Wills Ring

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. 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 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. They are only used for static face sealing, not moving glands. 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 metals including stainless steel and Inconel. Coatings of PTFE 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 of piston 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, and when these were re-tuned for racing, it was a regular practice to add this type of supplementary seal. 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 ring costing approx. $50 each, around the price of a high-performance head gasket for a four-cylinder engine. == References ==