Rubber O-Rings

Rubber O-Rings

In general, rubber O-Rings are not made of natural rubber but rather of a variety of synthetic rubbers also known as elastomers. Popular elastomers used to make O-Rings include Viton, Kalrez, Neoprene, silicone, flourosilicone, nitrile, hydrogenated nitrile, polyurethane, ethylene-propylene (EPDM or EPR), silicone, and flourosilicone.

Each type of rubber O-Ring has its own specific uses, based on its resistances to chemicals, pressure, and temperature.

Two of the most commonly used synthetic rubbers used to make O-Rings are nitrile and Viton. Nitrile O-Rings are among the most popular types of O-Rings, providing good resistance to oils and fuels. Hydrogenated nitrile is similar to regular nitrile, but it adds the added benefit of being ozone resistant. Viton O-Rings are very popular as well, mostly due to their resistance to chemicals and high temperatures; however, they may fail at very low temperatures.

O-ring From Wikipedia the free encyclopedia

Typical O-ring and application

An o-ring is a loop of elastomer with a round (o-shaped) cross-section used as a mechanical seal or gasket. They are designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.

The joint may be static, or (in some designs) have relative motion between the parts and the o-ring; rotating pump shafts and hydraulic cylinders, for example. Joints with motion usually require lubrication of the o-ring to reduce wear. This is typically accomplished with the fluid being sealed.

O-rings are one of the most common seals used in machine design because they are inexpensive and easy to make, reliable, and have simple mounting requirements. They can seal tens of megapascals (thousands of psi) pressure.

History

The o-ring U. S. patent claim was filed in 1937 by a then 72 year old Danish-born machinist, Niels Christensen [1]. He came to America in 1891 and soon after that patented an air brake system for streetcars. Despite his legal efforts, his intellectual property rights were passed from company to company until they ended up at Westinghouse [2]. During World War II, the US government commandeered the o-ring patent as a critical war-related item and gave the right to manufacture to other organizations. Christensen got a lump sum payment of US$75,000 for his efforts. Litigation resulted in a $100,000 payment to his heirs in 1971, 19 years after his death.

Theory and design

O-ring mounting for an ultra-high vacuum application . Pressure distribution within the cross-section of the O-ring. The red lines are hard surfaces, which apply high pressure. The fluid in the seams has lower pressure. The soft O-ring bridges the pressure over the seams.

O-rings are one of the most common yet important elements of machine design. They are available in various metric and standard sizes. The UK standards sizes are known as BS Sizes and typically range from BS001 to BS932. The most common standard sizes in the US are controlled by SAE AS568. In general o-rings are specified by the inside diameter and the cross section diameter (thickness). The o-ring is one of the simplest, yet most engineered, precise, and useful seal designs ever developed.

Typical applications

Successful o-ring joint design requires a rigid mechanical mounting that applies a predictable deformation to the o-ring. This introduces a calculated mechanical stress at the o-ring contacting surfaces. As long as the pressure of the fluid being contained does not exceed the contact stress of the o-ring, leaking cannot occur.

The seal is designed to have a point contact between the o-ring and sealing faces. This allows a high local stress, able to contain high pressure, without exceeding the yield stress of the o-ring body. The flexible nature of o-ring materials accommodates imperfections in the mounting parts. Maintaining good surface finish of those mating parts is still important, however, especially at low temperatures where the seal rubber reaches its glass transition temperature and becomes increasingly crystalline.

Vacuum applications

In vacuum applications the permeability of the material makes point contacts quite useless. Instead, higher mounting forces are used and the ring fills the whole groove. Also round back-up rings are used to save the ring from excessive deformation [3] [4] [5]. As the ring feels the ambient pressure only at the seals and the ring feels the partial pressure of gases only at the seal, their gradients will be steep near the seal and shallow in the bulk (opposite to the gradients of the point contact [6]).