3.3.2.1 O-ring seals and grooves
When vacuum technology components are detachably joined, seals must be used to prevent ambient air from flowing into the vacuum. To prevent this, there are, depending on the application and pressure range different types of seals.
O-rings are the most frequently used of all seals. They are available in different materials, usually elastomers with a hardness in the range of 65 to 80 Shore A. The unit Shore A is used for soft elastomers as a measure of malleability. The higher the number, the lower the deformation with the same applied force. The suitability of O-rings 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-free and scratch-free. The rings should be seamlessly pressed, the parting of the pressing tool should be on the level of the ring diameter and can be removed by grinding.
The O-ring can be coated with a thin film of a low vapor pressure grease (silicon grease, mineral oil-based or perfluoropolyether-based grease), depending upon the application in question. The vacuum grease smooth out small irregularities on the sealing surfaces and the surface of the O-ring and thereby improves, particularly at low degrees of deformation, the sealing effect. Here, the vapor pressure of the grease should be noted increases greatly with the temperature and should be well below the desired operating pressure. In addition, it must be examined whether components of the grease or low amounts of hydrocarbons are compatible with the application. 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. In addition, the degree of deformation should not be too small, to ensure good contact between the O-ring and sealing surfaces.
The cord diameter of the O-rings is usually 2 to 12 mm. The diameter for many connections is 5 mm resp. 5.33 mm in the area of the inch dimension. Generally speaking, O-rings are used as static seals. If dynamic stress is involved, precision O-rings that are manufactured especially for this purpose or alternatively mechanical seals or radial shaft seal rings, should be used.
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, by generally using a combination of one flat flange and one grooved flange. The grooves must be carefully dimensioned. There are no generally accepted dimensions for this. The dimensions listed in the tables of O-ring suppliers are only reference values are used for orientation. They must be checked by the user on each specific application and suitability (e.g. through tests). Elastomers swell, shrink, harden or can even crack, due to external influences, such as temperature, pressure or reactions with the fluids used. This must be considered when selecting the elastomer and the groove. In addition, the sealing effect must be sufficient during each operating condition to ensure that the O-ring is not excessively compressed. If due to an increase in volume of the O-ring the groove is overfilled, it can damage the O-ring or even distort the flange.
For static sealing the maximum compression for a cord thickness of 5 mm should be about 25 %. Smaller diameters can be compressed more, larger diameters less. In practice, a compression of less than 15 %, in particular for dry mounting, can cause an insufficient sealing effect. The deformation force is mainly determined by the cord thickness and the hardness of the elastomer. To compress an O-ring of 5.33 mm thickness by 20 %, approx 5 N per mm of sealing length is required for a hardness of 70 Shore A resp. 7 N/mm for 80 Shore A.
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. Elastomer rings can safely be stretched in length by 5 %. The maximum stretch is dependent on the material and the operating conditions.
Elastomeric seals with trapezoidal or similar cross-sections can be used, for example, for valve seats and for lids and doors of vacuum chambers. The trapezoidal opening should be dimensioned such that the O-ring is not damaged when inserted and on the other hand is not pulled out when the valve plate is lifted or when you open the chamber door. In addition, with large contact forces, as they occur e.g. for large chamber doors, the trapezoidal-groove must allow enough room for the deformed O-ring, so that its deformation is kept within limits.
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 chamfer of 45 ° at the upper end, into which the O-ring is inserted and then compressed by the surface of the screw. The seal should be lubricated, so it is not damaged during tightening. In addition, the installation space must be greater than the volume of the O-ring.
Flat gaskets in combination with planar sealing surfaces are to be avoided in vacuum technology as much as possible. For this, a hard to reach high contact force is required, so that the sealing material fills any surface irregularities. If flat gaskets are used, this usually happens with circulating locally raised sealing surfaces, for example, with the combination of CF flange with a FKM flat gasket.
Figure 3.6: O-ring seals in rectangular groove, trapezoidal-groove and in an angular position
