The choice of an O-ring is dependent on two factors: the end-use application and the vacuum/pressure range over which it is intended to operate. A typical O-ring is an elastomer – a polymer material with the property of elasticity – having a Durometer “hardness” in the range of 65–80 Shore (i.e. about the hardness of an automotive tire tread or soft skateboard wheel).
The secret to their success is their ability to adapt to the unevenness of mating surfaces. The O-ring must be smooth, crack or scratch-free and properly lubricated. A vacuum system in which the construction materials are chosen carefully, welded or brazed joints are sound, static and dynamic seals are designed properly, and all materials and components are designed to withstand bake-out temperatures can operate with a leakage of less than 10-10 cubic centimeters/second.
Application UsesIn general, the vapor pressure of most organic materials is inversely proportional to the temperature. It is, therefore, very important that O-rings be kept cool, especially when the vacuum system is being baked out. The best of the materials must be held below 250°F (120°C), and for minimum outgassing an even lower temperature is desirable. In fact, if the temperature is held below 65°F (18°C), it is possible to use carefully constructed O-ring joints at pressure down to as low as 10-9torr.
In most applications, O-rings are intended to be static seals. A static seal is accomplished by plastically deforming an elastic material (e.g., Buna-N, Viton® Neoprene®, silicone or Teflon®) into the non-uniform surface of a mating flange, thus reducing the leakage to an application-specific level. With proper precautions and tight tolerances, O-rings can be used in dynamic (moving) sealing applications as well.
In most cases, O-rings are placed in grooves and pressed between flanges, with one flat flange and one grooved flange being typical. The grooves must have close tolerances, a finish to at least 32 RMS and follow these rules:
- O-ring compression – the ratio of width to height – should be a maximum of 15-20% of the O-ring thicknesses of 0.196-0.393 inches (5-10 mm) and 30% for thicknesses under 0.125 inches (3 mm).
- The O-ring should fill 80-90% of the groove.
- For horizontal flanges, 25% compression on the O-ring is highly desirable (Fig. 1).
- For vertical flanges, an undercut groove is beneficial to hold the O-rings firmly in place (Fig. 2).
Types of SealsFlat seals should be avoided wherever possible because it is difficult to achieve the pressure required for the sealing material to cover all of the surfaces evenly.
Elastomer seals having a trapezoidal configuration are used for valve seals and for the covers and doors of large vacuum chambers. Deformation of the seals is typically kept within desired limits by attaching spacers in applications such as large chamber doors where high surface loading is possible.
Radial shaft seals (Fig. 3), or cap seals, are used where sealing of a rotating member is required. Care should be taken to ensure that only shaft sealing rings with a fully rubber-coated metal ring are used. With the shaft in motion, the leak rate will be significantly higher. Cap seals should only be used for manually rotated feed-through components.
Metal seals perform better in high-temperature applications (Table 1). These are made of materials such as copper, copper-nickel, aluminum, indium and even silver and gold (usually in the form of wire seals). Care must be taken to ensure that specific contact forces are maintained.
Common FallaciesHere are some of the most common misconceptions when working with O-rings:
1.One of the most common beliefs is that the vacuum grease applied to the O-ring surface is responsible for actual sealing and that the more grease present, the better (and longer) the seal will last. In reality, the layer of vacuum grease is intended only to act as a lubricant to seat properly under the applied compression forces (Fig. 4) and should be a very thin layer, so much so that when you move your finger along the O-ring surface it will slide or glide unhindered with no appreciable amount of grease buildup.
2.O-rings do not need to be re-greased after every run. An O-ring should be wiped down using a clean rubber glove. Running your fingers over the surface of the O-ring is an excellent way to detect minute particles of dirt or grit and reveal nicks or areas that should be cleaned and re-greased. Thoroughly cleaning the surface of the O-ring with alcohol, methyl-ethyl-keytone (MEK) or acetone before re-greasing (or applying vacuum grease initially) is critical.
3.The shelf life of a typical O-ring is forever. In reality, O-rings have a shelf life of only about six months.
4.O-rings cannot be spliced. Yes, they can as long as the part line is in the plane of the cross section. They must be joined together by gluing the material with a proper adhesive.
O-Ring FailuresO-ring seals often fail prematurely in applications because of either improper design or material selection. From the end-user’s point of view, a seal can fail in three general ways:
- Change in appearance
O-ring failures can be classified into the following general categories:
1.Abrasion (Fig. 5a) – The seal or parts of the seal exhibit a flat surface parallel to the direction or motion. Loose particles and scrapes are often found on the seal surface. Contributing factors include rough sealing surfaces, excessive temperature, process environment containing abrasive particles, dynamic motion and poor elastomer surface finish.
2.Flattening or over-compression (Fig. 5b) – The seal exhibits a flat-sided cross section with the flat sides corresponding to the mating seal surfaces. Contributing factors include excessive compression, excessive temperature, incompletely cured elastomer, elastomers with high compression set and excessive volume swell.
3.Degradation – The seal exhibits blisters, cracks, pits, voids or pockmarks on its surface. Absorption of gas occurs at high pressure and the subsequent rapid decrease in pressure. The absorbed gas blisters and ruptures the elastomer surface as the pressure is rapidly removed. Contributing factors include rapid pressure changes, low-modulus/hardness elastomers and incompatibility with the pressure/vacuum or thermal environment.
4.Extrusion (Fig. 5c) – The seal develops ragged edges (generally on the low-pressure side) that appear tattered. Contributing factors include excessive clearances, excessive pressure, low-modulus/hardness elastomers, excessive gland fill, irregular clearance gaps, sharp gland edges and improper sizing.
5.Installation damage (Fig. 5d) – The seal or parts of the seal may exhibit small cuts, nicks or gashes. Contributing factors include poor techniques, improper tools, sharp edges on glands or components, improper sizing of elastomer, low-modulus/hardness elastomer and elastomer surface contamination.
6.Over compression (Fig. 5e) – The seal exhibits parallel flat surfaces (corresponding to the contact areas) and may develop circumferential splits within the flattened surfaces. Contributing factor includes improper design (failure to account for thermal volume changes or excessive compression).
7.Thermal Degradation – The seal may exhibit radial cracks located on the highest temperature surfaces. In addition, certain elastomers may exhibit signs of softening (a shiny surface) as a result of excessive temperatures. Contributing factors include elastomer thermal properties, excessive temperature excursions or cycling.
8.Plasma Degradation (Fig. 5f) – The seal often exhibits discoloration, as well as powdered residue on the surface and possible erosion of elastomer in the exposed areas. Contributing factors include chemical reactivity of the plasma, ion bombardment (sputtering), electron bombardment (heating), improper gland design and incompatible seal material.
9.Other – Spiral failure in which the seal exhibits cuts or marks that spiral around the circumference. Contributing factors include difficult or tight installation (static), slow reciprocating speed, low-modulus/hardness elastomer, irregular O-ring surface finish (including excessive parting line), excessive gland width, irregular or rough gland surface finish and inadequate lubrication.IH
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