Seals

Seals  : The type of seals discussed here are not marine mammals or navy commandos. Seals are a means for keeping water out of pressure housings . Types of seals used in the marine environment include gaskets, shaft seals, lip seals, stuffing tubes and gland seals. The o-ring is the most commonly-used type of seal underwater. It is a solid piece of elastomeric material shaped like a doughnut or torus. When pressed between mating surfaces, an o-ring blocks the passage of liquids or gases. The oring is the most widely used seal due to its simplicity, low cost, ease of installation, and small space requirements without supporting structures. An O-ring can be considered an incompressible viscous fluid with very high surface tension. This “fluid” is forced by mechanical or hydraulic pressure to flow into the sealing cavity, blocking the flow of the lessviscous fluid being sealed. Properly installed, the O-ring is squeezed about 10% to 15% of its original cross-sectional diameter. The compression absorbs the tolerance stack up between mating surfaces (or between shaft and gland in dynamic applications) and forces the elastomer into microscopic surface grooves on mating parts.

Successful use of o-rings depends upon proper groove dimensions and selection of the right elastomeric compound. Compounds are chosen for their resistance to chemicals and temperatures. Common materials utilized include Nitrile, neoprene, flurocarbon (Viton), silicone, fluorosilicone, and urethane.

Today’s dynamic o-ring in a short rectangular groove was the result of experimental work in the early 1930s by Niels Christensen (see http://www.uh.edu/engines/epi555.htm ). In the early 1940s, the o-ring became the standard seal for U.S. Air Force hydraulic systems. This established the basic sizes and design information. Today, billions of o-rings are sealing every conceivable apparatus all over the world, in the air, on land and sea, and in outer space. (long page from alpha-section)