Abstract:
A sealing system for use in a chemical or gaseous delivery system has a delivery tube connected to a source of the material to be delivered and which intrudes into the revolving region where the material is to go. The sealing system has a secondary sealing nut affixed to the delivery end of the tube, and a primary sealing nut mounted on the secondary nut with a sealing member therebetween. When the region revolves, the two nuts are automatically tightened to hold the sealing member in firm and positive engagement with the housing surrounding the region.

Description:
RELATED APPLICATIONS 
     This invention is related to that shown in U.S. patent application Ser. No. 09/353,943 of Mueller et al., filed Jul. 15, 1999, and to U.S. patent application Ser. No. 9/383,780 of Mueller filed concurrently herewith. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a sealing apparatus for chemical delivery systems and, more particularly to the process of introducing materials into the interior of tubular members, such as glass starter tubes used in making optical fiber pre-forms. 
     BACKGROUND OF THE INVENTION 
     While the following discussion deals with starter tubes and sealing arrangements for optical fiber pre-forms, it is to be understood that principles of the present invention are applicable to other, different applications involving, generally, chemical delivery systems. For example, the sealing arrangement of the invention is usable with the starter tube sealed vacuum system for collapsing the tube into the desired pre-form, as is shown in the aforementioned Mueller et al. application. 
     Optical fiber of the type used to carry optical signals is fabricated typically by heating and drawing a portion of an optical pre-form comprising a refractive core surrounded by a protective glass cladding. Presently, there are several known processes for fabricating pre-forms. The modified chemical vapor disposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027 issued in the names of J. B. MacChensey et al. on Aug. 12, 1980 and assigned to Bell Laboratories, Inc. has been found to be one of the most useful because the process enables large scale production of pre-forms which yield very low loss optical fiber. 
     During the fabrication of pre-forms by the MCVD process, reactant-containing gases, such as SiCL 4  are passed through a rotating substrate tube which is made of silica glass. A torch heats the tube from the outside as the precursor gases are passed therethrough, causing deposition of submicron-sized glass particles on the inside surface of the tube. The torch is moved along the longitudinal axis of the tube in a plurality of passes to build up layer upon layer of glass to provide a pre-form tube. Once a sufficient number of layers have been deposited, the pre-form tube is then heated to cause it to be collapsed to yield a pre-form or pre-form rod as it is often called. The delivery system of the reactant gases to the starter tube interior is generally through a fixed metallic hollow tube connected to the source or sources of the gases. It is necessary that the space between the exterior surface of the delivery tube and the interior surface of the glass starter tube be sealed so that the critical gases do not leak out of the starter tube. 
     In the current method of manufacture, the apparatus which ensures sealed delivery of the deposition chemicals in the gases is a combination of a rotary union element, a structure for holding and sealing the starter tube, and a secondary face seal assembly for routing of purge gases through the structure. This is a complex apparatus that requires frequent maintenance. Existing systems also have the disadvantage of having inherently larger cavities for the accumulation of dead zones of flow, and a tendency to create particle contamination from the rotary union and face seal system. Inasmuch as the chemical delivery system is stationary, the current means of achieving delivery is via the rotary union, featuring a transition of the chemicals from a stationary pipe to a rotary pipe or to the inside of a supply coupling. The chemicals being delivered are at a pressure greater than atmospheric, and the face seal properties are the only restriction to the release of the chemicals to the atmosphere. The rotary union and secondary face seals generate a large quantity of particles from wear, and contribute to the contamination of the coupling. As the seals wear down, excessive leakage occurs both to the atmosphere and into the product (starter tube), resulting in lost product and requiring system maintenance. The complexity of the components involved requires skilled maintenance being performed using requalification through test of the system. Both material and labor costs are, consequently, high. 
     SUMMARY OF THE INVENTION 
     The present invention embodies an inventive sealing arrangement for introducing chemicals into the starter tube. 
