Deformation in O-rings: intended and harmful

Elastic Deformation: The Sealing Principle of the O-ring

An O-ring without elastic properties works only in exceptional cases. O-rings need the elasticity of the rubber to flow into the fine irregularities of the sealing faces. In addition, mechanical compression of the O-ring is required for this. In this way they achieve maximum tightness even between components that are not optimally machined.

Elastic behaviour is also required when sudden pressure surges change the position of the sealing faces relative to one another. Provided that manufacturing and storage are correct, elastomer O-rings have the necessary restoring force when installed to return to their original shape on their own after the pressure is released.

This recovery is physically anchored. The geometry of the O-ring is defined during the cross-linking of the polymer molecules in the moulding process. Under a force-induced strain, the molecules are stretched. When the force is removed, the molecules return to their original positions relative to one another. Entropic processes are responsible for this.

Permanent Set: When the Restoring Force Is Missing

In operation, extreme temperatures and aggressive media can greatly reduce the ability to recover. A permanent set then occurs, which severely limits the service life of the seal. The O-ring has adapted to a static position in the sealing system that is dictated by the compression of the cross-section between the sealing faces.

In addition, a medium such as cooling water or hydraulic oil can exert pressure on the sealing element. This too determines the position of the O-ring in the sealing gap and can cause further deformation of its cross-section. You can read more about behaviour under load in the article O-rings under pressure.

If the pressure conditions change, an elastic O-ring tends to return to its original geometry. If, for example, the bolted connection of the cover on a flange seal loosens, the pressure on the O-ring decreases. The axial compression of the cross-section is reduced. From the forced oval shape, the cross-section must return as far as possible to its round shape. In this way it fills the enlarged space between the groove in the housing and the flange cover.

With a permanent set, the O-ring often can no longer do exactly that. The pressure drops without the O-ring returning to its original shape. The cross-section does not relax sufficiently to close the enlarged sealing gap. A leak occurs. A lower permanent set generally indicates a higher quality of the O-ring material.

In dynamic applications, changes in the sealing system occur much more quickly. Here the arrangement of the components to be sealed is less fixed, so the forces acting on the O-ring change constantly. In the process industry there are often changing pressure conditions: liquids and gases are subjected to very high peak pressures that can, however, drop again very quickly. The O-ring has to absorb this as well.

Compression Set (CS) as a Key Figure

The compression set (CS) measures the ability of an O-ring to return to its original shape after the pressure is released. It is an important material-dependent key figure. It is always given in the data sheet of an O-ring material. The higher the compression set, the more strongly the seal is affected by a temporary pressure load. The O-ring then permanently retains a plastic deformation.

The technician determines the compression set as follows: the cross-section of the O-ring is compressed by 25 percent using a weight. This can be done in temperature-controlled air, but equally in all other relevant contact media. This is because temperature and media influence the compression set. In this way, laboratories can additionally evaluate the effects of swelling or extraction. The figure in the material data sheet, however, refers to storage in heated air in a heating cabinet.

After unloading, the sample is given time to relax again. It can then be measured to what extent the O-ring cross-section has returned to its original round shape. When designing a seal, it is important to take the compression set in the relevant contact medium into account.

The example of EPDM illustrates this: in air, the material already shows a high permanent set at 150 °C. However, EPDM O-rings are very well suited for use with hot water and steam. Here the seals retain a low permanent set even at higher temperatures.

In failure analysis, technicians recognise permanent set for example as follows: when removing an internally sealing piston seal, the missing radial contact force on the sealing face is noticeable. The O-ring has then permanently taken on the change in shape caused by the oversize of the piston.

Causes of Permanent Set

The mechanism behind the loss of elasticity is always this: under the influence of the environmental conditions, bonds in the molecular structure of the polymer are lost. At the same time, high temperatures can lead to post-cross-linking. In this process the molecules form additional bonds that increase the stiffness of the material. How heat changes the material is explored in more detail in the article Heat damage in O-rings.

In terms of material, the elasticity of an O-ring depends on the base polymer, the material formulation and sufficient vulcanization. But the design of the groove and the operating parameters can also contribute to a failure due to permanent set. The general target range for compression is between 15 and 30 percent. For certain materials, the recommendations differ. FFKM, for example, requires lower compression.

Important to know: at the same percentage compression and with increasing cord thickness, the deformation forces grow. The effect of a larger cord thickness must therefore be compensated by lower compression.

The permanent set indicates how well the elastic recovery capacity has been preserved. How long an O-ring with a permanent set maintains tightness, however, depends on the individual sealing system. In some cases, O-rings with a permanent set of 80 percent can still be tight, while other seals fail even at a considerably lower permanent change in shape. You will find further typical failure patterns in the overview of the causes of failure in O-rings.

Solutions to Counter Permanent Set

It depends on the entire sealing system how effectively permanent set has to be ruled out. When designing it, engineers must consider a large number of mutually influencing factors. Once the sealing point is given, the O-ring itself is the first to be put to the test.

Through material selection — that is, different base polymers and compounds — the best possible match between the O-ring and the operating conditions can be achieved. If that is not enough to create a stable sealing system, much more elaborate measures usually follow, such as reworking the groove or adjusting the operating conditions.

Frequently Asked Questions