Abstract:
A method of forming a brazed joint between an armature bar and a hydraulic header clip including the steps of: locating ends of a plurality of hollow strands and a plurality of solid strands within a cavity in an end fitting such that free ends of said hollow strands extend axially beyond free ends of said solid strands; and pre-placing an essentially phosphorous-free silver braze alloy around and between said ends of said hollow strands and said solid strands such that said braze alloy extends axially beyond the free ends of said solid strands.

Description:
CROSS RELATED APPLICATIONS  
       [0001]     This application is divisional of application Ser. No. 10/991,371 (&#39;371 application) filed Nov. 19, 2004, which is incorporated by reference herein. This application is related to commonly owned U.S. patent application Ser. No. 10/991,416, (U.S. Pat. No. 7,199,338) entitled “Method And System For Applying An Isolation Layer To A Brazed End Of A Generator Armature Winding Bar”, and U.S. patent application Ser. No. 10/991,501, entitled “Braze Chamber And Method For Applying An Isolation Layer To A Brazed End Of A Generator Armature Winding Bar” which were filed contemporaneously with the &#39;371 application. These related applications are incorporated by reference herein.  
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to brazing generator armature winding bars to hydraulic header clips, and to a method for sealing an armature winding bar to its header clips to prevent or reduce corrosion due to coolant water flowing through the end fitting and the armature winding bar.  
         [0003]     The armature windings on large steam-turbine generators are generally water-cooled. The armature windings comprise an arrangement of half coils or armature bars (collectively referred to as “armature bars” or “bars”) connected at each end through copper or stainless steel fittings and water-cooled connections to form continuous hydraulic winding circuits.  
         [0004]     Water-cooled armature winding bars are comprised of a plurality of small rectangular solid and hollow copper strands arranged to form a bar. The rectangular copper strands are generally arranged in rectangular bundles. The hollow strands each have an internal duct for conducting coolant through the bar. The ends of the strands are each brazed to a respective hydraulic header clip. The hydraulic header clip serves as both an electrical and a cooling flow connection for the armature winding bar.  
         [0005]     The hydraulic header clip is a hollow connector that includes an enclosed chamber for ingress or egress of a cooling liquid, typically deionized water. At one open end, the clip encloses the ends of the copper strands of the armature winding bar. A braze alloy bonds the end sections of the strands to each other and to the hydraulic header clip. The braze joints between adjacent strand ends and between the strand ends and the clip should retain hydraulic and electrical integrity for the expected lifetime of the winding. A typical life time of a winding is on the order of tens of years.  
         [0006]     Internal surfaces of the brazed joints between the clip and the ends of the strands are constantly exposed to the deionized, oxygenated water flowing through the clip and the hollow strands. The exposure of the brazed surfaces to the coolant can result in corrosion of the armature winding bar and hydraulic header clip. Corrosion tends to occur in the crevices of the joints between the hydraulic header clip and the strand ends of the armature bar, and in the crevices between the strand ends. Corrosion of a phosphorous-containing braze alloy and adjoining copper strand surfaces can occur if critical crevice geometry and crevice water chemistry conditions are present. Certain conditions promote crevice corrosion in the braze joints, such as: phosphorous, copper, suitable corrosion initiation sites and water. If any one of these conditions is eliminated from the clip to bar joints, crevice corrosion should be reduced or eliminated.  
         [0007]     The corrosion process can initiate if the braze joint surfaces contain surface crevices, pinholes, or porosity at or near the surface of the joint and the critical water chemistry conditions that support corrosion. The corrosion process can progress through the braze joints especially when critical crevice geometry and water chemistry conditions exist. Porosity within the braze joints can accelerate corrosion. If allowed to progress through a joint, corrosion will eventually result in a water leak through the entire effective braze joint length and compromise the hydraulic integrity of the clip-to-strand joint. Accordingly, there is a long felt need for a corrosion-resistant clip-to-strand braze joint. The benefits of crevice corrosion-resistant braze joint are expected to include improved generator availability and generator reliability.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0008]     A non-crevice-corroding clip-to-strand braze joint has been developed using a silver based braze alloy that is essentially phosphorous-free. A method to braze the joint and a brazing chamber assembly has also been developed. In preparation for brazing, strips of braze alloy are interleaved between tiered rows of the copper strands such that the strips extend beyond rows of short solid strands but not beyond the free ends of longer hollow strands. During induction heating, the braze alloy is briefly heated to above its liquidus temperature such that the alloy pools on the solid strand ends and in crevices between the strands and the internal surfaces of the hydraulic header clip. The pooled alloy when cooled forms a layer of braze alloy that isolates the solid strand ends, the joints between strand ends and the joints between strand ends and the clip from the coolant passage in the clip.  
