Patent Publication Number: US-2004051458-A1

Title: Discharge lamp

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The invention relates to a discharge lamp electrode assembly.  
       [0003] 2. Description of the Related Art  
       [0004] Conventionally, a discharge lamp has a bulb with hermetically sealed tubes on opposite ends of the arc tube which forms the emission space. Within the arc tube of this bulb, there are a pair of opposed electrodes (anode and cathode), and moreover, for example, it is filled with a rare gas and mercury. The electrodes are each mounted in the tip area of an upholding part of the electrode which extends along the tube axis from the hermetically sealed tubes of the bulb in a direction toward the arc tube and are supported by the latter.  
       [0005] As one measure for mounting the electrode on the tip area of the upholding part of the electrode, a process is known in which an open concave part is formed on the end of the electrode, into which the tip area of the upholding part of the electrode is fitted via a buffer component. This process is described specifically below.  
       [0006] As is shown in FIG. 5( a ), on the end  50   b  of the electrode  50 , a tapering concave part  50   a  is formed such that its diameter reduces in the direction from an open end toward the tip of the electrode  50 . Moreover, the tip area  51   a  of the upholding part  51  of the electrode is worked into a form which is matched to the concave part  50   a  of the electrode  50 , i.e., it tapers such that its diameter diminishes in the direction to the tip. Furthermore, a buffer component  52  is formed on the peripheral surface of the tip area  51   a.    
       [0007] Next, as shown in FIG. 5( b ), the tip area  51   a  of the upholding part  51  of the electrode is inserted into the concave part  50   a  of electrode  50 . Furthermore, as shown in FIG. 5( c ), the tip area  51   a  of the upholding part  51  of the electrode is force fitted such that the entire tip area  51   a  is held in the concave part  50   a  of the electrode  50 . In this way, the buffer component  52  is pressed flat between the tip area  51   a  and the concave part  50   a . The tip area  51   a  of the upholding part of the electrode  51  is fitted into the concave part  50   a  of the electrode  50  and attached. Such a process is described, for example, in Japanese patent disclosure document 2001-23566 (corresponds to U.S. Pat. No. 6,459,202) in conjunction with FIG. 7.  
       [0008] However, this process has the following disadvantages.  
       [0009] The buffer component  52  that is formed on that the tip area  51   a  of the upholding part  51  of the electrode is wound with a molybdenum foil, a tantalum plate or the like. The thickness of the buffer component  52  is adjusted, that is, the thickness, the number of windings and the winding position of the molybdenum foil, the tantalum plate or the like to be used are adjusted by repeated tests of the manufacturer. There is the disadvantage that the fit of the electrode  50  and of the upholding part  51  of the electrode is subject to variances and that the accuracy of the fit is low.  
       [0010] The tip area  51   a  of the upholding part  51  of the electrode which is fitted into the concave part  50   a  of the electrode  50  is also made tapered for the following reasons:  
       [0011] in conjunction with the adjustment of the thickness of the buffer component  52 , i.e., the thickness, the winding number, the winding position and the like of the molybdenum foil, the tantalum plate or the like which is to be used; and  
       [0012] the inside of the concave part  50   a  of the electrode  50  is made tapered.  
       [0013] In the case in which the tip area  51   a  is inserted into the concave part  50   a , the axis Y of the upholding part  51  of the electrode must actually agree with the axis X of the electrode  50 . However, there were cases in which the axes deviate from one another.  
       SUMMARY OF THE INVENTION  
       [0014] The present invention was devised in order to eliminate the above described disadvantages in the prior art.  
       [0015] A primary object of the invention is to devise a discharge lamp in which the tip of the upholding part of the electrode can securely fit into the open end of the concave part of the end of the electrode with extremely high accuracy of fit, in which variances of the accuracy of the fit among discharge lamps can be eliminated, and in which the axis of the upholding part of the electrode can be brought reliably into agreement with the axis of the electrode.  