     In more detail, the chemicals usually in the form of gases, are delivered into the starter tube by means of a stationary piping system. The rotary union is dispensed with and the delivery tube intruded into the interior of the starter tube. A stationary seal mounting member or hub is attached to the distal, open end of the delivery tube, preferably being threaded thereon, and forms the seal mounting point. A sealing member, sized to fit within the interior of the starter tube and forming a seal with the interior wall thereof is placed on the mounting member and secured thereto by a self-tightening fastener which is threaded onto the hub. The seal between the atmosphere and the interior of the starter tube is maintained by the physical properties of the seal and the deformation of the seal on the mounting member caused by the fastener. In operation, the seal fastener is constantly tightened by the rotation of the starter tube which, in turn, due to the friction between the seal and the inner wall of the starter tube, applies to the seal a torque which is transmitted to the seal fastener, causing it to be continuously tightened. The only wear surface is the contact between the seal and the inside wall of the glass starter tube, which is relatively minor in nature. Any particles generated by this interface wear tend to migrate to the atmosphere instead of into the glass tube inasmuch as the chemical gas pressure within the glass tube is greater than atmospheric. When wear does reach the point that the doping operation is compromised, the seal fastener is unscrewed and the seal discarded, to be replaced by a new seal. 
     In a second embodiment of the invention, the seal and fastener are combined into a single unit, which can be made completely of the seal material, such as Teflon® for example, and the entire unit discarded when leakage or wear becomes a problem. The mounting hub can be made integral with the end of the delivery tube and threaded to receive the fastener or the combined seal and fastener. 
     In still another embodiment of the invention, the seal can be made integral with the hub which is threaded to receive the fastener. In this case, the hub is threaded onto the end of the delivery tube, and the end is threaded to receive the fastener. 
     The numerous features and advantages of the present invention will be clearly presented in the following detailed description, read in conjunction with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of the apparatus for introducing gaseous elements into a starter tube and using the sealing arrangement of the present invention; 
     FIG. 2 is a perspective view of the seal forming elements of the present invention as assembled; 
     FIG. 3 is a front elevation view of the primary sealing nut of the invention; 
     FIG. 4 is a cross-sectional view along the line A—A of FIG. 3; 
     FIG. 5 is a front elevation view of the secondary sealing nut of the invention; 
     FIG. 6 is a plan view of the secondary sealing nut of FIG. 5; 
     FIG. 7 is a cross-sectional view along the line A—A of FIG. 5; 
     FIG. 8 is a cross-sectional view of the sealing member of the invention and of an optional resilient member for augmenting the resilient sealing members action; 
     FIG. 9 is a perspective, partially sectional view of a first embodiment of the invention; 
     FIG. 10 is a perspective, partially sectional view of a second embodiment of the invention; and 
     FIG. 11 is a perspective, partially sectional view of another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     In the fabrication of the pre-form for drawing clad glass fibers, generally, gas plane precursor reactants together with oxygen are introduced into a glass tube, i.e., a starter tube, in a constantly moving stream. The tube and its contents are heated to a homogenous reaction temperature within a moving hot zone produced by a moving heating element which continuously traverses the outside surface of the tube. Homogeneously produced glass particles, commonly called “soot” collect on the tube walls, and are fused into a continuous layer within the moving hot zone. In the modified vapor deposition (MCVD) process shown and described in the aforementioned MacChesney et al U.S. Pat. No. 4,217,027, the tube is continuously rotated about its longitudinal axis. 
     FIG. 1 is a diagrammatic view of the apparatus for preparing preforms as just discussed, and showing the sealing arrangement of the present invention. A lathe  11  has axially disposed between a tailstock  12  and a headstock  13  a starter tube  14 . Tube  14  is mounted in an arbor  16  having a cylindrical or hollow shaft  17  and which is driven by means not shown, but generally contained within the headstock, to rotate tube  14  in the direction of the arrow. It is to be understood that rotation may be clockwise or counter-clockwise. For the following discussion, it will be assumed that the direction of rotation is counter-clockwise as viewed from tailstock  12  toward headstock  13 . Within the arbor shaft  17  is a stationary delivery tube  18 , the stationary mounting arrangement for the tube being shown diagrammatically as mounting element  19 , which may take any of a number of forms. Delivery tube  18  is shown with a protruding end  21  which is coupled by a conduit  22  to a source  23  of the gaseous materials. Tube  18  intrudes into the end of starter tube  14  and has, at its intruding end  24  the sealing arrangement  26  of the invention, to be discussed more fully hereinafter. Sealing arrangement  26  is, in accordance with the invention, self tightening, i.e., as tube  14  rotates, it tightens the sealing arrangement  26  to insure maintenance of a seal with tube  14  that prevents the gases in tube  14  from leaking out. A heat or flame source  27  is movably mounted within lathe  11  for back-and-forth traversal of the length of tube  14 , as discussed hereinbefore, and as indicated by the arrows. 
