So, what are o rings used for, and why are they so popular? O rings have some amazing properties which makes them a crucial component of many precision engineered devices. Their natural propensity to return to their original shape when the cross section has pressure exerted on it means they are one of the most economical and reliable methods of making a strong seal possible.
The other reason o rings are so commonly used is down to the wide range of materials they can be made from. The majority of o rings are made from elastomers, which are a type of elastic polymer, but there are a huge variety of these elastomers available, each with different strengths, weaknesses and tolerances.
The application that the o ring is destined for will determine which type of material is most suitable, for example:. As you can see, for pretty much any application you can think of, there will be an o ring which can handle that environment. The sheets are fed into an extruder that heats the material and forces it through a die. The process produces the desired configuration of the material for being placed in the mold in lengths of cord.
The die selected for the extrusion process is selected according to the diameter of the finished O-ring. There are three molding processes used in the production of O-rings, which are compression, transfer, and injection.
Compression molding is used when there is a need for a high volume small non-standard O-rings. With compression molding, the extruded material is placed in the mold cavity and held at a high temperature under pressure, which forces it to take the shape of the mold. Transfer molding is a middle ground between compression and injection molding. In the transfer process, material is forced into the mold, while the mold is closed resulting in higher dimensional tolerances and less environmental impact.
Uniform pressure is used to completely fill the mold. The material for molding may be solid and be placed in the transfer pot from which it is forced into the preheated mold.
The injection process involves pre-heating the material, which is forced under pressure through an injection nozzle. The material enters the enclosed mold through a series of sprues. The molded material is left to cool and harden to the configuration of the mold cavity.
Post mold curing enhances the physical properties and performance of the molded O-ring. Post curing exposes the O-ring to elevated and increased temperatures as a means of improving its characteristics.
It assists in the cross linking process and improves tensile strength, flexibility, and the heat distortion temperature above what would happen if it were cured at room temperature.
After the O-rings are molded, they will have excess material around the sides where the molds meet. This material, known as flash, has to be removed for the O-ring to have the proper shape and size. Flash can be removed using three processes to give the O-ring its perfectly round shape.
Once the O-rings are deflashed, they need to be cured. How long the O-rings are in the curing oven depends on the type of elastomer and can vary from a few hours to a day. The purpose of this step is to stabilize the finished O-rings and drive off any by contaminants from the production process. Though the original material used to produce O-rings was rubber, in recent years the number of materials has grown extensively.
The choice of a specific material is dependent on the final application for the O-ring, which is to serve as a seal between two surfaces to prevent leakage of a gas or liquid. The choice of material is a major factor in designing an O-ring. Other considerations are the application, groove or gland design and size, surrounding conditions, and cross sectional diameter, or roundness, of the O-ring.
When examining the basic O-ring, the term design may not seem to fit since an O-ring is a circle made of an elastomer. In actuality, there are several considerations that have to be evaluated when producing an O-ring, which includes its inner diameter ID and cross sectional CS diameter, hardness of its material, durability, and shape.
Each of these factors is used to choose the correct O-ring for the application. As new applications for O-rings arise, different materials have been adapted to fit the increased need. The types of materials include several varieties of rubber, silicone, and polymers.
Materials that are chosen for use as O-rings all have the same basic qualities and characteristics, which is their elasticity and strength since O-rings are normally placed in critical and stressful conditions. They are naturally white in color and are valued for their ability to resist most chemicals, acids, oils, and steam.
They are tough and abrasive resistant, but cannot be easily compressed, leading to less secure sealing. Silicone is made from silicon, an element that is taken from quartz. It is produced by combining it with organic groups like methyl, phenyl, or vinyl.
The addition of these additional elements determines the properties of the silicone material. Silicone is resistant to the effects of oils, chemicals, heat, ozone, corona, and solvents. It is known to maintain its flexibility at low temperatures. Viton is a synthetic fluoropolymer elastomer rubber used for O-rings in stressful, harsh, and rigorous conditions. They are the main choice for applications that require an O-ring that can endure extreme heat and severe atmospheric conditions where oxygen, mineral oil, various fuels, hydraulic fluids, chemicals, and solvents are present.
