O-rings, also known as rubber rings and sealing rings are the most frequently used type of seal. It is due to the fact that installation of O-rings is very easy and that they require little space. If the selected material the design of the groove are suitable, sealing based on O-ring may play its role for a longer time (provided that the permitted temperature ranges, etc. are adhered to). The most commonly used materials for the manufacture of O-rings are NBR (nitrile butadiene rubber), FPM (fluoroelastomer), MVQ (methyl vinyl silicone rubber), EPDM (ethylene propylene diene monomer (M-class) rubber), etc. Among the less used ones is e.g. CR (chloroprene rubber).
The function of the O-ring inserted in a proper groove lies in the change of the circular section of the ring into an elliptic one. The pressed O-ring seals the gap between the contact elements or surfaces and the bottom of the groove. The diameter of the O-ring must always be larger than its depth.
The hardness of the material is not a sign of quality but is a property that plays an important role in the sealing process. The hardness must fit for example, the value of pressure in the system. The softer the O-ring, the easier deformation takes place due to pressure and installation into the gap. O-rings with lower hardness are more flexible and therefore the sealing effect occurs at a lower pressure and on an uneven surface.
For static and dynamic applications where sealing is provided by the internal diameter, it is necessary to select the dimensions of the O-ring in a manner that allows the internal diameter to be 1 - 3 % larger than the diameter of the groove (d6). The O-ring should be installed slightly depressed.
With internal pressure, the external diameter of the O-ring (d1 + 2d2) should be approx. 1 - 4 % larger than the diameter of the internal groove d7. In practice this means that the O-ring should be installed slightly depressed and it should bear upon the external wall of the groove.
With external pressure, the internal diameter of the O-ring (d1) should be approx. 1 - 4 % smaller than the internal diameter of the groove (d8). The O-ring should be installed slightly stretched and it should reach the internal wall of the groove.
The most important instructions for installation:
O-rings are very sensitive to contact with sharp edges. It is therefore necessary to file or round off all sharp edges that the O-ring might come into contact with during installation.
The physical properties of O-rings may change during long term storage. For example, the material may become too hard or too soft, The surface may appear cracked or damaged. This is caused by a combination of external factors e.g. oxygen, light, ozone, heat, humidity, oils and solvents, etc. The basic instructions for storage are defined in the DIN 7716 and ISO 2230 standards.
The optimal temperature for the storage of O-rings should range between +5 °C and +25 °C. It is important to eliminate direct contact with sources of heat such as heating elements. In addition, never expose stored O-rings to direct solar radiation.
Air humidity in the storage space should not exceed 70 %. Extremely humid or dry conditions may cause damage to the rubber.
O-rings should be protected against light sources, in particular against direct solar radiation and powerful artificial light sources emitting ultraviolet radiation.
O-rings should be protected against circulating air. Optimal protection is achieved if the seals are kept in the original package or in airtight packages.
O-rings should be kept loose and in their original form without being extended, depressed or otherwise deformed. To save space O-rings with large dimensions may be kept in a coiled condition.
Rubber |
Maximum storage period | Extension |
BR, NR, IR, SBR, PU |
5 years | 2 years |
NBR, XNBR, HNBR, CO, ECO, ACM, CR, IIR, BIIR, CIIR |
7 years | 3 years |
CM, CSM, EPM, EPDM, FPM, VMQ / MVQ, PVMQ, FVMQ |
10 years | 5 years |
if storage conditions pursuant to the DIN 7716 and ISO 2230 standards are adhered to, the storage period may be extended.
Hardness
[ShA]
|
Maximal pressure
[MPa]
|
O-rings made of elastomer according to |
60 | < 10 | PN 029281.2 (only for static seals) |
70 | < 10 | DIN 3770 – NBR70 |
80 |
< 10
|
PN 029280.2 and PN 029283.2 |
90 | < 20 | DIN 3770 – NBR90 |
90 | < 20 | PN 029080.9 – AU90 |
90 | < 32 | PN 029281.9 – AU90 (only for static seals) |
Hardness of a O-ring is chosen mainly by the size of the operating pressure and size of sealing joint. At higher pressure and bigger sealing joint, harder materials are chosen to avoid pushing the O-ring into the joint.