The simple elastomer round black ring provides a cost effective and reliable method of sealing pressurised fluid when applied correctly. O/rings, as their name suggests, are cylindrical in shape like a doughnut and come in a huge range of cross sections, diameters, and material compounds to provide a sealing action between two surfaces under static or dynamic applications.

When applying O/rings in pressurised hydraulic systems, there are several areas of consideration for the design engineer to take into account.

1. EXTRUSION GAP: The clearance present between the two sealing surfaces to which the O/ring is being potentially squeezed when pressure is applied. The maximum allowable pressure is inversely proportional to this gap dimension, as the pressure tries to extrude the O/ring into this gap. Applications that fail to consider this clearance are in jeopardy of significantly reducing the O/rings useful life expectancy. To increase operating pressures, the inclusion of a backup behind the O/ring will help to prevent the O/ring from extruding into this gap. There are various backup rings available that provide different operating characteristics.

2. BITE/CRUSH: For the O/ring to seal effectively, crush is applied to the cross section. The amount of crush depends on application parameters, but in general terms, 20% of the O/ring cross section in static application and 10% in dynamic. The crush is applied or reduced by varying the depth of groove. The length of the groove, the areas to which the O/ring will expand into, needs to be of a size large enough to accommodate the squashed O/ring. This again is dependant on application but, in general terms, is 30% longer that the O/ring's cross section. Detailed charts of these parameters are available from good O/ring suppliers or Hydraulic repair companies.

3. DIAMETER: Diameter is determined by the circumference or surface areas of items being sealed. O/rings are generally sized from their internal diameter. As the O/ring is stretchable in its nature, if the exact diameter is not available it is quite satisfactory to stretch an O/ring up to 6% of its diameter without any detrimental effects.  Due to the vast range available, this is normally not necessary.

4. MATERIAL: The O/ring material selection is dependant on the fluid medium being sealed. By far the most common material in hydraulic applications is Nitride, because of its compatibility with typical mineral hydraulic fluid. If fluid temperatures rise, a Viton compound is used because of its ability to handle temperatures above Nitrile's upper level of 110ºC. There is a large range of O/ring materials available with all good O/ring suppliers having charts, listing each materials operational parameters.

5. HARDNESS: Shore 'A' type durometer or duro for short, is the unit of measurement given to this elastomer product for its ability to withstand crushing. Lower hardness O/rings seal better under lower pressures as they flow more easily into the tiny surface finish grooves under less pressure activation. On the downside lower hardness O/rings are much more susceptible to extruding. Where harder O/rings are the converse of these two factors. Typical O/ring duro is between a softer 70 to a harder 90.

6. SURFACE FINISH: An O/ring is not going to provide adequate sealing if the surface it is sealing against exceeds the acceptable surface finish or roughness. An average surface roughness of between 16 and 32 Ra is satisfactory for most applications. This equates to a good finish achieved from the majority of turning processes.