Designs of static O-rings

The application determines the type of elastomer. The following criteria must be taken into account:

• media resistance,
• the elastomer must be resistant to possible extrusion,
• the maximum possible pressure must be taken into account,
• In addition, the retention of good physical properties over the entire temperature range should be ensured.

Use Kremer's O-ring installation calculation program , which we provide to you free of charge and without registration. The compression set, hardness , tensile strength , chemical resistance, thermal effects, pressure and the risk of extrusion influence the sealing function.

Compression set and compression

Compression set is the percentage of deformation that an elastomer permanently retains after a specified time at a specified temperature and a defined compression. Compression set is the measure of the expected loss of elastomeric resilience and is therefore an important indicator. It is usually determined in dry air and measures the percentage of the original cross-section. Although it is desirable to have as little compression set as possible, in some cases it is not as critical as it first appears:

An O-ring that has a compression set of 100% can still seal. In addition, volume swelling caused by the contact of the O-ring with the medium to be sealed can compensate for the compression set. If there is a high compression set and chemical shrinkage of the O-ring, this will lead to a failure of the seal. Exception: The seal was pressed very hard.

Important notes on compression set:

• A low compression set characterises a good permanent sealing function. However, the compression set increases with increasing temperature and time.
• For O-rings, the minimum compression set should be approximately 10%. The reason for this is that almost all elastomers quickly lose their elastomeric restoring forces at a very low compression set and achieve a compression set of 100%.
• Most O-ring applications cannot function at such a low compression of less than 10%, with the exception of non-contact seal designs in specialised pneumatic and rotary applications.
• The most common standards for determining compression set are DIN 53517 and ASTM D 395.

Please note that the value of the compression set changes over time and depends on the O-ring cord thickness. The adjacent table shows these differences based on values determined for the same compound after different periods of time. The test specimens were subjected to the test compressed by 25%.

>>> Tensile stress/elongation

The tensile strength is the maximum stress that is achieved when a test piece (either an O-ring or a dumbbell-shaped strip) is stretched. Elongation at break: The elongation or elongation at break is the sum of the expansion at the moment of tearing.

Modulus (also called ‘Modulus 100’) is the force required to achieve a certain elongation. In the case of modulus 100, this would be the force required to stretch a sample by 100%.

With elastomers, the necessary tension is not linear with the elongation. This means that the modulus is neither a quotient nor a constant gradient, but rather an indication of a specific point on the ‘stress-strain curve’.

Tensile tests are used to check product quality and to assess the effect of chemical and thermal influences on an elastomer. In the latter case, the retention of its physical properties is often more important than the absolute values of its maximum tensile stress, elongation at break or modulus.

>>> Tear resistance (tear propagation resistance)

The tear strength or tear resistance of most elastomers is relatively low. This test measures the force required to continue a notch or cut. Sealing materials with a weak tear strength fail quickly under further stress once a crack occurs.

A compound's low tear resistance is also an indication of poor abrasion resistance, which in turn can lead to premature failure of an O-ring in dynamic use.

>>> Volume change (volume swelling, volume shrinkage)

The volume change is the increase or decrease in the volume of an elastomer after it has been in contact with a medium. It is expressed as a percentage. An increase due to swelling or a decrease due to shrinkage in volume is almost always accompanied by a change in weight.

Volume swelling is caused by the absorption of a gaseous or liquid medium by the O-ring.

In static applications, extreme volume swelling can sometimes be tolerated more tightly.

In fact, an O-ring can only swell to 100% of the groove, so no further increase in volume is possible, regardless of how much volume swelling is observed in a dipping test. However, if free-state swelling exceeds 50 percent, a radially compressed assembly can be nearly impossible to disassemble due to the friction created.

>>> Thermal effects

All rubber is subject to aging at high temperatures. Volume swelling and compression set are affected by heat. The first effect of high temperature is to soften the compound . This is a physical change that goes back to normal as soon as the temperature drops.

In high-pressure applications and at increasing temperatures, the O-ring can still begin to flow into the sealing gap due to this softening (gap extrusion). Chemical changes occur as time and temperature increase.

>>> Thermal expansion

The linear thermal expansion coefficient is the quotient of the change in length per °C in relation to the original length at 0°C. The volumetric expansion coefficient of solids is approximately three times as high as the linear one. Roughly estimated, elastomers have a thermal expansion coefficient 10 times higher than steel. The thermal expansion coefficient is even higher for fluoroelastomers and perfluoroelastomers. This can be critical at high temperatures when the groove is almost full or at low temperatures when the compression is particularly low .

Seal failure can result in leakage if too little compression is achieved due to low temperatures. This generally results in an increase in hardness along with changes in volume and compression set as well as tensile strength and elongation at break. Because these changes are chemical in nature, they are not reversible.

Changes caused by low temperatures are mainly physical in nature and therefore reversible. An elastomer will regain almost all of its original properties when subsequently heated.

There are certain reactions that, under certain conditions, cause the O-ring to exert high forces against the groove sides. When the seal fills the groove 100% completely, the prevailing force is determined by the thermal expansion of the rubber.

The groove must always be large enough to accommodate the maximum expansion of the O-ring.

There have been cases where seals have torn due to thermal expansion of the steel grooves. As a precaution, it should therefore be ensured that the filling level of a seal groove is never more than 95%.

This should be taken into account especially when designing O-ring grooves for applications above 150°C.

Our O-ring installation calculation program helps you design O-ring installation situations.

Synonyms : Static O-rings, design