When vacuum technology components are detachably joined, seals must be used to prevent ambient air from flowing into the vacuum. There are different designs for this purpose, depending upon the application and pressure range in question.
O-rings / round rubber rings
O-rings are the most frequently used of all seals. O-rings are available in a variety of materials, usually elastomers with hardness ranging from 65 to 80 Shore. Their suitability as good vacuum seals stems from their ability to adapt to the minute unevenness of the mating surfaces. The surface of the o-ring must be free of releasing grease or talcum, smooth and crack- or scratch-free. In the low vacuum range the o-ring can be coated with a thin film of a low vapor pressure grease (silicon grease, Fromblin grease or mineral oil-based grease), depending upon the application in question. In the case of dry installation, particular attention must be paid to surface quality, the cleanliness of the mating surfaces as well as to the sealing material. The cross section diameter (thickness) of the o-rings can be 2 to 12 mm. 5 mm thicknesses are used for many joints, while rings with thicknesses of 8, 10 or even 12 mm are used only for very large seals. The o-rings should be seamlessly pressed. The parting line of the compression molding die is in the plane of the cross section diameter and is usually removed by abrasion.
Generally speaking, o-rings are used as static seals. If dynamic stress is involved, precision o-rings that are manufactured especially for this purpose should be used. A discussion on how to dimension the grooves for this purpose will not be presented here. O-rings can also be used in axial or radial grooves, in addition to being employed in conjunction with centering rings or sealing washers. In most cases, o-rings are placed in grooves and pressed between flanges, with one flat flange and one grooved flange typically being used. The grooves must be carefully dimensioned in accordance with the following criteria:
- Compression, i.e. the ratio (width / height)−1, should be a maximum of 30 % for o-ring thicknesses (cross sections) of less than 3 mm, and 20–15 % for thicknesses of 5–10 mm
- The groove fill factor should be between 79 and 91 %
- The inside diameter of the groove should be equal to or only slightly larger than the inside diameter of the o-ring
- The outside diameter of the groove may be larger than the outside diameter of the o-ring in its compressed state
If these conditions are maintained, the seals can be reused multiple times without any problem. If the groove is overfilled, the o-rings will be damaged and the flange might even bend, because the ring material is non-compressible. The table below shows groove dimensions, with the inside diameter of the groove and the inside diameter of the o-rings being equal.
To facilitate assembly, the diameter of the o-ring groove is usually selected somewhat larger than the diameter of the o-ring. This keeps the o-ring in the groove during assembly. There is no problem in stretching an o-ring by 5 % in length, however not more than 10 %. If the o-ring is used on a centering ring, e.g. in ISO-K and ISO-KF flange connections, the centering ring must be designed in such a manner that it properly positions the o-ring, supports it and limits its compression. There are centering rings that have inner support rings, outer support rings (for overpressure applications), as well as inner and outer rings.
To seal screws, e.g. oil filler screws or oil drain plugs, the o-ring is installed in an angular position. The thread has a 45Â° chamfer at the upper end, into which the o-ring is inserted. Here, too, the fill factor should be 79 – 91%, as in the case of axial installation. The o-ring is then compressed by surface of the screw. The o-ring should be lubricated for this installation method to prevent it from being damaged when the screw is tightened.
Elastomer seals having a trapezoid configuration or a similar cross section are used for valve seats and for the covers and doors of large vacuum chambers, for example, where they are tightly fitted to prevent them from being pulled out when the valve plate lifts or the chamber door is opened. Since enormous surface loads can occur in connection with large chamber dimensions, deformation of the seals is kept within the desired limits by attaching spacers in applications that involve large chamber doors.
Flat seals should be avoided wherever possible in the field of vacuum technology, because it is difficult to achieve the pressure required for the sealing material to fill our all surface unevenness.
Shaft seal rings / cap seals
Radial shaft ring seals or cap seals are used to seal rotating shafts (Figure 6.4). In this connection, care should be taken to assure that only shaft seal rings with a metal ring that is fully coated in rubber are used. While these seals are quite tight in the static state, the fact must be taken into consideration that their leakage rate will be significantly higher when the shaft is in motion. Cap seals are only suitable for slow-running feedthroughs, e.g. for manually rotated feedthroughs.
Metal seals must be used instead of elastomer seals in high-temperature applications (e.g. baking out vacuum chambers), for high radiation loads and wherever very low permeation rates are the priority. Materials that are frequently used for metal seals are copper, aluminum, indium and in some cases silver and gold. Gold, silver and indium are usually used as wire seals; in addition to wire form, aluminum can also used as a profile seal. In the case of all metal seals, care must be taken to ensure that the specific contact forces (up to 6000 N per cm of seal length) are maintained.
Cut-edge seals made of copper are used for UHV systems. They are placed between CF flanges. Silver-plated copper gaskets are used for temperatures of over 200 °C. In this case, the silver coating serves as a diffusion barrier against atmospheric oxygen to prevent the copper from oxidizing. Metal seals can be used only once. Indium is also employed as a metal seal, where it is placed between smooth flanges in the form of a wire. Although its ease of welding and its malleability are advantages, its low melting point prevents it from being baked out at high temperatures. In addition to temperature resistance, there can be other reasons for using metal is used as a seal material, e.g. resistance to radioactivity.
Greases are still being used as a full-fledged sealing material only in the field of glass technology or as a makeshift solution for slightly damaged seals. In addition to its use as a seal in oil-tight vacuum pumps, oil is being used virtually only as an aid in sealing detachable connections in the low and medium vacuum ranges.
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