Patent Publication Number: US-2004055137-A1

Title: Double layer electrode coil for a HID lamp and method of making the electrode coil

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention is directed to an electrode coil for a high intensity discharge (HID) lamp and to a method of making an electrode coil for a HID lamp.  
       [0002] As shown in FIG. 1, a conventional HID lamp includes a tube  10  with two electrode coils  12  therein that are typically placed at opposing ends of tube  10 . Tube  10  is filled with an appropriate gas and fill material, and sealed. Each electrode coil  12  includes a tungsten shank  14  with a tungsten wire coil  16  adjacent to a free end of tungsten shank  14  inside tube  10 .  
       [0003] Electrode coil  12  has been conventionally made with a coiled coil or by back winding tungsten wire to form a second layer of wire wound in a direction opposite to the winding direction of the first layer. These manufacturing methods have not proven entirely satisfactory.  
       [0004] The coiled coil method includes the steps of winding a primary tungsten wire around a primary tungsten mandrel and then winding the coiled wire and primary mandrel around a secondary molybdenum mandrel. The coiled coil is heat-treated, cut to length, and heat-treated again. The secondary molybdenum mandrel is dissolved in acid and replaced with a tungsten shank. An example of a coiled coil electrode coil is shown in FIG. 2.  
       [0005] The coiled coil method is generally cost effective because the manufacturing equipment is largely automated. However, the insertion of the tungsten shank can cause the primary tungsten mandrel to crack, which is a basis for rejecting the electrode coil.  
       [0006] The back winding method includes the step of winding a tungsten wire around a retractable steel pin. After a predetermined number of turns or distance, the winding direction is reversed (for example, from left-to-right to right-to-left) and the wire is wound back over itself to form a second layer. Subsequently, several turns of the first layer may be left exposed, the steel pin removed, the coil oriented properly, and the tungsten shank inserted. An example of a back wound electrode coil is shown in FIG. 3.  
       [0007] Although the back winding method produces fewer problems than the coiled coil method when the tungsten shank is inserted, the back wound coil does not hold its shape well. Moreover, the process is more labor intensive as the asymmetrical coil must be oriented properly on the tungsten shank. The orientation of the coil takes additional time and these machines quickly reach capacity limits.  
       [0008] U.S. Pat. No. 4,105,908 discloses a back wound coiled coil electrode. A coil wrapped around a primary mandrel is wrapped around a secondary mandrel and back wound over itself to form a two-layer coiled coil, such as shown in FIG. 4. However, manufacture of this electrode coil enjoys the problems of both the above-noted methods.  
       [0009] U.S. Pat. No. 2,523,033 is not related to the manufacture of electrode coils, but is of general interest because it discloses a double layer coil in a lamp. The lamp includes a filament that expands and contracts axially during use. A spring portion of the filament absorbs the stress of elongation and contraction. As shown in FIG. 5, an in-lead  18  for the spring is thicker than filament  20  and is connected to filament  20  by butt-welding  22  the ends of the small diameter filament  20  to the large diameter in-lead  18 . A first layer of wire  24  is wound around filament  20 . The wire  24  has a diameter equal to the difference between the radii of filament  20  and in-lead  18 . A second layer of wire  26  is screwed onto first layer  24  and onto in-lead  18 . The combination of first and second layers of wire  24  and  26  reinforces butt-weld  22  by absorbing some of the mechanical strain.  
       SUMMARY OF THE INVENTION  
       [0010] An object of the present invention is to provide a novel method of making an electrode coil for a HID lamp that avoids the problems of the prior art, specifically the problem of orienting the coil for insertion of the tungsten shank.  
       [0011] A further object of the present invention is to provide a novel method of making an electrode coil for a HID lamp in which two overlapping wires are wrapped in the same direction on a mandrel so that the second wire is entirely within a helical groove on an exterior of the first wire and in which the two coils formed by the first and second wires are generally the same length.  
       [0012] A yet further object of the present invention is to provide a novel method of making an electrode coil for a HID lamp including the steps of closely wrapping a first wire around a mandrel in a first direction to form a first coil with a helical groove on an exterior surface, closely wrapping a second wire in the first direction in the helical groove to form a second coil, where first and last turns of the second wire touch the first and last turns of the first wire, respectively, and dissolving the mandrel and replacing it with a tungsten core so that a free end of the tungsten core is adjacent to but spaced from a corresponding end of the first coil.  
       [0013] Another object of the present invention is to provide a novel electrode coil for a HID lamp that avoids the problems of the prior art.  
       [0014] Yet another object of the present invention is to provide a novel electrode coil for a HID lamp with two overlapping wires that are wrapped in the same direction so that the second wire is entirely within a helical groove on an exterior of the first wire and in which the two coils formed by the two wires are generally the same length.  