       [0016] This object is achieved in accordance with the invention by an electrode assembly for discharge lamp which has an electrode which is supported by an upholding part of the electrode, the electrode having an open concave part on one end which has a first concave portion with a smaller diameter which is located on the inside of the electrode, and a second concave portion with a larger diameter which adjoins the first concave part and which is located on the side of the electrode end, and furthermore, in the second concave portion, there is a cylindrical metallic buffer part with an inside peripheral surface which runs parallel to the electrode axis. The tip of the upholding part of the electrode has a cylindrical part which adjoins a tapering part and is fitted into the first concave portion of the electrode and supports this electrode such that the tapering part presses the buffer component flat. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017]FIG. 1 is a longitudinal cross-sectional view of the arrangement of a discharge lamp in accordance with the invention;  
     [0018] FIGS.  2 ( a ) to  2 ( d ) each show a step in the process by which an anode is mounted to fit on the tip of the upholding part of the electrode,  
     [0019]FIG. 3 is a perspective of the buffer component which is used in the fitting arrangement of the discharge lamp of the invention;  
     [0020]FIG. 4 is an enlarged schematic of the fitting arrangement of the discharge lamp according to the invention, and  
     [0021] FIGS.  5 ( a ) to  5 ( c ) each show a step in a conventional process by which an anode is mounted to fit on the tip of the upholding part of the electrode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0022]FIG. 1 is a schematic cross section of one example of an arrangement of a discharge lamp in accordance with the invention which has a bulb  10  made of silica glass that is formed with an ellipsoidal arc tube  11  and hermetically sealed tubes  12  which extend outward from opposite ends of the arc tube  11 . At each hermetically sealed tube  12 , a compressed part  12   a  of reduced diameter is formed near the arc tube  11 .  
     [0023] Within the arc tube  11  of the bulb  10 , an anode  13  and a cathode  14  made of tungsten are each mounted and held to fit on the tip of the molybdenum cylindrical upholding part  15  of the electrode in opposition to each other.  
     [0024] The respective upholding part  15  of the electrode extends within the respective hermetically sealed tube  12 , the ends of which project out of the hermetically sealed tube  12 . On the outer end of the hermetically sealed tube, the tube and the upholding part  15  of the electrode are welded to one another, forming a hermetically closed seal area  17 .  
     [0025] Within the hermetically sealed tube  12 , there is a sleeve component  16  of silica glass which has a through opening  19  which is matched to the outside diameter of the upholding part  15  of the electrode in the state in which is it penetrated by the upholding part  15  of the electrode. This sleeve component  16  is supported by the compressed part  12   a  of the hermetically sealed tube.  
     [0026] Within the arc tube  11  of the bulb  10 , there are a filler gas which is comprised of a rare gas, such as xenon, argon, krypton or the like or of a gas mixture thereof, and if necessary, an emission substance, such as mercury or the like.  
     [0027] The pressure of the filler gas during filling is, for example, 0.1 atm to 10 atm. In the case of using mercury as the emission substance, the filler amount is, for example, from 0.5 mg/cm 3  to 60 mg/cm 3  weight per unit of inside volume of the arc tube  11  of the bulb  10 .  
     [0028] The sequence with which the upholding part of the electrode is fitted into the electrode is described below using FIGS.  2 ( a ) to  2 ( d ).  
     [0029] As is shown in FIG. 2( a ), the electrode  13  has a concave part  13   a  which opens at the end  13   b . This concave part  13   a  has a first cylindrical concave portion  131   a  with a smaller diameter which is located on the inside of the electrode, and a second cylindrical concave portion  132   a  with a larger diameter which adjoins this first concave part  131   a  and is located at the electrode end  13   b . This means that the inside diameter of the first concave portion  131   a  is smaller than the inside diameter of the second concave portion  132   a.    
     [0030] The tip of the upholding part  15  of the electrode is made as a cylindrical part  15   a  which adjoins a tapering part  15   b  with a diameter which increases in the direction to the outer end of the upholding part  15  of the electrode. The tapering part  15   b  adjoins the body  15   c . The outside diameter of the cylindrical part  15   a  is smaller than the inside diameter of the first concave portion  131   a.    
     [0031] A buffer component  20 , as is shown in FIG. 3, is a partially cylindrical component of metallic tantalum which is provided with a gap  22  that extends in the axial direction, giving buffer component  20  an essentially C-shaped cross section. The buffer component  20  has an inner peripheral surface  21 .  
