1.Various seal failure problems on site. The main situations of external leakage caused by seal failure are as follows:

• The seal between the piston rod of the hydraulic cylinder and the end cover fails, resulting in external leakage.

• The seals at both ends of the directional control valve fail, causing oil to splash out during valve shifting.

• The seals at the joints between the valve and the valve plate, as well as between valves, fail, leading to external leakage.

• The seal of the rotating shaft of the hydraulic pump or hydraulic motor fails, causing external leakage.

• The oil pipes and pipe joints are loose, resulting in external leakage.

• The flexible oil pipes have external leakage due to damaged seals or installation errors.

• Various internal leakages are mainly caused by the damage of the seals between the internal liquid flow channels of hydraulic components. Such damage may be due to the wear of relevant parts or seals, or it may be caused by the intercommunication of flow channels due to casting blowholes, cracks or processing errors in valves or valve plates. The existence of internal leakage in the hydraulic system leads to a decrease in system pressure and flow rate, a reduction in efficiency, increased heat generation and an increase in oil temperature, resulting in disordered system operation and inability to work properly.

2. The causes of seal failure. The causes of seal faults are very extensive and can be summarized into four categories:
(1) Unreasonable product structure. Problems in this category include: wrong selection of sealing methods, wrong selection of seal shape and material, wrong parameter design, wrong design of the seal support surface or contact surface, etc.
(2) Poor processing and assembly quality. These problems are mainly manifested as the dimensional accuracy of seals exceeding the tolerance, the surface roughness exceeding the tolerance, causing eccentricity, scratching, insufficient tightening or excessive tightening at the sealing parts. The installation holes without chamfering and deburring cause the seals to be sheared and scratched during installation. Poor mechanical processing quality of parts leads to blowholes and cracks, uneven surfaces, and intercommunication of oil circuits, etc. In terms of assembly, the causes of seal failure are: the assembly environment is not clean, bringing impurities into the sealing parts; rough assembly damages the seals; the seals are not installed in the correct position, causing them to be crushed; insufficient installation accuracy causes eccentricity or uneven tightening on both sides; no sealing glue is applied to the pipe joints; some screws of valve components are not tightened; the sealing direction of the seals is installed in the wrong way, etc.
(3) Poor quality or improper selection of seals. Some seals have poor appearance quality, rough surfaces, and large dimensional and shape errors. The spring of the rotary shaft lip seal has a large dimensional error, which may cause looseness. The seal material has poor oil resistance, ages rapidly, has a short storage period, and produces permanent deformation after compression.
(4) Improper use. Harsh working conditions (high temperature, high speed, high pressure) cause the seals to age too quickly, lose their elasticity, and result in leakage. Maintenance is not carried out in a timely and thorough manner, and the seals that should be replaced are not replaced; the pressure is adjusted too high, and the hydraulic shock is too large, causing the pipe joints to vibrate loose. The seals are misused, such as errors in cross-section, diameter and sealing method, etc.