Shore hardness in O-rings simply explained

What is Shore hardness?

Shore hardness is a measure of the hardness of elastomers, plastics and other polymeric materials. It describes how strongly a material resists penetration by a standardized indenter. The less the indenter penetrates, the harder the material.

Hardness is measured with a device called a Shore hardness tester or durometer. A higher number means a harder material. Car-tyre rubber, for example, is around 70 Shore A, while a soft rubber hose is well below that.

In every hardness measurement the scale runs from 0 (very soft) to 100 (very hard). It was developed at the beginning of the 20th century by Albert F. Shore, who was looking for a standardized method of measuring hardness. This gave rise to several Shore scales, of which Shore A and Shore D are the most commonly used.

For O-rings, hardness is more than a technical parameter. It is one of the factors that determine how well a sealing ring seals, how easily it can be installed and how reliably it works under pressure.

How is Shore hardness measured?

The measurement is simple in principle. A standardized indenter is pressed into the material with a defined force. The penetration depth determines the hardness value. If the indenter penetrates deeply, the material is soft and the value is low. If it barely penetrates, the material is hard and the value is high.

Shore A provides a clear example. The indenter has an effective length of 2.5 mm. If the tip penetrates the full 2.5 mm, the result is a value of 0 Shore A. If it does not penetrate at all (0 mm), the result is a value of 100 Shore A.

How a measurement works

1. Prepare the material: The sample should be clean, flat and at a standardized temperature. Around 23 °C at stable humidity is typical.
2. Calibrate the device: The hardness tester is calibrated according to the manufacturer's specifications.
3. Carry out the test: The hardness tester is placed vertically on the surface and the indenter is held for a few seconds.
4. Read off the value: The scale shows the hardness value from 0 to 100.

For a reliable result, several measurements are taken in practice and averaged. Constant test conditions and a serviced, calibrated device are crucial here.

Test specimen or finished part

An important point for practice: the standardized measurement is carried out on the standardized test specimen under ideal conditions. A measurement directly on the finished part, for example on a finished O-ring, can deviate due to shape, size and curvature. The curvature of a thin cord cross-section makes it harder to seat the device cleanly. Hardness values from the data sheet therefore refer to the standardized test specimen.

Shore A and Shore D compared

Shore A and Shore D both serve to measure hardness, but they cover different ranges. The difference lies in the shape of the indenter and in the level of the applied force.

• Indenter: Shore A works with a rounded tip, Shore D with a more pointed shape. A sharp tip would deform soft materials too much, while a rounded tip would leave hardly any impression on hard materials.
• Range of application: Shore A is intended for soft to medium-hard elastomers, Shore D for hard plastics and very dense elastomers.
• Scale range: Both run from 0 to 100, but they refer to different hardness ranges.

The choice of the right scale depends on the material. If the wrong scale is used, the results are inaccurate or not meaningful at all. Classic Shore A materials are tyres, shoe soles and seals. Classic Shore D materials are hard plastics such as helmet shells or technical components.

Why hardness matters for the sealing effect

In an O-ring, hardness directly influences how the sealing ring works. It affects the deformation in the installation space, the sealing force and the behaviour under pressure.

Softer or harder, what does that mean?

• Softer O-rings adapt better to the surface. They fill small surface irregularities more easily and seal even at low compression. This is an advantage at low pressures and on rough surfaces.
• Harder O-rings withstand higher pressures better and are less prone to being pushed into the sealing gap. In return, they need a cleaner surface and a more precise installation situation in order to seal reliably.

Hardness and gap extrusion

Under pressure, an O-ring is pressed into the gap between the components to be sealed. If the pressure is too high or the gap is too large, material is forced into the gap. This effect is called gap extrusion. An O-ring that is too soft is more likely to be pushed into the gap at high pressure and can be damaged there.

A higher hardness increases the resistance to gap extrusion. For this reason, a harder material or an additional back-up ring is often chosen as the pressure rises. Hardness is thus a key lever in the design of seals for higher pressures.

The right hardness is always a compromise: soft enough for good adaptation to the surface, hard enough for resistance to the pressure. Which hardness is suitable depends on pressure, gap dimension, surface and material.

Typical hardness values of common materials

Every material has a typical hardness range. The following overview shows common elastomers and plastics with their usual Shore values.

The materials from NH O-RING

The following stated hardness values apply to the high-performance materials from NH O-RING:

At 75 Shore A, FKM covers a typical medium hardness range for O-rings. Depending on the type, FFKM is available from 70 to 90 Shore A and is therefore suitable for a range of pressure and installation situations. FEP-encapsulated O-rings, at 90 Shore A, are in the hard range. PTFE is measured as a solid material in Shore D and, at 60 Shore D, is the hardest material in the range.

Standards for Shore hardness testing

So that hardness values are internationally comparable, there are standardized test methods. They define the indenter, force, penetration depth and the test conditions. The most important standards are:

• DIN ISO 868: the authoritative standard in Europe for Shore A and Shore D. It defines the method so that results are internationally comparable.
• ASTM D2240: the American standard, widely used in the USA and internationally. It describes the method for determining the indentation hardness of rubber, elastomers and soft plastics.
• JIS K 6253: the Japanese counterpart, with requirements tailored to Japanese industrial practice.
• ISO 7619: a further international standard for the hardness measurement of vulcanized or thermoplastic rubber.

The standards are very similar, but they differ in details of the test conditions and the device specifications. Anyone comparing values from different sources should therefore pay attention to the underlying standard.

Frequently asked questions