Shore hardness of O-ring materials: values and effect

What the Shore hardness of an O-ring tells you

Hardness is an important parameter for the properties of O-rings. Above all, it indicates how well the material conforms to unevenness in the sealing surfaces under pressure. Less hard O-rings do this better, but under high pressure are also more prone to gap extrusion. Choosing the right hardness is decisive for the reliability of the seal.

By general definition, the hardness of a material describes the resistance it offers to the penetration of a harder body. Up to a certain degree, the harder body produces an elastic deformation of the softer one. After that, the object returns to its original shape once the load is removed. If the deformation is plastic, the body retains part of the deformation even when the pressure is no longer acting.

In addition, hardness testing serves to identify and quality-check elastomer compounds and finished parts. However, hardness provides only part of the necessary information. Although the parameter is important, on its own it has limited significance. Hardness is one of many important figures found in the technical data sheets of O-rings:

• Hardness
• Temperature range
• Elongation at break
• Tensile strength
• Compression set
• TR-10 value
• Density

Sealing effect through deformation

When O-rings are used as a sealing element, their elastic deformation is desired. To achieve this, the elements to be sealed exert pressure on the O-ring. As a result, it conforms to the contours and seals the gap between the sealing surface and the installation groove. In cross-section, the O-ring is then no longer circular but elliptical. This deformation is also called compression.

The softer an elastomer is, the less pressure is needed to achieve the deformation. Under low load, the O-ring then compensates even for larger surface defects in the sealing surfaces. These originate from the machining of the components. Cast parts, for example, exhibit burrs along the parting plane.

The quality of the finishing determines how much of these manufacturing-related marks is still present when the O-ring is installed. O-rings with higher hardness require more pressure to produce the necessary compression. For low-pressure applications, O-rings with low hardness are therefore better suited.

Choosing the correct Shore hardness for system pressure

But there is a further factor: the medium to be sealed can also exert pressure on the O-ring. Initially this is good for the sealing effect, because it increases the compression. However, if the pressure becomes too great, a counterproductive effect arises.

The O-ring moves to the side of the groove facing away from the pressure. It can then happen that the O-ring moves into the sealing gap. This so-called gap extrusion can cause permanent damage, so that the sealing effect of the O-ring is lost. We describe in detail how O-rings behave under pressure load in the article on O-rings under pressure.

For high-pressure media, the designer has two measures available: he can optimise the installation situation so that the sealing gap becomes as small as possible. Choosing an O-ring with higher hardness also helps with this problem. With dynamic seals, the friction and wear of the O-ring additionally play an important role. These too are related to the hardness of the O-ring. The hardness of the O-ring must therefore always be matched to the individual operating conditions.

Soft and hard O-rings compared

The widespread assumption that a hard O-ring is generally better than a softer one is a mistake. Rather, in practice it is about using an O-ring that is exactly as hard or soft as the application requires. Both ranges have their strengths and weaknesses.

In doing so, the engineer must take account of the complex interaction between the dimensions of the sealing gap, the properties of the O-ring and the pressure load. There is therefore no universally correct hardness value, only the one suited to the particular installation situation.

Shore hardness and IRHD: the test methods

The principle behind the test methods for hardness is simple: an indenter exerts pressure on the material under examination. What is measured is how deeply the harder body penetrates the softer one. Among other things, the shape of the indenter, the pressing force and the measuring time must comply with the standardised method. This is precisely where the widely used Shore and IRHD (International Rubber Hardness Degree) test methods differ. This also affects how the methods are applied in practice.

Shore hardness A and D

The specifications for measurement according to Shore are set out in DIN ISO 7619. With this method, the test engineer generally does not test the hardness on the finished part. Instead, a standard specimen with defined dimensions is used. This measured value later appears in the O-ring's data sheet. Two different variants are to be distinguished, Shore A and Shore D. For O-ring materials, testing according to Shore A is primarily applicable.

For testing on vulcanisates, as O-rings are, the standard specifies a specimen thickness of at least 6 millimetres. Under spring force, the conical indenter acts on the specimen for 3 seconds. In addition, temperature influences the material behaviour. A target temperature of 23 degrees therefore applies, from which no more than 2 degrees up or down should be deviated.

During evaluation, the tester converts the measurement result for the penetration depth. A depth of 2.5 millimetres corresponds to a hardness of 0 Shore A. If the indenter leaves no impression at all, the hardness is 100 Shore A. Very hard materials reach 90 points. The Shore hardness of finished parts can also be recorded, at least when the cord thickness of the O-ring is more than 3 millimetres.

In the Shore D hardness test for tougher elastomers, central parameters change with the pressure and dimensions of the indenter. There is therefore no linear relationship between the measured values for the two Shore hardnesses. A conversion is nevertheless possible.

Hardness testing according to IRHD

Hardness testing according to IRHD-M likewise gives the hardness in scale values from 0 to 100. The method serves above all to determine the hardness of finished parts. However, for cord thicknesses below 1.6 millimetres, IRHD measurement is not suitable either. Unlike the determination of Shore hardness, here a considerably smaller spherical indenter penetrates the O-ring. The highest point in the cross-section of the O-ring should be the point of application. After the indenter has acted on the O-ring for 30 seconds, the operator can read off the hardness.

Importantly, the hardness values determined on standard specimens and finished parts of an identical elastomer will almost always differ from one another. In addition, the results according to Shore A and IRHD-M cannot be converted into one another. This holds even though the results can lie relatively close together in the medium hardness range. Responsible for this are the different measuring methods, but also the shape-related deviations in properties.

Geometrically, the standard specimen of a material and the O-ring as a finished part have hardly anything in common. A differing material thickness and the curved surface thus influence the measurement. As the O-ring cross-section decreases, the precision of the methods drops sharply. Equally decisive is where on the finished part the tester places the indenter. The many possibilities for error represent a major problem in hardness testing. The results are generally much less reliable than the other measuring methods. A tolerance of 5 hardness points Shore A or IRHD up and down is therefore customary. High-quality finished parts keep within this relative to the figure in the data sheet.

Shore hardness of the NH materials at a glance

Which hardness an O-ring has depends on the material and the particular compound. The following overview shows the hardness values of our materials. The values apply with the customary tolerance of 5 hardness points up and down. Note that PTFE as a solid material is stated not in Shore A but in Shore D.

As you can see, even within the FFKM family ECOLAST the hardness spectrum ranges from 70 to 90 Shore A, depending on the chosen compound. This allows the hardness to be matched specifically to the pressure, sealing gap and type of motion of your application. To learn how the materials differ beyond this, read our articles on FKM O-rings and FFKM O-rings.

Design tips for practice

In practice, it is about using an O-ring that is exactly as hard or soft as the application requires. In doing so, the engineer must take account of the complex interaction between the dimensions of the sealing gap, the properties of the O-ring and the pressure load.

What is undisputed, however, is that the hardness of the O-ring as delivered should correspond as closely as possible to the target value specified in the order. This ensures that the O-ring can reliably fulfil its sealing task. Very good O-ring suppliers distinguish themselves from average ones by carrying out comprehensive measures to monitor series quality. In this way they ensure that their products meet the requirements of demanding applications.

Besides hardness, they also keep an eye on further material properties such as weight and tensile strength. Added to this are the behaviour of the O-ring at high and low temperatures as well as the interactions with the test media. Only in their entirety do these tests provide a comprehensive picture of the O-rings. You will find an in-depth overview of test methods and scales in our article on Shore hardness.

Frequently asked questions