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An O-ring is a simple sealing device with a circular cross section, from which the “O” in its name is derived. O-rings operate in both static (stationary) and dynamic (moving) applications and tolerate differences in stack-up dimensions between mating parts. O-rings are most commonly molded in one piece from an elastomeric (rubber-like) material. Elastomers are synthetic or natural materials with resilience or memory sufficient to return to their original shape after distortion.
O-rings are available in a variety of materials and durameters (hardness) for many different applications. The most common material is Nitrile rubber, which has excellent oil and fuel resistance as well as good wear-resistance. O-rings seal by blocking potential leak paths between two surfaces. The compression (or more accurately the deformation) of the O-ring provides part of the sealing function. An additional sealing function is realized when the O-ring is activated by the pressure of the gas or liquid that the O-ring contains.
The O-ring is most commonly placed in a machined groove in one of the surfaces to be sealed. As the surfaces are brought together, they squeeze the O-ring and deform the material to take the shape of the surfaces. The areas of contact between the O-ring and the surfaces create a barrier to block the fluid and create a seal. O-rings are also used in non-sealing applications, such as drive belts, tension bands, and spacers.
The majority of o-ring seals can be categorized into one of the three following physical arrangements:
- Male Gland Seal: an O-ring is installed in a groove that is machined into the OD of the piston. The piston and the installed O-ring are then inserted into the bore. The O-ring seals radially
- Female Gland Seal: an O-ring is first installed into a groove that is machined into the ID of the bore. The rod is then inserted into the bore through the O-ring. The O-ring seals radially.
- Face Seal: an O-ring is installed in a groove that is machined into a flat face around a hole. A second flat face then seals against the O-ring. The O-ring seals axially.
O-Ring Installation Guidelines
The following are general guidelines to avoid damage and leakage when installing an O-ring.
- The O-ring must not be stretched beyond its elongation limit
- Edges must be burr-free and all radii and angles should be applied smoothly
- Dust, dirt, metal chips, and other foreign material should be removed prior to installation of the O-ring
- Tips of screws and installation housings for other sealing and guiding elements should be covered by an assembly sleeve
- A suitable lubricant should be applied to the assembly surfaces and/or the O-ring
- All installation tools (mandrels, sleeves, etc.) should be made of a soft material and not have any sharp edges
- The O-ring should not be rolled over assembly surfaces
- Ensure that the O-ring is not twisted during installation into the groove
- D-rings are often used as an alternative to O-rings in reciprocating dynamic applications because of their resistance to spiral failure. They can be used for sealing on the ID or the OD.
- X-rings can be used as an alternative to O-rings in dynamic applications because of their reduced friction and their resistance to spiral failure.
- Square rings can be either molded or lathe-cut. The lathe-cut parts are often less expensive than comparably sized O-rings, especially for larger IDs.