O-RINGS MEssen | Design Simply explained

Everything you need to know about measuring O-rings

The right dimensions, the desired material behavior and a uniform surface quality: all of these properties together determine how reliably O-rings can perform their sealing task.

Even a single deficit is enough to significantly reduce the service life of a seal. Then, despite careful design, there is a risk of short maintenance intervals or sudden machine defects.

Quality control of O-rings is therefore an essential and ongoing task for manufacturers and suppliers. But which parameters are decisive for the quality of O-rings?

In our technical article, you can find out which properties are important and how they are measured.


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#1 The 5 most important O-ring properties

A reliable O-ring seal results from the sum of careful design, stable product properties and professional installation. Whether the O-rings supplied fulfill their part of the task can be checked primarily on the basis of these properties:

  • Inner diameter
  • Cord thickness
  • Hardness
  • Material composition
  • Compression set
Inner diameter

The inside diameter of O-rings, together with the groove base diameter, determines the elongation of a piston seal when installed.

It is crucial for the sealing effect that this is within the target range. For example, excessive elongation causes the cross-section of the O-ring to flatten. As a result, there may be less compression on the cylinder surface.

ZVarious measuring methods are available for determining the inside diameter. The range extends from simple measuring cones to laser scanning micrometers.

Cord thickness
Together with the inner diameter, the cord thickness describes the basic dimensions of an O-ring. The outer diameter of the sealing element is calculated from the inner diameter and the cord thickness.

Accordingly, a piston seal has a direct influence on compression. Cord thicknesses can be measured using probes or laser scanning micrometers.

Shore hardness
Harder O-rings are better suited for use under high pressure. There is a risk that the O-ring will be pressed into the sealing gap, especially if there is a combination of a large sealing gap width and high operating pressure.

A harder O-ring can withstand this stress for longer without shearing off material, for example. The common measuring principle consists of a truncated cone that presses on the material. Its penetration depth provides information about the hardness of the material.

The common Shore A and IRHD methods differ primarily in terms of the weight and shape of the test specimen. While Shore hardness is used for standard samples, IRHD is more suitable for measuring finished parts.

Material composition

The content of the polymer material in relation to other formulation components indicates how the media resistance or temperature behavior of an O-ring.

For example, the acrylonitrile content of NBR sealing elements varies between 18 and 50 percent. A higher compound content improves resistance to oil and fuels.

If the acrylonitrile content decreases, the elasticity improves in return. For reliable sealing, it is therefore crucial to rule out excessive fluctuations in the formulation.

The composition of a material sample can be examined using thermogravimetric analysis (TGA).

Compression set

As far as the temperature behavior of O-ring materials is concerned, the compression set (DVR) allows the best conclusions to be drawn.

To determine the value, the O-ring is stored in a temperature-controlled medium in a deformed state. The better the O-ring returns to its original shape after being relieved, the lower the compression set.

Accordingly, a lower value indicates that an O-ring is closing the sealing gap even under changing operating conditions.

Testing the compression set also provides information on the degree of vulcanization and therefore the elasticity of the O-ring.

#2 The most important O-ring standard: ISO 3601

Do the O-rings meet the requirements in terms of their properties?

For standard-compliant O-rings, this is decided by comparing the measured values with the relevant standard. The globally dominant standard ISO 3601 in its current version contains extensive specifications for the required properties of O-rings.

ISO 3601 Manufacturing tolerances
Part 1 of the standard refers to inside diameters and cord thicknesses and the associated tolerances. The valid tolerances are shown in two classes for each combination of inner diameter and cord thickness.

ISO 3601 Surface tolerances
Part 3 of ISO 3601 regulates the permissible deviations in shape and surface quality.

The section divides the common surface deviations into different categories. The permitted deviations in each category are given based on intervals of line thickness.

The standard also covers the requirements of different applications in the area of surface tolerances with various classes.

ISO 3601 Material properties
The section on material properties is a more recent addition to the standard.

Part 5 defines requirements for the properties of many standard materials for O-rings. These include NBR, HNBR, FKM and EPDM. The specifications for hardness and compression set are specified on the finished part.

This allows the properties of the O-rings actually supplied to be compared with the standard specifications. In the past, such a comparison was often made more difficult by the fact that values were specified on standard samples.

Even though ISO 3601 contains extensive specifications for tolerances of O-rings, the quality control of O-rings is not without its problems.

After all, O-rings are generally elastic products. For example, even with IRHD, the hardness test is not very meaningful for particularly small line thicknesses of less than 1.6 mm.

In addition, elastomers are usually not homogeneous over the entire circumference of the O-ring, so that local measurements are limited in their interpretability. On the other hand, better measurement methods are now available.

Where manual measurements used to inevitably lead to deviations, there are now economical alternatives for use in industry and commerce outside of specialized laboratories.

#3 Four other relevant O-ring properties

The five most important properties already allow a very good characterization of finished O-rings. However, other parameters may be relevant for particularly demanding applications.

  • Density
    Density is a material constant that is independent of the shape of the product. This is particularly useful when assessing O-rings. Users can deduce the base polymer and the composition of the respective O-ring from the density. In practice, this is an effective approach for incoming goods inspection. It is used, for example, to reliably detect incorrect deliveries and uncover recipe fluctuations.
  • Tensile strength
    Manufacturers specify the tensile strength of an O-ring as well as the density in the data sheet. It provides information on the force required to tear a sample. Except for unfavorable assembly processes, however, O-rings are hardly ever subjected to tensile strength in practice.
  • Tensile elongation
    This value indicates how great the elongation is at the moment a sample is torn. Like tensile strength, this property is particularly relevant for the assessment of critical installation processes.

  • TR-10 value
    Hardness and compression set describe the elastic behavior of O-rings. However, the TR10 test has also become established. A sample strip is frozen in a 100 percent stretched state. After release from the test fixture, it is observed how the sample returns to its original shape as the temperature equalizes. The TR10 point marks the point at which the elongation of the sample has decreased by 10 percent.

#4 Measuring O-rings: Classify results correctly

Measuring O-ring properties is a challenge in itself.

Hand-held devices usually do not have the necessary accuracy and therefore only provide a trend. In addition, the elastic products are manipulated during the measuring process. In addition to the equipment, compliance with the respective measurement standard also has a considerable influence on the measurement results.

To ensure that the results are reliable and can therefore be used, for example, in contact with suppliers, the requirements for the number of repetitions, temperatures or properties of standard samples must be taken into account. In practice, this often proves difficult.

In addition, the conflict between specifications on standard samples and measurements on the finished O-ring remains.

Many relevant properties of O-rings are subject to shape dependency, so that measured finished part values cannot be easily compared with data sheet specifications for standard samples.

This is why more and more users of O-rings are specifying the properties of the finished part. For example, ISO 3601 also provides for hardness and strength.

In addition, users of O-rings can reduce their own measurements by working with reliable suppliers who in turn ensure stable product quality.

“I am convinced that we should share our knowledge with the world. I hope I have been able to answer all your questions. If you have any further questions, please feel free to contact us at any time. We will be happy to help you.”

Luke Williams
Luke Williams

Lord of the O-rings
Author of the sealing academy

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