         [0009]     The brazing chamber includes a split hood that when closed and purged has an essentially oxygen free atmosphere. The armature bar is mounted vertically in the chamber such that the free ends of the copper strands are horizontal to allow liquid braze alloy to pool on the solid free ends. A cooled heat sink clamps the bar just below the hydraulic clip to chill the bar and solidify braze alloy flowing down between the strands. A hooked induction coil in the chamber heats the clip, strand ends and strips of braze alloy. A mechanical ram compresses the clip, strand ends and braze strips together during the brazing process in the chamber.  
         [0010]     The invention may be embodied as a brazed joint between an armature winding bar strand package and a hydraulic header clip end fitting comprising: a plurality of solid strands and a plurality of hollow strands arranged in a tiered array and forming the strand package, said plurality of hollow strands having free ends that extend axially beyond corresponding free ends of said solid strands; a cavity in the end fitting, said free ends of said plurality of hollow strands and said corresponding free ends of said solid strands extending to said cavity, and an essentially phosphorous-free silver braze alloy joining said free ends of said plurality of hollow strands and said corresponding free ends of said plurality of solid strands to each other and to interior surfaces of said end fitting, wherein said braze alloy forms an isolation layer over the free ends of said solid strands.  
         [0011]     Further, the invention may be embodied as a brazed joint between an armature bar and a hydraulic header clip comprising: a cavity in the end fitting, accessed by an opening; an array of solid and hollow strands received in said opening and arranged in a tiered array; and an essentially phosphorous-free braze alloy joining said solid and hollow strands to each other and to internal surfaces of said end fitting, said braze alloy covering free ends of said solid strands and leaving free ends of said hollow strands open and unobstructed.  
         [0012]     The invention may also be embodied as a method of forming a brazed joint between an armature bar and a hydraulic header clip comprising: locating ends of a plurality of hollow strands and a plurality of solid strands within a cavity in an end fitting such that free ends of said hollow strands extend axially beyond free ends of said solid strands; and pre-placing an essentially phosphorous-free silver braze alloy around and between said ends of said hollow strands and said solid strands such that said braze alloy extends axially beyond the free ends of said solid strands.  
         [0013]     The invention may also be embodied as a pre-braze assembly of an armature bar strand package and a hydraulic header clip comprising: a plurality of hollow and solid strands arranged in a tiered array and forming the strand package; a cavity in the end fitting, free ends of said plurality of strands extending through said opening and received in said cavity; and an essentially phosphorous-free braze alloy interleaved between the strands and extending beyond said free ends of said plurality of solid strands.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic illustration of a liquid-cooled stator winding arrangement illustrating the armature bars and hydraulic header clips coupled to inlet and outlet coolant headers.  
         [0015]      FIG. 2  is a perspective view of the end of an armature winding bar showing the tiered rows of hollow and solid strands, and interleaving sheets of braze material.  
         [0016]      FIG. 3  is a perspective exploded view of the end of an armature winding bar inserted into a hydraulic header clip, with braze material and a clip cover shown to the side of the clip.  
         [0017]      FIG. 4  is an end view of the strands of an armature winding bar within a hydraulic header end clip with a ram clamping the cover to the clip and a heat sink attached to the bar.  
         [0018]      FIG. 5  is a side view of the winding bar, end clip and ram shown in a cross-section taken along line  5 - 5  in  FIG. 4 .  
         [0019]      FIG. 6  is a perspective side view of a brazing chamber.  
         [0020]      FIG. 7  is an enlarged view of the interior of the brazing chamber that shows an induction heating coil and armature winding bar heat sink.  
         [0021]      FIG. 8  is a flow chart of an exemplary braze process.  
