1. Field of the Invention
The present invention relates to an arc tube, and a method of fabricating the arc tube for a discharge lamp in which a cylindrical shroud glass is welded on and integrated with an arc tube main body having a sealed glass bulb that serves as a discharge portion and is formed at a portion of the main body along the longitudinal direction thereof.
2. Description of the Related Art
Conventionally, as shown in FIG. 12, an arc tube is configured in a manner that a cylindrical shroud glass 8 for shielding ultraviolet rays is welded on and integrated with an arc tube main body 1 having a sealed glass bulb 2 which serves as a discharge portion and is formed at a portion of the main body along the longitudinal direction thereof, and the sealed glass bulb 2 is covered by the shroud glass 8. Reference numerals 8a, 8b depict welding portions of the shroud glass 8.
Electrodes a, a are provided in an opposite manner within the sealed glass bulb 2 sandwiched between the pinch seal portions 3a, 3b, and lead wires c, c coupled to molybdenum foils b, b are drawn from the pinch seal portions 3a, 3b at both ends of the glass bulb, respectively. Cylindrical portions 4a, 4b as non-pinch seal portions are formed at the front and rear portions of the pinch seal portions 3a, 3b so as to be extracted therefrom, respectively.
The shroud glass 8 cuts ultraviolet rays in a wavelength range that may be harmful to the human body among light emitted from the sealed glass bulb 2.
A sealed space 7, formed by the shroud glass 8 and surrounding the arc tube main body 1, suppresses devitrification generated at the arc tube. That is, since the lamp room in which the arc tube is disposed communicates with the outside of the lamp room through an air hole performing breathing operation, and the atmosphere within the lamp room contains a lot of moisture, the moisture causes the devitrification generated at the arc tube. Therefore, the arc tube main body 1 is covered by the sealed space 7 so that the arc tube main body 1 does not contact the atmosphere containing the moisture, thereby suppressing the generation of the devitrification.
In order to fabricate the arc tube shown in FIG. 12, first, the rod-shaped arc tube main body 1 having cylindrical portions 4a, 4b formed at both ends thereof is fabricated. Thereafter, the arc tube main body 1 is inserted within a shroud glass tube 9, then the front and rear end sides of the shroud glass tube 9 are heated to be molten and softened. After that, the softened portions are deformed by using forming rolls in a direction for reducing the diameter of the shroud glass tube (a direction shown by the arrows in FIG. 12) and pressed against the pinch seal portions 3a, 3b of the arc tube main body 1 at the inside of the glass tube and welded at the pinch seal portions. Then, the shroud glass tube 9 is cut at predetermined portions as necessary.
However, according to the conventional arc tube described above, there arises a problem that the devitrification phenomenon occurs despite the fact that the shroud glass 8 (the shroud glass tube 9) is welded to the arc tube main body 1 to form the sealed space 7.
The inventors of the present invention inspected the cause of the occurrence of the devitrification phenomenon and determined that the cause resides in the sectional shape of the arc tube main body (the pinch seal portions 3a, 3b) for welding the shroud glass 8 thereon. That is, although the cross section of the shroud glass tube 9 is cylindrical, the cross section of the pinch seal portion 3a (3b) is rectangular as shown in FIG. 13(a) since it is typically pinched by a pincher. Thus, in the welding process of the shroud glass, as shown by a phantom line in FIG. 13(a), when the shroud glass tube 9, which is molten and softened and pressed in the direction for reducing the diameter thereof along its radius direction, contacts closely to the surface (flat surface) of the pinch seal portion 3a, an opening S extending in the axial direction along the contact surface is formed (see FIG. 13(b)). As a result, the atmosphere (moisture) within the lamp room enters into the sealed space 7 around the arc tube main body 1 from the opening S formed at the welded portion, thereby causing devitrification.
The inventors of the present invention have determined that an opening is not formed at the contact surface between the arc tube main body 1 and the shroud glass 8 when the welded portion of the shroud glass at the arc tube main body 1 is formed in a circular shape in its cross section.
