Selecting O-Rings – Where to begin?

Selecting O-Rings – Where to begin?

If you have designed a piece of machinery that require seals, you are going to need to determine which seals are right for your chosen application. As a result, you should not overlook the process of designing in the right o-ring within an application.

If you choose the right o-rings, this will commonly help you to meet demands, some of which are often conflicting while they will help to improve performance as well as durability. What’s more, they will also help to reduce the risk of failure, but they are dependent on many different factors and so, we will explore these factors.

Choosing the O-Ring Material

It is vital that the compound material chosen is compatible with the working media that are used within the application. Therefore, if one or more of the following factors are incompatible then it can have a significant impact on the O-ring.

  • The operating fluid can cause the seal material to degrade dues to chemicals. As a result, it can mean that the wrong compound against compatibility could result in the O-ring swelling. This could then lead to a reduction in performance and even the risk of failure.
  • Temperature can also cause problems in the way in which an O-ring performs. Therefore, it is imperative that it remains with the optimum working parameters. If the temperature is too cold then this can reduce the elasticity, causing it to become brittle. This can then cause it to snap. If the temperature is too high then this can cause significant degradation.
  • The operating pressure is also a vital consideration, especially in a static sealing application. Where other factors are taken into account, a material that has a 70 shore hardness will often do but once the pressure increases to 1500 psi, a back up ring should be considered as a way of preventing extrusion. Along with this, it might also be advised to consider a shore hardness of 80-90 to prevent any problems.

Housing Design or Seal Grove

The performance of an O-ring will be determined by the finish of the mating surface but also the seal housing surface finish. This is down to the fact that extremes of smoothness or roughness are not suitable when it comes to creating a good seal. A rough surface, over time, might cause small cuts in the material and that can cause the o-ring to weaken and eventually fail, significantly reducing its lifespan. In contrast to this, a surface that is extremely smooth can cause a similar effect to aquaplane and that can eventually lead to leaks.

As a result, the general rule of thumb or guideline that should be followed is that a seal grove surface finish of 16 rms is suitable for sealing gases while up to 32 rms is considered suitable for fluids. Anything from 8-16rms is considered to be more suitable for dynamic applications as this can help to minimise the risk of abrasion as well as the potential for spiralling.

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