause oil leakage. For directional seals (such as V, Y and Yx seals), the lip should be placed against the pressure oil chamber during assembly, and attention should be paid to protecting the lip to avoid being scratched by the sharp edge and burr of the parts. For the sealing surface of rotary contact (such as the end of the hydraulic pump driving gear shaft), the double lip 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 seals should be strictly followed.
(8) Control oil temperature to prevent seal deterioration
An important reason for premature seal deterioration is high oil temperature. In most cases, when the oil temperature often exceeds 60 ° C, the oil viscosity greatly decreases, and the sealing ring expands, ages, and fails, resulting in leakage of the hydraulic system. According to research, the life of the seal will be halved for every 10℃ increase in oil temperature, so the oil temperature should be controlled within 65℃. To this end, the oil 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 right angle elbow should be used as little as possible on the pipeline. In addition, attention should be paid to the compatibility of oil and sealing materials, and the type and material of hydraulic oil and seals should be selected according to the instruction manual or relevant manuals.
(9) Attach importance to the repair and assembly process
The repair process of leakage prevention and control should be strengthened, such as the whole or part of the valve stem, piston surface and cylinder inner wall can be thickened by brush plating and electrostatic spraying, and then machined to the required size by lathe. When installing threaded fittings, tape of PTFE raw material should be wrapped around the threads. Casting or welding parts should be inspected and pressure tested before installation, the pressure of the pressure test is equivalent to 150% to 200% of its maximum 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.
