It is often necessary for mechanical movements, electrical current, light or optical signals and liquids to be transferred to the vacuum through vacuum-tight feedthroughs. If they are not integrated in the equipment itself, such as pumps or valves, these feedthroughs are usually installed in vacuum-tight flanges.
Although the o-ring is the simplest form of seal, it cannot be used for high speeds and continuous- duty operation due to the high and uncontrolled contact pressures involved.
What are used most frequently are radial shaft seal rings or cap seals, even though they require lubrication. Hydrocarbons in the form of vapors or crack products can penetrate into the vacuum system through the lubricant.
A magnetic coupling consists of a bell-shaped permanent magnet arrangement on the outside that rotates the rotor, which is rotatably mounted in the vacuum and is also equipped with magnets. The two components are separated by a can that forms a hermetic seal. In the case of slow rotation, dry-running ceramic ball bearings can be used in the vacuum. Fast-running shafts in vacuum can be magnetically levitated.
Figure 6.14 shows the design of such a feedthrough. The angled drive shaft (1), whose end is supported in a crank pin (3), rotates the driven shaft (4) in the vacuum. The hermetic seal consists of a non-rotating bellows seal (2) that performs a wobbling movement. UHV-suitable dry-lubrication ball bearings are used for mounting the drive shaft in the vacuum.
A major factor in engineering an electric feedthrough is the type of current and voltage for which it will be used and the requirements it will have to satisfy with respect to vacuumtightness and temperature resistance. The manufacturing processes for feedthroughs are discussed in Section 6.4. Feedthroughs with organic insulating materials can only be used for lower voltages. Simple cast-resin feedthroughs are frequently used for moderate current loads, e.g. for measurement currents. Epoxy resin is very well suited as an insulator and as a vacuum seal for moderate temperatures.
Multi-pole glass-molded feedthroughs to which the leads can be soldered on both sides are installed in small flanges. There are also versions with tubes through which leads can be inserted and soldered in place.
This is important when using thermocouples, for example, as solder joints could falsify the measuring voltage. The feedthroughs are cooled with water for high amperages.
With respect to their insulating resistance, feedthroughs with glass-to-metal fusings are suitable for high-voltage and weak-current feedthroughs for electronic devices. Feedthroughs with ceramic insulation offer greater mechanical stability and temperature resistance than glass. In addition, ceramic (e.g. aluminum oxide) can also be produced in an insulating form that is suitable for high voltage. This is why ceramic feedthroughs are superior to glass feedthroughs for high voltages and high performance. Only rigid metal-to-ceramic connections should be considered for the most rigorous electrical, thermal and vacuum technology requirements.
The voltage level must be taken into account for electric feedthroughs, because gas discharges and flashover can occur in the vacuum if there are small clearances between conductors with high voltage differentials. In the vulnerable pressure range between 10-3 und 10 mbar, appropriate clearances must be provided between high-voltage conductors. Potting with cast resin can also be useful in this regard.
Feedthroughs for liquids and gases
Problems can occur with these feedthroughs if media at extremely high or low temperatures (liquid air) must be advanced into the vacuum vessel. Feedthroughs can be employed in which the thermal conductivity between flange and tube is reduced far enough by a sufficiently long, thin-walled stainless steel tube that the flange remains at room temperature, with normal elastomer seals being used to seal the flange.
Sight glasses are primarily used to observe the interior of the vacuum chamber, even during the process. Consequently, normal window glass of an appropriate thickness is usually employed and installed in a sight glass flange by means of elastomer seals (ISO-K and ISO-KF sight glasses). The glass is metalized and soldered into the sight flange (CF sight glasses) for UHV applications and high temperatures.
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