Abstract:
A torch and mounting system therefor has a cylindrical main glass tube surrounded by a tough and rigid tube. The metallic tube is threaded at each end for receiving compression nuts, each having a ferrule such as an O-ring or a tapered ring inside. Each ferrule is a slip fit over the main glass tube and has an angled surface that butts against the end of the metallic tube. When the nuts are tightened, the sloped surfaces of the ferrules force the ferrules against the main glass tube, thereby fixing it in place within the metal tube. The metal tube in turn is clamped by a mounting clamp which is, in turn, mounted to an adjustment stage for optimum positioning of the torch. A support rail has adjustably mounted thereon one or more strain relieving clamps for holding gas or vapor delivery tubes in place to insure proper delivery of gases and vapors to the torch.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a torch assembly for use in a vapor axial deposition (VAD) process in forming optical fiber glass preforms.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the fabrication of optical fibers, it is customary to create a preform, which is a long glass rod having a central core and which is, in effect, a magnified or enlarged version of the optical fiber to be drawn therefrom. The preform consists of an inner core and an external cladding having an index of refraction profile that reproduces the index profile of the drawn fiber.  
           [0003]    There are three major processes for making preforms, the modified chemical vapor deposition (MCVD) process, the outside vapor deposition (OVD) process, and the vapor axial deposition (VAD) process. All of these processes utilize vapor deposition, in which a “soot” is deposited on the surface of a starting object such as the interior surface of a starter tube or a starting or target rod to form a glassy layer on the surface.  
           [0004]    In the VAD process, silica vapors flowing through a heating torch deposit the desired silica particle layers on the rod. As the sooty particles build up to a desired diameter, the target rod is moved upward to make room for further growth, while the torch or torches are fixed in position. When the build-up or deposition is complete, the rod is removed and the resulting preform is sintered or consolidated into a solid preform.  
           [0005]    Although the VAD process is widely used, in a production milieu, certain problems arise which prevent complete satisfaction with the method, more particularly, the apparatus. Thus, the glass torches, which must be precisely set for satisfactory and reproducible results, tend, over an extended period of use to become leaky, unstable, and not very consistent in producing repeatable results. For example, the various gases and vapors fed to the torch are generally transmitted through plastic tubing to Teflon fittings which, in turn, connect to the glass torch nipples. These Teflon fittings tend to loosen up over extended periods of use, which can, and often does, give rise to leakage, thereby destroying the calibrated delivery of the gases and vapors to the torch. The tendency to loosen is due, at least in part, to temperature fluctuations and also to lack of strain relief in the plastic delivery tubing. Lack of strain relief is a very important problem, since even small mechanical stress on the tubing can loosen the Teflon to glass interface and cause leaks. For example, during routine machine maintenance, the mechanic can accidentally push the tubing and thereby loosen the interface. A further problem, which can actually be more serious in that it can shut down the production line, is that heretofore the mounting of the tubular glass torch to the adjustable mounts, made necessary for precise positioning of the torch, can cause cracking or breaking of the glass of the torch, necessity replacement thereof, which can cause as much as a week&#39;s delay in production. The glass tube of the torch is, generally, clamped to the adjustment stages using a V-block type clamp. By nature, the glass torch is extremely delicate as well as expensive. In use, this type of clamping technique, if too tight a clamp can cause cracking or breakage of the glass and too loose a clamp results in process instability, thus a certain amount of operator skill is required in positioning and clamping the torch.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is a torch for use in, for example, the VAD process and includes a mounting assembly therefor which obviates the drawbacks of the prior art assemblies as discussed in the foregoing.  