     FIG. 2 is a perspective view of the self-tightening sealing arrangement  26  of the invention in one embodiment thereof, and FIGS. 3 through 8 are detailed views of the basic components thereof. As shown in FIG. 2, the sealing arrangement  26  comprises three basic parts: a conically shaped sealing member  28  which is made of any of a number of suitable materials characterized by long wear and flexibility. Such materials can be, for example, hard rubber, various plastics or material such as Teflon®, which has been found to make excellent seals. Sealing member  28  is clamped between a primary sealing nut  29  and a secondary sealing nut  31  which is mounted onto the intrusive end  24  of delivery tube  18 , by any of a number of suitable means. As shown in the figures, the secondary nut is threaded for mounting on the delivery tube  18 . However, other mounting arrangements are possible, such as a bayonet lock, or nut  31  may be a press fit on the end of delivery tube  18 . In the case of threads or bayonet lock, the direction of rotation of nut  31  for tightening is the same direction of rotation as starter tube  14 . In a similar manner, primary nut  29  is mounted to secondary nut  31  by threads or, for example, bayonet lock, with the tightening direction of rotation being the same as the direction of rotation of starter tube  14 . When primary nut  29  is mounted on secondary nut  31 , sealing member  28  is captured therebetween and held firmly in place with the nuts  29  and  31  being continuously subjected to tightening torque. 
     In operation, the sealing member  28  which is stationary bears against the interior wall of starter tube  14 , which is rotating. Thus, the friction between the two tends to impart a torque to the member  28  in a counter-clockwise direction, when the rotation is as shown in FIG.  1 . Thus, both nuts  29  and  31  are subjected to the same torque in that they grip member  28  tightly. This torque is in the direction to tighten nut  31  on tube  18 , and to tighten nut  29  on nut  31 . Hence, there is no danger of the sealing arrangement  26  working loose during operation. 
     FIG. 3 is a front elevation view of primary seal nut  29  and FIG. 4 is a cross-section thereof along the line A—A of FIG.  3 . Nut  29 , which may be made of stainless steel, for example, plastic, or the same material as sealing member  28  has a pair of flats  32 , 33  on the exterior thereof for initially tightening nut  29  on nut  31  and has an interior bore  34  extending therethrough to allow passage of the gaseous mixture into starter tube  14 . As seen in FIG. 4, a portion  36  of the length of the bore  34  is threaded for mounting nut  29  onto nut  31 . As pointed out hereinbefore, means other than threads may be used for mounting nut  29  provided that the direction of rotation for tightening corresponds to the direction of rotation of the starter tube  14 . In the case of the threads  36  shown in FIG. 4, and the counter-clockwise direction of rotation of the tube  14 , the threads  36  will be lefthanded threads. The rear shoulder  37  of nut  29  bears against the hub  38  of conical sealing member  28 , which is shown in cross-section in FIG.  8 . On the rear shoulder  37  is a circular ridge  35  which, when forced against the hub  38  of sealing member  28 , tends to distort it slightly, which in turn causes some expansion of the diameter of the outer edge of the seal and increases the sealing effect. Other configurations, such as radial ridges might also be used. Also shown in FIG. 8 is a conical resilient member  40 , which may be a leaf spring which applies restoring force to member  28 . 