     [0032] As is shown in FIG. 2( b ), the buffer component  20  is pressed from the outside into the second concave part  132   a  and is installed in a plastically deformed state. The inside diameter of this buffer component  20  is larger than the outside diameter of the cylindrical part  15   a  of the upholding part  15  of the electrode and smaller than the outside diameter of the body  15   c  of the upholding part  15  of the electrode. The inner peripheral surface  21  of the buffer component  20  runs parallel to the longitudinal axis X of the electrode.  
     [0033] Next, as is shown in FIG. 2( c ), the cylindrical part  15   a  of the upholding part  15  of the electrode and the tapering part  15   b  are inserted into the concave part  13   a  of the anode  13 .  
     [0034] The first concave part  131   a  is formed on the axis X of the electrode  13 . On the other hand, the cylindrical part  1  Sa of the upholding part  15  of the electrode is formed on the longitudinal axis Y of the upholding part  15  of the electrode. When the cylindrical part  15   a  is pushed into the first concave part  131   a , between the cylindrical part  15   a  and the first concave part  131   a , a gap is formed. Since this gap is very narrow, the first concave part  131   a  acts as a guide when the cylindrical part  15   a  is inserted. The cylindrical part  15   a  is thus located in the middle of the first concave part  131   a . As a result, the upholding part  15  of the electrode can be centered and the axis Y of the upholding part of the electrode  15  and the axis X of the anode  13  agree.  
     [0035] Furthermore, as is shown in FIG. 2( d ), the upholding part  15  of the electrode is fitted precisely into the anode  13  by the cylindrical part  15   a  of the upholding part  15  of the electrode and the tapering part  15   b  being pressed more deeply into the concave part  13   a  of the anode  13 .  
     [0036] This circumstance is described specifically below.  
     [0037] As is shown in FIG. 4, the inside peripheral surface  21  of the buffer component  20  is parallel to the axis X of the electrode  13 . The angle α which is formed by the tapering part  15   b  of the upholding part  15  of the electrode and the inside peripheral surface  21  is therefore constant over the entire circumference. When the tapering part  15   b  is pressed into the buffer component  20 , the tapering part  15   b  is pressed against the inside peripheral surface  21  of the buffer component  20  with a force which is directed essentially uniformly outward. In the buffer component  20 , the side of its inside peripheral surface  21  is pressed essentially uniformly flat by the tapering part  15   b , by which the upholding part  15  of the electrode is fitted reliably into the anode  13  and moreover with great accuracy of the fit.  
     [0038] The buffer component  20  is produced mechanically and has a certain shape, as is shown in FIG. 3. The buffer components  20  therefore have hardly any individual differences among one another.  
     [0039] Since, in the conventional process of winding the tip of the upholding part of the electrode with a metal foil, the thickness, the winding number and the winding position have been determined according to the experience of the manufacturer, there were variances in the accuracy of the fit between the upholding part of the electrode and the electrode for each discharge lamp. With the present invention, the experience of the manufacturer is not critical and the buffer component  20  is installed with a certain shape in the second concave portion  132   a . In this way, individual differences are lost between the states in which the buffer component  20  is pressed flat between the tapering part  15   b  of the upholding part  15  of the electrode and the second concave portion  132   a . Thus, in each discharge lamp, the variances of the accuracy of the fit between the upholding part of the electrode and the electrode can be eliminated and the accuracy of the fit can be kept constant.  
     [0040] The anode  13  was described above. However, the cathode  14  has the same arrangement and thus the same effect and the same action.  
     [0041] Test Example  
     [0042] According to the arrangement as shown in FIGS.  2 ( a ) to  2 ( d ), under the conditions described below, a total of ten assemblies were produced, each assembly having the anode  13 , the upholding part  15  of the electrode and the buffer component  20 .  
