O-Ring Quality Control: Inspection and Dimensions

Why quality control of O-rings is decisive

Users are placing ever greater emphasis on the careful design of O-ring seals. They check the operating conditions, select a suitable material in the correct dimensions and fit the O-ring without damage. This puts in place the most important measures for a reliable seal. One factor, however, often remains overlooked: the quality of the O-rings themselves.

Global procurement markets and the considerable influence of formulations and production processes on performance create uncertainty. Low prices and few objective distinguishing features mean that price dominates the purchasing decision. Too little focus on quality carries its risks, because when O-rings fail in operation, this can have considerable consequences.

• Entire product batches can become contaminated and have to be disposed of.
• Hazardous or environmentally harmful media can escape at the sealing point.
• In addition, there is possible consequential damage to machinery and equipment.

That the durability of O-rings is a critical factor is shown by a look at the fields of application: hydraulics, pneumatics, shipbuilding, heating technology, petrochemicals, the food and pharmaceutical industry, automotive engineering as well as general machinery and plant engineering. Stable product quality forms the basis for reliable and economical seals.

Quality-determining properties

The sealing effect of an O-ring results above all from the uniform compression of the round cross-section between the sealing faces. From this, requirements for dimensional accuracy and surface condition can already be derived.

For adequate durability, the O-ring must also offer a high restoring capacity and sufficient hardness. In this way it bridges changes in the sealing gap and withstands pressure peaks. Added to this is resistance to ageing in air and to chemical attack by the contact media.

The task of quality assurance is to minimise the deviations of the finished O-rings from the defined target state. Quality control addresses both processing-related and formulation-related defects in equal measure.

Relevant standards and technical characteristics

The common standards for O-rings reveal which properties are particularly important for function. DIN ISO 3601-1, for example, specifies target dimensions and permissible tolerances for the inner diameter and the cross-section. How these requirements are divided into tolerance classes is summarised by us under ISO 3601.

The DIN ISO 3601-5 standard in the 2015 edition is regarded as a major success. It defines internationally valid material properties for elastomer seals for the first time. The target values are specified on test specimens and on the finished product. This is new, because until now data sheet figures have referred above all to results on test plates. Due to the measuring methods, however, there can in some cases be significant deviations from the properties of the finished products. What you should look out for in the figures is explained by us under O-Ring data sheets.

In addition, there are many guidelines for specific fields of application. DIN EN 681, for example, refers to vulcanized rubbers for sealing water supply systems. This standard is representative of many fields of use with special requirements. Overall, the requirements can be divided into three categories:

• Selection of specific base elastomers
• Chemical resistance
• Mechanical properties

Quality tests ensure that the finished O-rings meet the respectively applicable requirements.

Practical examples of quality problems

With the choice of a base polymer, the essential properties of a sealing material are predetermined. Every rubber brings its individual profile in terms of temperature range, media compatibility and mechanical properties. Depending on the formulation and processing, however, this profile can be expressed differently in the finished O-ring. Three examples show how such factors affect durability.

Degree of cross-linking

With sulphur-cross-linked materials, variations in vulcanization can be compensated for by a thermal post-treatment. With some NBR, HNBR or EPDM compounds, however, this is barely possible. Fluctuations in the vulcanization process have a more direct effect on the final product here. Under-cross-linking leads to a lack of elastic restoring behaviour in the seal.

Volume shrinkage

NBR O-rings with hardnesses in the range around 70 Shore A can have a plasticizer content of up to 20 percent. In use, media leach the plasticizer out of the material. The result is a decreasing compression.

Hardness variations

Especially with O-ring materials based on VMQ, testing laboratories detect variations in hardness. These often result from post-processing steps. The hardness influences how well the O-ring adapts to the metallic sealing faces. O-rings that are too soft are prone to gap extrusion under pressure.

Practical quality assurance

Safeguarding the quality of sealing elements is a demanding task. It divides into two areas.

The first area is the mostly one-off verification of the material formulation. It is particularly extensive and lies with the manufacturer. This is followed by in-process series tests intended to ensure consistent product properties. This relates above all to the composition and the dimensional tolerances of the O-rings.

Laboratory testing at manufacturer and distributor

The technical possibilities for measuring O-rings and testing the material composition have developed considerably. Today they allow for significantly more efficient quality control. This applies to all relevant property fields: dimensions, surface condition, long-term behaviour and material properties.

There is major progress especially in the area of workpiece dimensions. Non-contact systems avoid deformation of the O-ring and thus lead to significantly better results. Optical measuring technology is just one example. How O-rings can be measured correctly is shown by us under Measuring O-rings. The following overview shows that common laboratory methods require a broad range of high-quality equipment.

The equipment alone, however, is not enough. Reliable results are barely possible without the necessary know-how on the part of the user. NH O-RING therefore works together with a proven specialist. The Richter O-Ring Testing Laboratory uses state-of-the-art methods to ensure the quality of the products. Here, new materials are comprehensively qualified for series use. In addition, the laboratory carries out ongoing tests to ensure series quality. With this competent partner, NH O-RING ensures that only flawless quality reaches the end customers.

Identity check at goods-in

The purchasers of O-rings usually carry out a simplified identity check at goods-in. It mostly consists of three components.

• Density testing
• Hardness testing
• Compression set

The aim here is above all to rule out a mix-up of materials in the upstream process chain. In addition, some operations also apply tensile tests or swelling tests. The mechanical tests can also reveal strong processing-related deviations between different batches. Added to this are tests of the dimensional tolerances and the surface condition.

Conclusion: integrated quality assurance across the value chain

End users can effectively protect themselves against the risks of qualitatively inadequate O-rings. To do so, they should rely above all on transparent supply chains and reliable partners.

Manufacturers and suppliers of O-rings for commercial use carry out extensive quality tests in order to maintain a high standard. In doing so, they not only take the risk off their customers, they also save costs. Because the more customers can trust the quality of the O-rings supplied, the lower the effort for internal tests.

Provided that partners are reliable, a lean process at goods-in provides sufficient certainty. Anyone who increasingly incorporates quality criteria into the procurement decision achieves lower overall costs.

Frequently asked questions