     FIG. 5 is a front elevation view of the secondary sealing nut  31 , FIG. 6 is a plan view thereof, and FIG. 7 is a cross-sectional thereof along the line A—A of FIG.  5 . Nut  31  has an axial bore  39  extending therethrough with a threaded portion  41  intermediate the ends of nut  31  for mounting the nut  31  on the end of the delivery tube  18 . As is the case with nut  29 , other mounting means, including but not limited to bayonet lock or press fit, may be used, so long as the aforementioned torque from sealing member  28  is in the direction to tighten nut  31  or tube  18 . Nut  31  also has an extended portion  42  having threads  43  on the outside thereof for receiving nut  29 . The rear of hub  38  of sealing member  28  butts up against a shoulder  44  formed on portion  42  so that sealing member  28  is sandwiched between the two nuts  29  and  31 . 
     As is the case with nut  29 , nut  31  has a pair of flats  46  and  47  to facilitate initial tightening of the nut. 
     The elements of the sealing arrangement  26  as depicted in FIGS. 3 through 8 are shown assembled and in use in FIG. 9, which is a perspective, partially sectional view of the assembly in a first embodiment of the invention. It can be seen that the conical sealing member  28  is sandwiched between the shoulders  37  and  44  of nuts  29  and  31  respectively, as discussed hereinbefore. It can also be seen that sealing member  28  is slightly deformed where it contacts the inner surface of the starter tube  14 . As pointed out hereinbefore, as tube  14  rotates, nuts  29  and  31  are tightened, thereby squeezing sealing member  28  therebetween. As member  28  is so squeezed, it tends to flatten slightly, thereby increasing its diameter and insuring sealing contact with the inner wall of tube  14 . 
     During prolonged use, there will naturally be some wear of the periphery of member  28  inasmuch as it remains stationary while tube  14  revolves. The squeezing action will act to some extent to prolong the sealing action, but, eventually, member  28  will have to be discarded and replaced. Because of the extremely simple construction of the sealing arrangement, replacing member  28  can be accomplished in a matter of minutes. 
     FIG. 10 is a perspective, partially sectional view of a second embodiment of the invention wherein nut  29  and sealing member  28  are made of the same material and are integral with each other. As pointed out hereinbefore, Teflon® is an excellent material for use in such a construction, having excellent sealing properties and strength. When wear of member  28  requires replacement, the entire nut  29  sealing member  28  integral combination can be quickly removed and discarded, and a new nut  29  with integral sealing member  28  mounted onto secondary nut  31 . 
     FIG. 11 is a view similar to those of FIGS. 9 and 10 of still anther embodiment of the invention, wherein sealing member  28  and secondary nut  31  are made of the same material, e.g., Teflon® and are integral with each other. In this case replacement of the sealing member  28  requires removal of nut  29  before nut  31  can be removed and discarded. Also, in this embodiment, it is not desirable for nut  31  to be a press fit on tube  18  inasmuch as removal would be too difficult. 
     The invention, by virtue of the seal and nest arrangement being self tightening in operation, is extremely simple and economical compared to prior art arrangements and the seal is quickly and easily replaced. Because the parts are simple, discarding one or more of them in case of excessive wear or leaks due to other causes, is a minor economic factor. The added complications of rotary union and two or more sealing arrangements are dispensed with. 
     The invention has been shown and described as it is used with an apparatus for delivering gas mixtures to optical fiber starter tubes. It should be readily apparent that the invention is amenable to a wide variety of air, gas, or chemical delivery systems wherein sealing is an important factor. Because the sealing member itself is readily replaceable, and can be sized to fit a wide range of internal diameters, the adaptability of the arrangement of the invention to other types of delivery systems is unique. 
     In conclusion, it should be noted that it will be obvious to those skilled in the art that many variations and modifications may be made to the preferred embodiment or embodiments without departure from the principles of the present invention. For example, whenever threads or bayonet locks are used, it might be feasible to use O-rings to enhance the sealing. All such variations and modifications are intended to be included herein as being within the scope of the present invention. Further, in the claims hereafter, the corresponding structures, materials, acts, and equivalents of all means or step plus function elements are intended to include any structure, material, or acts for performing the functions with other specifically claimed elements.