         [0022]      FIGS. 9 and 10  are end and cross-sectional side views respectively of the hydraulic header clip brazed to an armature bar.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]      FIG. 1  illustrates a liquid-cooled armature winding arrangement for a stator in a typical liquid-cooled generator. A stator core  10  having stator core flanges  12  and core ribs  14 . Armature winding bars  16  (also referred to as stator bars) pass through radially extending slots in the stator core and are capped at opposite ends by hydraulic header clips  18  fitted to the ends of the bars. Inlet hoses  22  connect an inlet clip  18  to an inlet coolant header  24 . Outlet hoses  26  connect an outlet clip  18  to an outlet coolant header  28 . A copper or stainless steel fittings  20  connect adjacent ends of pairs of armature bars and clips to form complete armature coil elements.  
         [0024]      FIG. 2  is a perspective end view of an armature winding  16  bar without a hydraulic header clip. The bar is a rectangular array of solid  34  and hollow  36  copper strands.  FIG. 3  is a perspective view of the armature winding bar  16  inserted in a clip  18  with braze strips  30  and a braze sheet  50  and a clip cover  32  shown to the side of the clip. In  FIG. 2 , the braze strips  30  are shown interleaved between tiered rows of solid the copper strands  34  and rows of hollow strands  36  of the bar  16 .  
         [0025]     Each armature winding bar  16  includes a plurality of solid copper strands  34  and hollow copper strands  36 . The strands  34 ,  36  may also be constructed of metals other than copper, such as copper-nickel alloys or stainless steel. The ends of the strands  34 ,  36  form the end of the armature winding bar  16 . The free ends of the hollow strands  36  (and optionally some of the ends of the solid strands) extend axially beyond the free ends of short solid strands  34 . For example, the free ends of the hollow strands extend approximately 0.31 inch (10 to 500 mils) beyond the free ends of the solid strands.  
         [0026]     In the armature winding bar  16  shown in  FIGS. 2 and 3 , the extended hollow strands  36  form tiered rows with respect to the shorter rows of solid strands  34 . A four-tier array is shown in  FIG. 2 . It will be appreciated that various numbers of tiers are possible in an armature bar. The particular configuration of solid strands  34  and hollow strands  36  within the armature winding bar  16  is a matter of design choice. There may be a one to one ratio of solid to hollow strands or a ratio of 6 solid strands to one hollow strand. The ratio may be greater or smaller depending on the capability of the bar design to remove heat during generator operation.  
         [0027]     Braze alloy strips  30  and sheets  50  of a rolled, essentially phosphorous-free, silver based braze alloy are placed between the tiers of strands and between the strands and the internal surfaces of the hydraulic header clip  18 . The silver braze alloy of the strips  30  and sheets  50  may contain other elements, such as tin, zinc or nickel, that can result in solidus and liquidus modifications to suit specific applications. The thickness of the alloy strips  30  and sheets  50  is a matter of design choice. For example, the strip  30  thickness may be 0.060 inches and the sheet  50  thickness may be 0.020 inches.  
         [0028]     The braze alloy has minimal phosphorous. The phosphorous-containing metallurgical phases of earlier braze alloys are susceptible to crevice corrosion. Braze alloys with less than 500 ppm (or 0.05 weight percent) phosphorous are considered phosphorous-free. The benefits of using a phosphorous free braze alloy include reduced corrosion and hence improved generator availability and reliability.  
         [0029]     The pre-braze positioned braze alloy strips extend beyond the ends of the short solid strands. After brazing, the braze alloy forms a braze alloy isolation layer  52  over the end of the armature bar (but not the end of the hollow strands). The isolation layer shields the solid strand ends and the joints from the coolant passage in the clip. The braze alloy also bonds the clip to the strands and the strand ends to each other.  
         [0030]     The strips  30  inserted between the tiers of strands may be rectangular as shown in  FIG. 3 . The braze strips are shaped to fit between the strand rows. The edges of the braze strips may be trimmed into alignment with the outer surfaces of the strands of the bar  16 . Substantially square braze sheets  50  may be fitted between the sides of the armature winding bar and the internal sides of the header clip. The height of the alloy pre-positioned before brazing is selected so that the braze alloy will entirely melt during the braze process and not flow into the open ends of the extended hollow strands.  
