Magnetic fluid feedthrough primer — page 1/4

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Introduction

Magnetic fluid feedthroughs are vital components in many modern vacuum systems. They are also used in other process systems. Yet, for many users, a certain mystery has surrounded these devices. This primer seeks to clarify any mystery by describing the technical foundations on which these feedthroughs stand.

Several factors should be considered when selecting the most appropriate feedthrough for a given application. This primer provides an overview of these technical issues. In many cases, this information will be sufficient to make a good choice.

These products have been used in many complex applications. Rigaku engineers are ready to help customers find the most appropriate feedthrough for a given application. In some cases, a custom design may be needed.

Basic sealing principle

Dynamic sealing is accomplished by a hermetic liquid seal that permits free rotation of the shaft. Several different arrangements of housing, shaft, bearings, magnets and fluid have been used over the years. Figure 1 illustrates the patented design employed in Rigaku's RMS family of products. Other designs will be discussed in later sections. 

Magnetic fluids are stable colloids comprising a base liquid, ferromagnetic particles, and a dispersing agent that suspends the particles in the base liquid. The ferromagnetic particles interact directly with an external magnetic field. Because of the strong coupling to the magnetic particles via the dispersing agent, the base liquid interacts indirectly with the magnetic field. Hence magnetic fluids can be pushed, pulled, and shaped by magnetic fields. In feedthroughs, narrow rings of fluid (shown here in red) form liquid barriers filling the annular spaces (or gaps) between a rotating shaft and the tips of a stationary pole piece. Radially, the fluid rings are bounded by the shaft and the pole piece tips. Axially, the rings are free surfaces at gas-liquid interfaces, restrained only by magnetic forces. Magnetic flux density at the pole tips is very large. Hence, any axial displacement of a liquid ring away from the pole tip results in a force that resists the displacement. The isolated volumes between adjacent rings are important in the functioning of the device. 

Maximum sustainable pressure difference across a single liquid ring is less than 1 atmosphere. Consequently, practical devices require a series of separate rings with small, isolated gas volumes between each pair of rings. The total pressure difference (typically 1 atmosphere in vacuum applications) is divided over several stages. 

Dimensions and locations of shaft and pole tips are critically important for reliable operation. A complete system of shaft, pole piece, bearings, housing, and other parts provides the necessary precision in a single package that is easily integrated into a vacuum system. Note that a single a-ring provides static sealing between the pole piece and housing.

Figure 1

Figure 1

Design improvements pioneered by Rigaku

Figure 2 shows how the earliest commercial products used a single ring magnet in combination with two smooth-walled pole pieces and a shaft that contained multiple grooves. This type of design is still used by some manufacturers (but NOT by Rigaku), despite the fact that it has several significant drawbacks:

  • The grooves weaken the shaft in two ways. First, the shaft diameter at the bottom of the grooves is reduced. Second, the corners of the grooves act as stress concentration points. In shafts of small diameter, the reduction in shaft strength is quite substantial.

  • Because this early design uses only a single magnet, the entire device has a significant overall magnetic polarity. In some applications the external magnetic field can be a serious problem. At a minimum, it is an annoyance.

  • The two separate pole pieces require each one to have its own O-ring seal to the housing. A leaky vacuum-end O-ring will escape detection by an ordinary helium leak test, since the other O-ring will not allow helium to pass. In such a case, the feedthrough would contain a large virtual leak (the region between the two O-rings, including the magnet) and would also have reduced pressure capacity since the inner fluid sealing stages would be bypassed by the virtual leak region. 

By comparison, the patented Rigaku design of Figure 1 has these advantages:

  • Full shaft diameter is maintained throughout the magnetic sealing region, for full strength. 
  • Four ring magnets (two pairs) are arranged with alternately opposed polarity. One magnet of each pair opposes and substantially nulls out the external field of its partner. So the entire feedthrough has only a very small residual external field. 
  • At the same time, the opposed-pair arrangement results in exceptionally high fields in the sealing region, thereby maximizing the force holding the sealing fluid in place against the external pressure gradient.
  • Because the magnets and pole rings are constructed as an integral pole piece unit, only one 0ring static seal to the housing is required. This O-ring is located at the vacuum end of the pole piece. Helium passes freely to the O-ring and all around the pole piece during leak testing. No virtual leak can exist because there is no trapped internal region.
Figure 2

Figure 2



→ Continue to "SuperseaL" - A breakthrough in feedthrough design

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