Magnetic fluid feedthrough primer — page 3/4



Impact of feedthrough on process

Feedthroughs can make undesirable secondary contributions to the vacuum process environment because of the presence of magnetic fluids, bearings, and magnetic fields. Users should consider whether any of these contributions might be significant in a particular application. 

Magnetic fluids are the most mysterious of these elements for most users; their properties are discussed more fully in a later section. With respect to their possible impact on the process environment, the main considerations are vapor pressure and chemical composition. Each feedthrough contains a small free surface of fluid iu contact with the vacuum space. The area of this surface is approximately given by the relationship:

A ~ 0.3 × D²

where

  • A = area of free surface, mm²
  • D = shaft diameter, mm

Fluid evaporates from this free surface at a rate proportional to vapor pressure (IE-10 to IE-12 torr at 20°C for typical fluids). Not much can be done to reduce the area of the free surface, but it is possible to select fluids with the lowest vapor pressures. However, fluids with the lowest vapor pressure typically have very high viscosity, leading to an increased torque requirement to operate thc fccdthrough. At high speeds, this leads to increased self-heating, raising the local temperature (and the vapor pressure) of the fluid. The most common fluids are based on hydrocarbons with vapor pressure in the neighborhood of IE-10 torr (20°C). Fluids based on perfluorinated polyether (PFPE) materials can also be obtained (vapor pressure approximately IE-12 torr).

Bearings can contribute particles to the process because most feedthroughs employ a "straddle" design in which one ball bearing is placed on each side of the magnetic seal system. The vacuum-side bearing generates particles that depend on load, speed, and lubrication. Grease lubricants with low vapor pressure (PFPE materials) arc commonly used. When exceptionally low particulate generation is required, dry-lubricated bearings can be specified, but load-carrying capacity is substantially reduced and cost is increased. In some cases (e.g., very high speeds) ceramic bearings may be justified (very expensive). Sometimes a "cantilever" bearing arrangement is employed in which both bearings are located on the atmosphere side of the seal. This isolates the bearings and lubricants from the process, but also increases the size and cost of the feedthrough.

Impact of process on feedthrough

Most operating environments (except where high temperatures are present) are benign, with no adverse impact on these devices. It is important to be aware of the unusual cases in which the environment threatens the feedthrough, especially as a result of foreign matter finding its way into the bearings or the fluid region. Four such cases are discussed here. 

Reactive process gases wreak havoc with hydrocarbon-based magnetic fluids, causing the feedthrough to leak. In some cases, reactive gases also lead to gummy deposits on bearings, causing mechanical failure. Cantilever bearing arrangements (both bearings on the atmosphere side of the seal) protect the bearings, but not the sealing fluid. 

Particulate matter generated by the process can find its way into the seal region. Small amounts of this can become jammed into the narrow magnetic gap between the shaft and pole piece, causing the shaft to vibrate as it rotates. Labyrinth or lip seals are sometimes used to reduce the ingress of material. Lip seals used in this way necessarily result in small, isolated volumes located between the lip seal and the fluid seals. In some processes, the resulting virtual leaks can be tolerated. 

Solvents used during cleaning of the system can destabilize the colloid system of the magnetic fluid, leading to instantaneous major leakage. Cleaning solvents must be used with care, limiting the quantities employed in the neighborhood of the feedthrough. When the feedthrough is located at the bottom of the system, with vertical shaft orientation, the chances of flooding are greatly increased. 

Solvents sprayed on the atmosphere end of the feedthrough while hunting for gross leaks have damaged many feedthroughs. This leak-testing technique must not be used on feedthroughs.

Material considerations

Most applications arc well served by feedthroughs of standard design. However, users should be aware of the materials used in constructing feedthroughs to be sure they are compatible with the system environment in which the feedthrough will operate. Table 1 lists the materials commonly employed in these feedthroughs. Rigaku can provide more specific information.

Material 

Where used

Comments 

Stainless steel, nonmagnetic

Housing, flange

303 and 304

Stainless steel, magnetic

Shaft, pole piece, Superseal housing 

17-4PH (SUS 630) and 416

Bearing steels

Balls, rings

SAE 52100 (SUJ 2) 

Retainers (ball cages) 

Usually pressed from carbon strip steel. Stainless steel available on special order.

Magnet alloys 

Magnets 

Magnets are normally isolated from the vacuum space by some of the fluid rings, so are not usually an issue with respect to process compatibility.

Rigaku uses SmCo and NdFeB. Other manufacturers often use AlNiCo. 

Elastomers

Static O-ring seals

Viton® O-rings provide static vacuum sealing between housing and pole piece.

Hydrocarbons, fluorocarbons 

Magnetic fluids

Formulations are proprietary. Major constituent is  the base oil, which is typically alkylnaphthalene, or PFPE in Rigaku feedthroughs. See Table 2.

Adhesives 

Epoxies and thread-locking compounds 

Epoxies, if used, are normally isolated from the vacuum space by some of the fluid rings, so are not usually an issue with respect to process  compatibility. 

If threaded retainers are present on the vacuum face, it is possible that some locking compounds have been used. Consult manufacturer for details in such cases. 

Table 1: Materials used in feedthroughs

Typical characteristics of magnetic fluids are summarized in Table 2. Since these fluids are proprietary materials, this table must be understood as a very broad characterization. 

For many applications, hydrocarbon-based fluids are entirely satisfactory. PFPE fluids offer lower vapor pressures, but present significantly higher drag. It is important to ensure that motors and drive systems can handle the starting and running torque of the feedthrough.

 

Hydrocarbon 

Fluorocarbon 

 

Vapor pressure, torr at 20°C

6E-10

2E-11

Fluorocarbon fluids (Krytox®, Fomblin®) have lower vapor pressure

Vapor pressure, torr at 100°C

 2E-5

 5E-7

Viscosity, centipoise at 20°C

300

 5,000

Hydrocarbon fluids have lower viscosity, lower drag

Magnetization, gauss

450

200

 Hydrocarbon fluids are "magnetically stronger", can provide higher pressure  capacity with fewer sealing stages. However, SuperseaL technology minimizes the practical effect of this difference

Table 2: Typical magnetic fluid material properties


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