Electric lamp and discharge devices – With gas or vapor – Having electrode lead-in or electrode support sealed to...
Reexamination Certificate
2001-12-04
2003-11-25
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having electrode lead-in or electrode support sealed to...
C313S318070
Reexamination Certificate
active
06653785
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an arc tube and a method for forming an arc tube for a discharge bulb, and more particularly, to an arc tube and method for forming an arc tube that can be used as a light source such as for an automobile headlight.
2. Description of the Related Art
An arc tube is generally used to provide high luminance irradiation. Recently, therefore, an arc tube is frequently used also as a light source for an automobile headlight.
As shown in
FIG. 17
, an arc tube which is used in an automobile headlight or the like is usually configured by an arc tube body
104
which is made of quartz glass, and in which pinch seal portions
104
b
are formed in ends of a luminous tube portion
104
a
constituting a discharge space
102
. The arc tube includes a pair of electrode assemblies
106
in which a tungsten electrode
108
and a lead wire
110
are connected and fixed to each other via a molybdenum foil
112
. Each of the electrode assemblies
106
is pinch sealed into the arc tube body
104
in the corresponding pinch seal portion
104
b
with protruding a tip end part of the tungsten electrode
108
into the discharge space
102
. As a result of this pinch sealing, the molybdenum foil
112
is joined to the arc tube body
104
in a state where the foil is embedded in the arc tube body
104
.
The connection and fixation between the tungsten electrode
108
and the molybdenum foil
112
are performed by welding together the members in a partially overlapped state. In the periphery of the overlapped portion, however, it is not easy to sufficiently ensure a force of joining the molybdenum foil
112
and the arc tube body
104
. In a conventional arc tube, therefore, the molybdenum foil
112
is easily peeled off while the arc tube is in use.
When such peeling occurs, the arc tube body
104
cracks, typically starting from an edge of a joining face between the molybdenum foil
112
and the arc tube body
104
. As the crack grows, it causes a leakage between the discharge space
102
and the external space. Consequently, the life period of a conventional arc tube becomes relatively short.
SUMMARY OF THE INVENTION
The invention has been conducted in view of such circumstances. It is an object of the invention to provide an arc tube and a method for forming an arc tube in which leakage due to peeling of a molybdenum foil is effectively suppressed so that the life period of the arc tube can be prolonged.
The present inventors have discovered that when consideration is given to dimensional relationships among components in the periphery of the overlapped portion of a tungsten electrode and a molybdenum foil, occurrence of peeling of the molybdenum foil may be effectively suppressed without adversely affecting other characteristics. In view of this discovery, an object of the present invention is attained by defining these dimensional relationships.
The arc tube of a first aspect of the invention is an arc tube comprising an arc tube body which is made of, for example, quartz glass, and in which pinch seal portions are formed in ends of a luminous tube portion constituting a discharge space, respectively; and a pair of electrode assemblies in each of which a tungsten electrode and a molybdenum foil are joined together, such as by welding, in a partially overlapped state, and which are pinch sealed into the arc tube body in the pinch seal portions, respectively, with protruding a tip end part of the tungsten electrode into the discharge space, wherein
an overlapped length L
1
of the tungsten electrode and the molybdenum foil is set to have the following relationship with a diameter D of the tungsten electrode and a width W of the molybdenum foil:
2D≦L
1
≦0.8W.
The arc tube of a second aspect of the invention is an arc tube comprising an arc tube body which is made of quartz glass, and in which pinch seal portions are formed in ends of a luminous tube portion constituting a discharge space, respectively; and a pair of electrode assemblies in each of which a tungsten electrode and a molybdenum foil are welded together in a partially overlapped state, and which are pinch sealed into the arc tube body in the pinch seal portions, respectively, with protruding a tip end part of the tungsten electrode into the discharge space, wherein
a distance L
2
between the molybdenum foil and the discharge space is set to have the following relationship with a width A and a thickness B of the pinch seal portion:
B≦L
2
≦0.8A.
The above-mentioned “tungsten electrode” is an electrode in which tungsten is a principal component.
The above-mentioned “molybdenum foil” is a foil in which molybdenum is a principal component.
Further, the range of “overlapped length L
1
” in the first aspect of the invention, and that of “distance L
2
” in the second aspect of the invention may be set to both pinch seal portions of the sides of the luminous tube portion, or to only one of the pinch seal portions.
As shown in the above configuration, in the arc tube of the first aspect of the invention, the overlapped length L
1
of the tungsten electrode and the molybdenum foil which constitute an electrode assembly of the arc tube is set to be 2D≦L
1
≦0.8W with respect to the diameter D of the tungsten electrode and the width W of the molybdenum foil. Therefore, the arc tube can attain the following functions and effects.
In each of the electrode assemblies which are pinch sealed into an arc tube body, minute gaps are formed between the electrode assembly and the arc tube body in the periphery of a portion where the tungsten electrode and the molybdenum foil are welded together. The molybdenum foil is peeled off starting from such gaps. As the overlapped length L
1
becomes longer, the proportion of the gaps becomes greater, and hence peeling of the molybdenum foil occurs more easily. As a result of temperature changes due to turning-on and turning-off of the arc tube, the tungsten electrode expands and contracts with respect to the arc tube body. As the overlapped length L
1
becomes longer, the expansion and contraction of the tungsten electrode produce a greater stress. Also from this viewpoint, peeling of the molybdenum foil occurs easily. Therefore, reduction of the overlapped length L
1
to a value which is as small as possible is effective in suppression of occurrence of leakage due to peeling of the molybdenum foil.
By contrast, when the overlapped length L
1
is extremely reduced, it becomes physically impossible to join, for example, by welding, the tungsten electrode to the molybdenum foil. In this welding, moreover, there is the possibility that the molybdenum foil is broken in the portion overlapped by the tungsten electrode. Even in the case where the molybdenum foil is not broken in the welding step, when the overlapped length L
1
is extremely reduced, the strength of the periphery of the welded portion of the molybdenum foil becomes largely reduced. Therefore, there is the possibility that the molybdenum foil is broken by the pinch seal pressure in the subsequent pinch sealing step, or that, when the arc tube is turned on, the current flow is concentrated in the welded portion to fuse off the welded portion.
When, based on the results of experiments conducted by the present inventors, the overlapped length L
1
of the tungsten electrode and the molybdenum foil is set to a value within a range of 2D≦L
1
≦0.8W with respect to the diameter D of the tungsten electrode and the width W of the molybdenum foil as set forth in the first aspect of the invention, occurrence of peeling of the molybdenum foil can be effectively suppressed while effectively suppressing occurrence of breakage of the molybdenum foil. Therefore, occurrence of leakage due to peeling of the molybdenum foil can be effectively suppressed, so that the life period of the arc tube can be prolonged.
In the arc tube of the second aspect of the invention, the distance L
2
between the molybdenum foil which constitutes an electrode assembly of the arc tube, and
Fukuyo Takeshi
Kasuya Nobuyuki
Kobayakawa Yukihiro
Ohshima Yoshitaka
Shido Masaya
Koito Manufacturing Co. Ltd.
Patel Ashok
Sughrue & Mion, PLLC
Zimmerman Glenn
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