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.
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.
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.
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.
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