Measuring O-rings correctly

The five most important O-ring properties

A reliable seal results from the sum of careful design, stable product properties and proper assembly. Whether the delivered O-rings do their part of the job can be checked above all by means of these five properties:

Inner diameter d1

In a piston seal, the inner diameter d1 together with the groove base diameter determines the stretch in the installed state. That this stretch lies within the target range is decisive for the sealing effect. Excessive stretch, for example, causes the cross section of the O-ring to flatten. As a result, reduced compression at the cylinder surface can occur.

Various measuring methods are available to determine d1. The range extends from a simple measuring cone to the laser scan micrometer.

Cord thickness d2

Together with the inner diameter, the cord thickness d2 describes the basic dimensions of an O-ring. The outer diameter of the sealing element also follows from d1 and d2. Accordingly, in a piston seal there is a direct influence on the compression. Cord thicknesses can be recorded using a measuring probe or laser scan micrometer.

Shore hardness

Harder O-rings are better suited for use under high pressure. Especially when a larger sealing gap width and a high operating pressure coincide, there is a risk that the O-ring is forced into the sealing gap. A harder O-ring withstands this stress for longer without material being sheared off.

The common measuring principle consists of a truncated cone that presses onto the material. Its penetration depth provides information about the hardness. The Shore A and IRHD methods differ above all in the weight and shape of the indenter. While Shore hardness is used for standard test specimens, IRHD is better suited for measuring finished parts.

Material composition

The content of the polymer material in relation to other formulation components indicates how the media resistance and temperature behaviour of an O-ring turn out. In sealing elements made of NBR, for example, the acrylonitrile content varies between 18 and 50 percent. A higher blend proportion improves resistance to oil and fuels. If the acrylonitrile content decreases, the elasticity improves in return.

For a reliable seal, it is therefore crucial to rule out excessive formulation variations. A material sample can be examined for its composition by means of thermogravimetric analysis (TGA).

Compression set

As far as the temperature behaviour of O-ring materials is concerned, the compression set (CS) allows the best conclusions. To determine the value, the O-ring is stored in a deformed state in a temperature-controlled medium. The better the O-ring returns to its original shape after being relieved of load, the lower the compression set turns out.

A lower value indicates that an O-ring reliably closes the sealing gap even under changing operating conditions. The test also provides information about the degree of vulcanization and thus the elasticity of the O-ring.

The most important O-ring standard: ISO 3601

Do the properties of the O-rings meet the requirements? For standard-compliant O-rings, this is decided by comparing the measured values with the relevant standard. In its current version, the globally dominant ISO 3601 standard contains far-reaching specifications for the required properties of O-rings. Our dimensional standards for O-rings give an overview of the relevant sets of rules.

Part 1: Manufacturing tolerances

Part 1 of the standard refers to inner diameters and cord thicknesses as well as the associated tolerances. For each combination of inner diameter and cord thickness, the applicable tolerances are specified in two classes.

Part 3: Surface tolerances

Part 3 of ISO 3601 governs the permissible deviations in shape and surface finish. The section divides the common surface deviations into different categories. The permitted deviations in each category are specified on the basis of cord thickness intervals. Here too, the standard covers the requirements of different application cases with various classes.

Part 5: Material properties

The section on material properties is a more recent addition to the standard. Part 5 defines requirements for the characteristics of many standard materials for O-rings. These include NBR, HNBR, FKM and EPDM. The specifications for hardness and compression set are defined on the finished part.

This makes it possible to compare the properties of the O-rings actually delivered with the standard specifications. In the past, such a comparison was often made more difficult by the fact that values were specified on standard test specimens.

Even though ISO 3601 contains extensive specifications for tolerances, quality control remains demanding. After all, O-rings are as a rule elastic products. With IRHD, for example, hardness testing is not very meaningful for particularly small cord thicknesses below 1.6 mm. In addition, elastomers are usually not homogeneous over their entire circumference, so local measurements are limited in their interpretability. On the other hand, economical alternatives for industry and trade are now available outside specialized laboratories as well.

Four further relevant properties

The five most important properties already allow a very good characterization of finished O-rings. For particularly demanding application cases, however, further parameters can be relevant.

Density

Density is a material constant that is independent of the shape of the product. This is particularly advantageous when assessing O-rings. From the density, users can infer the base polymer and the composition of the respective O-ring. In practice, this is an effective approach for incoming goods inspection, for example to reliably detect incorrect deliveries and uncover formulation variations.

Tensile strength

Manufacturers specify the tensile strength of an O-ring in the data sheet, just like the density. It provides information about the force required to rupture a sample. Apart from unfavourable assembly processes, however, O-rings are rarely stressed to their rupture strength in practice.

Elongation at break

This value indicates how large the elongation is at the moment a sample ruptures. Like tensile strength, this property is relevant above all for assessing critical installation processes.

TR10 value

Hardness and compression set describe the elastic behaviour of O-rings. In addition, the TR10 test has become established. In it, a test 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 moment at which the elongation has decreased by 10 percent.

Interpreting measurement results correctly

Measuring O-ring properties is in itself already a challenge. Handheld devices usually do not have the necessary accuracy and therefore only reflect a tendency. Added to this is the handling of the elastic products during the measuring process. Besides the equipment, adherence to the relevant measuring standard also has a considerable influence on the results.

For the results to be reliable and usable, for example in dealings with suppliers, the requirements regarding the number of repetitions, temperatures or the characteristics of the standard test specimens must be observed. In practice, this often proves difficult.

In addition, the conflict between specifications on standard test specimens and measurements on the finished O-ring remains. Many relevant properties are subject to a shape dependence, so that measured finished-part values cannot readily be compared with data sheet figures for standard test specimens.

Therefore, more and more users rely on specifying the properties on the finished part. This is what ISO 3601 provides for hardness and strength, for example. In addition, users can reduce their own measurements by working with reliable suppliers who in turn ensure stable product quality.

Frequently asked questions