A Wills Ring or Cooper Ring is a form of all-metallic

O-ring 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 par ...

seal. They are used for extremely arduous service, such as sealing the

head gasket In an internal combustion engine, a head gasket provides the seal between the engine block and cylinder head(s). Its purpose is to seal the combustion gases within the cylinders and to avoid coolant or engine oil leaking into the cylinders. Le ...

of high performance

piston engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common feat ...

s. Hollow, metallic Wills Rings are used as they have better springback than yielding soft metal seals and higher temperature limits than elastomer O rings.

Construction

A Wills Ring is an O-ring in the form of a hollow metallic tube. They are pressurised internally by gas pressure The advantages of this construction are that the rings can seal against a wide range of conditions. Metallic construction makes them robust against high or low temperatures and chemical attack. Softer coatings may be applied to provide even better compatibility, or to seal against roughly machined surfaces. Internal pressurisation makes them compliant, both for the range of movement required and with their speed of response being faster than

elastomer An elastomer is a polymer with viscoelasticity (i.e. both viscosity and Elasticity (physics), elasticity) and with weak intermolecular forces, generally low Young's modulus and high Deformation (mechanics), failure strain compared with other mate ...

ic O-rings. Their flexure is also unaffected by temperature.The

Space Shuttle Challenger disaster On January 28, 1986, the broke apart 73 seconds into its flight, killing all seven crew members aboard. The spacecraft disintegrated above the Atlantic Ocean, off the coast of Cape Canaveral, Florida, at 11:39a.m. EST (16:39 UTC). It w ...

was an accident caused by the poor dynamic response of cold elastomers. Rings are pressurised by either a static gas fill during manufacture, or may be pressure-actuated by the applied pressure on the joint in service. These pressure-actuated rings have vents on the pressure side to allow gas ingress. The 4 MPa

Atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...

is approximately 0.1 MPa pressure limit of the unpressurised ring is exceeded by the 40 MPa of the internally pressurised ring and 200 MPa or 1,000 MPa for the externally pressurised forms. Wills Rings are manufactured in five overall types: ; Hollow, internally gas pressurised : A high performance seal suitable for extreme pressures and still sealing when pressure is released. ; Hollow, slotted pressure-actuated : Best springback performance. ; Hollow, vented : Suitable for even higher pressures than the slotted form. ; Hollow, unpressurised : Lower cost and higher clamping forces ; Solid : Limited springback and pressure capacity, these are mostly used as a non-

outgassing Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (which ...

metallic seal for use in

vacuum system Vacuum engineering deals with technological processes and equipment that use vacuum to achieve better results than those run under atmospheric pressure. The most widespread applications of vacuum technology are: * Pyrolytic chromium carbide coatin ...

s. The vented and slotted forms are usually vented to the inside, for carrying an internal pressure, but are also available with external vents, for use in high external pressures, such as in undersea use.As vacuum systems will develop a maximum of 1 Bar pressure differential, they use the internally pressurised or solid forms instead. They are only used for static face sealing, not moving

gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Structure De ...

s. The thin tube walls are sensitive to damage from rough handling, point tools and crimping during assembly. The pressure-actuated form is considered to be slightly more resistant to point damage, but more at risk of folding and crimping, as they are not supported by internal pressurisation during assembly. Rings are installed in machined grooves in, usually one of, the mating faces. The correct perpendicular depth of these grooves (rather than their width or cross-sectional area) is an important factor that controls seal performance. Unlike the grooves for elastomer O-rings, the width and shape of the groove is unimportant, just the depth. It is even possible for this 'groove' to become an open-sided groove or aperture, provided that this free height is maintained. Wills Rings are constructed in a variety of

refractory metal Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definition of which elements belong to this group diff ...

s including

stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...

and

Inconel Inconel is a registered trademark of Special Metals Corporation for a family of austenitic nickel-chromium-based superalloys. Inconel alloys are oxidation-corrosion-resistant materials well suited for service in extreme environments subjected ...

. Coatings of

PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chem ...

or varying thicknesses of silver plating are also available.

Applications

Cylinder head gaskets

One of the best-known applications for pressurised Wills Rings, or Cooper Rings as they were originally known, is for sealing the

combustion chamber A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process. Intern ...

s. These are not widely used, owing to their high cost and the good performance of other modern gaskets, but they have a place for high-performance engines. One of the first applications in this case was for the

Coventry Climax Coventry Climax was a British forklift truck, fire pump, racing, and other specialty engine manufacturer. History Pre WW1 The company was started in 1903 as Lee Stroyer, but two years later, following the departure of Stroyer, it was reloca ...

racing engines. Coventry Climax's more mundane utility engines did not require these seals, but they were adopted to production car engines such as the

Hillman Imp The Hillman Imp is a small economy car that was made by the Rootes Group and its successor Chrysler Europe from 1963 until 1976. Revealed on 3 May 1963, after much advance publicity, it was the first British mass-produced car with the engine bl ...

, and when these were re-tuned for racing, it was a regular practice to add this type of supplementary seal. Some Coventry Climax designs, such as the FWMV V8, used an open-deck block with wet liners. These were located in place at the base of the liner, rather than near the cylinder head. They were thus susceptible to problems from differential

thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kin ...

, giving rise to coolant leakage, where the block and liner expanded at different rates: specifically when the liners contracted more rapidly as engine load was reduced. The solution to this was to redesign the liner so that they were located at the top instead, reducing the distance susceptible to expansion. The high compliance of the Wills Ring, thanks to their internal pressurisation, allowed reliable sealing, even though the short clamping distance now had little elasticity and so the applied clamping force from the head bolts could vary substantially.

Cost

Wills Rings are expensive in comparison to other seals, such as elastomer O-rings or metallic foil head gaskets. A typical comparison is one ringMotor-racing quality, rather than aerospace. costing approx. $50 each, around the price of a high-performance head gasket for a four-cylinder engine.

References

{{Reflist, colwidth=35em, refs= {{Cite web , title=Wills Rings: Overview , publisher=Busak & Shamban , url=http://www.busakshamban.us/pdf/willsrings.metal.orings.product.overview.pdf , archiveurl=https://web.archive.org/web/20060916101133/http://www.busakshamban.us/pdf/willsrings.metal.orings.product.overview.pdf , archivedate=2006-09-16 {{Cite web , title=Wills Rings: Catalogue , publisher=Busak & Shamban , year=2004 , url=http://www.busakshamban.us/pdf/wills.rings.complete.catalog.busak.shamban.pdf , archiveurl=https://web.archive.org/web/20060922222743/http://www.busakshamban.us/pdf/wills.rings.complete.catalog.busak.shamban.pdf , archivedate=2006-09-22 {{Cite web , title=Wills Rings: Installation , publisher=Busak & Shamban , url=http://www.busakshamban.us/pdf/willsrings.installation.construction.pdf , archiveurl=https://web.archive.org/web/20060919125522/http://www.busakshamban.us/pdf/willsrings.installation.construction.pdf , archivedate=2006-09-19 Seal rings