Magnetic fluid seals
Nov 24, 2022
Composition of magnetic fluid
Magnetic fluid, invented by Papell (USA) in 1995, is a colloidal liquid in which a microfine powder (about 100Å) with strong magnetism such as magnetite is stably dispersed in water, oil, ester, ether and other liquids. This fluid has the property that it can be attracted by a magnet by not settling and coagulating but also by subjecting itself to magnetism under the action of usual centrifugal force and magnetic field.
The magnetic fluid consists of three main components: 1. solid ferromagnetic particles (Fe3O4); 2. a surfactant (stabilizer) that coats the particles and prevents them from coalescing with each other; and 3. a carrier liquid (solvents).
Properties of magnetic fluids
Magnetic fluid is a kind of solution called colloid. As a magnetic fluid for sealing, its performance requirements are: good stability, no coalescence, no precipitation, no decomposition; high saturation magnetization strength; high initial magnetic permeability; low viscosity and saturation vapor, other requirements such as freezing point, boiling point, thermal conductivity, specific heat and surface tension.
The main factors affecting the stability of magnetic fluids are: the size of the particle strength, surfactant and carrier fluid and their reasonable ratio. Stability is a prerequisite for the existence of various properties of magnetic fluids.
Working principle of magnetic fluid seal
The magnetic circuit formed by the circular permanent magnet, the pole shoe and the shaft, under the action of the magnetic field generated by the magnet, concentrates the magnetic fluid placed in the gap between the shaft and the top of the pole shoe, forming a so-called "O" shaped ring, blocking the gap channel and achieving the purpose of sealing. This type of seal can be used in both cases where the shaft is magnetic and where the shaft is non-magnetic. In the former case, the magnetic beam is concentrated at the gap and passes through the shaft to form a magnetic path, while in the latter case, the magnetic beam does not pass through the shaft, but only through the magnetic fluid in the sealing gap to form a magnetic path.
Limiting Conditions
The magnetic fluid seal is limited in its operation by the following conditions: 1. Evaporation. The magnetic fluid consists of 3 parts: magnetic particles, surfactant and carrier fluid. The evaporation of the carrier fluid is the main factor that determines the ultimate rotation frequency and service life of the seal. Because the seal works on a limited amount of magnetic fluid. For this reason, a carrier fluid with low vapor pressure should be selected so that the evaporation loss is a minimum value. 2. Temperature rise. Temperature rise can lead to magnet demagnetization and evaporation of magnetic fluid. Because the temperature rises, the viscosity decreases, and the power consumption decreases, which is a favorable side. But the temperature of, rising, magnetic saturation strength decreases, but also may make the seal pressure resistance is somewhat reduced, therefore, the magnetic fluid temperature should generally not be higher than 105 ℃, otherwise cooling measures should be used. 3. Ultimate vacuum. Magnetic fluid seal ultimate vacuum depends on the volatility of the carrier fluid, the carrier fluid made of dibasic lubricant can meet the requirements of 1.333 × 10-7Pa ultra-high vacuum technology. 4. Peripheral speed. Generally, magnetic fluid seal is applicable to the operation of high circumferential speed above 30m/s, without limit mark. However, taking into account the temperature and heat dissipation, the circumferential speed should be limited to 60-80m/s, when the ultimate pressure resistance is also considered.
