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Publication Date: 2014-Nov-21
Document File: 10 page(s) / 1M

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The Prior Art Database

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I.          Background

A ferrule is utilized to provide a fluidic seal between two interfaces, such as to make a fluid-tight junction between two conduits.  A ferrule is commonly required in analytical environments such as gas chromatographs as well as in other operating environments.  Achieving a leak-free seal is necessary to prevent air contamination and loss of sample when analyzing a substance at a user-specified concentration.  In a typical application, a column, tube or needle is inserted into a union or other type of fitting of an instrument, and the ferrule is swaged to permanently deform to the outer diameter of the column and make an air-tight seal between the column, tube or needle and the contacting interface of the union or fitting. 

An example of the use of ferrules to make fluid-tight seals is shown in Figure 1.  Each ferrule has a conical outer surface that is brought into contact with a conical inner surface of the union.  On either side of the union, a nut is threaded into the union, axially displacing the ferrule into bearing contact with the inner surface of the union, creating fluid-tight interfaces whereby fluid conducted from the first column, tube or needle can be transferred into the second column, tube or needle without any appreciable loss of fluid in the union. 

With the increasing variety in union or fitting designs, standard ferrules are sometimes insufficient in providing a seal or meeting other customer needs.  Geometric compatibility of the ferrules and unions, and the resulting contact points between the two, are crucial in optimizing the sealing capability.

Figure 2 is a cross-sectional view of a typical implementation in which a ferrule of known design and standard size (0.32 mm inside diameter) is swaged to a needle in a typical union.  The narrow cone angle of the ferrule, in comparison to the angle of the hole in the union, creates poor contact regions for a fluidic seal.  Because such "seals" still tend to leak after torqueing to the appropriate degree of swage, the nut is often over-tightened by the user in an attempt to form a tighter seal.  However, over-tightening causes the ferrule to be over-swaged.  This can damage threads of the fitting or nut, deform the ferrule to the point of it getting stuck in the fitting, and damage the torque tools.

An over-sized hole in the fitting for the standard ferrule causes the ferrule to be extruded into the union cavity as the nut is tightened.  Figure 3 is a photograph showing an example of a ferrule with an extruded tip, which takes the shape of the internal surface of the union.  When it is time to remove the needle from the union, the deformed surface of the ferrule causes it to become stuck, making it very difficult to withdraw from the fitting. 

A ferrule having the current geometries as described above was evaluated by performing finite element analysis (FEA) using FEA model...