An O-Ring is a circumferentially closed, ring-shaped sealing element, which prevents unwanted discharge or loss of media. It requires little installation area and can be easily mounted. Therefore, the O-Ring is the most widely used seal. Other advantages are its functional reliability and very cost-effective manufacturing.
ttv O-Rings are manufactured and controlled in accordance to DIN 3601 (formerly 3771). They are produced seamlessly in a heated injection or compression mould by vulcanization. As for the material variations of elastomer materials are used.
The designation of the O-Ring is composed of three elements:
» d1 = Dimensions of the inner diameter (mm)
» d2 = Cord thickness (mm)
» Material designation and its hardness (elasticity)
The O-Ring marks a strikingly simple form and reliable function. The sealing action of the O-Ring is formed by the deformation of its cross-section d2 in a groove.
Thereby, the sealing gap at the groove base and the contact or sealing surface is sealed. As a result a surface pressure is generated, which makes the sealing effect possible. The maximum deformation of the O-Ring cross section depends mainly on the depth of the groove. With the right groove design and material selection, a dynamic or static seal can then be employed within the temperature limits of the material.
In the operating state the pressure of the medium increases the deformation and therefore its sealing function. If this pressure drops to „zero“, then the deformation is close to reaching installation condition once more.
The cord diameter d2 must always be greater than the installation area.
The compression is specified as a percentage. Designated as pressing is that percentage of the cord diameter d2, by which it is compressed in the installation state. The compression is consequently directly related to the depth of the groove. Equal percentage of compression force and increase deformation in accordance with the increasing cord strength d2. To compensate for this, the percentage of the compression will be reduced with increasing cord diameter.
Existing pressure can be advantageous for the sealing. This is additionally deforming the O-Ring; the pressure will be supported in in some areas. Pressure bears down on the O-Ring on the pressure-remote groove side. In order to avoid gap migration at the O-Ring, this should be kept to a minimum. For a radial sealing tolerance of H8 / f7 is anticipated, for an axial sealing it is H11 / h11.
If this cannot be ensured, or high pressures are to be expected, a high material hardness of the O-Ring should be chosen. Otherwise, a gap migration?/?extrusion may occur resulting in the destruction of the O-Ring.
O-Rings can be applied in two areas:
» static sealing in resting machine parts
» dynamic sealing in moving machine parts
O-Rings are very suitable for the sealing of machine elements, which do not move comparatively to each other. Here, with O-Rings pressures up to 1000 bar are sealed, as long as the installation area is carried out properly, the application and critical design areas are accurate and correct material has been selected (Additional Backup-Rings are to be used if in doubt).
In dynamic applications, O-Rings are used successfully as a sealing element. Here, however, more likely at lower pressures, velocities or in small installation areas. Because it comes to frictional resistance with e.g. the movement in hydraulic or pneumatic components, a smaller compression of the O-Ring is selected as for static sealing. To prevent friction or premature wear of the O-Ring caused by dry running, good lubrication should always be guaranteed.
For the translational (back-and-forth) motion and for spiral motion the installation areas are the same. In the application fields of hydraulics and pneumatics, however, they differ in air pressure and lubrication conditions.
The primary criterion for selection of materials comes down to the operating temperature and the media resistance. Since they contribute towards the life of the seal, the mechanical properties of an elastomeric composition need to be taken into account.
The ttv resistance guide gives information on the chemical resistance of different materials. Technical rubber materials are subject to an exact recipe. In comparison of all the media to be sealed containing mixed components, the polymer is in relation to its chemical resistance the weakest of the components. The selection of the right material is dependent on the right choice of the base polymer. In practice, further recipe-related influences in accordance to the nature and amount of the plasticisers and fillers, would decisively alter the characteristics.
The polymer compatibility alone is no guarantee for reliable sealing, but it is an important prerequisite.
|Auxiliary processing means||1.3|
|Nitril NBR||-30°C||+120°C||Hydraulic oil, grease, hydrocarbons, oils, lubricants, vegetable oil, water, butane, compressed air|
|HNBR||-35°C||+150°C||Ozone, UV, hot water, sulphurous oils|
|Chloropren CR||-40°C||+120°C||Air, ozone, water up to 80 °C, vegetable oils, oxygen, caustic soda, fatty alcohol, chlorine, refrigerant gas, alimentary applications, CO2|
|Ethylen / Propylen EP||-45°C||+110°C||Food resistant (when peroxide cross-linked): water, beverages, use with inflammable liquids, vapour, diverse acids, caustic soda, glycols, ozone, hot water|
|Silikon VMQ||-60°C||+225°C||Low and high temperatures, air, oxygen, inert gas, low concentrated bases and acids, ozone|
|Fluorenkohlenstoff FKM||-15°C||+240°C||Good oil resistance, hydraulic liquids, solvents, use with inflammable oils and chemicals, ozone|
|PTFE||-150°C||+260°C||Excellent chemical resistance, electric insulator, low friction coefficient|
Since elastomeric materials typically point out "anti-sliding“ and "sticky“ surfaces, it is often necessary to improve the coefficient of friction of an O-Ring. Various methods can reduce slip friction and for easier assembly and even achieve a lifetime extension.
|Type of treatment||Description||Colour/Aspect|
|Short term||Siliconisation||A silicone film is sprayed onto the parts to be treated||Glossy, greasily, transparent|
|Intensification of sliding||Adding a molybdenum disulfide coating||Molybdenum powder coating by tumbling||Silver-like|
|Long term||Talcum powder coating||Talcum powder coating by tumbling||Dry, white|
|PTFE powder coating||PTFE powder coating by tumbling||Dry, white|
Even better friction reduction over a long period can be achieved by intensifying sliding additives in elastomer compositions, such as molybdenum disulfide (MoS2) or PTFE.
The tendency to extrusion?/?gap migration mostly depends on the gap measure within the machine parts. The gap is dependant on the processing, manufacturing procedure and tolerance.
It is advisable to implement the clearance as small as possible.
A larger seal clearance can result in the destruction of the O-Ring by gap migration.
O-Rings with a hardness of 90 Shore A allow for a slightly larger gap than standard-O-Rings in 70 Shore A.