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O Rings
O-Rings are strange little things preferring a pressurised existence than a life of idleness. I find it quite mystifying how that simple bit of rubber can seal reliably against quite immense pressures and yet leak when the system is not used.
O-rings are a form of seal made from varying grades of flexible materials which display chemical resistance and elastomeric behaviour over a wide range of temperatures. They are often grouped under a general heading of rubber. They are usually toroidal in shape and circular in cross section although this can vary, particularly on sliding or rotating seals. A typical example for routine to arduous applications would be a low compression set flourocarbon elastomer such as Viton. This would be made from a blend of the base polymer, filler, plasticers and curing agents.
During manufacture, the o-ring is first moulded then subjected to a curing cycle that will, in conjunction with the base materials and additives, determine the final properties. In use the O ring is fitted in to a groove typically rather larger in cross section one one axis and smaller on the other than that of the O ring itself. This allows for the O ring to compress and distort freely during assembly and for high-pressure fluid to act directly on the side of the ring.
The mechanical compression between components and just as importantly the compression due to fluid pressure on the ring results in a sealing force between the elastomer and metal surfaces. This compression coupled with thermal cycling causes the O ring to develop a permanent set which increases with operating hours and greatly reduces the sealing force. The result would be leakage but luckily, this effect is balanced by seal swelling with operating temperature and by the swelling action of the lubricant as fractions become absorbed into the elastomer.
The O ring eventually reaches an equilibrium condition between compression set and seal swelling, typically after about 1000 hours operation on a single lubricant. This last point has importance when an oil grade or manufacturer is changed as different oil formulations can change the amount of swelling in the elastomer. This swelling action is primarily influenced by the lubricant base stock and by the degree of lubricant degradation in proximity to the seal (especially on high temperature applications such as gas turbines). In this respect the elastomer acts like a molecular sieve allowing lower molecular weight materials to be preferentially absorbed.
If swelling becomes excessive then the O ring may be extruded from the groove resulting in almost certain leakage. Another common cause of failure is gradual reaction with the lubricant causing hardening (polymer cross linking) of the elastomer surfaces. The effect is often visible as hard-flattened surfaces, often having a crazed appearance or cracking when flexed. So, elastomers need fluid pressure for an effective seal. This is why they may leak when idle but not when in use. The higher the pressure of the working fluid the better the seal. They will distort with time and swell in contact with a lubricant. Swelling rates can change and the surfaces can become hardened with time, temperature and incompatible lubricants.
What is the best policy when assembling equipment after overhaul? - Fit NEW O rings!
O-rings are a form of seal made from varying grades of flexible materials which display chemical resistance and elastomeric behaviour over a wide range of temperatures. They are often grouped under a general heading of rubber. They are usually toroidal in shape and circular in cross section although this can vary, particularly on sliding or rotating seals. A typical example for routine to arduous applications would be a low compression set flourocarbon elastomer such as Viton. This would be made from a blend of the base polymer, filler, plasticers and curing agents.
During manufacture, the o-ring is first moulded then subjected to a curing cycle that will, in conjunction with the base materials and additives, determine the final properties. In use the O ring is fitted in to a groove typically rather larger in cross section one one axis and smaller on the other than that of the O ring itself. This allows for the O ring to compress and distort freely during assembly and for high-pressure fluid to act directly on the side of the ring.
The mechanical compression between components and just as importantly the compression due to fluid pressure on the ring results in a sealing force between the elastomer and metal surfaces. This compression coupled with thermal cycling causes the O ring to develop a permanent set which increases with operating hours and greatly reduces the sealing force. The result would be leakage but luckily, this effect is balanced by seal swelling with operating temperature and by the swelling action of the lubricant as fractions become absorbed into the elastomer.
The O ring eventually reaches an equilibrium condition between compression set and seal swelling, typically after about 1000 hours operation on a single lubricant. This last point has importance when an oil grade or manufacturer is changed as different oil formulations can change the amount of swelling in the elastomer. This swelling action is primarily influenced by the lubricant base stock and by the degree of lubricant degradation in proximity to the seal (especially on high temperature applications such as gas turbines). In this respect the elastomer acts like a molecular sieve allowing lower molecular weight materials to be preferentially absorbed.
If swelling becomes excessive then the O ring may be extruded from the groove resulting in almost certain leakage. Another common cause of failure is gradual reaction with the lubricant causing hardening (polymer cross linking) of the elastomer surfaces. The effect is often visible as hard-flattened surfaces, often having a crazed appearance or cracking when flexed. So, elastomers need fluid pressure for an effective seal. This is why they may leak when idle but not when in use. The higher the pressure of the working fluid the better the seal. They will distort with time and swell in contact with a lubricant. Swelling rates can change and the surfaces can become hardened with time, temperature and incompatible lubricants.
What is the best policy when assembling equipment after overhaul? - Fit NEW O rings!
