Premier Distributor of Parker O-Rings, Parker Seals and Parker Products
Premier Distributor of Parker O-Rings, Parker Seals and Parker Products
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| Face Sealing Glands | Dovetail Grooves | Half-Dovetail Grooves | Static Vacuum Seal Glands | Reciprocating Vacuum Packing Glands |
Vacuum Weight Loss
It is particularly important in many space and other vacuum applications that optical surfaces and electrical contact surfaces remain clean to serve their intended purpose. Some rubber compounds contain small quantities of oil or other ingredients that become volatile under high vacuum conditions and deposit as a thin film on all the surrounding surfaces. Table indicates the weight loss of several Parker Seal compounds due to vacuum exposure. Where sensitive surfaces are involved, the higher weight loss compounds should be avoided. In those compounds which show low weight loss, the small amount of volatile material that is indicated is primarily water vapor. It is not likely to deposit on nearby surfaces.
Vacuum Seal Considerations
The rate of flow of gases from the pressure side to the vacuum side of an elastomeric seal depends to a great extent on how the seal is designed. Compound B0612-70 has been tested in face type O-ring seals, using grooves that provided 15%, 30% and 50% squeeze. It will be seen from the results that increasing the squeeze reduced the leak rate dramatically. Lubricating the O-rings with a high vacuum grease also reduced the leakage of the lightly squeezed (15%) rings significantly, but the effect of the grease was considerably less at 30% squeeze. At 50% squeeze the effect of the grease was not detectable. Several other compounds were tested in this way with similar results. Increased O-ring squeeze reduces permeability by increasing the length of the path the gas has to travel (width of ring) and decreasing the area available to the entry of the gas (groove depth). Increasing the squeeze also tends to force the rubber into any small irregularities in the mating metal surface, and thus prevents leakage around the seal. The vacuum grease aids the seal by filling these microscopic pits and grooves, thus reducing leakage around the ring, and at the same time it may be changing the surface tension favorably with the effect of a reduced rate of surface absorption. It is recommended, therefore, that face type O-ring grooves be used whenever possible for static vacuum seals, using a silicone grease as a seating lubricant and surface coating in addition to a heavy squeeze of the O-ring cross section. When a radial seal is required, or when a heavy squeeze is not possible for some other reason, it becomes more important to use a vacuum grease. As an example of the benefit of high squeeze, we have found that Gask-O-Seals and Integral Seals both make effective Vacuum seals because of the generous squeeze that is built into them. Gask-O-Seals have the added advantage of a high percent fill of the groove together with a shallow depth which reduces the seal area that can be exposed to the effects of vacuum, and prevents the rubber sealing element from moving due to vibration or pressure changes. An additional benefit of high percentage confinement is the fact that increased temperatures do not increase the leak rate as much as normally expected with a lesser confinement. Although a very heavy squeeze is necessary to reduce leakage to an absolute minimum in an O-ring seal, this kind of design may require heavy construction. When such a shallow gland is desirable, it must be wide enough to receive the full O-ring volume.
