18.104.22.168 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.
Figure 3.25: Bellows-sealed UHV rotary feedthrough (cattail principle)
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. Alternatively, for high vacuum applications that are resistant to small amounts of hydrocarbons, bearings can be lubricated with vacuum-suitable grease.