O-rings: Most Common Damages
| Extrusion | Compression Set | Spiral Deformation | Abrasion | Working Environment Impact | Exposure to Ozone |
In order to seal properly, the O-ring must maintain its elasticity when pressed between the seal surfaces. When exposed to a very high pressure, the O-ring gets sqeezed into its housing seat and the seal area with a strength proportional to the pressure applied. In addition to pressure, the degree of extrusion also depends on the gap size between the two surfaces. It is essential to keep this gap as small as possible; otherwise, pressure would cause the ring to slide out of its housing seat into this gap. This processes often causes significant mechanical damage onto the ring surface, including peeling or cracking of the elastomer. O-Rings of 90 Shore A hardness permit slightly larger gaps than standard-O-Rings of 70 Shore A. Silicone O-rings require special attention because high temperature reduces their endurance to pressure.
Most elastomeric seals are designed to operate within ambient pressure to about 1 500 psi (~10 342 kPa). At very high pressures, the seal must have sufficient strength to resist extrusion into the clearance gap. The chart at right illustrates the recommended limits of the combination of clearance gap (diametral), seal hardness, and pressure differential.
Techniques to avoid extrusion in high-pressure applications include decreasing the clearance gap, increasing the elastomer hardness and the use of backup rings. Backup rings can be made of many rigid polymeric materials and are used on the low-pressure side within the seat to help prevent extrusion.
1 psi = 6,895 kPa = 0.06895 bar
1 inch = 2,54 cm.
Compression set is the loss of compound elasticity, which causes permanent flattening, deformation and weakening of sealing qualities of the O-ring. Among the reasons causing compression set are: excessive compression, excessive temperature, incompletely cured elastomer, incorrect use of compound with high compression set or incorrect choice of working fluid which results in excessive volume swell in chemical. It is by far the most common damage in O-rings. Compression set comes as a result of breaking molecular structure due to external factors. Lower quality compounds are in general more susceptible to compression set, even though this deformity can be seen in all O-rings to some extent. In other words, once mounted and used, a sealing can no longer regain its original shape.
Spiral deformation is very easily recognizable by its spiral cuts on the O-ring surface and in due time it would eventually lead to the sealing's irreversible damage. It occurs when any one or more of the following is present: tight installation, slow reciprocating speed, low hardness of the compound, irregular O-ring surface finish, excessive housing seat width, irregular or rough hosuing surface finish, inadequate lubrication. Anotherreason for the occurrence of spiral deformation may be the large inside diameter to cross-section ratio: the O-ring doesn’t have enough strength to resist the twisting forces that naturally develop during dynamic movement.
To prevent such damage, pay extra attention to mounting procedures and ensure adequate lubrication the the O-ring surface.
Because all elastomers are porous, when an O-ring is used in a gas media under high pressure, gas can get trapped inside the seal’s micropores. With subsequent rapid decrease in pressure, this trapped gas rapidly expands in an effort to match external pressure causing blisters, pits or pocks to appear on the O-ring surface. The amount of structural damage done to the O-ring depends on the volume of the trapped gas and the hardness of the material. Smaller volumes (especially in soft compounds) may only cause surface blisters which can disappear as pressure equalizes. Larger volumes (particularly in hard compounds) can cause deep cross-section ruptures or even total O-ring disintegration.
Not all gasses have the same impact: lighter gasses such as CO2 or helium are more likely to cause elastomer swell and rupture than heavier gasses. Explosive decompression is rarely witnessed at an operating pressures of less than 30 bars. To prevent this damage, we recommend using harder compounds and allowing for longer decompression periods.
Failure due to abrasion is most likely to occur in dynamic seals--the bigger the friction on the O-ring surface, the higher level of abrasion.
Among the causes for abrasion are: heavy dynamic loads, lack of proper lubrication, rough sealing surfaces or poor elastomer seal finish, excessive temperatures, excessive compression, operating fluid containing abrasive particles. As opposed to compression set which causes flattening on both sides of the cross section, abrasion leads to a flat surface parallel to the direction of motion. Wear lines may also appear on the flattened part of the O-ring.
To avoid or limit abrasion, we recommend ensuring the O-ring surface is smoothly finished, as well providing sufficient lubrication upon installation.
One of the most common reasons for O-ring failure is the working environment impact. Even if only one of the elements of the working environment falls outside of its proper levels, this can easily cause irreversible damage. As previously mentioned, all elastomers are porous. If an elastomer is not suited for contact with an aggressive chemical within its working medium, such a contact could cause a chemical reaction, such as absorption and/or extraction of elements out of the compound's original molecular structure. The change on molecular level bring about physical swelling or shrinking of the O-ring, often leading to significant decrease of sealing qualities of the O-ring. To avoid this, one must carefully choose the O-ring compound taking into account all fluids/gasses that might come in contact with the O-ring within its working environment. Such information can be obtained under the Resistance tables section.
Another important environmental factor which needs to be monitored is the working temperature. If this temperature is not kept within the recommended range, it could cause secondary curing and/or hardening of the material (the compound becomes brittle and complete loses its sealing ability). Please bear in mind that excessive heat might come as a result of dynamic friction.
Excessive O-ring stretching (a stretching of more than 6-10% of the O-ring's internal diameter, depending on the compound), as well as direct sunlight, both increase the negative effect of ozone. Exposure to ozone and other atmospheric contaminants can cause tiny cracks to form on the O-ring's surface. These cracks form rapidly under pressure and attack the molecular structure of the elastomer. This is usually observed with compounds non-resistant to ozone and other atmospheric elements (such as NBR).
Make sure that the O-rings you are installing have not been previously damaged; stretching damaged O-rings makes them more susceptible to ozone impact. High temperature and high levels of oxygen or ozone accelerate this process. Ozone attack on intact surfaces results in the so called ozone frosting--characterized by whiting or graying of the vulcanized surface. In comparison, ozone attack on previously damaged surfaces causes typical cracks running perpendicular to the direction of stress, thus completely destroying the O-ring.
To limit possible damage by ozone exposure, it is recommended to store O-rings in their original plastic packages and into purpose-suited storage rooms, only to be taken out prior to installation.
| Home| About us | O-rings | Oil seals | UN seals | Other products | Contact|
© 2004-2013 Satellite Nikis. All rights reserved. Last change on Jan 22, 2013 by RS