Seal for a joint or juncture – Seal between relatively movable parts – Circumferential contact seal for other than piston
Reexamination Certificate
1999-04-07
2003-05-06
Browne, Lynne H. (Department: 3679)
Seal for a joint or juncture
Seal between relatively movable parts
Circumferential contact seal for other than piston
C277S553000, C277S569000, C277S572000
Reexamination Certificate
active
06557857
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to seals for rotary shafts and more particularly, to an improved radial lip seal and seal housing for sealing around a rotatable shaft.
BACKGROUND OF THE INVENTION
Radial lip seal assemblies are used to prevent fluids (liquids or gasses) from leaking along a rotatable shaft from a relatively higher pressure region in an apparatus in which the shaft is mounted to a relatively lower pressure region. Such radial lip seal assemblies typically include a circular or annular seal ring or body which is sealably fit within a seal bore formed within the seal housing surrounding the rotatable shaft. A sealing lip extends radially inward from the seal body to slidably contact against the rotatable shaft and provide a leak-tight seal therewith
Where practical, a continuous or single piece radial lip seal is used. The seal is slid over the shaft and secured to the seal housing. However, in some applications, the continuous radial lip seal cannot be used or is simply not practical. For example, when sealing against a shaft with an oversized end, the seal may not stretch sufficiently to pass over the enlarged end. Similarly, disassembly and reassembly of complex and enlarged components such as flanges and gears, to get access to the shaft, may not be practical. To accommodate these applications, multi-piece or split seals are used. Radial lip split seals may also be used in applications where single piece or continuous radial lip seals would be difficult or time consuming to replace. The radial split seal may be separated into its two or more semi-annular pieces and then reassembled around an appropriate location of the shaft.
Known lip seal technology and particularly, radial split seal technology, centered around the use of elastomer rubbers. However, these elastomer seals have a very limited temperature and velocity range in which they can operate as radial lip seals. Elastomer rubber radial lip seals also have a limited shelf life and have uses that are limited to a narrow range of compatible fluids and gasses.
With regard to split seals, split packings and split mechanical seals are also used for sealing between a seal housing and a rotatable shaft. However, split packings and split mechanical seals typically require complex hardware designs to ensure adequate sealing and seal life. In addition, these split packings and mechanical seals require additional space, assembly and maintenance. The complex hardware design and assembly often results in additional costs.
In an effort to overcome these problems of the known elastomer rubber split seals as well as the inconveniences and expenses of the split packings and split mechanical seals, an effort has been made to produce radial lip split seals made from fluoroplastic or fluoropolymeric materials. The fluoroplastic materials provide a greater temperature operating range for the seal as well as increasing the number of compatible fluids and gasses. However, the development of fluoroplastic radial split seals has been limited due to a number of disadvantages. Some of the disadvantages include flow creep of the seal material, deformation of the seal due to compression set and resizing of the seal due to thermal and compression effects. These disadvantages all tend to cause leakage across the seal.
Flow creep, compression set, and resizing are characteristics exerted by almost all plastic seals which are exposed to heat while the seal is restricted from expanding in its natural radially outward direction. The application of heat leads to thermal expansion of the seal and particularly to an increase in the seal's outer diameter. This radial outward expansion is typically restricted by the seal housing, which leads to a buildup of compressive stress within the seal. These stresses tend to distort and/or resize the seal. Seal size and shape, temperature range, time at which the seal is held at the various temperatures, the thermal expansion characteristics of the particular plastic material used as well as its mechanical properties, as well as the amount of expansion that is actually restricted, are just a few of the key parameters that define the amount of permanent deformation or resizing a seal will experience.
Resizing and deformation, as described above, can lead to shrinkage of the physical size of the seal. This includes a shrinkage of the inside diameter. With radial split seals, this shrinkage leads to a separation of the individual semi-annular sections of the seal as the seal is maintained tight against the shaft. This separation breaks the continuous radial seal barrier around the shaft and leads to leakage at the interfaces between the split ends of the semi-annular seal sections.
As an example of a resizing problem and particularly, a problem when using a conventional fluoroplastic radial split seal, consider a two piece semi-annular radial seal split placed into a seal housing bore. Typically, the seal housing bore has a slightly smaller diameter than the outside diameter of the seal. This configuration creates a slip fit or slight press fit, sufficient to maintain the radial seal within the housing bore and fluidly seal therebetween. A shaft is then inserted through the inside diameter of the radial split seal to create a complete fluid barrier.
In this configuration, positive sealing is expected across the seal lip with only slight leakage at the split joints. If the seal is required to perform at typical ambient temperatures and relatively low rotational velocities, the desired positive sealing may be achieved. However, most applications see various temperature swings, which can be extreme, throughout their life cycle. In addition to the application temperatures, additional heat is generated when the shaft rotates and creates a friction between the rotating shaft and the stationery sealing lip. As the shaft rotational speed increases, this heat generation can become substantial.
As previously mentioned, heat tends to thermally expand the seal. This thermal expansion results, in part, to an increase in the overall outer diameter of the seal. However, the fixed diameter bore formed within the seal housing physically restricts this radially outward expansion and causes compressive stresses to build within the fluoroplastic seal body. These internal stresses cause the seal to physically alter its shape and can even lead to a resizing of the seal to a new diameter.
After the seal is allowed to cool and thermally contract, the seal outer diameter will measure slightly smaller than when initially installed. This diametrical shrinkage and resizing can create leakage problems. For example, the resized seal may leak between the outer diameter of the seal and the seal bore. Leakage may also occur at each of the split joints. This is a particular concern where the shaft is applying load to the sealing lip in an outward direction. The outward load causes the seal to push radially outward and thus, open at the split joint locations. Since the seal has shrunk, the housing bore is no longer in contact with the seal outside boundary and the split joint is allowed to open.
SUMMARY OF THE INVENTION
The present invention overcomes the aforementioned problems by providing a radial lip seal and seal housing which allows the seal to thermally expand and compensate for different heats without substantial physical restriction. When subject to application temperatures and heat, the radial lip seal of the present invention is not restricted from thermally expanding by the seal bore of the seal housing. This nonrestrictive configuration eliminates the shrinkage and resizing problems of the known fluoroplastic radial lip seals. By providing a specifically configured seal outer diameter which freely floats within a compatibly configured seal housing, the seal is free to thermally expand and contract without substantial restriction.
The present invention is generally directed to a radial lip seal for sealably mounting in a seal housing having a circular seal bore and for sealably surrounding a shaft.
Browne Lynne H.
Christie Parker & Hale LLP
Hewitt James M.
Saint-Gobain Performance Plastics Company
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