Hot Topic Nose Rings – Metal O-rings will be the metallic variants of typically used elastomeric O-rings. They are typically used in stationary sealing applications which are incompatible with all the physical and chemical characteristics of elastomeric O-rings. All these O-rings have high tolerance to radiation as well as high vacuum, and are generally employed for flanged applications. Ideally, metal O-rings are placed in both closed and open grooves and axial pressure is applied between two parallel surfaces. Furthermore, grooves that hold the O-rings are made to be vertical to the direction of the fluid flow. When placed between surfaces and compressed to a predetermined height, deforms and generates contact between the jointing and sealing faces. Inherently, O-rings’ metallic elasticity displays a reasonable deflection and generates maximum sealing contacts through surgery to make sure the sealing action. In addition to elastic deflection of the metal, structural builds of the O-rings also play a significant role in creating the sealing force. Although most metals may be used for fabricating metal O-rings, the choice of metals must be such that the linear growth co-efficient of the O-ring metal varies slightly from the linear growth co-efficient of the metal(s) of jointing surfaces. Metal O-rings are vulnerable to rusting unlike elastomeric O-rings, and therefore, are coated with the right substance to avoid rusting. This material is softer than the base metal and serves another purpose of negating the effect of surface abnormalities. The coat materials which are ordinarily used are soft metals, such as silver, and non-metals such as PTFE.
Metal O-rings are primarily classified according to their fundamental structural characteristic, they are categorized as follows:
- Solid Metal O-Rings
- Hollow Metal O-Rings
Solid Metal O-Rings
Solid metallic rings are wires of circular profile connected like a ring. These types of O-rings need specific grooves for their effective performance. This requirement of specific grooves limits the use of strong metallic O-rings to specific applications, such as engine cylinder heads. Solid metallic rings are generally made in aluminum and appropriate for application in nuclear reactors, for checking radiation leaks. These O-rings can manage pressures of around 4MPa and temperatures hitting 500°C and are acceptable for moderate strain and moderate vacuum applications.
Hollow metallic O-rings are structurally more sophisticated in comparison with solid metallic O-rings and formed by circular tubes bent into the form of a ring. All these O-rings are sub-classified based on the characteristics of the hollow sector of the ring.
These are hollow metallic O-rings with nitrogen in atmospheric pressure or vacuum within them. Plain hollow O-rings are employed in low stress or low vacuum applications. Though plain hollow O-rings show similarity in pressure and temperature handling capabilities with solid metallic O-rings, they offer substantially superior spring-back and sealing capabilities. Normally, plain hollow metallic O-rings are manufactured in Inconel 750 and garnished with silver.
Self Energized Hollow O-Rings:
Self-energized hollow O-rings have vents or holes in their surfaces to utilize the system pressure or atmospheric pressure to sealing action. Based upon the orientation of such grooves, the O-rings are categorized into internally vented O-rings and externally vented O-rings. Internally vented O-rings made of light steel and garnished with PTFE can withstand pressures of around 3500 kg/sq cm and temperatures of 600°C. Externally vented O-rings are employed in applications where the external pressure is higher than the pressure within the seal.
Stress stuffed hollow O-rings are modified plain hollow rings using pressurized inert gasses inside them. These O-rings may be used in tongue and groove flanges and can handle temperatures from cryogenic temperatures up to 850°C. Stress filled O-rings are employed in vacuum cleaning applications.
- Metal O-rings can’t be used in highly dynamic sealing applications
- All these O-rings are rendered useless after they are used once since they can’t keep their shape after single use
- Broad tolerances of the groove can’t be adapted by the metallic O-rings, particularly in larger sizes
- Careful fitting ought to be done to avoid their deformation until their seal is tightened.