Many vacuum processes in research and industrial production require the movement of samples or components in the evacuated area. This can be linear position changes in the orientation of the three spatial axes and the revolution around the axis, as a single movement or as combinations of movement types among each other.
Manipulators and mechanical feedthroughs allow translations and rotations in the vacuum, their drive is located on the atmospheric side and the movements are transferred in a vacuum-tight way. Several different action and transmission principles are used, which are in accordance with the respective and vacuum and application conditions. Metalic membrane and spring bellows, special elastomer seals, magnetically coupled systems or differentially pumped seals can be used as sealing elements.
The designs with two open flange connections and an open passage are called manipulators, and components that have an actuator installed in a vacuum-tight flange are called mechanical feedthroughs. The latter are characterized by a compact design. Manipulators can be combined and extended among each other. In addition, mechanical feedthroughs can be attached to them, so that diverse types of movements transfer into the vacuum and with that provide solutions for various motion tasks. Additionally, other feedthroughs can be attached to the inlet flange, for example, for electricity or fluids and these can be guided by the manipulator directly to the application.
Especially with larger nominal diameters, the flanges of manipulators are under considerable forces due to the atmospheric pressure. So that the flanges are stable under vacuum, great emphasis must be placed on their inherent rigidity during the design of manipulators. The particular design challenge is to bring the highly precise transmission of movements in accordance with the external load and other loads by built-in components. The built-in components must be able to be baked out, in order to ensure the use even in the UHV range.
When selecting a suitable manipulator or a mechanical implementation, the user should know the underlying principle, to be able to assess the technical parameters regarding the suitability for its tasks.
3.7.1.1 Translation sealed by diaphragm bellows
Membrane bellows are made from individual lamellas welded together. With a small spring rate, the axial expansion is highly flexible. Bellows ensure a hermetically sealed metal enclosure and are suitable for the highest UHV requirements.
The selection and design of the bellows is in accordance with the required movement task, taking into account the operating conditions: Service life (number of movement cycles), operating temperature, bakeout temperature, differential pressures. The lifetime can be up to 0.5 million cycles with membranes made from the material 316L and up to 10 million movement cycles for the more elastic but magnetizable material 350AM.
Membrane bellows within the specified number of cycles are maintenance free. After they reach the calculated motion cycles they need to be replaced. They are not suitable for environments with dust and dirt.
Membrane bellows are used in the Z-axis, XY-axis and XYZ-axis precision manipulators, and port aligners and bellows sealed rotary feedthroughs.
When baking out units with diaphragm bellows, it is important to ensure an even heating. Heater bands on diaphragm bellows should be avoided. They lead to a strong local heating, as the diaphragm bellows have a low mass and a small area for the heat conduction. Temperature controlled heating sleeves are the better, though more elaborate alternative.
3.7.1.2 Bellows-sealed rotation
The so-called cattail or wobble principle allows the transmission of a rotational movement with bellows, i. e. rotary motion with a hermetic separation between vacuum and atmosphere. Figure 3.25 shows the design of such a feedthrough. The angled drive shaft (1), whose end is supported in a crank pin (3), rotates the drive shaft (4) in the vacuum. The hermetic seal consists of a non-rotating bellows seal (2) that performs a wobbling movement. Driven and drive shaft are supported by stainless steel ball bearings, which are coated with a vacuum-suitable dry-lubrication.
The dry lubrication increases not only the service life and possible maximum speed, it also prevents that the balls get stuck during the bake out at high temperatures. In addition to dry lubrication, hybrid bearings with ceramic balls can also be used for UHV applications. Their high price justifies a use only for the highest standards. Alter natively, for high vacuum applications that are resistant to small amounts of hydrocarbons, bearings can be lubricated with vacuum-suitable grease.
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Figure 3.25: Bellows-sealed UHV rotary feedthrough (cattail principle)
3.7.1.3 Magnetically coupled rotation and translation
Magnetic couplings are used for hermetically sealed rotary or linear feedthroughs. They consist of an array of permanent magnets on the outside that drive a rotor also equipped with magnets that can rotate/move in a vacuum. Both parts are completely vacuum sealed off from one another by a thin walled pipe. The distance between the magnets of the inner and outer rotor should be as low as possible, so that the coupling force between them is as large as possible. The rotor inside is connected to the application to be moved, the outer magnetic rotor is manually moved or motor-driven. The internal bearings are lubricated with a dry lubrication that are suitable for vacuums.
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Figure 3.26: Magnetically coupled UHV rotary feedthrough
Often high-performance magnets made from metals are used for rare earths, such as samarium-cobalt magnets. They ensure highest possible coupling strength with simultaneously excellent resistance to high temperatures, such they occur during bake out. Such magnetically coupled systems are suitable for UHV applications. Even though the magnetic fields are shielded for the most part, the dispersion of the magnetic field lines cannot be avoided completely. The user must therefore check whether its application is sufficiently resistant to the residual magnetic field.
With magnetically coupled systems, rotational and linear motion and simultaneous rotation/linear motion can be implemented. The transferable displacement forces and torques are limited by the number of magnets and their arrangement. The achievable accuracy depends on the mass to be moved. The magnetic fields act as a spring between the two coupling partners. The higher the external force or torque, the greater the deviation of the position between the inner and outer rotors. If the external forces and moments are low compared to the maximum allowable values, the movements can be transferred very accurately.
3.7.1.4 Sealed elastomer rotation and translation
For vacuum-suitable seals subjected to dynamic stress, special elastomer seals are used, which are often made of the FKM (fluoroelastomer) material. The adjustment of the limiting size between housing, seal and actuator shaft must be done carefully, the same goes for the design and implementation of surface finishes.
For frequent movements and for the increase of the sealing effect, elastomer seals must be lubricated on the moving contact points, which reduces friction, prevents premature wear and evens out small irregularities on the surfaces. When selecting a suitable vacuum grease or oil for maximum operating and bakeout temperature, their properties must be considered, particularly the vapor pressure.
Elastomer-sealed feedthroughs are only partially bakeable. In addition, it must be examined whether components of the grease or low amounts of hydrocarbons are compatible with the application. In consideration of these limitations, elastomer-sealed mechanical feedthroughs represent an economical alternative to more expensive devices with functional principles, if highest demands on vacuum quality are not priority.
An advantage of the continuous actuator shaft is the direct connection of the vacuum-side application with the drive on the atmosphere side. There is no play or load-dependent positioning error. Depending on the dimensioning and mounting of the actuator shaft, large loads can be moved as well.
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Figure 3.25: Bellows-sealed UHV rotary feedthrough (cattail principle)
With elastomer-sealed feedthroughs, rotational and linear motion and simultaneous rotation/linear motion can be implemented.
3.7.1.5 Rotation via sliding gaskets with pumped interspaces
The only way to manufacture a manipulator with a free passage and flanges that are freely rotatable to one another, is by using sliding gaskets. Since a sliding gasket alone does not provide a UHV-tight separation, multiple are wired in series and the interspaces are evacuated. Generally three special PTFE sliding gaskets are installed in series. For the first interspace one backing pump is sufficient. If UHV conditions should prevail in the interior, an additional interstage pumping is required, to which a high vacuum pump will be connected.