Viton O-rings maintain exceptional performance in extreme temperature conditions. NBR is known as acrylonitrile butadiene or Buna-N.
It is a synthetic rubber copolymer made from butadiene and acrylonitrile. NBR has good mechanical properties and wear resistance, which is influenced by the percentage of the various compounds from which it is produced. The higher the nitrile content, the better is its resistance to the effects of oil and fuels. It is used in applications that have dilute acids, alkalis, and salt solutions present and comes in a wide variety of colors.
EPDM is a terpolymer made from ethylene and propylene with a monomer such as diolefin to activate vulcanization. It has resistance to ozone, sunlight, and weathering with good flexibility at low temperatures.
EPDM is used for O-rings due to its resistance to dilute acids, alkalis, and certain solvents as well as its electrical insulation properties. It comes in a variety of colors for applications that require sealing phosphate ester based hydraulic fluids and glycol based brake fluids.
Polyurethane rubber is a thermoplastic elastomer that is made by reacting a polyol with a diisocyanate or polymeric isocyanate with some form of catalyst. It has high strength and is resistant to tears and abrasions with excellent preventative leakage ability. The many features of polyurethane O-rings include resistance to hydraulic oil, gasoline, hydrocarbons such as propane, grease, water, oxygen, and aging. It is frequently used for hydraulic, cylinder, and valve fittings as well as pneumatic tools and firearms.
CSM O-rings are made by treating polyethylene with a mixture of chlorine and sulfur dioxide in the presence of UV radiation. The combination of these elements helps in the vulcanization process, which affects the strength of the final product. CSM O-rings are resistant to dilute acids, alcohol, ozone, oxidation, and weathering. They are mainly used for static applications since they have a low compression resistance. The first U. Niels studied rubber in the shape of rings in the quest to find a sealing solution.
He discovered, in , that rubber rings set in a groove that measured one and a half times the minor radius of the ring created an ideal seal for applications like pistons and cylinders. He was granted a patent in after two years of working through the application process. Although O-rings are typically circular, different shapes are used for various applications including squares, X-shapes and others.
O-Rings are produced using a variety of manufacturing techniques like extrusion, compression molding, injection molding, transfer molding or machining. Depending on the application, they can be made from a plethora of materials: nitrile rubber, silicone, polyurethane, neoprene, fluorocarbon as well as other elastomers. O-Ring design considers quality, quantity, cost, application temperature, sealing pressure, chemical compatibility, movement, action, lubrication and other requirements.
Friction of O-ring seals under low pressures may exceed the friction of properly designed lip type seals, but at higher pressures, developed friction compares favorably with, and is often less than, the friction of equivalent lip type seals. Synthetic rubber can be made for continual use at high or low temperatures, or for occasional short exposure to wide variations in temperature. At extremely low temperature the seals may become brittle but will resume their normal flexibility without harm when warmed.
Prolonged exposure to excessive heat causes permanent hardening and usually destroys the usefulness of the seal. The coefficient of thermal expansion of synthetic rubber is usually low enough so that temperature changes present no design difficulties. Note: This may not be true for all elastomer compounds. Chemical interaction between the seal and the hydraulic medium may influence seal life favorably or unfavorably, depending upon the combination of seal material and fluid.
Excessive hardening, softening, swelling, and shrinkage must be avoided. O-ring seals are extremely dependable because of their simplicity and ruggedness. Static seals will seal at high pressure in spite of slightly irregular sealing surfaces and slight cuts or chips in the seals. Even when broken or worn excessively, seals may offer some measure of fl ow restriction for emergency operation and approaching failure becomes evident through gradual leakage.
The cost of O-ring seals and the machining expense necessary to incorporate them into hydraulic mechanism designs are at least as low as for any other reliable type of seal. O-ring seals may be stretched over large diameters for installation and no special assembly tools are necessary. Irregular chambers can be sealed, both as fixed or moving-parts installations. It has been brought out in the foregoing discussion that there are certain definite limitations on their use, i.
Disregard for these limitations will result in poor seal performance. Piston rings, lip type seals, lapped fits, flat gaskets and pipe fittings all have their special places in hydraulic design, but where the design specifications permit the proper use of O-ring seals, they will be found to give long and dependable service.
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