O-ring and Seal Design Considerations

A basic manufacturing component that has to be right or an entire project can be ruined.

By Christopher M. Chiodo, Apple Rubber Products, Inc.

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 considerations to take into account when designing with seals and 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, de­termines the effectiveness of a seal.

Pressure Considerations

Sealing Pressure - Sealing pres­sure is the force exerted by compres­sion of the seal between the mating surfaces forming the gland itself and external forces (i.e. the fluid being sealed) transmitted through the seal. A physical barrier is formed when the force from compressing the seal and pressure applied by the fluid being sealed causes the seal to fill all leak paths.

Low Pressure Sealing - In a low pressure sealing application, system pressure does not cause a noticeable increase in the seal's sealing force. Because of this, all sealing force must come from the ability of the seal to de­form and create a barrier by compres­sion of the seal. Surface finish on both the seal and gland are critical to an ef­fective seal.

High Pressure Sealing - In a high pressure sealing application, system pressure causes a noticeable increase in the seal's sealing force to fill all leak paths. Because this pressure activates the seal, the compression of the seal and surface roughness are not as im­portant as in low pressure sealing.

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.”

Material 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

The selection of elastomeric mate­rial is contingent upon a number of de­sign factors such as: dynamic or static application; chemical compatibility; the O-ring's ability to resist pressure, heat, cold and friction, and consideration of material availability and cost. (In addi­tion to design factors, of course, one must also consider the nature of the installation, the use of the product and the kind and degree of maintenance that may reasonably be expected.)

As an example, silicone is a popular choice of designers and engineers for a wide variety of medical applications. Silicone has a wide temperature range (-94°F to 400°F) with excellent long-term temperature resistance and low temperature flexibility. It offers hydro­phobic properties, is biocompatible and inert, and has a neutral odor and taste. In addition, FDA, ISO 10993 or USP Class 6 grades are available.

With such a multiplicity of design factors to consider, it is possible for several O-ring materials to meet, or nearly meet, your specified applica­tion requirements. In the case where several O-ring materials meet your re­quirements, cost and availability may be the deciding factor.

In the case where several materi­als nearly meet your application re­quirements, a compromise material selection may have to be made. In such cases, due to the complexity of interacting forces, it is highly recom­mended that your O-Ring material selection be rigorously tested in the actual application. Performance as­sumptions must then be checked so that you are certain that all variables have been carefully considered. TMD

If you require assistance for a seal­ing application, Apple Rubber's Web site has an Engineering Assistance Request form, which can be filled out and e-mailed to the company for more detailed analysis. Visit www.applerub­ber.com.