           [0007]    In a preferred embodiment of the invention, the glass torch comprises an outer main tubular body within which may be a plurality of coaxial glass tubular bodies of diminishing size for creating a plurality of gas and/or vapor delivery passages. Such nested nozzles are shown, for example, in U.S. Pat. No. 4,627,866 of Kanamori et al., and in accordance with the invention each of the concentric delivery passages is attached to Teflon fittings connected to glass nipples formed on the nested nozzles. The main tubular glass body is mounted within a metal or other rigid and non-fragile type material torch clamp tube and has an outer diameter less than the inside diameter of the torch clamp tube so that there is clearance therebetween. Within the torch clamp tube, at each end thereof, is a tapered or round ring or ferrule made of somewhat resilient and high temperature material, for example, Teflon, whose inner diameter is, preferably, a slip fit over the outer main glass torch tube, and which serve to hold the main glass tube in suspension within the clamp tube. Each end of the clamp tube has external threads formed thereon and first and second compression nuts are screwed thereon. The compression nuts, when tightened, bear against the ferrules to tend to push them longitudinally relative to the clamp tube. The tapered or round, where an O-ring is used, surface of the ferrules bears against the inner surface of the main tube ends causing them to exert an increasing clamping force on the main nozzle body, thereby affixing it concentrically in position within the clamp tube. The stress thus induced in the glass main nozzle of the torch is evenly distributed over an area around the entire torch periphery and is well below critical levels for glass. It has been found that hand tightening the compression nuts is more than adequate to secure the torch firmly within the glass clamp tube. Thus, the danger of too much stress being applied to the torch main tube even during temperature fluctuations, which could produce cracking or breaking, is no longer a consideration. The torch clamp tube, and not the thin glass tube, is used to mount the torch to the adjustment stages. The mounting arrangement of the invention, therefore, is more robust and safe, and provides several other benefits in addition to those just discussed. For example, the ferrule is made of elastic and resilient material such as Viton or Teflon, or other high temperature plastic and acts as a temperature compensating member during expansion and contraction of the glass and the metal. It also acts as a heat isolator allowing the gases within the torch to stay warm; acts as a vibration and shock isolator/damper, thereby protecting the glass torch; and it provides easy rotary and linear (in/out) coarse torch alignment adjustment.  
           [0008]    Another feature of the preferred embodiment of the invention is directed to the mounting system for the torch and the torch clamp tube to the adjustment stages and to inclusion in the assembly of the gas delivery system. The mounting system comprises a torch support rail having tracks or slots machined or extruded therein holding strain relief clamps for the gas delivery tubing. The tracks in the support rail and the clamps make possible both longitudinal and lateral positioning of the strain relief clamps and, therefor, the gas delivery tubing. Each of the concentric torch nozzles has a nipple to which is attached a temperature compensating Teflon fitting, to which a gas delivery tube is connected. Adjustment of the position of each strain relieving clamp relieves its associated tubing of any kinks, sharp bends, or other stress inducing positions, and the Teflon fittings and glass nipples are isolated from the rest of the system by the clamping action of strain relieving clamps.  
           [0009]    At one end (the large torch end) of the rail member is mounted to a support block or bottom clamp having an arcuate section for receiving the torch clamp tube which is clamped therein by a top clamp half, thus holding the torch clamp tube. A second mounting clamp grips the torch clamp tube and is mounted to a mounting stage for coarse longitudinal adjustment of the torch position. At the opposite or rear end of the rail member is a rear mounting tube plate which has a bore therein through which a single, axially aligned supply tube passes to a Teflon fitting and a longitudinally extending torch glass nipple. The bore has a strain relieving clamp surrounding it for clamping the supply tube in position. As will be apparent hereinafter, the strain relieving clamps for the supply tubes are angled in a manner to position the tubes optimally as they lead into the corresponding Teflon fittings. Thus the tubes are fixed in position without inducing any stress on the tubing and fittings, so that leakage at the fittings does not result as the assembly is moved or during machine maintenance.  
           [0010]    As a consequence of the unique assembly construction, uniform clamping pressure is placed on the glass torch body, thereby lessening the occurrence of cracking or breaking; the assembly is robust and stable and much more immune to the effects of continued use, and of temperature fluctuations; the torch mount is in effect temperature compensated, and all glass to pipe fittings are virtually leak proof; and no load is placed upon the glass nipples, thereby eliminating the risk of cracking or torch damage.  
