Electric lamp and discharge devices – With gas or vapor – Having electrode lead-in or electrode support sealed to...
Reexamination Certificate
2000-06-26
2002-09-17
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having electrode lead-in or electrode support sealed to...
C313S332000, C313S631000
Reexamination Certificate
active
06452334
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to an arc tube for a discharge lamp unit structured such that two electrode assemblies are disposed opposite to each other in a central sealed chamber in which light emitting substances are enclosed. Each electrode assembly includes an electrode rod, molybdenum foil and a lead wire, and is sealed in a pinch seal portion adjacent to the central sealed chamber. Each pinch seal portion includes a residual-compressive-stress layer. The present invention also relates to a method for manufacturing an arc tube with a residual-compressive-stress layer.
2. Prior Art
FIG. 6
shows a conventional discharge lamp unit that incorporates an arc tube
5
having a front end supported by one lead support
2
projecting forward from an insulating base
1
. A recess
1
a
of the base
1
supports the rear end of the arc tube
5
. A metal support member S, secured to the front surface of the insulating base
1
holds a portion of the arc tube adjacent to the rear end of the arc tube. A front lead wire
8
, extending from the arc tube
5
, is welded to the lead support
2
, while a rear lead wire
8
penetrates a bottom wall
1
b
having the recess
1
a
of the base
1
formed therein. Then, the rear lead wire
8
is, by welding, secured to a terminal
3
provided for the bottom wall
1
b
. Symbol G represents an ultraviolet-ray shielding globe arranged to remove an ultraviolet-ray component in the wavelength region harmful to the human body. The ultraviolet-ray shielding globe forms a cylindrical shape and is integrally welded to the arc tube
5
.
The arc tube
5
has a sealed chamber portion
5
a
formed between a pair of front and rear pinch seal portions
5
b
. The sealed chamber portion
5
a
has electrode rods
6
disposed opposite to each other and contains light emitting substances. In the pinch seal portions
5
b
, the sealed molybdenum foil
7
connects the electrode rod
6
projecting into the sealed chamber portion
5
a
to the lead wire
8
extending from the pinch seal portion
5
b
. Thus, the pinch seal portions
5
b
remain airtight.
Preferably, the electrode rod
6
is made of tungsten exhibiting excellent durability. Tungsten has a coefficient of linear expansion that is considerably different from that of the quartz glass that constitutes the arc tube. Worse, only unsatisfactory conformability with quartz glass is permitted and the permitted airtightness is unsatisfactory. Therefore, the molybdenum foil
7
having a coefficient of linear expansion similar to that of quartz glass and exhibiting relatively satisfactory conformability is connected to the tungsten electrode rods
6
. Moreover, the pinch seal portion
5
b
seals the molybdenum foil
7
. Thus, pinch seal portions
5
b
remain airtight.
Referring to FIG.
7
(
a
), a method of manufacturing the arc tube
5
is illustrated. An electrode assembly A comprises an electrode rod
6
, molybdenum foil
7
and a lead wire
8
. The components are integrally connected. The electrode assembly A is initially inserted into an end of either opening of a cylindrical glass tube W having a spherical expanded portion w
2
disposed at an intermediate position of a straight extending portion w
1
. Then, adjacent position q
1
of the spherical expanded portion w
2
undergoes a primary pinch-seal operation.
Referring to FIG.
7
(
b
), a light emitting substance P and the like are introduced into a spherical expanded portion w
2
through the other end opening of cylindrical glass tube W. Referring to FIG.
7
(
c
), a second electrode assembly A is inserted. A secondary pinch sealing operation seals the spherical expanded portion w
2
, while simultaneously cooling the spherical expanded portion w
2
by using liquid nitrogen to prevent both vaporization of the light emitting substance P and heating the adjacent position q
2
of the spherical expanded portion w
2
. The final result is an arc tube
5
having the chipless sealed chamber portion
5
a.
Referring to FIG.
7
(
b
), the primary pinch-sealing operation uses inactive gas (in general, which is low-cost argon gas or nitrogen gas) as forming gas into the glass tube W in order to prevent oxidation of the electrode assembly A. Referring to FIG.
7
(
c
), in the secondary pinch-sealing operation, the ends of the openings in cylindrical glass tube W are closed and cooling with liquid nitrogen prevents vaporization of the light emitting substance P. Therefore, a state of near vacuum is necessary for the pinch-sealing operation.
Since a large temperature change occurs between a state where the arc tube is turned on and a state where the arc tube is turned off, thermal stress occurs between the electrode rod and the glass layer. The electrode rod and the glass layer each have considerably different coefficients of linear expansion when the arc tube is turned on. In recent years, the arc tube structure now lights instantaneously. Therefore, a high temperature-rise ratio is realized. After repeated cycling, a crack forms in the pinch seal portion (the glass layer) for sealing the electrode rods
6
. Thus, the sealed substances leak, thereby causing a defect in the lighting of the arc tube and shortening its life.
In view of the foregoing, the inventor has repeatedly performed experiments and studies to solve the foregoing problems experienced with the conventional technique. As a result, the inventor discovered that retention of compressive stress produced in the pinch seal portions
5
b
during the arc tube manufacturing process causes a thermal stress in the glass layer in the pinch seal portion to disperse due to rise in the temperature occurring after turning the arc tube on. Therefore, prevention of the formation of a crack in the glass layer in the pinch seal portion will extend the life of the arc tube.
BRIEF SUMMARY OF THE INVENTION
The present invention solves the problems experienced with the conventional technique and in accordance with the inventor's discovery. An object of the invention is to provide an arc tube for a discharge lamp unit that is free of crack formation in the pinch seal portion when the thermal stress changes due to arc tube cycling.
To achieve the object, an arc tube for a discharge lamp unit comprising at least two electrode assemblies, each of the electrode assemblies comprising an electrode rod, a foil and a lead wire integrally connected in series, a tube having a central sealed chamber enclosing light emitting substances, and further comprising pinch seal portions disposed at opposite ends of the chamber, each pinch seal portion enclosing an electrode assembly such that the electrode rod projects into the chamber and the lead wire projects from the pinch seal portion, and a residual-compressive-stress layer facing a glass layer region in each of the pinch seal portions, the residual-compressive-stress layer hermetically contacting the electrode rod, wherein the residual-compressive-stress layer and the glass layer region extending only along the electrode rod.
According to another aspect of the invention, the residual-compressive-stress layer is formed for a length greater than or equal to 30% of the axial length of the glass layer region that only contacts the electrode rod.
According to another aspect of the invention, the residual-compressive-stress layer is formed in an angular range of about 180° or larger in the circumferential direction of the electrode rod.
According to another aspect of the invention, the residual-compressive-stress layer is formed for a length greater than or equal to 30% of the axial length of the glass layer region that only contacts the electrode rod and in an angular range of about 180° or larger in the circumferential direction of the electrode rod.
No thermal stress is produced in the boundary between the glass layer and the electrode rod immediately after the pinch-sealing operation. When the temperature returns to room temperature, the boundary between the electrode rod (made of tungsten) and the glass (quartz glass) encounters gene
Irisawa Shinichi
Ohkawai Nobuo
Ohshima Yoshitaka
Koito Manufacturing Co. Ltd.
Patel Ashok
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