1 The role of the filter in the hydraulic system
The role of the filter in the hydraulic system is:
1) Filter the dirt particles in the oil, prevent the moving parts from jamming, parts scratches, oil passage blockage and other failures, and improve the reliability and stability of the system.
2) Filter out fine particles of impurities in the oil, prevent abrasive wear of parts, and extend service life. For this reason, the filtration accuracy should meet the sliding clearance that needs to be protected.
3) By analyzing the accumulated pollutants in the filter, analyzing the hidden trouble, finding the worn parts, and monitoring the operation of the equipment.
2 Filter type
The filter used in the hydraulic system is divided into three types, namely surface type, depth type and adsorption type, according to the filtration mechanism of the filter material.
1) Surface type filter
Filtering is achieved by a geometric surface.
The filter element material has a uniform calibration hole, which can filter out particles larger than the size of the hole. The filtered particles are trapped on the surface of the filter element on the upstream side of the oil.
Because the particles gather on the surface, the filter element is easily blocked and needs to be cleaned regularly.
Mesh filters and wire gap filters belong to this category.
The filter element is composed of copper wire mesh wrapped on the core frame. The filtration accuracy is low, which is related to the mesh size and the number of mesh layers. There are 80, 100, 180μm. For pressure pipeline, commonly used 100, 150, 200 copper wire mesh; For hydraulic pump suction pipe, commonly used 20 ~ 40 copper wire mesh. Pressure loss does not exceed 0.004MPa. Simple structure, large circulation capacity, convenient cleaning.
The filter element is composed of metal wire wound on the core frame, and the small gap between the lines is used to block the passage of particles. High filtration accuracy, 30, 50, 80μm and other three grades. For low pressure lines. Pressure loss does not exceed 0.03 ~ 0.06MPa. The structure is simple, the flow capacity is large, the strength of the filter material is low, and it is not easy to clean.
2) Depth filter
The filter element is composed of porous permeable material and has a zigzag channel inside. Particles larger than the surface aperture are trapped on the outer surface; Smaller particles enter the material and are adsorbed by the channel wall. Paper, felt, sintered metal, ceramics, various fiber products and other filter elements of the filter belongs to this category.
① Paper core filter. The structure is the same as the line-gap filter, but the filter element is made of plain and corrugated chemical fiber or wood pulp microporous filter paper. In order to increase the filtering area, the paper core is often made into a folded shape. In order to enhance the strength of the filter element, its inner and outer sides are usually reinforced with metal mesh. High filtration accuracy, up to 5 ~ 20μm. The pressure loss is about 0.01 ~ 0.4MPa.
② sintered filter. The filter element is sintered from metal powder, and micropores between metal particles are used to block the particles in the oil. The filtration accuracy is high, up to 5 ~ 10 μm, and the filtration accuracy can be changed by changing the size of the metal powder particles. Suitable for fine filtration.
3) Adsorption filter
The filter material adsorbs the particles in the oil onto the filter. Magnetic filters are adsorption filters, made of permanent magnets, which can absorb iron filings, iron powder or magnetic abrasives in the oil. It is usually combined with other types of filters to make a compound filter. Magnetic filters also monitor the wear of mechanical equipment.
Magnetic filter is divided into ordinary magnetic filter and high gradient magnetic filter. Ordinary magnetic filters use the magnetic field of the magnet to directly capture ferromagnetic pollution particles in the oil. Among them, the permanent magnet filter has the advantages of simple structure, convenient manufacturing, reliable operation and low operating cost, so it is widely used, and its main disadvantages are poor particle capture effect, and the particles adsorbed on the magnetic pole are difficult to clean. The high gradient magnetic filter can effectively overcome the above shortcomings, so that the performance of the magnetic filter is greatly improved.
The attraction of ferromagnetic particles is proportional to the external magnetic field intensity H, and is proportional to the gradient gradH of the magnetic field intensity.
Ordinary magnetic filter is to improve the magnetic attraction by increasing the external magnetic field strength H, so as to improve the filtration capacity; The high gradient magnetic filter is to increase the attraction by increasing the gradient gradH of the magnetic field through the magnetic medium to obtain the effect of efficient filtering of pollutants.
The arrangement of polymagnetic porous media in the magnetic field will make the magnetic field H and magnetic gradient gradH near the magnetic medium much larger than that without magnetic medium, especially gradH is inversely proportional to the magnetic medium section diameter d, when d is very small, the gradH value can be very high. Therefore, the high gradient magnetic filter based on this principle is also called the high efficiency magnetic filter.
The high gradient magnetic filter uses the magnetic medium magnetized by the magnet to capture the pollution particles, and the magnetic medium can greatly improve the magnetic field gradient around the medium, so the filtration performance of the high gradient magnetic filter is significantly higher than that of the traditional magnetic filter.
