Is Durometer Hardness the Best Way to Specify the Stiffness of a Rubber Mount?
There’s more than one way to measure hardness. You may have relied on durometer hardness readings in the past as a means to check the stiffness of polymers, elastomers and rubbers. The question looms though as to whether providing a durometer hardness value is the best way to specify the stiffness of a rubber mount. The easiest way to determine this is to first understand how a Durometer works.
What is a Durometer and How Does it Work?
A Durometer is an instrument used to measure the hardness of various plastics and rubbers based on the depth of an indentation in the material being tested. The depth of the indentation is determined using a specific force on a standardized pressure foot and results in a durometer hardness reading.
The use of a Durometer is governed by ASTM D2240 Standard Test Method for Rubber Property – Durometer Hardness. This specification allows for a measurement of initial hardness or the indentation hardness after a given period of time, taken in a consistent manner, and without shock. It specifies that readings are done on flat, parallel surfaces that are at least 6.4mm (0.24 inch) thick. There are twelve types of durometer hardness measurements specified in ASTM D2240 based on material type and hardness.
So, is Durometer Hardness the Best Way to Specify the Stiffness of a Rubber Mount?
Since rubber mounts rarely have a flat surface, it is extremely challenging to get an accurate and repeatable durometer hardness reading. It is difficult to find a surface on a rubber mount that conforms to the criteria specified in ASTM D2240.
For this reason, the engineers at LORD prefer using a spring rate value, also known as load-deflection testing, rather than a durometer hardness value when specifying the stiffness of a rubber mount. There are many conditions where accurate readings are not possible using a durometer. Additionally, small surface deformations rarely relate to the ability of a rubber mount to function as intended in its application, which is an important factor to remember.
LORD recommends static load-deflection testing to verify the stiffness of a rubber mount. In static load-deflection testing, a given load is applied at a given deflection in compression or in shear to determine the spring rate which is normally in newtons per millimeter (N/mm). Both load-deflection testing and durometer testing provide data but performing a static load-deflection test to determine a spring rate is much more indicative of how a rubber mount will perform in an application.
There is no correlation between the durometer reading and the physical size and shape of the part you are measuring. However, if you use a spring rate test, you can zero in on the specifics – a much more accurate way of determining the correct part for your application.
Check out our video on measuring hardness using a durometer and stay tuned for a future blog on calculating the natural frequency of rubber.