Accordingly, an object of the present invention is to provide an arc tube for a discharge lamp and a method for fabricating the arc tube in which the welded portion of the shroud at an arc tube main body is formed in a circular shape in its cross section thereby preventing the forming of an opening at the contact surface between the arc tube main body and the shroud glass.
In order to attain the aforesaid object, an arc tube for a discharge lamp according to the present invention is arranged in a manner such that the arc tube includes an arc tube main body at which a sealed bulb, for example, a glass bulb, serving as a discharge portion sandwiched by front and rear pinch seal portions is formed at a portion of a tube along a longitudinal direction thereof, and a cylindrical shroud which is welded on and integrated with the arc tube main body so as to cover the sealed bulb to form an airtight sealed space around the arc tube main body,
the front and rear end portions of the shroud are welded on shroud weld portions with circular cross sections provided at front and rear end sides of the arc tube main body, respectively. In the embodiments described herein, the shroud may be formed of glass, and the arc tube body may be formed from a glass tube. of course, other materials known to those skilled in the art may be substituted without departing from the scope of the present invention.
A method of fabricating an arc tube for a discharge lamp according to the present invention includes an arc tube main body fabricating process for fabricating an arc tube main body at which a sealed glass bulb serving as a discharge portion sandwiched by front and rear pinch seal portions is formed at a portion of a glass tube along a longitudinal direction thereof, and a shroud glass welding process for welding and integrating a cylindrical shroud glass on and with the arc tube main body so as to cover the sealed glass bulb, wherein
in the arc tube main body fabricating process, shroud glass welded portions with circular cross sections are formed on front and rear end sides of the arc tube main body, respectively, and
in the shroud glass welding process, the arc tube main body is inserted into the shroud glass tube, predetermined positions of the shroud glass having been heated, molten and softened are modified in a direction of reducing diameters thereof, and the predetermined positions are welded on the shroud glass welded portions on the front and rear end sides of the arc tube main body, respectively.
At the time of welding the shroud glass to the arc tube main body, the predetermined positions of the shroud glass tube having been heated, molten and softened are modified inside so as to reduce their diameter in a radial direction. As shown in FIG. 9 (a diagram showing a state where the shroud glass is shrink-sealed) in an embodiment of the invention, each of the outer peripheral surfaces of the shroud glass welded portion of the arc tube main body (a shrink seal portion 15a and a cylindrical portion 14a) and the inner peripheral surface of the molten diameter-reduced area of the shroud glass tube 20 may have a circular shape almost matching to each other. Thus, the inner peripheral surface of the molten diameter-reduced portion of the shroud glass tube 20 is molten and welded on the outer peripheral surface of the shroud glass welded portion of the arc tube main body (the shrink seal portion 15a and the cylindrical portion 14a) uniformly along the peripheral direction thereof without causing any space therebetween, so that such a space for releasing the sealed space around the arc tube main body to the atmosphere is not formed at the welding portion between the arc tube main body and the shroud glass tube.
In particular, when inactive gas, adjusted to be a negative pressure such that a pressure becomes about 1 atm. upon lighting and heating the arc tube, is supplied within the airtight sealed space formed around the arc tube main body, the arc tube main body is prevented from contacting moisture in the atmosphere.
Further, in a method of fabricating an arc tube according to the present invention, in the arc tube main body fabricating process, a cylindrical non-pinch seal portion is formed in an extended manner at a backward portion of the pinch seal portion on the rear end side of the arc tube main body, and a shrink seal portion is formed adjacent to a forward portion of the pinch seal portion on the front end side of the arc tube main body, and
in the shroud glass welding process, the rear end side of the shroud glass tube is welded on the cylindrical non-pinch seal portion on the rear end side of the arc tube main body, and the front end side of the shroud glass tube is welded on the shrink seal portion on the front end side of the arc tube main body.