           [0011]    These and other features and advantages of the invention will be more readily apparent from the following detailed description, read in conjunction with the accompanying drawings.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic depiction of a VAD system;  
         [0013]    [0013]FIG. 2 is a perspective view of the VAD torch and mounting assembly of the present invention;  
         [0014]    [0014]FIG. 3 is a cross-sectional end view of an array of glass tubes usable in the torch of the invention;  
         [0015]    [0015]FIG. 4 is a cross-sectional elevation view of a portion of the secondary glass tube arrangement of the torch of the invention;  
         [0016]    [0016]FIG. 5 is a cross-sectional view of the mounting arrangement of the invention for the main glass tube of the torch;  
         [0017]    [0017]FIG. 6 is a cross-sectional elevation view of the typical mounting ferrule of the invention;  
         [0018]    [0018]FIG. 7 is a plan view of the ferrule of FIG. 6;  
         [0019]    [0019]FIG. 8 is a cross-sectional view of a gas or vapor delivery fitting of the invention;  
         [0020]    [0020]FIG. 9 is a perspective view of a side tube strain relief clamp of the invention;  
         [0021]    [0021]FIG. 10 is an elevation view of the right hand version of the clamp of FIG. 9;  
         [0022]    [0022]FIG. 11 is a top plan view of the clamp of FIG. 9;  
         [0023]    [0023]FIG. 12 is a side elevation view of the support rail of the invention; and  
         [0024]    [0024]FIG. 13 is an end view of the support rail of FIG. 12. 
     
    
     DETAILED DESCRIPTION  
       [0025]    [0025]FIG. 1 is a schematic view of a VAD system  11  comprising a starting rod  12 , which may be of silica glass, one end of which is mounted in a chuck (not shown) and rotated as indicated by the arrow. Initially the end of the rod is directly in the flame of one or more oxygen-hydrogen torches  13  and  14 . Vapors from a chemical delivery system  16 , shown in dashed lines, flow into and through the glass torches  13  and  14  where they react via flame hydrolysis to form sub-microscopic particles which are deposited on the starting rod  12 . As the sooty particles build up, the starting rod  12  is moved upward to make room for new growth and continuously rotate to maintain cylindrical symmetry. A position control comprising, for example, a laser  18  and detector  19  may be used to insure proper location of the flame from the torches relative to the starting rods, and, where the torch and rod combination is contained in a housing  21 , a housing exhaust  22  may be, and usually is, included. After a soot preform  17  of proper size is built, it is removed from the chuck, dehydrated, and the preform  17  is consolidated, which removes trapped gases and water vapor, to produce a solid glass preform  17  ready for subsequent operations, such as, for example, rod in tube formation and then drawing into fiber.  
         [0026]    As discussed hereinbefore, during production runs over extended periods of time, numerous problems arise, and it is to the reduction or elimination of these problems that the present invention is addressed. FIG. 2 is a perspective view of the VAD torch  23  of the present invention, shown mounted to an adjusting stage  24  for proper positioning of the torch  23 . Torch  23  comprises a main glass tubular member  26  which, as will be discussed in greater detail hereinafter, is contained in a clamping tube or sleeve  27  of suitable signed and non-fragile material, preferably aluminum, which does not tend to contaminate glass, even at elevated temperatures. As will be apparent hereinafter, the clamping sleeve  27 , which is mounted to the adjusting stage  24  by a two piece clamp having a bottom portion  28  and a top portion  29 , effectively isolates the main glass tubular member  26  from high mounting clamping pressures, thereby protecting it from possible breakage. Each end of the clamping sleeve or the tube is threaded to receive nuts  31  and  32 , the function of which will be discussed hereinafter.  
         [0027]    An elongated support rail  33  is mounted to the clamping sleeve  27  by a two part support clamp comprising a bottom portion  34  and a top portion  36  and functions to support, a plurality of strain relieving side tube clamps  37 , which hold the numerous gas and vapor delivery tubes, such as tube  38 , without stressing or other undue distortion, with both transverse and longitudinal adjustment.  
