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Rotary Feedthrough
The characteristic of Rotary Feedthrough

What is a Rotary Feedthrough?

A Rotary Feedthrough is a kind of sealing device: a specialized part that prevents leakage of liquids or gases.
The Rotary Feedthrough is a seal unit for rotating parts that uses the remarkable property of magnetic fluid which is held by magnetic field of magnets.
If you're interested in the components of magnetic fluid and their properties, we invite you to read the article What is a magnetic fluid?
Seals come in a wide variety of types, with a wide range of properties.
You can read a general explanation of the types of seals here.

A magnetic fluid is held in the gap between the Shaft and the Pole Piece in a Rotary Feedthrough. The magnetic fluid flows along the Magnetic flux lines generated by the magnet.
The magnetic fluid maintained in the gap by the magnetic force functions like an O ring, keeping fluid from escaping even in the presence of a pressure differential.
The strength with which the magnetic fluid is held is determined by the magnet. The stronger the magnetic force is, the greater the magnetic fluid's resistance to pressure in the ring will be.
By constructing a multistage ring of Pole Pieces, it is possible to build a Rotary Feedthrough strong enough to withstand even quite large pressure differentials.

Basic structure of a Rotary Feedthrough


The Rotary Feedthrough uses liquid to create a seal; there is no solid-solid contact in this design. This characteristic creates the following properties:

  • Clean: Because liquid is used for the sealing material, no solids come into contact between the Shaft and the Pole Piece, preventing particle generation due to friction.
  • High vacuum performance: By using the magnetic fluid of low vapor pressure, it can be used even in ultra high vacuum region of less than 10-6Pa.
  • Long Service Life: With no solid parts in contact between the Shaft and Pole Piece, virtually no loss from wear occurs, resulting in a low-maintenance seal with a long service life.
  • Other: Because the seal is formed using liquid, little torque is lost, so high-speed rotation is enabled.

Rigaku developed this extraordinary Rotary Feedthrough and its magnetic circuit exclusively, ensuring that the Rotary Feedthrough and the Magnetic Seal it creates deliver excellent performance.
For the secret of the Rigaku Magnetic Seal Unit, and the clean vacuum it generates, please read about Magnetic Circuit Configuration unique to Rigaku.

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Types of Seal

"Seal" is a generic term for any part used to prevent gas or liquid leakage, or the incursion of foreign matter from outside. Seals can be broadly classified according to function into dynamic seals and static seals.

"Dynamic seals" is a generic term for seals used to seal moving parts, such as rotating or reciprocating parts. They are also called "packing."
Similarly, "static seals" is the generic term used for seals applied to fixed parts, such as pipe flanges. They are also called "gaskets."

Although there are a wide variety of seal types, only five are widely used to facilitate rotation in vacuum environments: Rotary Feedthroughs, oil seals (Wilson seals), O-ring seals, bellows seals and magnetic-coupling seals.
The following section provides a convenient guide to the functional principles, structure, characteristics and other details of typical dynamic seals used in vacuum.

  Rotary feedthrough Oil seal
(Wilson seal)
O-ring seal Bellows seal Magnetic-coupling seal
Vacuum environments in which the seal can be used Ultra-high vacuum High vacuum High vacuum Ultra-high vacuum Ultra-high vacuum
Rotational speed High Low Low Low Low
Transfer torque High
(dependent on shaft strength)
High
(dependent on shaft strength)
High
(dependent on shaft strength)
Low Low;
lost motion
Dust generation Extremely little Great Great Little Little
Service life Long Short Short Short Short

*"Lost motion" is loss of tracking generated during rotation, acceleration or deceleration.

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Typical dynamic Seals Used in Vacuum

O-ring seals

Principles

A rubber O-ring is inserted into a groove with an appropriate degree of pressure, causing the rubber to push back.
The repellent force (contact pressure) so generated in the rubber O-ring creates the seal.

Advantages

The area of attachment has low volume, so the seal takes up little space.
The structure is simple, so it can be used in a relatively wide range of vacuum equipment.

Disadvantages

The contact pressure between the rubber and the shaft are what form the seal. This pressure creates considerable rotational resistance.
Friction is great, so service life is short.


Oil seals (Wilson seals)

Principles

The oil seal consists of a rubber lip, a metal reinforcing ring and a spring. The spring presses the edge of the lip against the shaft, forming the seal.

Advantages

This seal is simple in structure and relatively easy to handle.
It can be used in a wide range of situations, including cases where the shaft eccentricity is high and at high speeds.

Disadvantages

The seal is filled with grease or oil, which tends to leak, making the surrounding area dirty.
The contact pressure between the rubber and the shaft are what form the seal. This pressure creates wear from friction, so working life is short.


Rotary feedthrough

Principles

A magnetic fluid is held in the gap between a shaft and a pole piece along magnetic flux lines generated by a magnet.
The magnetic fluid held in the gap by the magnetic force does not flow out even in the presence of a pressure differentials, so it works like a liquid O-ring to form a seal.

Advantages

The seal is formed using a liquid, so contact between solids is avoided.
This structure virtually eliminates friction, yielding long service life, eliminating particle generation and supporting high rotational speeds.
Moreover, the vacuum-side bearing is moved to the atmosphere side, creating a cantilever structure that further ensures a clean ambient environment.

Disadvantages

The seal is for rotational movement only. It cannot be used as a translatory motion seal.


Bellows seal

Principles

The bellows seal uses the principle of elasticity to transfer rotational force from the atmosphere side of the seal to the vacuum side.

Advantages

The atmosphere and vacuum sides are isolated from each other by the welded bellows. Rubber is not used, so baking at high temperatures can be performed. This type of seal is used in ultra-high vacuum equipment.

Disadvantages

The bellows and welded section undergo repeated expansion, contraction and twisting, causing damage from fatigue that shortens working life. The vacuum side must be fitted with a bearing.


Magnetic coupling

Principles

The atmosphere side and vacuum side are separated by a thin barrier wall. The magnetic force of the magnet on the atmosphere side rotates the shaft on the vacuum side.

Advantages

The vacuum and atmosphere sides are shut off from each other by a nonmagnetic barrier wall. Since rubber is not used, this type of seal can be used in high-temperature baking environments. This seal is widely used in ultra-high vacuum equipment.

Disadvantages

Motion is transferred by magnetism, causing idling if the load is great. Moreover, a bearing must be installed on the vacuum side.


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