What is the Shore Hardness Scale?

Mar 19, 2020

In the basic sense, hardness relates to the way in which a material reacts to intrusion as well as permanent deformation from a harder body. When it comes to seals, it is a property that is a significant consideration, especially in relation to function and specification.

The hardness of seal materials such as rubbers, plastics and elastomers are measured in units of Shore or IRHD (International Rubber Hardness Degrees).

When comparing two materials, Shore hardness offers a reference point and there are 12 different scales that are used for measuring a range of substances. However, each scale will rank the hardness of a substance between 0 and 100. The higher the value, the harder the material.

The most common scales are the A and D scales, with the Shore A scale being used for elastomers and the Shore D scale is used to measure rigid plastics.

A durometer test instrument is used to measure shore hardness. The test method designation is ASTM D2240 while related methods include the like of ISO 7619.

So, the ASTM stipulates that the test method focuses on the penetration of a certain type of indentor when it is pushed into the chosen material under certain conditions. As a result, the indenter shape and the force that is applied will alter by scale and test.

The Shore Hardness gauge will have a needle mounted on a spring. This needle is then placed on the material before the pressure is applied. After the gauge has been placed against the material firmly and the needle has penetrated as much as possible, the needle will provide a measure of hardness that will relate to the Shore Hardness Scale

So, when it comes to seals, the Shore Hardness Scale will make it possible to determine which material should be used. The required hardness will be determined by the application

The Shore Durometer Hardness results are a great way of measuring the resistance of a number of materials. Despite this, the test is not used to predict a range of other property such as strength or resistance to scratches, abrasion or wear, and so, when it comes to design specifications, it should not be relied upon solely.

So, seal hardness is vital to fluid power system designers. The softer a seal is, the more easily it will stretch and that means that they will work their way into microfine surface imperfections while they will also provide a better seal on rough surfaces. This is ideal for lower system pressures. However, where pressures are higher, the hardness of a seal is important. A harder seal will have the ability to deal with abrasion and dynamic friction while it will also resist gap extrusion.

Therefore, the Shore Hardness Scale is imperative when it comes to ensuring the correct material is chosen for the application.