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.

Application Methods

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).

Specifications

Imporant specifications for O-rings include size, material, hardness rating and features.

Size

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
• AS871
• Custom sizes

When custom o-rings are required, the width or thickness, insider diameter, and outside diameter are important to consider.

Materials

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.

Hardness Rating

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

Features

Common features for o-rings include:

• FDA approved or 3A sanitary classification
• AMS/MIL spec
• Peroxide cured
• Chlorinated

Standards

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

Image credits:

Applied Industrial Technologies | CT Gasket & Polymer Co., Inc.