A Comparison of Rubber Hardness and the Indenters Used

Isolation is defined by the level of reduction of vibration passed between the oscillatory system and its surroundings. For example, anti-vibration mountings act as an isolator and as a result, minimise the level of vibration transmitted from an oscillatory system to its surroundings. The rubber hardness of an anti-vibration mount is a critical factor in order to provide isolation. The hardness has a direct impact on the “stiffness” of a part, ultimately affecting how a component will perform in an application.

In order to measure hardness, a rubber hardness scale is used; this comparison guide will provide an overview of the different scales that can be used. In addition, we will also touch on how our very own GMT Rubber engineers measure hardness.

Rubber hardness comparison

There are several scales to measure the hardness of an elastomer, used for materials with different properties, shown in the diagram below:

Two such scales, using slightly different measurement systems, are the Shore A and Shore D scales. The A scale is for softer materials such as rubber, while the D scale is for harder, less elastic materials.

The Shore A scale, used by GMT, is the most prevalent with typical readings ranging from 20 to 95 degrees, as indicated in the diagram.



What is the unit of rubber hardness?

Shore hardness is measured with a spring loaded indentation device known as a Durometer. Each rubber Durometer has a truncated cone indenter, which is pressed onto the sample against an internal spring. Each 0.001 inch of deflection of the indenter is shown as 1 degree Shore (A). Therefore, the harder the material, the lower the deflection and the higher the number overall. Harder elastomers use a more pointed conical indenter (Shore D scale), which will give a wider range of readings, shown in the durometer hardness test below.


Correction Factor

Often it is not feasible to test certain parts to measure their hardness. For instance, when a customer requests theoretical stiffness characteristics of a slightly softer or harder part, it wouldn’t be efficient to go through the whole testing process. So, in these instances GMT is able to extrapolate known data to estimate how a part will perform in a different hardness, using a table similar to the one shown below. This calculation uses a ‘Correction Factor’ and is just one of the many ways GMT Engineers utilise their expertise, to assist customers on a day to day basis.


If you have any further question regarding rubber hardness, please feel free to contact us at GMT. If you are the start of the rubber selection process, then check out our guide on how to choose the best rubber compound for specific applications, or keep reading our blog for more expert tips on rubber compound selection and components.

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