Wills Ring

Last updated

A Wills Ring or Cooper Ring is a form of all-metallic O-ring seal. They are used for extremely arduous service, such as sealing the head gasket of high performance piston engines.

O-ring mechanical, toroid gasket that seals an interface

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 parts, creating a seal at the interface.

Head gasket gasket that sits between the engine block and cylinder head(s) in an internal combustion engine

A head gasket is a gasket that sits between the engine block and cylinder head(s) in an internal combustion engine.

Contents

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

The yield point is the point on a stress–strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically whereas yield point is the point where nonlinear deformation begins. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.

Construction

A Wills Ring is an O-ring in the form of a hollow metallic tube. They are pressurised internally by gas pressure [1] The advantages of this construction are that the rings can seal against a wide range of conditions. [2] 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. [2] Internal pressurisation makes them compliant, both for the range of movement required and with their speed of response being faster than elastomeric O-rings. Their flexure is also unaffected by temperature. [note 1]

An elastomer is a polymer with viscoelasticity and very weak intermolecular forces, and generally low Young's modulus and high failure strain compared with other materials. The term, a portmanteau of elastic polymer, is often used interchangeably with rubber, although the latter is preferred when referring to vulcanisates. Each of the monomers which link to form the polymer is usually a compound of several elements among carbon, hydrogen, oxygen and silicon. Elastomers are amorphous polymers maintained above their glass transition temperature, so that considerable molecular reconformation, without breaking of covalent bonds, is feasible. At ambient temperatures, such rubbers are thus relatively soft and deformable. Their primary uses are for seals, adhesives and molded flexible parts. Application areas for different types of rubber are manifold and cover segments as diverse as tires, soles for shoes, and damping and insulating elements. The importance of these rubbers can be judged from the fact that global revenues are forecast to rise to US$56 billion in 2020.

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 [note 2] 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. [3]

Wills Rings are manufactured in five overall types: [2]

Hollow, internally gas pressurised
A high performance seal suitable for extreme pressures and still sealing when pressure is released. [3]
Hollow, slotted pressure-actuated
Best springback performance. [3]
Hollow, vented
Suitable for even higher pressures than the slotted form. [3]
Hollow, unpressurised
Lower cost and higher clamping forces
Solid
Limited springback and pressure capacity, these are mostly used as a non-outgassing metallic seal for use in vacuum systems.

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. [note 3]

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. [4] The correct perpendicular depth of these grooves (rather than their width or cross-sectional area) is an important factor that controls seal performance. [5] 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. [5]

Wills Rings are constructed in a variety of refractory metals including stainless steel and Inconel. [5] Coatings of PTFE or varying thicknesses of silver plating are also available. [5]

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. [5] 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 racing engines. [6] 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. [4] 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, 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. [7] 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 [note 4] costing approx. $50 each, [8] around the price of a high-performance head gasket for a four-cylinder engine.

Related Research Articles

Piston moving component of reciprocating engines that is contained by a cylinder and is made gas-tight by piston rings

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder.

End-face mechanical seal type of seal utilised in rotating equipment

An end-face mechanical seal, or a mechanical end-face seal, also referred to as a mechanical face seal but usually simply as a mechanical seal, is a type of seal utilised in rotating equipment, such as pumps, mixers, blowers, and compressors. When a pump operates, the liquid could leak out of the pump between the rotating shaft and the stationary pump casing. Since the shaft rotates, preventing this leakage can be difficult. Earlier pump models used mechanical packing to seal the shaft. Since World War II, mechanical seals have replaced packing in many applications.

Gasket type of mechanical seal

A gasket is a mechanical seal which fills the space between two or more mating surfaces, generally to prevent leakage from or into the joined objects while under compression.

Bottle cap

A bottle cap seals the top opening of a bottle. A cap is typically colourfully decorated with the logo of the brand of beverage. Plastic caps are used for plastic bottles, while metal with plastic backing is used for glass; the metal is usually steel. Plastic caps may have a pour spout. Flip-Top caps like Flapper closures provide controlled dispensing of dry products. Caps for plastic bottles are often made of a different type of plastic from the bottle.

A gate valve, also known as a sluice valve, is a valve which opens by lifting gate out of the path of the fluid. Gate valves require very little space along the pipe axis and hardly restrict the flow of fluid when the gate is fully opened. The gate faces can be parallel, but are most commonly wedge-shaped.

Viton is a brand of FKM, a synthetic rubber and fluoropolymer elastomer commonly used in O-rings, chemical-resistant gloves, and other molded or extruded goods. The name is a registered trademark of The Chemours Company.

Seal (mechanical) device that helps join things together by preventing leakage

A mechanical seal is a device that helps join systems or mechanisms together by preventing leakage, containing pressure, or excluding contamination. The effectiveness of a seal is dependent on adhesion in the case of sealants and compression in the case of gaskets.

