How magnetic fluid seals work

First, the magnetic fluid

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1. Composition of magnetic fluid

The magnetic fluid invented by Papell in the United States in 1995 is a stable dispersion of a fine magnetic powder (about 100 Ã…) such as magnetite in a liquid such as water, oil, ester or ether. a colloidal liquid. This liquid has the characteristics of being attracted to the magnet under the action of the usual centrifugal force and the magnetic field, neither sinking nor agglomerating, but also subjecting itself to magnetism.

The magnetic fluid consists of three main components:

1 solid ferromagnetic particles (Fe3O4);

2 a surfactant (stabilizer) which coats the particles and prevents them from agglomerating with each other;

3 carrier liquid (solvent).

2, the 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 aggregation, no precipitation, no decomposition; high saturation magnetization; high initial permeability; low viscosity and saturated vapor, others such as freezing point, boiling point, heat conduction There are also certain requirements for rate, specific heat and surface tension.

The main factors affecting the stability of magnetic fluids are: particle size, surfactant and carrier liquid, and their reasonable ratio. Stability is a prerequisite for the existence of various properties of magnetic fluids.

Second, the working principle of magnetic fluid seal

The circular permanent magnet, the magnetic circuit formed by the pole piece and the rotating shaft, concentrates the magnetic fluid placed between the shaft and the tip of the pole piece under the action of the magnetic field generated by the magnet to form a so-called "O" shape. The ring blocks the gap channel to achieve the purpose of sealing. This sealing method can be used in the case where the rotating shaft is a magnetic body and the rotating shaft is a non-magnetic body. The former magnetic flux concentrates on the gap and penetrates the rotating shaft to form a magnetic circuit, and the latter magnetic flux ratio does not pass through the rotating shaft, but constitutes a magnetic circuit by sealing the magnetic fluid in the gap.

Third, the limit conditions

Magnetic fluid seals are subject to the following conditions during operation:

1. Evaporation. The magnetic fluid consists of three parts: magnetic particles, surfactant and carrier liquid. The evaporation of the carrier liquid is the main factor determining the ultimate rotational frequency and service life of the seal. Because the seal works with a limited magnetic fluid. For this reason, a carrier liquid with a low vapor pressure should be selected to minimize the evaporation loss.

2, temperature rise. An increase in temperature causes demagnetization of the magnet and evaporation of the magnetic fluid. This is an advantageous side because the temperature rises, the viscosity decreases, and the power consumption decreases. However, if the temperature rises and the magnetic saturation strength decreases, the pressure resistance of the seal may also decrease somewhat. Therefore, the temperature of the magnetic fluid should generally not be higher than 105 ° C, otherwise cooling measures should be adopted.

3. Ultimate vacuum. The ultimate vacuum of the magnetic fluid seal depends on the volatility of the carrier liquid. The carrier liquid made of the diester lubricant can meet the requirements of 1.333×10-7 Pa ultra-high vacuum technology.

4, weekly speed. Generally, the magnetic fluid seal is suitable for operation with a high peripheral speed of 30 m/s or more, and there is no limit mark. However, considering the temperature and heat dissipation, the peripheral speed should be limited to 60 to 80 m/s. At this time, the ultimate withstand voltage capability should be considered.

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