A back-up ring is a rigid ring that holds an

elastomeric An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus (E) and high failure strain compared with other materials. The term, a portmanteau of ''elas ...

seal Seal may refer to any of the following: Common uses * Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly: ** Earless seal, also called "true seal" ** Fur seal ** Eared seal * Seal ( ...

or plastic (such as

Polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...

) connection to its designed shape and in its correct place. Back up rings are commonly used with

O-rings An O-ring, also known as a packing or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more part ...

, lip seals, and as reciprocating shaft seals. They are also used for piping connections joining two different materials - typically one flexible and one rigid.

Seals

When sealing the piston inside a

pneumatic cylinder Pneumatic cylinder, also known as air cylinder, is a mechanical device which uses the power of compressed gas to produce a force in a reciprocating linear motion. Like in a hydraulic cylinder, something forces a piston to move in the desired ...

, a soft and flexible material is required to prevent leakage, but those same properties may leave the seal material vulnerable to being pulled out of its seat and then pinched or torn in the narrow space between piston and cylinder wall. If the joint cannot be redesigned, or a more resistant elastomer used, then the solution may be direct reinforcement with a stiffer material - in the form of a hard inner ring in this case. A second function can be to hold the elastomer in place while a machine is being assembled, as geometry may prevent the seal from being directly checked after assembly.

Rubber Contoured Back-Up Ring Size Chart

Contoured back-ups are routinely produced in a 90 durometer

nitrile In organic chemistry, a nitrile is any organic compound that has a functional group. The name of the compound is composed of a base, which includes the carbon of the , suffixed with "nitrile", so for example is called " propionitrile" (or pr ...

. This provides sufficient elasticity to permit stretching over the major diameter of a piston and then snapping back into the

gland A gland is a Cell (biology), cell or an Organ (biology), organ in an animal's body that produces and secretes different substances that the organism needs, either into the bloodstream or into a body cavity or outer surface. A gland may also funct ...

groove cut into the piston. 90 durometer has sufficient hardness to resist extrusion of the softer elastomeric O-ring performing the actual sealing function against the high pressure liquid or gas. * 000 Series O-rings 0.070" ±0.003" - .045" Thick * 100 Series O-rings 0.103" ±0.003" - .045" Thick * 200 Series O-rings 0.139" ±0.004" - .040" Thick * 300 Series O-rings 0.210" ±0.005" - .060" Thick * 400 Series O-rings 0.275" ±0.006" - .096" Thick

Pipe Connections

Back-up rings are used in Polyethylene (PE) pipe connections, such as HDPE pipes, where they are connected to steel pipe or pipe fittings. Flange Adapter and Back-up Ring

The back-up ring is matched to the steel pipe flange specification and is placed behind the HDPE flange adapter - an end piece on the HDPE pipe that looks much like a steel flange at the end of the HDPE pipe. As the flange adapter does not have much structural strength, the back-up ring supports and compresses the HDPE flange adapter against the steel flange and creates an effective connection between the two piping materials.

References

External links

"Rubber Contoured Back-Up Ring Size Chart"Technical Note PP 811-TN PE Flange Connections
Seal rings {{engineering-stub