Recent developments in O-ring sealing technology
By D. M. Ashby
The shape is familiar, but this 60-year-old seal design has changed in many subtle ways and continues to evolve today.
What's new in O-rings? In one respect, not much; in other respects -- namely user expectations -- quite a bit. The familiar toroidal (or doughnut-shaped) geometry, first patented by Neils Christensen in 1939, is still one of the most widely used sealing solutions in use today, some 60 years later. In the 1940s and early 1950s, O-rings were used primarily in military applications, where they became the preferred seal geometry for hydraulic systems. Gradually, O-rings found a place in the industrial hydraulic sector, and the rest, as they say, is history. Today, O-ring seals are used in almost every facet of industry, ranging from jewelry to plumbing to the Space Shuttle.
Although the basic shape of the O-ring has not changed, there have been a number of dramatic improvements in O-ring sealing technology over its history. Specifically, the most prominent areas have been material development, testing, and characterization. From the successes in these areas have come higher expectations -- sometimes unreasonable -- by users.
Let's explore the increased level of awareness by seal users. Simply stated, most users have read or been told that leakage is no longer a necessary evil in hydraulic systems. (While modern sealing technology makes this statement true, it does not just happen automatically.) In any case, the user's expectation of zero leakage is a reality, and in many cases, a prerequisite for doing business. These expectations mark a departure from the not-too-distant past when an acceptable mean time between failures was the user's goal.
Users also are moving away from material specifications toward performance specifications for seals. This creates a need for new elastomeric seals based on performance of the component in a simulated application rather than merely meeting the material specification. As a corollary, there is a trend toward statistically based, six-sigma material properties instead of the pass/fail criteria that were the norm for many years. There is a need for elastomers that survive wider temperature ranges than ever before. In addition, enhanced chemical resistance of all polymer families has become a very important requirement for new compound developments.
All elastomeric sealing compounds are a combination of many chemical ingredients. These ingredients can be classified as:
* polymers (or elastomers)
* inert fillers, such as carbon black or mineral fillers, and reinforcing agents -- to improve physical properties
* accelerators, activators, retarders, and curing agents to assist in vulcanizing (or curing)
* anti-degradients to inhibit undesirable chemical reactions during compounding
* plasticizers to decrease stiffness and improve low-temperature properties
* process aids that help in the formation of finished parts, and
* special additives, such as pigments, flame retardants, etc.
Seal materials can be compounded to enhance specific engineering properties for specific applications. Much research is being conducted in each of the above categories to expand the useful envelope for each ingredient involved. Some of the most exciting material developments are in the polymer area. More than 20 engineering polymers are available to the rubber chemist today. (These are described in ASTM D1418.)
There also has been much work done by polymer manufacturers in modifying existing technologies to incrementally enhance their products' performance. For example, by varying the percentage of fluorine in fluorocarbon-based polymers, their relative chemical resistance and thermal stability can be enhanced. Some of these variations on the fluorocarbon polymer have only been available in the last year. In fact, this entire additional product offering in fluoropolymers is in answer to the increased demands from users.
The same can be said for each of the areas of rubber chemicals used to manufacture seals. New or revised technologies are being offered to the seal community in answer to more demanding applications. Material technology advances are available to the user today, but because of the multitude of possibilities, discussions with seal design experts should be held to assist in choosing the best technology available.
As stated previously, nothing has changed in the shape of the basic O-ring. However, the engineering tools to refine that shape have changed considerably. A variety of PC-based software packages are currently available (including Parker Hannifin's InPHorm Series) to assist the designer in creating a robust O-Ring design. In addition, there are many designs that use redundancy in the seals to facilitate lower-leakage results. True zero-leakage designs are being described in a number of technical journals.
Nonlinear finite element analysis has been applied to elastomeric seal designs with great success. The improved capabilities of the various FEA packages allow the nonlinearity of elastomers to be more accurately explored.
In the last few years there have been numerous advances in the analytical characterization of elastomers. Techniques that were proved in other areas of material technology -- such as metals, composites, plastics and fluid dynamics -- have been successfully translated to characterize O-rings and other sealing products. They have been used for predictive analysis as well as failure-modes-effects analysis. The information that is being obtained is invaluable in engineering a better sealing solution for the user community.
More sophisticated testing capabilities have been developed recently to more accurately describe the deleterious effects of hostile environments on elastomers. Specifically, functional-type test stands that emulate the end-use application have become much more commonplace. Stress-relaxation testing on elastomers has seen a resurgence -- with significant work being applied to improving the testing techniques and equipment available.
The goal of these efforts is to more accurately predict the potential failure modes of elastomers before they actually occur. The whole area of seal-life prediction analysis has emerged as the modus operandi for advanced research in seal technology. A cooperative environment with academia exists today that was nonexistent as recently as a few years ago in this area.
This discussion has attempted to bring to light some of the developments that are taking place on a number of fronts ranging from material technology to seal-life prediction. The rate of change in these disciplines is increasing dramatically -- making for some exciting research, and bringing design solutions to users. The prognosis for the future is bright. Additional developments in the three areas discussed will result in increased satisfaction as we strive to develop a zero-leakage seal for every user's application. This ever-challenging work will continue to evolve.
Dale M. Ashby is material-technology/research-and-development manager, Parker Hannifin Corp., O-Ring Div., Lexington, Ky. Contact the division for more information by phone at 606 / 269-2351 or fax at 606 / 269-3816.