         [0028]    In the VAD process, it is usual that several gases, such as oxygen and hydrogen (for the burner flame), and several soot or vapor mixtures are applied through the torch  23  to the starting rod  12  and the preform  17  during the deposition operation. To this end, there is a plurality of nested secondary glass tubes  39 ,  41 ,  42 ,  43 ,  44 ,  46 , and  47 . Main glass tubular member  26  and each of the secondary tubes has a vapor (or gas) delivery nipple,  48 ,  49 ,  51 ,  52 ,  53 ,  54 ,  56 , and  57  respectively, to which is mounted a temperature compensating fitting to which are attached the gas or vapor delivery tubes  38 , only one of which is shown. The fittings form substantially leak proof connections to the nipples. This nesting of the secondary tubes as shown in FIG. 3 is shown and explained in U.S. Pat. No. 4,627,866 of Kanamari et al. FIG. 4 is a cross-sectional diagram of a portion of the nested tubes of FIGS. 2 and 3, demonstrating how each tube supports the next outer tube and provides vapor or gas annular delivery nozzles  68 ,  69 , and  71 . Each pair of adjacent rings forms such an annular delivery nozzle to which the respective nipples, integral with its tube, are connected. The smallest diameter secondary tube  47  is its own nipple  57 , and is connected to an in line temperature compensating fitting  67 .  
         [0029]    [0029]FIG. 5 is a cross-sectional view of the mounting arrangement for the main glass tubular member  26  of the torch of the invention. As can be seen in FIG. 5, the clamping sleeve or tube  27 , which is preferably made of aluminum, has an inside diameter that is greater than the outside diameter of the tubular member  26 . Sleeve  27  is, as discussed in connection with FIG. 2, mounted to adjusting stage  24  by means of a clamp which comprises a bottom portion  28  and a top portion  29  (not shown). Also, support rail  33  is mounted or affixed to sleeve  27  by bottom portion  34  and top portion  36  of a clamp member. A locating pin  72  which fits within a hole  73  in sleeve  27  serves to maintain sleeve  27  and support rail  33  in fixed relationship. At each end of sleeve  27  is a compressible mounting ferrule  74  and  76 , each having at least a portion of its outside diameter tapered, as best seen in FIGS. 5 and 6, it is also possible to use O-rings and to taper the inside lip of the sleeve  27 . Each of ferrules  74  and  76  is preferably made of Teflon or Viton and is a light slip fit on main tubular member  26  and the tapered surface thereof bears against the corresponding end of sleeve  27 . The outside surface of sleeve  27  at each end thereof is threaded to receive a compression member such as threaded nut  31  and  32  the inner end of which bears against the rear surface of mounting ferrule  74  or  76 , as seen in FIG. 5. Nuts  31  and  32  may be made of suitable material such as aluminum or brass. As the nut  31  or  32  is tightened, the tapered surface of the corresponding mounting ferrule  74  or  76  is forced against the corresponding end of sleeve  27 , thereby applying a radial inward pressure on tubular member  26 . As pointed out hereinbefore, hand tightening of the nuts  31  and  32  is sufficient to cause tubular member  26  to be firmly held within sleeve  27 , without the danger of cracking or breaking. In addition, the thermal expansion and mechanical properties of the ferrules cause them to act as a temperature compensating member between the aluminum sleeve and the glass tube. Since the coefficient of thermal expansion of glass is ˜3-6 10-6  in/in/° F., the Teflon is ˜47-55 10-6  in/in/° F. and brass/aluminum is ˜9-13 10-6  in/in/° F., one can see that the Teflon will expand the most. Since aluminum is on the outside, it will limit the expansion and the Teflon will be forced toward the glass. Since it is ductile, once against the glass, it expands axially, thus maintaining a tight connection.  
