Identifying and preventing causes of failure in O-rings
Swelling, heat damage, gap extrusion or assembly errors: we show you the most important failure patterns in O-rings, their causes and the right countermeasures so that your seals last longer.
- O-ring failures fall into four categories: media exposure, temperature and ageing, physical stress and manufacturing defects.
- Mechanical and physical influences such as faulty assembly and poorly designed grooves are the most common cause of failure.
- Data sheet figures for temperature resistance apply without pressure and without media. Pressure and media shift the real-world limit downwards.
- Swelling, embrittlement and a mushy feel point to media attack, peeled material to gap extrusion.
- Training in design and assembly, together with careful material selection, prevents most failures.
Many failures are avoidable
Users barely notice a seal that works well. It reliably does its job over a long period. When a seal does eventually attract attention, the consequences are usually unpleasant. If a machine comes to an unplanned standstill because of a leak, high revenue losses loom.
This risk is especially high in highly interconnected production plants, such as those that prevail in the chemical, pharmaceutical and food industries. There, O-rings also face challenging operating conditions: particularly high or low temperatures, aggressive media and high pressures put the seals to the test.
Machine failures are not the only problem. Harder to quantify are consequences such as product recalls for goods manufactured under undefined production conditions, or environmental damage from escaping hazardous substances. A look at the most common causes of failure, however, shows that in practice many failures can be avoided with simple measures.
Why failure analysis is so important
Anyone who wants to increase the reliability of seals should look for the cause when a failure occurs. Often a clear reason for the defect can be identified. In more complex cases, the interplay of several causes leads to the leak. Even then, failure analysis at least makes it possible to rule out unrelated factors as the cause.
Assembly, installation space and operating conditions stress the O-ring in use. To check whether the right solution was chosen for the sealing task, all these factors need to be examined. In addition, the sealing elements may already be exposed to unwanted influences during manufacturing and storage. An expert eye often provides valuable clues and, in many cases, makes elaborate laboratory analyses such as spectroscopy or chromatographic methods unnecessary.
Experts divide the causes of seal defects into four categories: damage from the action of process media, weakening of the material through temperature and ageing, manufacturing-related defects, and impermissible physical loads.
| Failure pattern | Typical cause | Remedy |
|---|---|---|
| Swelling or shrinkage | Unsuitable pairing of material and media | Choose a resistant material, if in doubt the next grade up |
| Embrittlement, cracks | Exceeding the continuous temperature, ageing | Factor in a safety margin, upgrade the material |
| Gap extrusion | Excessive pressure, too much sealing clearance, wrong groove | Check gap dimensions, use a harder material or a back-up ring |
| Assembly damage | Insufficient lubrication, sharp edges, twisting | Assembly aids, training, chamfers on installation edges |
Damage from media exposure
An unsuitable combination of sealing material and media can cause severe swelling of the O-ring. This happens when the medium penetrates the material. In the opposite case, shrinkage is also possible: the medium then leaches mixture constituents out of the material.
Media exposure is not limited to ongoing operation. Unsuitable assembly oils or assembly greases can chemically damage O-rings even before they are put to use. Both phenomena change the material properties. The rubber elasticity suffers as a result, that is, precisely the property that makes elastomers so suitable for sealing applications.
Cracks, embrittlement or permanent deformation often occur. Another indicator is the feel: under media exposure the surface structure changes, and the O-ring then feels somewhat mushy.
An impermissible pairing is not always due to the resistance list being ignored. Operating conditions that deviate from the plan, or a mix-up in storage, are also possible causes. Even FFKM, known for its particularly broad range of applications, shows severe deformation after just a few weeks in contact with hot water above about 150 °C with common formulations.
Damage from temperature and ageing
Common failure patterns in this category include crack formation and embrittlement. The cause is often a significant exceedance of the upper continuous temperature for which the seal is approved. Important: the temperature resistance stated on a data sheet was determined without pressure and without media, that is, in air. In real applications, pressure and media have a negative effect on temperature resistance.
Heat damage is usually easy to recognise because the feel changes markedly. The O-ring feels brittle and sooty and, in extreme cases, can be broken apart under only slight pressure.
The cause of failure known as ageing covers the effect of substances from the ambient air as well as of heat. For example, the reaction of NBR with ozone from the air leads to irreparable damage within a few days. These deep cracks, known as ozone cracks, always form perpendicular to the direction of the stress acting on the O-ring in its installed state. A coating or a switch to an ozone-resistant rubber provides a remedy.
Damage from impermissible physical stress
By a clear margin, faulty assembly is the most common reason why an O-ring seal fails after some time. This includes almost all defects that occur without prior damage to the network structure. The encouraging part: precisely these failures are the easiest to avoid through appropriate training.
The main source of error lies in the assembly process, in the design of the installation space, and in too little or too much compression. Often it is insufficient lubrication during installation, as well as missing or unsuitable assembly aids, that cause permanent damage. Excessive twisting also damages the O-ring, but is hard to avoid with large seals that have a small cord thickness.
The error can also occur as early as the design stage. Sharp-edged groove cuts, inappropriate compression or too much diametral clearance are among the common causes. A frequently seen phenomenon: due to poorly designed grooves or excessive pressure, the O-ring is forced into the sealing gap and peeled away over time. This gap extrusion is one of the most common causes of failure and is easy to recognise by the peeled rubber material. Permanent deformation and abrasion also belong in this category.
Manufacturing-related damage
Much like O-rings damaged by assembly errors, sealing elements with manufacturing defects often fail after only a fraction of their expected service life. The two causes of failure are therefore hard to separate.
A key indication of a manufacturing defect is the exceedance of the limit dimensions defined in DIN ISO 3601. Clear deviations from the target condition of an unused O-ring are also incipient cracks and their precursor, radial flow lines. Peroxide-cross-linked materials such as FKM are considerably more prone to defects in this respect than NBR.
In addition, there is under-vulcanization, in which the molecular cross-links in the elastomer do not form completely. The result is an undefined development of the desired rubber-elastic property.
Preventing premature failures
Quantitative studies show that mechanical and physical influencing factors are most often responsible for failure. Next come defects in the temperature and ageing category. Incompatibility of the material with the media, as well as manufacturing defects, are by contrast considerably rarer.
Because assembly-related damage and inadequately designed installation spaces dominate the most common group of faults, one thing becomes clear: anyone who values a fault-free assembly process as well as careful design and manufacturing of the groove noticeably extends the service life of many seals. Training employees in assembly and design contributes to this, as do suitable assembly fixtures and auxiliary materials.
The consequences of a failing seal are often grossly disproportionate to its procurement cost. This is precisely why careful supplier selection pays off. At NH O-RING you gain not only a flexible supplier of high-quality seals, but also access to the expertise of experienced staff who work with you to design the right seal for your application.
Frequently asked questions
What are the most common causes of failure in O-rings?
How do I recognise heat damage on an O-ring?
What is gap extrusion in O-rings?
Do the temperature figures in the data sheet apply to every application?
How can O-ring failures be avoided?
What happens with media incompatibility?
