O-Ring Basics and Seal Thinking™

Elastomer seals are unlike any other materials that design engineers confront. Metal or plastic parts, for instance, are probably failing if visibly distorted. But, an O-ring MUST be deformed to function properly. In fact, an O-ring that is not squeezed and stretched in its application is the wrong O-ring. Here are some basic facts to review on O-rings.

The definition of an O-ring

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

As the illustration shows, 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 gland.

Designed to deform, the O-ring "flows" to fill the diametrical clearance and blocks any further leakage. The following illustration shows a progressive application of pressure and the effect it has on the seal. Pressure, as well as many other considerations, determine the effectiveness of a seal.

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.

Cross Section : When calculating the cross section (C.S.) of an O-ring, you need to consider the size of the gland to be filled as well as the amount of squeeze needed to create a good seal. Virtually every gland has a slight gap between the two mating surfaces, termed "diametrical clearance."

Other Considerations

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. Often there are a number of materials that are appropriate for a particular application. Consideration should be given to the full range of environmental and cost factors. Your final selection will usually be a compromise in the sense that you have to balance all of these considerations

ALL of these points of O-ring design are covered in detail within the Seal Design Guide sections of our web site. Or for more information on any of these points, call 1-800-828-7745.