An O-ring is a doughnut-shaped object or torus.
The opposite sides of an O-ring are squeezed between
the walls of the cavity or “gland” into which the O-ring
is installed. The resulting zero clearance within the
gland provides an effective seal, blocking the flow of  liquids or gases through the gland’s internal passage. An O-ring is defined by its dimensions (based on inside [hole] diameter and cross section), durometer
(Shore A hardness), and material composition.

Why an O-Ring Works

A properly designed sealing system incorporates some degree of initial O-ring compression. At atmospheric pressure, only the resiliency of the compressed O-ring provides the seal. However, as system pressure activates the seal, the O-ring is forced to the low pressure side of the sealing surface. Designed to deform, the O-ring “flows” to fill the diametrical clearance and blocks any further leakage.

Dimensional Considerations

Inside Diameter

To provide an effective seal, the O-ring’s inside diameter (I.D.) must be smaller than the piston groove diameter, so that the O-ring is slightly stretched, fitting snugly in the groove. This stretch should be between 1%-5% with 2% as the ideal in most applications. A stretch greater than 5% is not recommended. The resulting stress on the O-ring will cause accelerated aging and cross section reduction. Exception to this rule is a floating seal. These are O-rings that are allowed to sit in grooves freely or “float”. These are used in applications where some leakage is allowed and less friction is required.

For optimum sealing performance, correct O-ring selection is the direct result of a number of design considerations. These considerations include: size, squeeze, stretch, chemical compatibility, and the ability to resist pressure, temperature, and friction.  Your final selection will usually be a compromise in the sense that you have to balance all of these considerations.

The content for this portion of this post was adopted from an article entitled “O-Ring Basics”.

Types of O-Ring Elastomers

Nitrile (Buna-N)

The most widely used O-Ring elastomer. Excellent resistance to petroleum products. Excellent compression, set, tear and abrasion resistance. Does not have good resistance to ozone, sunlight, or weather (unless specifically compounded). Should not be stored in direct sunlight or near motors or other electrical equipment which may generate ozone.
Temperature range: – 40° to +250°F.

Fluoroelastomer [Viton® (E.I. duPont)]
Excellent mechanical and physical properties. Good resistance to petroleum products, low compression set, and high temperature resistance. Wide spectrum of chemical compatibility. Good for vacuum service and low gas permeability. APG stocks the Type “A” Grade Fluoroelastomer. Other grades are available ─ please inquire.

Chloroprene (Neoprene)

Moderately resistant to petroleum products. Good ozone and weather resistance. Good compression set. Excellent for sealing refrigeration fluids such as Freon®. Temperature range: -65° to +250°F.

Recommended for applications requiring a wide temperature range and good dry heat resistance. Good weather and ozone resistance. Limited oil resistance. Not normally recommended for dynamic sealing applications due to relatively poor tensile and tear strength and relatively low abrasion resistance. Temperature range: -80° to +450°F.

Highly Saturated Nitrile (HSN, HNBR)
Better resistance to high temperatures, superior physical properties, and improved chemical resistance over traditional nitrile compounds. It also has better resistance to ozone, sunlight, and other atmospheric conditions. Excellent resistance to compression set. Green HSN is used in refrigerant R134a applications. Temperature range: -40° to +325°F.

Ethylene Propylene (EPR, EPDM, EPM)
Excellent resistance to Skydrol® fluids used in commercial aircraft hydraulic systems. Also recommended for hot water, steam, and phosphate ester type hydraulic fluids. Resistant to some acids, alkalies, and ozone. Not recommended for petroleum fluids or diester lubricants. Temperature range: -65° to +300°F.

Aflas® (Asahi Glass Co., Ltd.)
Advantageous combination of high temperature, chemical, and electrical resistance properties. Recommended for oils and lubricants, hydraulic and brake fluids, transmission and power steering fluids, sour oil and gas (H2S), amine corrosion inhibitors, ozone, steam, acids, bases, alcohols, and a variety of other chemicals. Temperature range: -25° to +450°F. (+500°F short term).

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