Hardness

Throughout the seal industry, the Shore A type durometer scale, manufactured by a variety of manufacturers, is the standard instrument used to measure the hardness of most rubber compounds. It should be noted that there are other hardness scales used to describe elastomers (B, C, D, DO, O, OO) but these are typically not used by the rubber seal industry.

The durometer has a calibrated spring which forces an indent or point into the test specimen against the resistance of the rubber. The indicating scale reads the hardness of the rubber. If there is no penetration, the scale will read 100, as on a flat glass or steel surface. (For specimens that are too thin or provide too small an area for accurate durometer readings, Micro Hardness Testing is recommended).

In the O-ring industry, another hardness scale is used due to the curved surface of the O-ring cross-section causing problems with accurately reading Shore A. The scale is IRHD (International Rubber Hardness Degrees). The size and shape of the indentor used in IRHD readings is much smaller, thus allowing for more accurate measurements on curved surfaces such as an O-ring cross-section. Unfortunately, there is not a direct correlation between the readings of Shore A and IRHD Scales.      

Softer sealing materials, with lower hardness readings, will flow more easily into the microfine grooves and imperfections of the mating parts (the gland, bore, rod or seal flanges). This is particularly important in low-pressure seals because they are not activated by fluid pressure. Conversely, the harder materials offer greater resistance to extrusion. Referring back to the O-ring seal diagrams, as shown below, it can be seen that a harder O-ring will have greater resistance to extrusion into the narrow gap between the piston and bore. 

O-ring installed	O-ring under pressure	O-ring extruding	O-ring failure

In dynamic applications, the hardness of the O-ring is doubly important because it also affects both breakout and running friction. Although a harder compound will, in general, have a lower coefficient of friction than a softer material, the actual running and breakout friction values are actually higher because the compressive load required to achieve the proper squeeze and force the harder material into a given O-ring cavity is so much greater.

For most applications, compounds having a Shore A durometer hardness of 70 to 80 is the most suitable compromise. This is particularly true of dynamic applications where 90 durometer or harder compounds often allow a few drops of fluid to pass with each cycle, and 50 durometer compounds tend to abrade, wear, and extrude very quickly.

Normally durometer hardness is referred to in increments of five or ten, as 60 durometer, 75 durometer, etc. - not as 62 durometer, 66 durometer or 73 durometer. This practice is based on:

(1) The fact that durometer is generally called out in specifications with a tolerance of ±5 (i.e., 65±5, 70±5, 90±5);

(2) The inherent minor variance from batch to batch of a given rubber compound due to slight differences in raw materials and processing techniques; and

(3) The human variance encountered in reading durometer hardness. On a 70-durometer stock, for example, one person might read 69 and another 71. This small difference is to be expected and is considered to be within acceptable experimental error and the accuracy of the testing equipment.