O’rings

o’Rings
Introduction

O’rings are the most commonly used seal worldwide due to their simple, economical and efficient design. An o’ring is a torus shaped ring (like a doughnut) that is used to prevent leakage of fluids and gases by filling a clearance gap between two mating hard surfaces with a softer material (usually rubber or plastic).

Despite their simple design, there are many significant factors to be considered in the successful design of an o’ring seal… Material compounds, sizing, groove design and environmental aspects are vitally important to regard when designing an o’ring seal. Get it wrong and the consequences are potentially catastrophic and expensive. The space shuttle Challenger disaster of 1986 is a key example of what can go wrong with a failed o’ring seal; a combination of temperature incompatibility and poor groove design being the primary causes of that incident.

O’ring Compounds

O’rings are typically manufactured in various rubber compounds such as nitrile, fluorocarbon (Viton®), neoprene, silicone and thermosetting plastics like polyurethane and Teflon. Within each of these compounds, there are many different grades of compound which affect the o’ring’s physical properties and chemical resistance. Grading of compounds comes about from the mixing of various fillers, vulcanizing agents, accelerators, aging retardants and other chemical additives to the base polymer which modify and improve the physical properties of the polymer to meet either the requirements of a specific application or for a broad spectrum of applications.

There are different grades for hardness, colour and temperature range. There are grades for contact with food, drinking water and sewerage. Grades for pharmaceuticals, semiconductors, resistance to radiation, resistance to explosive decompression and many more. Careful planning prior to specification is vital for a successful o’ring seal design. Contact us for more info.

Selecting a compound

There are important factors to consider when selecting a suitable o’ring compound for an application. Operating environment and the physical characteristics of the elastomer must be considered. Incompatibility can shorten seal life, increase machine downtime and contribute to a catastrophic failure.

Operating environment factors

In order to mitigate issues with incompatibility, the following environmental factors must be taken in consideration when selecting a compound.

• Chemical compatibility: The effects of an incompatible chemical on an o’ring elastomer are generally evidenced by volume change. The o’ring may absorb the media, causing it to swell in size and soften. Or the media may extract plasticisers and other additives from the o’ring compound causing the seal to shrink, harden and become brittle. Sometimes slight volume swell is desirable as it compensates for variables such as compression set. Shrinkage on the other hand, can intensify an existing compression set problem, cause leakage and contaminate the sealing medium with the extracted particles.
• Temperature: Temperature incompatibility can severely shorten seal life. High temperature exposure initially softens an elastomer, but after extended exposure, the o’ring can cook and cause irreversible changes to the chemical structure of the elastomer where it can harden and become brittle, possibly releasing contaminating particles into the medium. Unlike high temperatures, changes to the elastomer from excess low temperatures are generally not permanent. Extended exposure to very low temperatures generally increases an elastomer’s hardness, but will return to a softer state as temperature rises. Temperature can also change the characteristics of a chemical medium being sealed where a normally compatible chemical at ambient temperature, becomes incompatible as temperature rises.
• Pressure: Effects of excess pressure on an o’ring seal is generally evidenced by the o’ring extruding into the clearance gap of the o’ring groove or a complete rupture of the seal itself. This may require the use of a harder compound, or a review of o’ring groove dimensions. Another effect of excess pressure is permeability where a gaseous medium diffuses into and passes through the elastomer. A sudden change of pressure or explosive decompression may rupture the o’ring as pressurised gas suddenly tries to escape from within the elastomer.
• Exposure to UV and radiation: Depending on compound, effects of low level ultra violet radiation such as sunlight on an elastomer can have a minimal effect, but prolonged exposure will shorten shelf life. Therefore o’rings should be stored away from direct sunlight and sources of ultra violet light. Gamma radiation can have a severe effect on an elastomers compression set characteristics (see below), allowing the elastomer to lose its compressive memory, making the seal susceptible to leaks. But generally in these applications, gamma radiation is at low levels and environmental factors other than radiation will be more significant.

Physical Characteristics of an elastomer

Also to be considered are the mechanical and physical properties of the o’ring elastomer, especially of concern in a dynamic application where movement is involved. The following physical characteristics of elastomers should also be taken into consideration when selecting a compound:

• Abrasion Resistance: The elastomer’s ability to resist wear from scraping or rubbing on a surface in a dynamic application.
• Compression set: The elastomer’s ability to return to its uncompressed state after compression.
• Tear resistance: The elastomer’s ability to resist tearing, nicking or cutting as the o’ring passes over a port or drawn over sharp edges and burrs during installation.
• Tensile strength: The measurement of an elastomer that is stretched until it ruptures or breaks.
• Permeability resistance: The elastomer’s ability to resist gas that passes into or diffuses through the elastomer due to the porous nature of rubber. A sudden loss of pressure or explosive decompression (ED) may result the o’ring rupturing as pressurised gas suddenly escapes from within the elastomer, requiring the use of ED resistant compounds.

Go to our Elastomer Application Guide for information on o’ring compounds and their common applications. Or contact us for more information.

