O-ring grooves: designing dimensions and fill level

What are O-ring grooves?

Grooves are the installation space for O-rings. Consider a piston seal: a piston slides along the running surface inside the cylinder. What has to be sealed is the lateral face of the piston against the cylinder running surface. This task is performed by the O-ring, which sits in a groove in the piston.

Temperature and pressure differences between the two regions in the cylinder must be maintained, as must the separation of different media. Besides the piston seal, this also applies to other installation types.

Flange seal: A flange seal is used for mostly stationary connections such as pipe joints or inspection openings. The compression therefore acts in the axial direction, and the groove is arranged accordingly. The necessary pressure is created by screwing on a cover.

Piston seal: Here the O-ring sits in a groove that runs around the outer diameter of the piston. The groove creates space for the sealing element and produces the sealing effect through the correct elastic compression between the contact faces. The decisive factors are the dimensional ratios of the inner and outer sealing face, the depth of the groove and the cord thickness of the O-ring.

Rod seal: With a rod seal, the O-ring is compressed along the radius, as with a piston seal. However, the groove is not located in the inner part but in the outer part.

These dimensions define the groove

The groove depth has the most direct effect on the compression of the O-ring. However, there are further parameters that influence the long-term durability of a seal just as strongly. Depending on the load profile of the specific operating conditions, they have a greater or lesser effect.

Groove depth: The distance between the inner and outer face — the groove depth — together with the cord thickness of the O-ring produces the compression of the sealing element. It must lie within a target range that depends on the seal material. The correct value also varies depending on whether the application is a static or dynamic seal.

For a piston seal, the groove depth should be chosen so that the O-ring is stretched about 1 to 6 % over the groove base diameter. For a rod seal, by contrast, the O-ring must have an oversize of about 1 to 3 % relative to the outer diameter of the installation space. This too must be taken into account when determining the groove depth.

Groove width: The width of the groove channel must first of all be dimensioned so that an O-ring with the appropriate cord thickness fits. Designers take into account that the O-ring deforms elliptically due to the compression and takes up more space in the groove. In pressure applications, a sufficient groove width should ensure that the pressurized medium can enter the installation space. This achieves a uniform pressure load on the O-ring.

However, a groove dimensioned too wide is disadvantageous, especially under changing pressure conditions. The O-ring then repeatedly migrates to the edge of the groove facing away from the pressure and can suffer mechanical damage in the process.

Groove radius: Edges that are touched by the O-ring during assembly and in the installed state must always be rounded. Otherwise they pose a major risk of mechanical damage. Under pressure in particular, the O-ring is pressed against the upper edge of the groove. If this edge is sharp, material can very easily be sheared off. This behaviour is part of the failure pattern of gap extrusion and directly reduces the compression.

Sealing gap: The sealing gap is decisive above all in connection with the operating pressure. This gap dimension defines the distance between the two sealing faces. As a general rule: as the pressure increases, the sealing gap should be designed smaller. Depending on the tolerances, this can push the manufacturability of the components to its limits.

For particularly heavily stressed seals, back-up rings are therefore used. If the sealing gap has already been reduced to the technically and economically feasible minimum, they provide additional safety. They are made of a harder material and prevent the O-ring from being pushed into the sealing gap.

Three groove shapes dominate in practice

The arrangement of the groove depends on the installation type. In detail, there are additional design options for the shape of the groove.

Rectangular groove: Rectangular grooves are the standard case for installation spaces of O-rings. They are manufactured by turning or milling into metallic components. Thanks to the simple geometric shape, this is a particularly economical option.

Trapezoidal groove: Because it is more complex to manufacture, a trapezoidal groove is only used when the installation space must have special properties. For example, the dovetail shape can hold the O-ring in position when the sealing faces are separated from each other for maintenance. Important detail: the groove width is measured before the radii are applied to the edges. Sufficient radii are particularly important in order to avoid damage to the O-ring. There is also a version as a half trapezoidal groove.

Triangular groove: The triangular groove is an option for flange and cover seals. Due to its shape, the O-ring presses against three contact faces in its deformed state. One disadvantage is that this can result in a varying compression. However, it is suitable for confined space conditions.

What matters in groove design

In many cases, the installation space can only be adapted to changed requirements with some additional effort. It is all the more important to take all influencing factors into account when designing the groove.

Frequently asked questions