rotary feedthrough


Rotary feedthroughs are used to transmit rotations from the outside to the inside of the vacuum chamber, for example for moving a shutter or something similar. A typical rotational feedthrough is shown below.


The trouble with feeding a rotation inside the vacuum system is, of course, that a rotating shaft going through a hole would cause problems: It would be impossible to make the shaft seal with the hole so tightly that UHV could be maintained. The feedthrough above avoids this problem altogether: the outside rotation drives a bent shaft, insulated in a flexible bellows. The off-centre motion of this shaft then drives the rotation inside the vacuum.

This trick is impossible if you need something to go through the centre of the rotational axis. In the picture below, the top flange holds a tube that can be filled with liquid nitrogen for cooling a sample, mounted to the closed bottom of the tube. In a case like this there is no choice but to have a vacuum-tight seal between the rotating part and the fixed outside part. Here this is achieved by differential pumping. There is one viton seal between the UHV part of the chamber and the pumping flange going out on the right hand side and there is another seal between there and ambient pressure. In this way, the first seal does only have to be tight between UHV and 10-3 mbar or so.


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