Compound Hardness

Rubber o’rings are available in various hardness’s. There are two common standards for measurement of rubber hardness: Shore Durometer (specifically Shore A Durometer) and International Rubber Hardness Degrees (IRHD). Both Shore A and IRHD hardness values are comparable, the difference being IRHD is assumed to have a higher accuracy. Measurement is taken with a device which uses a ball or cone indenter connected to a spring and gauge. This device is pressed against the rubber. As the rubber deflects, a value is given. The higher the value or Duro, the harder the material (i.e. less deflection equals a higher value).

Shore A Durometer of various common materials:

• 25 Duro:   Rubber band
• 55 Duro:   Door seal, soft o’rings
• 70 Duro:   Most o’rings, automotive tyre tread
• 90 Duro:   High pressure o’ring, hydraulic u-seal
• 100 Duro: Ebonite rubber, skateboard wheels:

The majority of o’ring rubbers have hardness’s between 60 and 90 Duro. The most common hardness found is 70 Durometer. Sealing Australia stocks a wide range of o’rings from 40 Duro to 90 Duro.

Size Standards

There are many imperial and metric o’ring size standards, but few are commonly found in use.

Imperial

The most common imperial standard is British Standard BS1806, which is equivalent to American Aerospace Standard AS568A. Both have the same three digit size reference (e.g. BS-230 is also AS568-230, and also known as 2-series; e.g. 2-230). There’s also boss seal o’rings for SAE straight thread fittings (also known as 3-series or dash-9; e.g. 3-905). There are non-standard sizes, some known as Dowty sizes and others based on OEM’s, but they’re uncommon.

Metric

Compared to imperial, there are quite a number of metric size standards from around the globe such as; Japanese, British, German, Swedish, French and Italian metric standards. French and Italian standards are predominantly based on British Standard imperial with slight variations. The most common metric size standard in Australia is the Japanese JIS-B2401 standard with piston, gland and vacuum sizes (P, G & V). Then there’s non-standard metric o’rings, which are just as common, if not more common as standards based sizing. These are primarily based on rounded numbers (e.g. 30 x 3 instead of 29.4 x 3.1 as found in JIS).

Please see our O’ring Size page for a comprehensive list of BS1806/AS568A, SAE and JIS metric sizes.

Groove Design

As important as the o’ring seal itself is the groove that the o’ring seats into. The groove must be designed to accommodate not just the o’ring size, but also its intended usage; be it dynamic or static operation, radial or axial loading, vacuum or high pressure.

Please see our Groove Design page for tables of suggested groove dimensions for various applications of imperial and metric o’rings.

Storage

The shelf life of an o’ring varies depending on elastomer and storage conditions. To ensure maximum shelf life of an elastomer, o’rings should be stored:

• In ambient temperature not exceeding 49°C (120°F)
• Away from sources of sunlight, artificial sources of ultra violet light and radiation such as fluorescent tubes, compact fluorescent lamps (energy saver bulbs) and gas discharge lights commonly found in factories and warehouses
• In an airtight container or sealed polyethylene bags to prevent oxygen, ozone and other sources of contamination
• Away from electrical devices like D.C. motors, gas discharge lights and electrical arcing which create ozone gas. In applications where ozone cannot be avoided, protective antiozonant coatings like paraffin and para-phenylenediamines (PPD’s) should be used before storage and installation.

Go to our Elastomer Application Guide for information on elastomer shelf life.

Pressure Rating and Extrusion

We are often asked what is the pressure rating of an o’ring. Pressure limits on o’rings are highly dependent on variables such as groove design, clearance gaps and o’ring compound hardness. Situations of over-pressure often result in what is called extrusion failure; where the o’ring is forced into the clearance gap of a groove, nibbling the o’ring and forcing it to blow out as illustrated below. Extrusion can be caused by over-pressure, excess clearance gap, flexing/expansion of the cylinder wall under pressure or a combination of the above.

There are a number of ways to prevent o’ring extrusion failure. The most common is to specify a harder compound o’ring (90 Shore A Durometer for example) or if the groove allows, the addition of one or two back-up rings (aka anti-extrusion rings) made from a harder compound than the o’ring. These essentially provide a zero clearance. Go to our Back-up Rings page for more information on o’ring back ups.

Stiffening the cylinder wall may also help if excess pressure is causing the cylinder to expand, opening up clearance gaps.

The table below may be used as a rough guide in determining the pressure rating of o’rings and whether back-up rings should be used or not. Please note, there are many variables in determining the pressure rating of o’rings and if using this guide, it is best to apply a reasonable safety factor for any potentially unknown conditions.

Basis for Curves

1. 100,000 pressure cycles at the rate of 60 per minute from zero to the indicated pressure

2. Maximum testing temperature 71°C

3. No back up rings used.

4. Total diametrical clearance must include cylinder expansion due to pressure.

5. Apply a reasonable safety factor in practical applications to allow for excessively sharp edges and other imperfections and or higher temperatures.

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