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.
Types of Seals
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). Straight threads used with O-rings provide a better seal than tapered threads used alone.
A boss seal is also an O-ring, however it does not 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.
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).
Imporant specifications for O-rings include size, material, hardness rating and features.
O-rings are typically available in standard sizes per industry standards. These include:
- AS568 standard (001-475)
- AS568 boss gasket (901- and up)
- British Standard metric
- JIS standard sizes
- JASO standard
- Custom sizes
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)
- 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
- >90 durometer
Common features for o-rings include:
- FDA approved or 3A sanitary classification
- AMS/MIL spec
- Peroxide cured
A-A-55801 - Rubber, silicone; packing, preformed (o-ring)
ASTM D1414 - Standard test methods for rubber o-rings
BS ISO 3601-1 - Fluid power systems - o-rings part 1
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