O-rings are the simplest, most readily available type of seal used across every industry and market. They are arguably the best seal for many applications, but perhaps daunting to an engineer with no experience in seal design. The O-Ring Basics chapter of Parker's eHandbook provides an overview of what an O-ring is, how it works, and finally, the benefits over other seals.
One very important consideration is the hardness of a rubber material. A customer may need to seal a high-pressure fluid, which would call for a harder material. On the other hand, he might want to minimize insertion force, or the O-ring may be sealing some delicate hardware, such as thin plastic or a sapphire plate. This may call for a softer material. For testing the hardness of the material, Parker recommends Shore A hardness. It is an industry-standard test for rubber, and it is an acceptance criterion for every batch of O-ring material mixed by Parker.
Shore A hardness testing is performed using a durometer, which was developed by Albert Ferdinand Shore in the 1920s. The Shore A hardness measurement is often colloquially called “durometer” or “Duro”. This test is performed on cured rubber sheets per ASTM D2240, and on finished O-rings per ASTM D1414.
The device consists of a hardened steel rod with a truncated cone at the tip. The steel rod is spring-loaded and actuates a dial with a scale of 0 to 100. The test specimen is placed directly underneath the truncated cone, and the device is pressed down onto the part until the flat metal plate on the bottom is flush with the rubber specimen.
The more the cone deforms the rubber material, the lower the hardness measurement. The less the cone deforms the rubber material, the higher the hardness measurement.
Parker compounds utilize a distinct numbering system, which consists of one or two letters to designate material, a three or four-digit identification number, and a two-digit number for the Shore A hardness. Two examples of this are N0674-70 and NM304-75. Of particular interest are the last two digits, which indicate the nominal hardness of the material. Compound N0674-70 has a Shore A hardness of 70, while NM304-75 has a Shore A hardness of 75. The tolerance on the Shore A hardness is +/-5 for all Parker compounds.
For further information, check out the Material Selection Guide which provides a compound family overview including a description of compound advantages, typical temperature maximum/minimum, compatible fluids, and material durometers. A list of incompatible fluids is also detailed, allowing the user to receive a concise summary of the most common, and even the more obscure elastomer types. And cIf you have any questions, please feel free to contact Parker applications engineering at 859-335-5125.
This article was contributed by
David Mahlbacher, Parker O-Ring Division, applications engineer
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