What is 'Shore' hardness?

What is 'Shore' hardness?

The Guide to Shore Hardness

Anybody that has used a seal or o-ring for any type of use knows that selecting the right fit, the right size and the right material are crucial in keeping your equipment functioning perfectly.

While finding the right size and material for any seal is relatively straightforward, you also need to consider the hardness of the polymer you are using. As each seal type has its unique hardness, it is important to understand what the benefits are, how shore hardness is tested and how the results can help you find the best seals for your specific needs.

What is shore hardness?

Shore hardness is the measure of a particular material’s resistance to penetration or indentation. Basically, it tells you how hard a material is and how much force it can take.

The Shore hardness scale was invented by Albert Ferdinand Shore in around 1920 and has been used ever since.


How is shore hardness tested?


Shore hardness is tested using a durometer, which is a needle-like instrument that measures the depth of an indentation in the seal created by a standard amount of force. Similar in looks to a tire-pressure gauge with a protruding needle, it is placed against the material before pressure is applied.

durometer shore hardness indentation guide

As the durometer is pressed into the material, it makes a mark of varying depth. This depth depends on the hardness of the material, meaning that you can compare the difference in the indentation across all materials.

Shore hardness is scored on the International Rubber Hardness Degree scale of 0 to 100, with a higher number indicating a hard material.


The types of shore hardness scales

In order to provide users with a more accurate assessment of the hardness of all materials (including seals), there are shore hardness scales. These scales can be confusing so we have put the common types together in one diagram.

shore hardness scales durometer rubber plastic

Shore 00 Hardness Scale measures materials such as foams, gels, sponges and very soft rubber.

Shore 0 Hardness Scale measures soft to medium rubbers and also soft plastics

Shore A Hardness Scale measures the hardness of mold rubbers. These are softer and more flexible materials in general, but the scale gives an indication of whether they are extremely soft or if they are more rigid. In addition to soft rubbers, semi-rigid plastics can also be tested on the Shore A scale, but at the higher level.

Shore B Hardness Scale measures mid range rubbers and plastics, this can be a good choice to measure if you deal with a variety of materials.

Shore C Hardness Scale measures medium medium rubbers up to hard plastics.

Shore D Hardness Scale measures hard rubbers, semi-rigid plastics and hard plastics. Generally, the rubbers are on the low end of the scale, while the hardest plastics receive the highest Shore D ratings.

Shore Hardness and Seals

The shore hardness scale of o-rings is important to discovering the best seal material for certain surfaces.

Softer compounds are best suited for sealing rough surfaces, as their flexibility and malleability means that they are able to adapt to the uneven areas, allowing for a tight seal.

Harder compounds provide more resistance to abrasions and extrusions but offer less flexibility, making them the best choice for sealing even areas that are subject to high pressure. Hard compounds such as polyurethane is used for Hydraulic seals and a good scale to measure these on would be Shore C or D.

Where do seals rank on the shore hardness scale?

Seals can measure everywhere from 1 to 99 on the shore hardness scale, however the standard number for most nitrile o-rings is 70 on the Shore A scale (70A).

In fact, a majority of the polymers used for seals fall into the 60-90 range on the shore A scale, as they offer the necessary resistance for extrusion and pressure. Especially true in dynamic applications, the materials falling higher on the shore hardness scale are important due to their effect on both running friction and breakout.

Seals with the ratings below 50 offer the ability to stretch into imperfections and fine grooves of the parts. This means that they are great for any uneven, rough or grooved surfaces, providing the sealing properties that hard plastics cannot. The drawback of seals on the low end of the shore hardness scale is that they extrude and wear out much faster than their harder counterparts.

Levels of shore A hardness

To offer you an idea of where different o-rings fall on the shore A hardness scale and what they offer in terms of sealing properties, we have detailed them in this list:

  • 10A Shore Hardness: Extremely soft material that can catch throughout the grooves, creating a ‘snake skin’ style that can push excess material out of the application. While it doesn’t impact the functionality, it does wear out quickly.
  • 20A Shore Hardness: Similar to that of a rubber band, it extrudes smoothly and is able to seal rough areas and grooved, uneven applications.
  • 40A Shore Hardness: A flexible and slightly more durable material that has the hardness of a pencil eraser, the seals can be used in applications that are rough and do not undergo too much pressure.
  • 70A Shore Hardness: The combination of malleability, hardness and durability makes this the standard for most seals. Able to flex when needed but also withstand pressure, these o-rings have the same feel as a tire tread.
  • 90A Shore Hardness: These seals are harder, able to withstand more pressure and extrusion. These materials are longer-lasting and can be used in dynamic applications.

Finding the right shore hardness for you

No matter what type of application you have or the style of seal required, our expert team will help you find the perfect material based on shore hardness.

Just contact us today and our specialists will offer valuable insights that get the right seals for your needs.

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