WHAT IS DUROMETER?The quick answer, as it relates to seals, is that durometer is the hardness of the seal material, however, there is much more information that is critical when selecting the correct seal for your application. Durometer is more commonly referred to as a Durometer Scale. Although there are several scales of durometer, the two most commonly used scales for measuring the hardness of polymers, elastomers and rubbers, are the ASTM D2240 type A and type D scales.
ORIGINS OF SHORE DUROMETERThe Shore hardness standard refers to a measuring device that was developed in the 1920’s by an American metallurgist, Albert Ferdinand Shore. In the case of Shore, his was not the first to measure hardness or to be called durometer, but today the name usually refers to Shore Hardness. Furthermore, there are several other devices and scales used to measure hardness, for instance the Rockwell Scale.
METHOD OF MEASUREMENT
“Durometer, like many other hardness tests, measures the depth of an indentation in the material created by a given force on a standardized presser foot. This depth is dependent on the hardness of the material, its viscoelastic properties, the shape of the presser foot, and the duration of the test. ASTM D2240 durometers allows for a measurement of the initial hardness, or the indentation hardness after a given period of time. The basic test requires applying the force in a consistent manner, without shock, and measuring the hardness (depth of the indentation). If a timed hardness is desired, force is applied for the required time and then read. The material under test should be a minimum of 6 mm (0.25 inches) thick”  “Rubber Hardness”. National Physical Laboratory, UK. 2006. Retrieved 2006-07-22.
HARDNESS SCALEThe below Hardness scale illustrates a comparison between the Shore A, Shore D and Rockwell R scale and how some measurements overlap each other. For example, a measurement of 90 durometer Shore A is comparable to 40 durometer shore D.
HOW DOES DUROMETER AFFECT SEAL PERFORMANCE?
The majority of sealing applications use an elastomeric seal (NBR/Buna, Viton, Silicone, EPDM) ranging in hardness between 70 and 90 Durometer-Shore A. There are several factors within the application that will dictate the durometer of the seal to be used. For example, pressure. Generally, lower durometer seals perform very well in low pressure applications as they produce less resistance to the surface of whatever is being sealed. On the other hand, the higher the pressure the higher the durometer or hardness of seal will be required. In high pressure applications the likelihood of extrusion through the clearance gap increases. (see chart below). Higher pressure combined with higher durometer seals will increase the friction at the sealing point of contact. Many factors must be considered when selecting the proper durometer and seal material. The graph below shows the relationship between pressure, durometer and diametrical clearance (the radial clearance if concentricity between the piston and cylinder is maintained)
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