         [0031]      FIG. 4  is a cross-sectional end view of the hydraulic header clip  18 , the free ends of the solid  34  and hollow  36  strands, a ram  54  pressing the clip cover  34  into the clip and an induction heating coil  66  to heat the assembly of the clip, strand and braze strips  30  and sheets  50 . The hydraulic header clip  18  (also referred to as a stator bar clip) is formed of an electrically conductive material, such as copper. The clip  18  is hollow and includes a rectangular collar  38  that slides over the outer side surfaces of the end of the armature winding bar  16 . A rectangular slot  39  in the collar receives the end of the armature winding bar and interleaved strips  30  of the braze alloy. The clip cover  32  fits into the matching rectangular slot  39  in the side of the collar  38 . At the other end of the clip  18  is a cylindrical coupling end  40  that is configured to connect to the coolant circuit.  
         [0032]      FIG. 5  is a cross-sectional side view of a hydraulic header clip  18  receiving an armature winding bar  16  and the ram  54  to press the clip cover  32  into the clip slot  39  during brazing. The solid and hollow copper strands  34 ,  36  are disposed in a side-by-side and superposed relation one to the other, in a generally rectangular, multi-tier array. The array may be compressed within the hydraulic end fitting or header clip  18  by means of the side cover  32  fitted within a similarly shaped slot  39  of the header clip. Ram  54  presses the clip cover  32  into the collar  38  and compress together the ends of the strands  34 ,  36  and interleaved braze strips.  
         [0033]     The clip is seated in an induction heating coil  66 . Mica spacers  76  separate the coil from the clip and the ram  54  from the clip cover. The mica spacer between the coil and clip may be 0.060 inches and the spacer between the ram and clip cover may be 0.030 inches. A cooled heat sink clamp  74  grasps the bar  16  just below the clip during the brazing process.  
         [0034]     Each hydraulic header clip  18  includes an internal manifold chamber  42  within the clip collar  38 . The manifold chamber  42  receives the strand ends  34 ,  36  of the armature bar and provides a conduit for coolant flowing through the clip  18  to enter or be discharged from the hollow strands  36  of the armature bar  16 . Within the clip, the manifold chamber  42  is internally open to a necked down internal chamber section  56  and to an expanded sub-chamber  58 , which is aligned with the hose coupling  40  and configured to receive coolant flowing into or out of a hose. The external and internal shapes of a clip may vary to suit different armature bar configurations that are present in large liquid cooled turbine generators.  
         [0035]     When the bar  16  is brazed to the hydraulic header clip  18 , the free ends of the solid copper strands  34  are generally flush with a back wall  48  of the manifold chamber  42 . The free ends of the hollow copper strands  36  extend partially into the manifold chamber  42 . The ends of the hollow copper strands  36  may extend about 10 to 500 thousands of an inch beyond the ends of solid strands  34  and into the chamber  42 .  
         [0036]     The differential lengths of the solid and hollow strands may be achieved by any suitable means including the use of a cutting tool to shorten the solid strands. The alloy strips  30  between the tiers of the solid and hollow strands do not generally extend axially beyond the ends of the hollow strands  36  so that liquid braze when liquefied does not plug the open ends of the hollow strands. In addition, filler metal  44  and the braze alloy sheets  50  ( FIG. 3 ) are pre-placed along the interior walls  46  of the clip to surround the enclosed ends of the hollow and solid strands. The filler metal  44  may be a copper-silver alloy that is positioned between the outer strands and the interior of the clip.  
         [0037]     At the end of the brazing process, a braze alloy isolation layer  52  ( FIG. 9 ) extends axially along and between all sides of each of the strands  34 ,  36  in the array, and also covers the ends (or faying surfaces) of the solid strands  34  while leaving the ends of the hollow strands  36  open and unobstructed for free flow of coolant through the hollow strands.  
         [0038]     The braze joint can be made with the axis of the armature bar in either a horizontal or a vertical orientation. The vertical orientation is preferred because it aids alloy retention in the joint and permits pieces of the alloy to be more easily pre-placed on the surface of the assembly inside the hydraulic header clip, thereby providing a source of additional braze alloy and/or filler metal that will melt and flow over the bar  16  end surfaces to create a thicker layer of braze isolation layer  52  ( FIG. 9 ).  
         [0039]      FIG. 6  is a side view of a brazing chamber  60  assembly. The braze chamber  60  is used to form a brazed connection of a liquid-cooled armature bar strand package to the hydraulic header clip  18  with a corrosion resistant braze alloy that is not susceptible to crevice corrosion initiation and provides for an alloy layer at the liquid-cooled interface surface of the brazement.  
