Home News What is Shore Hardness? O-Ring Hardness Selection

What is Shore Hardness? O-Ring Hardness Selection

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So, you’ve determined that you are in the market to use o-rings for your sealing needs. You have the product uses identified, you know the general parameters and requirements for your o-ring selection, and you are on the way to finding the ideal selection. But there’s one more characteristic that has you uncertain: shore hardness.


What Does Shore Hardness Mean?

Shore hardness describes the hardness of the material of which the o-ring is made. Most o-rings are made from rubber, so the majority of shore hardness references tie back to the specs for that rubber material.

O-rings are often impacted by materials within the machine they are sealing, so knowing the level to which they will be indented or misshapen is important to ensure proper seal retainment, while also keeping in mind the surrounding materials and their characteristics.


Shore Hardness Testing and Scales

An o-ring’s durometer is the measurement of that specific material’s hardness. This scale is also measured by an instrument called a durometer gauge, which uses a spring-loaded rod to indent the material and provide a reading. The durometer scale runs numerically from 0-100, with lower numbers indicating softer materials and higher numbers for harder materials.

There are two main shore hardness scale categories, each indicating a different type of material. A Shore scale measures a wide range of materials, from the very soft to the hard and semi-rigid materials. Shore A is the most commonly used category, as it provides a wider variance of hardness scores, from extra soft to hard materials falling in this category. Shore D scales measure very hard rubbers to plastics, taking over around the medium-hard range all the way to extra hard.

What Is the Strongest O-Ring Material?

An o-ring can be used to block a path that may allow gas or liquid to escape. Therefore, you need to know how to do an o-ring hardness selection.

Fluorocarbon o-rings are among the most versatile and durable due to their high-temperature resistance and chemical composition. Other particularly strong materials include silicone, polyurethane, nitrile butadiene rubber, buna nitrile, and ethylene-propylene.

How to Use a Durometer

Durometers that measure shore hardness are available in a range of scales, but they all work the same way.

  1. Choose the right durometer for the sample material. There are a dozen durometer measurements, with the D and A measurements being the most used.
  2. Position the test material on a horizontal, hard, flat table or countertop.
  3. Grip the durometer between your hands over the test material so that the indentor presses it.
  4. Press down straight until the presser foot is in contact with the test material.
  5. Record the measurement.

The reading is influenced by the following factors:

  • Duration of the force applied to the surface
  • Real hardness of the material
  • Viscoelasticity of the material
  • Shape of the indenter

Other Methods of Measuring Shore Hardness

Besides the durometer, there are other conventional testing techniques that are used to measure shore hardness. ISO 868 and ASTM D-2240 are some of the most common. These scales generally measure materials within the range of 20 to 90 Shore A.

ISO 868:2003 defines a way of calculating the indentation hardness of ebonite and plastic. It is a measure of the indentation resistance of soft or elastomeric substances by determining the depth of piercing of a conical indentor.

ASTM D2240 is used to examine the hardness of a material by utilizing a succession of tests. It enables the measurement of the indentation hardness, or the initial hardness, after a specified duration. ASTM D2240 is commonly used to determine the shore hardness of gel materials, elastomeric materials, vulcanized rubber, thermoplastic elastomers, and some plastics.

Applications of Shore Hardness

In many industries, measuring the shore hardness of materials is important when deciding which choice will offer optimal performance. For instance, you may want to secure a high-pressure gas or liquid, which would require a harder material. Alternatively, you may need to reduce the insertion force of some fragile hardware, such as a sapphire plate or thin plastic. This would require a softer material.

O-Ring and Seal Selection Factors

While it may seem that going for the middle range durometer-rated materials would always be the best selection for o-ring and seal selection, that is not always the case. For instance, if your project involves high pressures or high impacts, a harder material may be best. On the other end of the spectrum, there may be a need to seal more fragile or flexible materials, leading to looking lower on the durometer scale.

For most applications, a 70-durometer hardness is a good place to start. This sits in the middle section of the scales and provides a balanced combination that often encompasses the best of both sides. Going softer will provide more stretch and flexibility, while also allowing for easier seals on uneven or rough surfaces. Harder material compounds are more resistant to scratches, impacts and other abrasions.


Wyatt Seal Can Help

Whatever your o-ring needs or questions you may have, we’re here to assist. Wyatt Seal is proud to offer a variety of Parker O-Rings, Kalrez® o-rings and custom o-rings to support every project’s unique needs. Browse our extensive inventory of o-rings, and feel free to reach out to our support unit for developing o-ring prototypes or questions regarding fulfillment and support for your manufacturing needs.


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