The metal or other surface over which the O-ring will move also becomes critical. It must be hard and wear resistant. It also must be sufficiently smooth so that it will not abrade the rubber, and yet there must be small microfine“pockets” on the moving surfaces to hold lubricant.

Dynamic O-ring sealing applications are considerably more involved than static applications due to the implied motion against the O-ring seal interface. Resistance to fluids must be more carefully scrutinized than in conventional static seal designs since a volumetric increase in the O-ring in excess of approximately 20% may lead to friction and wear difficulties and only a minimum of shrinkage (at most 4%), can be tolerated.

In principle, the same conditions for a hydraulic seal apply to a pneumatic seal except that the effects of certain extreme conditions are more serious.

An O-ring has proved to be a practical rotary shaft seal in many applications. With the correct design, Parker O-Ring rotary seal compound N1090-85, will provide satisfactory service at surface speeds up to 1500 feet per minute.

The design conditions are most critical for rotary seals, as would be expected. Relatively high durometer compounds, close control of tolerances, and minimum cross section are required.

Modern elastomeric sealing compounds generally contain 50 to 60% base polymer and are often described simply as “rubber.” The remaining balance of an elastomeric compound consists of various fillers, vulcanizing agents, accelerators, aging retardants and other chemical additives which modify and improve the basic physical properties of the base polymer to meet the particular requirements of a specific application.

An O-ring is a torus, or doughnut-shaped ring, generally molded from an elastomer, although O-rings are also made from PTFE and other thermoplastic materials, as well as metals, both hollow and solid. This handbook, however, deals entirely with elastomeric O-ring. O-rings are used primarily for sealing. The various types of O-ring seals are described in the section called “Scope of O-ring Use.” O-rings are also used as light-duty, mechanical drive belts. More information, including design criteria on O-ring drive belts and their application can be found in the O-Ring Applications Section. So what is an O-Ring Seal? An O-ring seal is used to prevent the loss of a fluid or gas. The seal assembly consists of an elastomeric O-ring and a gland. An O-ring is a circular cross-section ring molded from rubber (as shown above). The gland - usually cut into metal or another rigid material - contains and supports the O-ring. The combination of these two elements; O-ring and gland - constitute the classic O-ring seal assembly.

Millions of O-rings are used very successfully in reciprocating hydraulic, pneumatic, and other fluid systems which employ long stroke, large diameter seals.

The AS568-901 through -932 O-ring sizes (Parker’s 3- series) are intended to be used for sealing straight thread tube fittings in a boss.

Parker recommends utilizing our inPHorm design software to guide the user through the design and selection of an O-ring and corresponding seal gland. Parker’s inPHorm not only addresses standard O-ring sizes, but allows the user to custom design O-ring glands and seals specifically for their application. To obtain inPHorm download from www.parkerorings.com. If inPHorm is not readily available manual calculations can be performed using the following guidelines.

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The greatest dynamic use of O-rings is in reciprocating hydraulic rod and piston seals. These are discussed first, but many of the ideas expressed are also applicable to other dynamic applications.

Elastomers used in producing seals, and particularly those used in O-ring, will usually provide reliable, leak-free function if fundamental design requirements are observed. What exactly makes a rubber compound behave the way it does? The magic is in "cross-linking". Bridges tie together the polymer chains forming bonds during the vulcanization process, as depicted in the image below. Cross-linking of the molecules changes the rubber from a plastic-like material to an elastic material.

Temperature in and around a system is significant to the longevity and performance of the seal. Both high and low temperatures must be considered.

It has been said that O-ring are “the finest static seals ever developed.” Perhaps the prime reason for this is because they are almost human proof. No adjustment or human factor comes into play when O-ring are assembled originally or used in repairs if the gland has been designed and machined properly. O-ring do not require high bolting forces (torque) to seal perfectly. O-ring are versatile and save space and weight. They seal over an exceptionally wide range of pressures, temperatures and tolerances. Once seated, they continue to seal even though some feel that they theoretically should not. In addition, they are economical and easy to use. Therefore, we agree that the O-ring is “the finest static seal ever developed.

The basic core polymer of an elastomeric compound is called a rubber, produced either as natural gum rubber or manufactured synthetically by the chemical industry. Today, more than 32 synthetic rubbers are known, the most important ones are listed here.

When using an O-ring as a face seal, a groove is cut in a flat surface, such as a flange. The O-ring is placed in the groove and a second flat surface compresses the O-ring.

Elasticity is what allows a rubber compound to function as a seal. When a rubber compound is compressed, the "cross-linked" polymer chains are pushed close together. These cross linked bonds want to extend to their original state acting as springs, pushing the rubber compound outwards. The rubber conforms to the gland surfaces and creates a barrier, preventing fluid from crossing, and thus creating a good seal.

Friction is a complex subject and can be divided into sub-categories that include running friction, breakout friction, wear, lubrication, and more. The full edition of the Parker O-Ring Handbook contains many details addressing each of these topics, as well as in depth calculations for estimating frictional properties.