         [0040]     A split braze chamber has left and right side hood sections  62  that laterally separate to receive the armature winding bar. Once the bar  16  is mounted vertically in the left hood section, the right hood section closes against the left hood to form a closed chamber. Windows  64  in the hood sections allow the braze process to be viewed. The hood can withstand a brazing temperature of 1,000 degrees Celsius (1,832 degrees Fahrenheit) or more.  
         [0041]     A controlled gas atmosphere is pumped into the chamber to purge oxygen and form an internal substantially oxygen free atmosphere within the chamber. The controlled gas atmosphere may comprise mixtures of nitrogen and hydrogen or 100 percent hydrogen. After purging, the oxygen level is preferably less than 500 parts per million (ppm) oxygen in the chamber. A substantially oxygen free atmosphere allows the brazing process to proceed without unwanted oxidation of the braze.  
         [0042]      FIG. 7  is a perspective view of the interior of the left hood  62  of the chamber  60 , without an armature bar or clip seated in the coil  66 . The induction heating coil  66  heats the clip and bar to a predetermined brazing temperature for a prescribed time period. The temperature profile of the heating coil is a design choice and depends on the brazing process being performed.  
         [0043]     A hook-shaped induction heating coil  66  receives the bar end and hydraulic header clip  18 . An upper guide  71  aligns the top of the hydraulic header clip such that the collar is between the legs  78  of the induction coil  66 . A heat sink clamp  74  secures the armature bar vertically within the braze chamber and prevents liquid braze from flowing down between the strands of the bar. The ram  54  presses the clip cover  32  and strand ends  34 ,  36  into the clip during the braze process. A pneumatic drive cylinder  55  moves the ram and applies a compressive force to the clip cover.  
         [0044]     The bottom wall  68  of the chamber includes a seal to receive the armature bar and prevent leakage of the gas atmosphere in the chamber. The inert gases in the chamber may be maintained at an above-atmospheric pressure to ensure that oxygen does not leak into the chamber.  
         [0045]     Multiple temperature indicators  70  in the chamber and are located at various positions inside the brazing chamber. An oxygen sensor  72  within the chamber generates a signal in real time of the oxygen level in parts per million in the chamber atmosphere. The oxygen signal may be provided to a programmable logic controller  73  for the brazing process.  
         [0046]     The programmable logic controller (PLC)  73  automates the braze process protocol. The PLC controls the induction coil and monitors the temperature and oxygen level in the chamber during the brazing process. The PLC may also control the force applied by the ram  54 ,  55  and the linear movement of the ram. The control program executed by the PLC may include multiple time and temperature cycles for heating the coil and the clip and armature bar assembly.  
         [0047]     The heat sink  74  is a straight bar clamp that is spring loaded and grasps the bar  16  just below the clip. The heat sink is water cooled to ensure that the armature winding bar  16  below the clip is cooler than the liquidus temperature of the braze alloy. The cool armature bar at the clamp point causes liquid braze alloy flowing down between the bar strands to solidify.  
         [0048]      FIG. 8  is a flow chart of exemplary steps for brazing. In step  80 , the armature bar  16  and clip  18  assembly is seated in the induction heating coil. Mica insulation sheets  76  may separate the clip from the induction coil. In step  81 , ram  54  is positioned against the cover  32  of the clip to force the cover and bar into the clip. The armature bar is mounted vertically such that the free ends of the solid strands  34  are horizontal during the brazing process. An upper stop guide  71  in the left hood ( FIG. 7 ) provides an alignment stop for the free end of the clip. In general, the clip and bar are seated such that the legs  78  of the induction coil  66  are in the same plane as are the extended free ends of the hollow strands  36 . In step  82 , a heat sink, e.g., a cooled bar clamp, is applied to the armature winding bar  16  at a location below the clip  18 . The heat sink cools the armature bar below the clip to prevent liquid braze alloy from flowing down between the bar strands.  
         [0049]     In step  84 , the hood sections  62  of the braze chamber  60  are closed. The closed chamber is purged to an oxygen free atmosphere, such as less than 500 parts per million of oxygen. The control gas may be a mixture of hydrogen and nitrogen, or alternatively be 100% hydrogen or have some other composition that allows for a good braze joint.  