         [0030]    A similar clamping arrangement is subject to temperature fluctuations, such as Teflon fitting  67 , as shown in FIG. 8, which is a cross-sectional view of the fitting  67 . Inasmuch as both ends of the fitting are substantially identical, corresponding parts at each end bear the same reference numerals. Fitting  67  is mounted on nipple  57  of secondary tube  47 , and has a bore  77  extending therethrough and external threads  78  at the nipple receiving end thereof and at the gas delivery tube  38  end. A mounting ferrule  82 , which is virtually a miniature version of tapered ferrule  76 , has a sloped or angled surface  83  adapted to bear against a sloped surface  84  of bore  77  when the ferrule  82  is mounted on nipple  57 , as shown. A compression nut  86  screws onto the threaded portion of bore  77  and the end thereof bears against the back of ferrule  82 , as shown. During operation, fitting  67  is subjected to wide swings in temperature which will cause it, over time, to loosen on nipple  57 , thereby creating possible leakage. Teflon expands with heat more than glass, and this differential expansion can cause the leakage, as well as altering the grip of ferrule  82  on nipple  57 . In such case, compression nut  86  can be forced against the rear of ferrule  82 , and the angled surfaces  83  and  84  will force ferrule  82  into tighter gripping engagement with nipple  57  sufficiently to eliminate the leaks. Bolt  86  is preferably made of a material such as stainless steel, which expands less with temperature than does Teflon, thus, when screwed into contact with ferrule  82  to force a tighter grip on nipple  57 , it tends to maintain the grip because of the lesser expansion change of the bolt  86 . In the same manner, gas delivery tube  38  is retained in fitting  67 , and it is to be understood that other arrangements for preventing leakage might also be used.  
         [0031]    The same leakage prevention scheme may be used with the other fittings  58 ,  61 - 64 , and  66 , however, these fittings are not heated and are less prone to developing leaks inasmuch as they are not subjected, in the configuration shown in FIG. 2, to temperatures fluctuations.  
         [0032]    [0032]FIG. 9 is a perspective view of a side tube strain relief clamp  37  and FIG. 10 is an elevation view thereof. As pointed out hereinbefore, clamp  37  is designed to position and hold a gas or vapor delivery tube (such as tub e  38 ) so that there is no stress in the Teflon tubing between the strain relief and the Teflon fitting, thereby insuring a smooth vapor (or gas) delivery to the associated nozzle. Also, the second function of the strain relief is to isolate and protect the glass torch nipple from accidental Teflon tubing movement during torch adjustment or machine maintenance, which could potentially cause nipple cracking or breakage. To this end, clamp  37  comprises a plate member  87  having first and second elongated slots  88  and  89  therein, and an extension  91  upon which is mounted as by welding, preferably at a 45° angle, as shown in FIG. 11, a split C-clamp collar  92 . C-clamp collar  92  is designed to grip a gas or vapor delivery tube, such as tube  38  shown in FIG. 2, and has an adjusting clamp bolt  93  for optimum gripping force. Plate  87  and clamp  92  are preferably made of stainless steel, although other suitable materials might readily be used.  
         [0033]    As shown in FIG. 2, the clamps  37  are mounted on support rail  33 , which is shown in greater detail in FIGS. 12 and 13, of which FIG. 12 is a side elevation view and FIG. 13 is an end elevation view thereof. As best seen in FIG. 13, rail  33  comprises three X-shaped sections which form three top tracks  94  and three bottom tracks  96 , which extend the length of rail  33 . The mounting for clamp  37  on support rail  33  is shown in FIG. 13 and comprises a bolt  97  which extends through one of the slots  88  or  89 , with its head  99  riding in and free to move in an enlarged section of a track  96 , and having an adjustable nut  98  for clamping plate  87  to rail  33  by preventing further movement of the bolt  97  in track  96 . It can be appreciated that slots  88  and  89  permit a great deal of lateral movement of clamp  37  when bolt  97  is loosened, as well as longitudinal movement along track  33 . After the desired lateral and longitudinal positions of clamp  37  are realized, bolt  97  is tightened by nut  98  and clamp  37  is firmly fixed in place. A certain amount of angular positioning of clamp  37  is also realizable when bolt  97  is loosened.  
         [0034]    From the foregoing description of the preferred embodiment of the invention, it can be seen that the torch and mounting arrangement of the invention inherently prevents the application of excessive clamping forces on the glass torch body; that the assembly is both robust and stable and hence less subject to the affects of continued use and excessive temperatures, as well as temperature fluctuations; and that all glass to pipe fittings are virtually leak proof, and not subject to breakage due to accidental Teflon tubing movement.  
         [0035]    It is to be understood that the various features of the present invention, as set forth, might be incorporated into other types of torch arrangements and that other modifications and/or adaptations might occur to workers in the art. All such variations and/or modifications are intended to be included herein as being within the scope of the present invention as set forth. Further, in the claims hereinafter, 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 in combination with other elements as specifically claimed.