O-ring materials and compounds

Shore A is the indentation hardness scale for rubber; a higher number is a harder compound. The families supplied run from 30 to 96 Shore A, with 70 to 75 A the common grade in most families: fluorocarbon from 60 to 90 A, nitrile 50 to 90 A, EPDM 50 to 80 A, fluorosilicone 60 to 80 A and polyurethane 70 to 95 A. A harder ring resists extrusion better, but above 70 A it also runs with more friction at the same squeeze.

Both are fluorinated, but FFKM is fully fluorinated, so it resists chemistry that swells or attacks FKM; FFKM runs -26 C to 320 C on the Parker ranges, and named grades on file reach 327 C (Kalrez 7075) and -42 C (Kalrez 0040), each on its maker's sheet. FFKM is supplied in 70, 75 and 90 Shore A, including a peroxide-cured 75 A grade made to AMS7257 by a listed manufacturer. It is specified where the medium or the temperature rules the other families out.

When the seal sees fuel or oil at silicone temperatures. Silicone has poor resistance to petroleum fluids; fluorosilicone keeps most of the low-temperature flexibility (Parker ORD 5700 lists the family at -73 C to 177 C) and adds fuel and oil resistance. It is the aerospace fuel-system material, specified through MIL-DTL-25988 and its SAE successor AMS-R-25988 in classes such as M25988/1 and M25988/3, in grades from 60 to 80 Shore A.

Each element is a test requirement. M means SI units, 2 is the grade, B is the heat-aging temperature (70 hours at 100 C), G is the oil-swell class (40 percent maximum volume change in IRM 903 oil), 7 is 70 plus or minus 5 Shore A, 10 is a 10 MPa minimum tensile strength, and B14 adds a compression-set limit after 22 hours at 100 C. The type letter is an aging test, not a service temperature, and a letter pair is an envelope rather than a material: BG covers both nitrile and polyurethane.

Because the range belongs to the compound, not the family. On the compound chart the nitrile grades start at different cold ends compound by compound, against the 100 C standard and 121 C short-term ceiling on the Parker ranges, the EPDM grades differ at the cold end and run to 150 C, and the fluorocarbon grades start at -26 C standard or -46 C in low-temperature types on the Parker ranges. State the coldest start and the hottest continuous temperature on the request with the compound your print calls out, and ask for the datasheet with the quote.