Bridgman seal

A Bridgman seal, invented by and named after Percy Williams Bridgman, can be used to seal a pressure chamber and compress its contents to high pressures, without the seal leaking and releasing the pressure

Hydraulic machinery machinery and tools that use liquid fluid power to do simple work

Hydraulic machines are machinery and tools that use liquid fluid power to do simple work, operated by the use of hydraulics, where a liquid is the powering medium. In heavy equipment and other types of machine, hydraulic fluid is transmitted throughout the machine to various hydraulic motors and hydraulic cylinders and becomes pressurised according to the resistance present. The fluid is controlled directly or automatically by control valves and distributed through hoses and tubes.

A beadlock or bead lock is a mechanical device that secures the bead of a tire to the wheel of a vehicle. Tires and wheels are designed so that, when the tire is inflated, the tire pressure pushes the bead of the tire against the inside of the wheel rim so that the tire stays on the wheel and the two rotate together. In situations where tire pressure is insufficient to hold the bead of the tire in place, a beadlock is needed.

Piston ring Part of a reciprocating engine

A piston ring is a split ring that fits into a groove on the outer diameter of a piston in a reciprocating engine such as an internal combustion engine or steam engine.

A hydraulic seal is a relatively soft, non-metallic ring, captured in a groove or fixed in a combination of rings, forming a seal assembly, to block or separate fluid in reciprocating motion applications. Hydraulic seals are vital in machinery. Their use is critical in providing a way for fluid power to be converted to linear motion.

Piping and plumbing fitting piece that fits or connects pipes and tubes; used in pipe systems to connect straight pipe or tubing sections, adapt to different sizes or shapes and for other purposes

A fitting is used in pipe systems to connect the straight pipe or tubing sections, adapt to different sizes or shapes and for other purposes, such as regulating fluid flow. "Plumbing" is generally used to describe the conveyance of water, gas, or liquid waste in domestic or commercial environments; "piping" is often used to describe the high-performance conveyance of fluids in specialized applications. "Tubing" is sometimes used for lighter-weight piping, especially that flexible enough to be supplied in coiled form.

Oil pump (internal combustion engine) oil pumps

The oil pump in an internal combustion engine circulates engine oil under pressure to the rotating bearings, the sliding pistons and the camshaft of the engine. This lubricates the bearings, allows the use of higher-capacity fluid bearings and also assists in cooling the engine.

Junk head

A junk head is a form of piston engine cylinder head, where the head is formed by a dummy piston mounted inside the top of the cylinder. In most other engine designs, the cylinder head is mounted on top of the cylinder block. That form has also been termed a "poultice head".

Monobloc engine

A monobloc or en bloc engine is an internal-combustion piston engine where some of the major components are formed, usually by casting, as a single integral unit, rather than being assembled later. This has the advantages of improving mechanical stiffness, and also improving the reliability of the sealing between them.

A dry-seal Wiggins gasholder is a device designed to hold gas.

Bonded seal

A bonded seal is a type of washer used to provide a seal around a screw or bolt. Originally made by Dowty Group, they are also known as Dowty seals or Dowty washers. Now widely manufactured, they are available in a range of standard sizes and materials

References

  1. ↑ The Space Shuttle Challenger disaster was an accident caused by the poor dynamic response of cold elastomers.
  2. ↑ Atmospheric pressure is approximately 0.1 MPa
  3. ↑ As vacuum systems will develop a maximum of 1 Bar pressure differential, they use the internally pressurised or solid forms instead.
  4. ↑ Motor-racing quality, rather than aerospace.
  1. ↑ "Wills Rings". Trelleborg Sealing Solutions.
  2. 1 2 3 "Wills Rings: Overview" (PDF). Busak & Shamban. Archived from the original (PDF) on 2006-09-16.
  3. 1 2 3 4 "Wills Rings: Installation" (PDF). Busak & Shamban. Archived from the original (PDF) on 2006-09-19.
  4. 1 2 Roy Mc Bride (23 November 2010). "A Hillman Imp Wills Ringed cylinder Head". CKD Boats.
  5. 1 2 3 4 5 "Wills Rings: Catalogue" (PDF). Busak & Shamban. 2004. Archived from the original (PDF) on 2006-09-22.
  6. ↑ Whitelock, Mark (2006). 1½-litre GP Racing 1961–65. Veloce Publishing. p. 299. ISBN   184584016X.
  7. ↑ Hammill, Des (2004). Coventry Climax Racing Engines. Veloce Publishing. pp. 113–115. ISBN   978-1-903706-83-1.
  8. ↑ "Wills ring for SOHC Pinto engine". Burton Power.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.