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ABSTRACT
Dynamic sealing is defined as preventing any fluid leakage between two mechanical parts moving relative to each other. It can be divided into two categories, reciprocating motion and rotational motion. Its role is crucial in hydraulic and pneumatic systems because a simple fluid leak can affect the smooth running and safety of the systems. This paper describes the different types of rotary dynamic seals and specifies the criteria for their selection according to the expected dynamic sealing.
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Abdelghani MAOUI: Design and services engineer - Fluid and Sealing Technologies Department, Cetim, Nantes, France - Editor's note: This article is an updated reprint of the chapter on dynamic sealing in article [B 5 420] "Sealing in mechanics", written by Jean MARTIN in 2004.
INTRODUCTION
In mechanical engineering, sealing is said to be dynamic when it is applied between two parts in relative motion, which may be rotational and/or translational. When there is no movement between the parts, sealing is said to be static. The latter is covered in
Dynamic sealing is considerably more involved than static sealing. A dynamic seal must seal at rest, just like a static seal. It must also be capable of performing this task in motion, taking into account frictional forces and the heating of dynamic surfaces.
Containing a gas or liquid, by its very nature, is not easy. The permissible leak rate depends on the nature of the fluid to be sealed; leakage of expensive, toxic, corrosive, explosive or flammable fluids must be kept to a minimum. Seal life and reliability are also major concerns for users, to reduce equipment and process downtime.
Seal failure can have multiple consequences, in terms of equipment availability, operation and safety. Common examples show that highly complex assemblies, such as landing gear, can experience major problems as a result of a simple leak. Good dynamic sealing reliability is not often easy to achieve.
In fact, dynamic sealing calls on many notions of physics and chemistry, in which the properties of materials and dynamic contact surfaces play an important role. In addition, the problems to be solved are varied, involving a large number of parameters that are difficult to optimize simultaneously. The best solutions for mechanical sealing in general, and dynamic sealing in particular, are almost always the result of skilful compromises.
This article is dedicated to dynamic rotary shaft seals. It describes the different technologies of dynamic seals for rotating shafts according to the different degrees of sealing: rigorous, controlled and relative. It complements
At the end of the article, readers will find a glossary and a table of the notations used.
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KEYWORDS
dynamic seals | leakage | dynamic sealing | rotary seals
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Dynamic mechanical sealing
Definition of dynamic rotation tightness
Bibliography
Standards
- Mechanical seals – Main dimensions, designations and material codes. - NF EN 12756 - Avril 2001
- Bagues d'étanchéité à lèvres pour arbres tournants incorporant des éléments d'étanchéité en élastomère. Partie 1 : dimensions nominales et tolérances. - ISO 6194-1 - 2007
- Lip seal rings for rotating shafts incorporating elastomer sealing elements – Part 2: vocabulary. - NF ISO 6194-2 - 2009
- Geometric product specification...
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