O-rings are solid-rubber seals that are shaped like a doughnut. When pressed between two mating surfaces, O-rings block the passage of liquids or gases. O-rings can form a static or dynamic seals. A static seal is where the O-ring does not move and is used simply for containing pressure or maintaining a vacuum. Dynamic seals can be reciprocating (like a piston and cylinder), or rotating (shaft rotating in a housing). Axial squeeze and radial squeeze are two methods for applying an O-ring. An axial squeeze is when the ring is compressed parallel to a line drawn through the center or axis of the ring. In a radial squeeze the ring is compressed between the internal diameter (ID) and overall diameter (OD). Also, a boss seal is an O-ring, however it just doesn't fit the standard sizes for an O-ring. A boss is a cylindrical projection on a casting or forging. The end of that projection is machined to provide a flat, smooth surface for sealing. Straight threads used with O-rings provide a better seal than tapered threads used alone.
O-rings are typically available in standard sizes per industry standards. These include "AS568" standard sizes (001-475), "AS568-" boss gasket sizes (901- and up), British Standard metric sizes, JIS standard sizes, JASO standard sizes, AS871, and custom. When custom o-rings are required, the width or thickness, insider diameter, and outside diameter are important to consider. Common materials of construction for o-rings include butyl (-75° to 250° F), cast polyurethane (-40° to 125° F), ethylene propylene (EPDM) (-70° to 275° F), fluorosilicone (-100° to 350° F), fluorocarbon/fluorosilicone blend (-25° to 400° F), highly-saturated nitrile (-40° to 300° F), Hydrin (-60° to 300° F), neoprene (chloroprene) (-45° to 230° F), nitrile (Buna-N) (-30° to 275° F), polyacrylate (-25° to 350° F), polyurethane (Millable) (-40° to 180° F), SBR (BUNA-S) (-30° to 275° F), silicone (-65° to 450° F), and Thiokol (-30° to 300° F). Exact specifications should be clarified by the vendor, as chemical and physical properties of an O-ring can differ based on manufacturing technique.
The hardness rating on a Shore A scale for O-rings, can be <30 durometer, 30 durometer, 35 durometer, 40 durometer, 45 durometer, 50, durometer, 55 durometer, 60 durometer, 65 durometer, 70 durometer, 75 durometer, 80 durometer, 85 durometer, 90 durometer, and >90 durometer. Common features for o-rings include FDA approved or 3A sanitary classification, AMS/MIL spec, peroxide cured, and chlorinated.
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