       [0015] Still another object of the present invention is to provide a novel electrode coil for a HID lamp with a first wire closely wrapped in a first direction to form a first coil with a helical groove on an exterior surface, a second wire closely wrapped in the first direction in the helical groove to form a second coil, and a tungsten core with a free end adjacent to but spaced from a corresponding end of the first coil, where first and last turns of the second wire touch the first and last turns of the first wire, respectively. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1 is partial pictorial view of a conventional HID lamp with electrode coils in opposing ends.  
     [0017]FIG. 2 is a pictorial view of a conventional coiled coil electrode coil.  
     [0018]FIG. 3 is a pictorial view of a conventional back wound electrode coil.  
     [0019]FIG. 4 is a pictorial view of a known back wound, coiled coil electrode coil.  
     [0020]FIG. 5 is a pictorial view of a known butt-weld reinforcement technique.  
     [0021]FIG. 6 is cross section of an embodiment of the electrode coil of the present invention.  
     [0022]FIG. 7 is a pictorial view with phantom lines showing the coiling arrangement of an embodiment of the present invention.  
     [0023]FIG. 8 is a pictorial view with phantom lines showing the coiling arrangement of a known back wound electrode coil.  
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0024] The present invention provides a more stable layer of coils during manufacture by front winding, instead of back winding, the layers of wire. That is, two lengths of wire are wound, one atop the other, in the same direction on a mandrel. This means that the second layer of wire is entirely within a helical groove on the exterior surface of the first layer of wire. This arrangement is particularly stable and permits more rapid insertion of the shank after removal of the mandrel.  
     [0025] With reference now to FIG. 6, an embodiment of the present invention is an electrode coil for a HID lamp. The electrode coil  30  may include a tungsten core  32  with a free end  34  adapted to be placed in a HID tube. A first wire  36  is wrapped on tungsten core  32  in a first direction (for example, left to right, as shown by direction “A” in FIG. 6) with each turn  38  of first wire  36  touching at least one other turn  38  of first wire  36 . First wire  36  forms a first coil  40  that has an exterior surface with a helical groove therein. Free end  34  of tungsten core  32  is adjacent to but spaced from a corresponding end  42  of first coil  40 , with an exterior of tungsten core  32  touching an interior of first coil  40 . A “turn” of wire extends once around the mandrel.  
     [0026] A second wire  46  is wrapped in the first direction directly on first wire  36  entirely in the helical groove in the exterior of first coil  40 . Second wire  46  may be second length of wire separate from first wire  36 . Second wire  46  forms second coil  48  whose interior touches the exterior of first coil  40 . First coil  40  and second coil  48  may have substantially the same length; that is, a first turn of second wire  46  may touch a first turn of first wire  36  and a last turn of second wire  46  may touch a last turn of first wire  36 , such as shown in FIG. 6. Each turn  50  of second wire  46  may touch two turns  38  of first wire  36  and at least one other turn  50  of second wire  46 .  
     [0027] The method of making the electrode coil of FIG. 6 may include the steps of closely wrapping first wire  36  around a mandrel (not shown, but is similar in size and shape to shank  32 ) in a first direction to form first coil  40  with a helical groove on an exterior surface. Thereafter, second wire  46  is closely wrapped in the first direction in the helical groove to form second coil  48 , where a first turn of second wire  46  touches a first turn of first wire  36  and a last turn of second wire  46  touches a last turn of first wire  36 . The mandrel is then removed and replaced with tungsten core  32  so that free end  34  of tungsten core  32  is adjacent to but spaced from corresponding end  42  of first coil  40 . After wrapping second wire  46  and before replacing the mandrel, first and second coils  40  and  48  may be heat-treated, cut to a desired length, and heat-treated again.  
     [0028] The result of this coiling arrangement is shown in FIG. 7. As shown therein, second coil  48  fits into the helical groove in the exterior of first coil  40  over an entire length of second coil  48 . In contrast, as shown in FIG. 8, the lower layer of wire wound in direction “A” periodically is crossed by the upper layer of wire wound in direction “B” so that an entire length of the upper layer of wire is not in the helical groove in the exterior of the lower layer.  
     [0029] The present invention provides the advantage that the two layers of coiled wire are substantially more stable than the two layers of coiled wire in the prior art. A more stable coiled wire is easier to handle and allows the tungsten core to be more easily inserted into the position vacated by the mandrel during manufacture. This stability decreases production time and reduces the number of rejected electrode coils.  
     [0030] In further embodiments, second wire  46  may have the same length as the helical groove, and first and second wires  36 ,  46  may both be tungsten wires with the same diameter. First wire  36  may be attached to tungsten core  32  to discourage unraveling and second wire  46  may be attached to first wire  36  for the same purpose. The ends of the first and second wires may be flattened. The mandrel may be removed conventionally, such as by dissolving in acid.  
     [0031] While embodiments of the present invention have been described in the foregoing specification and drawings, it is to be understood that the present invention is defined by the following claims when read in light of the specification and drawings.