What Is a Magnetic Seal?
A magnetic seal is a seal unit for rotary motion that uses a magnetic fluid attracted by a magnet to seal.
They are sometimes referred to as Magnetic Fluid Seals or Vacuum Seals. A rotary seal seals and seals the hydraulic fluid that allows equipment to function over the long term.
Magnetic seals are primarily used in atmospheres, dust, oil mist, gases, and vacuums.
Uses for Magnetic Seals
The primary use for magnetic fluidic seals is in the bearings of HDDs in personal computers and other hardware. Magnetic fluidic seals improve durability and reduce noise from rotation.
Other applications include semiconductor manufacturing equipment, analytical equipment, and other equipment where a vacuum field must be maintained. However, a single vacuum seal is not sufficient to seal the vacuum field. Therefore, a vacuum is maintained by configuring multiple vacuum seals in a multi-stage configuration.
In addition, magnetic seals using rare earth magnets can strongly hold magnetic fluid due to the strong magnetic field. Vacuum seals that can maintain a difference of 1 atmospheric pressure are used for CVD equipment, RVD equipment, ion implantation equipment, etc.
Principle of Magnetic Seals
The mechanism of the magnetic seal is based on the property that magnetic fluid is attracted to a magnet. A permanent magnet and magnetic fluid are placed around the axis of rotation to be sealed, and the magnetic fluid is held in place around the axis of rotation to create a seal.
The stronger the magnetic force, the greater the pressure resistance of the magnetic fluid, and it is common to design a single magnetic seal with a multi-stage structure of 10 to 20 stages. By forming multiple stages, a magnetic seal with a large pressure resistance is completed.
There are three types of magnetic fluid: water-based, hydrocarbon oil-based, and fluorocarbon oil-based. The hydrocarbon-based type is commonly used. Water-based magnetic fluids can evaporate, and hydrocarbon-based magnetic fluids are more suitable from the standpoint of type and price. Fluorocarbon oil-based magnetic fluids are used when chemical stability, low vapor pressure, and heat resistance are required.
Other Information on Magnetic Seals
1. Advantages of Magnetic Seals
High sealing and non-dusting properties
Since magnetic seals are sealed with a liquid, gaps are difficult to form. Also, since it is not a contact between individuals, it does not generate any wear dust due to friction.
Low Friction Torque
Since the contact is not between individuals with contact pressure like an oil seal, the loss resistance can be kept extremely low.
Low Heat Generation and Low Noise
The friction generated by the contact between the individual and the liquid generates almost no heat or noise.
Sealed in Both Rotating and Stationary Conditions
The sealing performance of an individual oil seal is due to the pumping effect caused by the rotation of the shaft. The seal is maintained as the shaft rotates. Magnetic seals seal regardless of whether the shaft is rotating or stationary, as long as the magnetism is maintained.
Relatively Easy to Design
Individual oil seals require a detailed design of the lip shape in contact with the rotating shaft. In contrast, magnetic seals are relatively easy to design.
2. Disadvantages of Magnetic Seals
Limited Heat Resistance
Magnetic fluid has the property of becoming less magnetic as temperature increases, and once it reaches the Curie point, it loses its magnetism completely. The pressure at which it can be sealed decreases at higher temperatures.
Not Suitable for Sealing Liquids
Since magnetic fluid is a liquid that seals, there is a risk that the magnetic fluid may dissolve depending on the liquid to be sealed.
Not Suitable for Reciprocating Seals
While individual O-rings can be used for reciprocating sealing, magnetic seals are not suitable for reciprocating sealing because the magnetic fluid may be taken out.
Not Suitable for High-Pressure Seals
Magnetic fluid increases sealing pressure by reducing the gap between the rotating shaft and the magnetic pole pieces that provide magnetic force to the magnetic fluid. However, it cannot seal as high pressure as an individual oil seal.
Since the pressure that can be sealed decreases with temperature and rotational speed, it is important to design for the operating environment.