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The 'O' ring seal

seals-gasketsI know it's hard to credit it, but before 1937 the 'O' ring didn't exist. Patented in that year by Danish immigrant to the US, Neils Christensen, an 'O' ring, while eminently simple in concept, is in practice a very powerful sealing mechanism. No wonder they can be found in any number of critical applications inside most purpose-designed race engines. At the base of the cylinder liner, around the body of the fuel injector and at the top deck of the cylinder block sealing oil and coolant as it passes into the cylinder head - these are all critical areas where a reliable seal is imperative.

To call it a ring is perhaps not totally correct. For although most are circular in both section and plan view, 'O' rings are perhaps better referenced as a torus, a shape formed by rotating an enclosed section about a line in the same plane but not intercepting it. And although generally circular in cross-section and made from elastomer-type materials, 'O' rings can be used to seal circular holes as well as many irregular but planar apertures.

'O' rings can be described by three main characteristics - outside diameter; inside diameter which, assuming a circular cross-section, specifies the cross-sectional diameter; and the material from which it is made. Held in a groove of specified dimensions, sealing is achieved by the compression of the section against the fourth surface, which is clamped to the other by mechanical means. The circular cross-section therefore distorts, ensuring the maximum sealing pressure with the minimum of clamping load. With an 'O' ring the maximum sealing pressure is equivalent to the pressure created by the seal deformation at the contact point.

Whereas the external and internal diameter of the seal is normally dictated by the size of the hole to be sealed, the thickness of the ring is to some extent quite arbitrary. A smaller cross-section will be more compact and lighter in weight and, being smaller, inevitably less expensive, particularly if the more expensive elastomer materials are used. On the other hand, a larger 'O' cross-section will be less prone to compression 'set', less prone to swell characteristics of the elastomer and will allow for larger machining tolerances on the ring groove. So, for a race engine, smaller is not always necessarily better.

seals-gaskets pic1-o-ring

That other characteristic, the elastomer, is defined as either a synthetic or natural product that has enough resilience to return to its original shape after some level of distortion. It is this property alone that makes them work as seals. The design parameters that reflect this ability are referred to as compression squeeze and compression ratio.

Compression squeeze is the difference between the compressed thickness and the original cross-sectional diameter of the seal, while the compression ratio is compression squeeze expressed as a ratio with the original cross-section, and is essentially the elastomer compression expressed as a percentage of the seal cross-section. Compression squeeze is expressed as a minimum dimension (in mm or in) while compression ratio is generally desired to be between 5% and 20% at all times. The next major decision is the choice of elastomer material.

Elastomers have to be selected according to their operating environment. In an engine, therefore, the ability to withstand temperatures of up to 200 C and be unaffected by modern engine oils, fuels (ethanol in particular) and water-glycol mixtures are paramount. At high temperatures, rings can harden and crack, while engine oils and hydrocarbon fuels can create some level of seal swell in most elastomers. Some elastomers are highly resistant to hydrocarbons, while others are easily attacked by acids or alcohols (methanol/ethanol). Some types are also affected by the glycols used in cooling systems.

seals-gaskets pic2-compress

Selecting the correct elastomer is therefore critical. Fluoroelastomers of the FPM category (of which the trademarked name Viton is an example) would seem to have the best overall properties at a reasonable price. For greater temperature performance FFKM (Perfluoroelastomer) elastomers might be slightly better but cost more.

Simple it may be, but the 'O' ring is not without its own share of design issues and potential pitfalls.

Fig. 1 - The 'O' ring
Fig. 2 - Compression squeeze

Written by John Coxon

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