Hardness, in basic terms, is the resistance of a material against intrusion and permanent deformation by a harder body. It is one property that is important for the proper specification and function of seals and other critical components in fluid power systems.

Hardness of seal materials like rubbers, elastomers and plastics are typically measured in units of Shore or IRHD (International Rubber Hardness Degrees). The Shore scales are widely used in the United States and were devised by Albert F. Shore in the 1920s.

Shore Hardness values provide a common reference point when comparing different materials. There are 12 different scales for measuring substances with varying properties, including thermoplastic elastomers, thermoset rubber, elastomeric materials, cellular materials, gel-like materials and some plastics. Each scale ranks hardness values between 0 and 100, with higher values indicating higher hardness.

Most common are the A and D scales. The Shore A scale is suitable for materials ranging from soft and flexible ones to semi-rigid, relatively inflexible plastics. The Shore D scale measures hard rubbers and semi-rigid and rigid plastics. The scales overlap to some extent. For example a material with a Shore A hardness of 80 is also rated approximately 30 on the Shore D scale. Experts generally recommend that measurements be made with the type D durometer when the value is above 90 A, and with a type A durometer when the value is less than 20 D.

Shore Hardness is measured with a durometer test instrument to determine how a given material resists permanent indentation. The test method designation is ASTM D2240 and is generally used in North America. Related methods include ISO 7619.

According to ASTM, this test method is based on the penetration of a specific type of indentor when forced into the material under specified conditions. The shape of the indenter and the force applied vary by scale and test. Indentation hardness inversely relates to the penetration and depends on the elastic modulus and viscoelastic behavior of the material.

The Shore-type durometer is a spring-loaded indentation device. A steel rod with a cone-shape indentor extends and presses onto the sample. For example, for the A scale, each 0.001 inch of deflection of the indentor is gaged as 1 degree Shore A; therefore, the harder the material, the more the deflection and the higher the number. Force is rapidly applied without shock, and the hardness reading is made after 15 sec duration. Instantaneous readings can also be specified.

This method is an empirical test intended primarily for control purposes. No simple relationship exists between indentation hardness determined by this test method, and the Shore test doesn’t relate well to other fundamental characteristics. Experts say the Shore durometer test results do not necessarily serve as a predictor of other properties such as strength or resistance to abrasion or wear. Thus, it is only one of many considerations for product design specifications. Shore hardness is, however, often used as an indicator of an elastomer’s flexural modulus. The correlation between Shore hardness and flexibility holds for similar materials, especially within a series of grades from the same product line, but this is an empirical and not a fundamental relationship.

Why is seal hardness important to fluid power system designers? Softer seals stretch more readily and work more easily into microfine surface imperfections and seal better on rough surfaces. This is beneficial at lower system pressures. At higher pressures and in dynamic applications, seal hardness is equally important. Generally, for the same gland depth, a harder seal better withstands abrasion and dynamic friction and resists gap extrusion. Back-up rings are also frequently used in piston or rod seals to prevent gap extrusion. With O-rings, for instance, Shore A 70-durometer hardness is a good baseline starting point in many applications.