Sorting out static seals

April 1, 2004
Square-section seals and O-rings both provide leaktight sealing.
O-rings works well in static-sealing applications. Square cross-section seals are a viable option when designs demand flexibility, such as when working with nonstandard glands.
Tests show that the maximum contact pressure for comparable O-rings and square cross-section seals is nearly identical.

Chris Couchell
David Lancaster

Applications Engineers
Parker Hannifin Corp.
TechSeal Division
Spartanburg, S.C.

O-rings have long been considered the standard solution for static-sealing applications. But designers should also consider square cross-section seals. Both types have desirable characteristics, depending on the situation. A round cross section makes O-rings hard to beat in radial or dynamic applications. And they are widely available, over the counter, in many standard sizes. However, the flexibility and performance of square cross-section seals (sometimes referred to as lathe-cut seals) often make them the better choice for static sealing the face of a gasket.

Sealing fundamentals

Creating a face seal is not terribly complicated. In most sealing systems, the objective is to prevent fluid from leaking from high to low-pressure regions through a gap. Static seals are typically preloaded to help accomplish this task.

A seal whose height exceeds the sealing gap produces preload. The seal elastically deforms when installed, generating internal stresses and reaction forces on the top and bottom of the sealing gap. Fluid pressure further increases internal stress and supplements the preload. To prevent leaks, contact pressure between the gasket and sealing interfaces on both sides of the sealing gap must exceed the fluid pressure.

Preload can be specified in terms of percent axial squeeze Sa, defined as:

Sa = (Ho-Hc) / Ho

where Ho = original seal height and Hc = compressed seal height.

Typically, axial thickness should be such that the installed seal deflects 25% from its original height. Also account for the manufacturing tolerances of both the seal and seal gap. The recommended range of axial squeeze, considering tolerances, is 10 to 40%.

Gland fill Gf is another important parameter indicating the percent of seal-groove volume that a seal occupies.

Gf = Vs / Vg

where Vs = seal volume and Vg = gland volume.

This is important because overfilling the gland can damage the seal and sealing surfaces. Maximum recommended gland fill, considering manufacturing tolerances, is 95%.

The last characteristic that helps define seal geometry is the desired fit. Typically, if higher pressure acts on the inside of a seal, experts recommend an OD interference with the gland. Conversely, with high pressure on the outside of the seal, an ID interference with the gland works best. Interference should range from 1 to 5%, including manufacturing tolerances. Sometimes automated assembly or other special considerations make an interference fit impractical. In these cases, design in as little clearance as possible.

Square seals and O-rings

The geometric difference between an O-ring and square cross-section ring is that the latter has one more degree of freedom. An additional degree of freedom lets the square ring work in many applications where an O-ring will not. Two such situations are a wide but shallow groove, and a groove with one end higher than the other. The extra degree of freedom lets thickness vary to better fit the groove.

O-ring seals generally focus the seal force onto a thin sealing line, whereas square-cut seals provide a much wider contact area. As the accompanying graphic illustrates, the maximum contact pressure for comparable O-rings and square-cut seals is almost identical. Sealing ability is therefore comparable, although overall compressive load is higher for a square-cut seal.

With nearly equal sealing performance in face-seal applications, other characteristics may be considered to determine which seal to use. One of the most important is design flexibility. Square cross-section seals are manufactured using a highly flexible, extrude-and-cut process. Simply changing forming dies on the extruder modifies the outer and inner diameters of the square cut. Adjusting the cutting machine modifies axial thickness. This flexibility saves on tooling costs when working with nonstandard glands. The extrude-and-cut process for manufacturing a square cross-section seal produces a consistently high-quality seal.

O-rings and extruded seals are manufactured from most common polymers, along with a host of specialty materials that are FDA, USP Class VI, UL, and NSF approved for applications in the medical, telecommunications, automotive, and water-systems markets.

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