     [0043] (Anode  13 )  
     [0044] Material: tungsten  
     [0045] Diameter: 15 mm  
     [0046] Length in the axial direction: 27 mm  
     [0047] Inside diameter of the first concave part: 3.2 mm  
     [0048] Length in the axial direction of the first concave part  131   a:  7.0 mm  
     [0049] Inside diameter of the second concave part  132   a:  3.8 mm  
     [0050] Length of the second concave part  132   a  in the axial direction: 5.0 mm  
     [0051] (Upholding Part  15  of the Electrode)  
     [0052] Material: tungsten  
     [0053] Outside diameter of the cylindrical part  15   a:  3.0 mm  
     [0054] Length of the cylindrical part  15   a  in the axial direction: 5.0 mm  
     [0055] Slope of the tapering part  15   b:  3 mm/10 mm  
     [0056] Length of the tapering part  15   b  in the axial direction: 3.3 mm  
     [0057] Outside diameter of the body  15   c:  4.0 mm  
     [0058] (Buffer Component  20 )  
     [0059] Material: tantalum  
     [0060] Outside diameter: 3.78 mm  
     [0061] Inside diameter: 3.2 mm  
     [0062] Thickness: 0.29 mm  
     [0063] Length in the axial direction: 5.0 mm  
     [0064] Using the anode  13 , the upholding part  15  of the electrode and the buffer component  20 , after the steps shown in FIGS.  2 ( a ) to  2 ( d ), the upholding part  15  of the electrode is fitted precisely into the concave part  13   a  of the anode  13  with a force of roughly 1 ton.  
     [0065] In order to measure the amount of fit between the anode  13  and the upholding part  15  of the electrode, the force for pulling the upholding part  15  of the electrode out of the anode  13  (tensile strength) was measured. In this case, it was roughly 250 kgf to 350 kgf. The following was confirmed.  
     [0066] Between the products, there were only small variances in the accuracy of the fit. Furthermore, the accuracy of the fit between the anode  13  and the upholding part  15  of the electrode was extremely large.  
     [0067] (Comparison Test)  
     [0068] According to the arrangement shown in FIGS.  5 ( a ) to  5 ( c ), under the conditions described below, a total of ten assemblies were produced, each assembly having the anode  50 , the upholding part  51  of the electrode and the buffer component  52 .  
     [0069] (Anode  50 )  
     [0070] Material: tungsten  
     [0071] Diameter: 15 mm  
     [0072] Length in the axial direction: 27 mm  
     [0073] Slope of the concave part 50: 0.6 mm/20 mm  
     [0074] Diameter of the tip area of the concave part 50: 4.06 mm  
     [0075] Diameter of the opening on the side on the end  50   b  of the concave part 50: 4.1 mm  
     [0076] Length of the concave part  50  in the axial direction: 12 mm  
     [0077] (Upholding Part  51  of the Electrode)  
     [0078] Material: tungsten  
     [0079] Diameter of the tip of the tip area  51   a:  3.9 mm  
     [0080] Slope of the tip area  51   a:  0.06 mm/20 mm  
     [0081] Length of the tip area  51   a  in the axial direction: 16 mm  
     [0082] (Buffer Component  52 )  
     [0083] Material: tantalum foil  
     [0084] Height: 10 mm  
     [0085] Width: 6 mm  
     [0086] Thickness: 0.05 mm  
     [0087] Winding method: 1.5 turns  
     [0088] Using the anode  50 , the upholding part  51  of the electrode and the buffer component  52 , after the steps shown in FIGS.  5 ( a ) to  5 ( c ), the upholding part  51  of the electrode was fitted precisely into the concave part  50   a  of the anode  50  with a force of roughly 1 ton.  
     [0089] In order to measure the amount of fit between the anode  50  and the upholding part  51  of the electrode, the force for pulling the upholding part  51  of the electrode out of the anode  50  (tensile strength) was measured. In this case, it was in the range from roughly 40 kgf to 250 kgf. There were great variances between the products. The accuracy of the fit between the anode and the upholding part of the electrode compared to the accuracy of the fit between the anode and the upholding part of the electrode of the invention was less.  
     [0090] Action of the Invention  
     [0091] As was described above, in the discharge lamp of the invention, the cylindrical portion of the upholding part of the electrode is pushed into the first concave part with a smaller diameter of the electrode, the cylindrical metallic buffer component is located in the second concave part with a greater diameter and the upholding part of the electrode is fitted precisely into the concave part of the electrode such that the side of the inside peripheral surface of this buffer component is pressed flat by the tapering part of the upholding part of the electrode. The upholding part of the electrode is therefore securely mounted in the electrode, and moreover, in a state in which extremely great accuracy of the fit is maintained. Furthermore, the variances of the accuracy of the fit among discharge lamps is eliminated. Because the first concave part of the electrode acts as a guide when the cylindrical part of the upholding part of the electrode is inserted, the cylindrical part is mounted along the middle of the first concave part. Therefore, the axis of the upholding part of the electrode can be reliably brought into agreement with the axis of the electrode.