         [0050]     In step  86 , the clip is heated by the induction coil to: braze the strand ends together, braze the clip to the strands, and to form a isolation layer  44  ( FIG. 9 ) over the solid ends of the clip. To reduce liquation of the braze alloy, the braze assembly is held at a temperature just below solidus of the braze alloy to allow equalization of temperature within the braze chamber for a period of, for example, 30 to 600 seconds, in step  88 . Thereafter, the power applied to the induction coil  66  is increased to quickly raise the temperature to above the liquidus temperature of the braze allow but below the maximum allowable braze temperature for the specific alloy, during step  90 . This higher temperature is held for a period of, for example, 5 to 100 seconds. At the higher temperature the braze alloy bonds to the strands and to the clip. In addition, at the higher temperature the braze alloy strips extending beyond the solid strands melts and pools on the ends of the solid strands.  
         [0051]     At the higher temperature, the assembly of clip, strands and braze alloy softens and partially liquefied. The ram  54  pressing against the clip cover  32  causes the cover to slid further into the slot  39  of the clip, in step  91 . The hold time above the liquidus temperature may be controlled by the amount of displacement experienced by the clip cover. When the desired displacement is met, the braze cycle is terminated. Accordingly, the controller  73  monitors the displacement of the ram against the cover in step  92 . The controller terminates the high temperature induced by the coil when the ram displacement exceeds a predetermined level, e.g., up to 0.25 inch, in step  94 . The volume and placement of the alloy force applied by the ram to the clip cover, and the temperature profile in the chamber may be controlled by the PLC controller  73  and selected to assist the capillary flow of the liquid braze alloy between the strands and create the desired layer  52  on the ends of the solid strands and between the extended hollow strands.  
         [0052]     To control the alloy flow at liquidus within the hydraulic header clip, the clip to cover clearance may preferably be between 0.001 to 0.005 inch between mating surfaces. The liquid cooled heat sink  74  adjacent to the clip on the strands also controls liquid alloy flow by solidifying the braze alloy below the clip. To allow proper alloy flow between the strands, a faying surface allowance of preferably between 0.001 and 0.010 inch is used and a braze assembly force preferably of 100 to 1,800 pounds of force is applied by the ram  54  to the assembly during the braze operation.  
         [0053]     During heating to above liquidus temperature (step  90 ), the liquid braze alloy pools on top of the ends of the solid strands. The pooled alloy forms a corrosion-resistant isolation layer  52  on the end of the armature winding bar. In addition to the braze strips and sheets between adjacent strands and between the strands and the clip, braze material, e.g., braze rods or strips, may be prepositioned on the ends of the solid strands or may be added during the braze process to ensure sufficient braze material pools on the end of the solid strands.  
         [0054]     When heated to its melting temperature, the braze alloy flows and fills in the spaces between the solid and hollow strands  34 ,  36  and between the strands and the interior surfaces of the header clip, including at the opening of the header clip into which the strands are inserted. At its melting temperature, the alloy remains sufficiently viscous that it does not flow substantially to the free ends of the hollow strands. The extended length of the hollow strands  36  provides a safety margin in that the excess alloy material does not flow out as far as the ends of the hollow strands, precluding the possibility of plugging the cooling passages in the hollow strands.  
         [0055]     The layer  52  has sufficient thickness and quality to fill the area between the extended hollow strands and over the ends of the short solid strands. Capillary flow draws the liquid braze alloy into the faying surfaces between the strands and between the strands and manifold chamber  42  of the clip. The layer  52  produces a corrosion resistant isolation layer on the water inlet end surface of the armature winding bar. The isolation layer seals the liquid-cooled stator armature bar strands to the hydraulic header clip.  
         [0056]     The brazed clip and strands remain in the control atmosphere of the hood until the temperature decreases to a temperature below which no appreciable oxidation forms on the metal surfaces. Thereafter, the hood sections are separated and the armature bar and clip assembly is removed from the braze chamber.  
         [0057]      FIGS. 9 and 10  are end and cross-sectional side views respectively of the hydraulic header clip brazed to an armature bar. The hollow  36  and solid  34  end strands are brazed to the collar  38  of the clip  18  such that the free ends of the hollow strands are open to the manifold chamber  42 . A braze alloy isolation layer  44  has formed over the free ends of the solid strands  34  and in the crevices between the strands and between the strands and internal surfaces of the manifold chamber of the clip  18 . The minimum thickness of the braze alloy isolation layer  44  may be at least 0.050 of an inch.  
         [0058]     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.