Working principle of magnetic fluid seal
1. Magnetic fluid
1. Composition of magnetic fluid
The magnetic fluid invented by American Papell in 1995 is a stable dispersion of magnetite and other highly magnetic fine powder (about 100?) In liquids such as water, oils, esters, ethers, etc. A colloidal liquid. This liquid has the characteristics of not being settled and condensed under the action of the usual centrifugal force and magnetic field, but also able to withstand the magnetism itself and can be attracted by the magnet.
Magnetic fluid consists of 3 main components:
â‘ Solid ferromagnetic particles (Fe3O4);
â‘¡A surfactant (stabilizer) that coats the particles and prevents them from agglomerating;
â‘¢ Carrier liquid (solvent).
2. Characteristics of magnetic fluid Magnetic fluid is a kind of colloidal solution. As a magnetic fluid for sealing, its performance requirements are: good stability, no condensation, no precipitation, no decomposition; high saturation magnetization; high initial magnetic permeability; low viscosity and saturated vapor, other such as freezing point, boiling point, thermal conductivity Rate, specific heat and surface tension also have certain requirements.
The main factors affecting the stability of the magnetic fluid are: particle strength, surfactant and carrier liquid, and their reasonable ratio. Stability is a prerequisite for the existence of various characteristics of magnetic fluids.
2. The working principle of the magnetic fluid seal The annular permanent magnet, the magnetic circuit formed by the pole shoe and the rotating shaft, under the action of the magnetic field generated by the magnet, the magnetic fluid placed between the gap between the shaft and the top of the pole shoe is concentrated to make it A so-called "O" ring is formed to block the gap channel and achieve the purpose of sealing. This sealing method can be used for two occasions where the rotating shaft is magnetic and the rotating shaft is non-magnetic. The former magnetic flux is concentrated at the gap and penetrates the rotating shaft to form a magnetic circuit, while the latter magnetic flux ratio does not pass through the rotating shaft, but only forms a magnetic circuit by sealing the magnetic fluid in the gap.
Third, the limit conditions The magnetic fluid seal will be limited by the following conditions when working:
1. Evaporation. The magnetic fluid is composed of magnetic particles, surfactant and carrier liquid. The evaporation of the carrier liquid is the main factor that determines the limit rotation frequency and service life of the seal. Because the seal works with limited magnetic fluid. For this reason, carrier fluids with low vapor pressure should be selected to minimize evaporation losses.
2. Temperature rise. An increase in temperature will cause demagnetization of the magnet and evaporation of the magnetic fluid. Because the temperature increases, the viscosity decreases, and the power consumption also decreases, which is an advantage. However, the increase in temperature and the decrease in magnetic saturation strength may also reduce the pressure resistance of the seal. Therefore, the temperature of the magnetic fluid should not be higher than 105 ° C, otherwise cooling measures should be adopted.
3. Ultimate vacuum. The ultimate vacuum degree of the magnetic fluid seal depends on the volatility of the carrier liquid, and the carrier liquid made with the diluent lubricant can meet the requirements of 1.333 × 10-7 Pa ultra-high vacuum technology.
4. Weekly speed. The general magnetic fluid seal is suitable for the operation with a high peripheral speed of 30m / s or more, and there is no limit mark. However, considering the temperature and heat dissipation, the peripheral speed should be limited to 60 ~ 80m / s, at this time, the ultimate pressure capability must also be considered.
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