Industrial Applications of Rubber O-Rings
Most people have never heard of the o-ring before the Challenger disaster of 1986. Today, rubber o-rings have wide ranging applications and are found in equipment, devices and heavy machineries in practically every industry.
Some of the most common uses of the o-ring are found in industries such as:
Construction and heavy machineries
Food and drug manufacturing
Medical and pharmaceutical industries
Hydraulic and pneumatic applications
Aerospace and defense
Potable processing and distribution
The Mechanism of Sealing
The design of the rubber o-ring or its selection will have to take into account the mechanical mounting within which it is installed. When compressed, a calculated stress is applied to the o-ring against the contact surface of the mounting, and leakage is prevented so long as the contained fluid does not exert a pressure exceeding the stress at the contact point of the o-ring. The material of the rubber o-ring, which is highly incompressible, acts as a medium of transfer for the pressure of the contained fluid. An increase in pressure from the contained fluid correspondingly results in a rise in contact stress on the o-ring, thereby enabling the o-ring to maintain its sealing integrity even at high pressures.
Extrusion of rubber o-ring material through the mating components is a common mode of failure. Service conditions varies with each application, and material selection will have to take into consideration factors such as temperature extremes, chemical contact media, resistance to abrasion and wear, and pressure conditions in both static and dynamic environments. Material swelling is an indication of material compatibility with the contact fluid, and chemical attack can also cause brittle cracks and reduce the tensile properties of the material. Failure of rubber o-ring joint design can also be a simple matter of selecting the wrong o-ring size for a particular groove, or specifying an inappropriate material for a given application.
Modes of Failure
Extrusion: Extrusion can be the result of using a rubber o-ring material that is too soft, and selecting a higher modulus elastomer may be the solution. Other contributing factors include excessive clearances, material degradation or using an incorrect size of o-ring.
Chemical Degradation: Failure is identified as swelling, cracking, discoloration and blistering. Chemical degradation is often the result of incompatibility of the elastomeric material with the contact chemical media.
Thermal Degradation: Radial cracks may be seen on the surface of the o-ring that is exposed to the highest temperature. Cause may be due to repeated temperature excursions and thermal properties of the material. Selecting a seal with higher temperature compatibility may resolve this problem.
Plasma Degradation: Seal displays discoloration with a powdery surface deposition. Result of plasma incompatibility of seal material. Perfluoroelastomer (FFKM) is the material of choice for applications in plasma environments.
Abrasion and Wear: Flat surface is observed in an otherwise circular rubber o-ring cross-section. The deformed surface is the result of abrasion and wear on the contact surface of the seal, and is in a direction parallel to the line of motion.
Compression Set: The o-ring or rubber seal exhibits a deformed flat surface on its otherwise circular cross section, on the side that is in direct contact with the mating surfaces. Caused by exceedingly high compression, volume swell, excessive temperature as well as improperly cured elastomers.
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