At the rear end portion of the shroud glass, the circular inner peripheral surface on the rear end side of the shroud glass tube which is molten, softened and modified in a direction of reducing the diameter thereof matches almost with the outer peripheral surface of the cylindrical non-pinch seal portion on the arc tube main body side, and so the inner peripheral surface of the molten shroud glass is molten and adhered to the outer peripheral surface of the non-pinch seal portion uniformly along the peripheral direction thereof without causing any space therebetween.
In contrast, at the front end portion of the shroud glass, since the outer peripheral surface of the shrink seal portion has a circular shape, the circular inner peripheral surface on the front end side of the shroud glass tube which is molten, softened and modified in a direction of reducing the diameter thereof adheres to the circular outer peripheral surface of the shrink seal portion on the arc tube main body side uniformly along the peripheral direction thereof without causing any space therebetween.
Incidentally, the welded portion with a circular cross section on the front end side of the arc tube main body may be, for example, a cylindrical portion (see FIG. 11) serving as a non-pinch seal portion extending forward of the front end side pinch seal portion; a pinch seal portion with a circular cross section provided adjacent to the forward portion of the front end side pinch seal portion with a rectangular cross section; a shrink seal portion provided adjacent to the forward portion of the front end side pinch seal portion; the pinch seal portion with the circular cross section and the cylindrical portion (see FIG. 10); or the shrink seal portion and the cylindrical portion (see FIG. 1). The shrink seal portion to which the shroud tube is welded can be formed in the following manner.
The arc tube main body may be fabricated in a manner that the predetermined filling material is supplied to the glass bulb of the glass tube which one end side is subjected to the primary pinch sealing, and thereafter the other side of the glass tube is subjected to the secondary pinch sealing. Then, the secondary pinch sealing process is performed in a manner that the seal expected area near the glass bulb is heated, molten and softened while the glass bulb of the glass tube is cooled by using cooling medium. In this respect, prior to the secondary pinch sealing using a pincher, the seal expected area having been heated, molten and softened deforms and shrinks in the diameter reducing direction due to the negative pressure within the glass tube (the negative pressure formed by condensing the filling material such as inactive gas etc. within the glass bulb) and so the shrink seal portion with the circular cross section is formed. In other words, the secondary pinch seal expected area of the glass bulb is entirely shrink-sealed. Then, the glass bulb side of the shrink seal portion is pinch-sealed with the predetermined width (a portion of the shrink seal portion closer to the glass bulb is pinch-sealed so that the shrink seal portion with the predetermined width remains), whereby the shrink seal portion with the circular cross section (shroud glass welded portion) is formed adjacent to the pinch seal portion with the rectangular cross section.
The width (length) of the shrink seal portion serving as the shroud glass welded portion may be in a range of L/6 to L/2, where L represents the entire length of the seal portion (that is, the pinch seal portion and the shrink seal portion). The inventors have determined that when the width is equal to or less than L/6, it becomes difficult to weld the shroud glass and a space is generated at the welding surface. In contrast, when the width is equal to or more than L/2, the length of the pinch seal portion becomes shorter, so that the property of the adhesion between the glass layer and the electrode assembly at the seal portion may be degraded and the airtightness of the sealed glass bulb may not be secured.
Further, in a method of fabricating an arc tube for a discharge lamp according to the invention,
in the arc tube main body fabricating process, a cylindrical non-pinch seal portion provided with a circular flange portion on an outer periphery thereof is formed in an extended manner at a backward portion of the pinch seal portion on the rear end side of the arc tube main body, and
in the shroud glass welding process, the rear end side of the shroud glass tube is welded on the circular flange portion on the rear end side of the arc tube main body.
The circular flange portion serving as the shroud glass welded portion is disposed closely to the inside of the rear end portion of the shroud glass tube, and the rear end portion of the shroud glass tube having been heated, molten and soften is molten and welded smoothly on the circular flange portion on the inside.
Also, the arc tube main body fabricating process may include a glass bulb forming process for forming a glass bulb at a portion of the glass tube; a primary pinch seal process for inserting an electrode assembly from one end side of the glass tube provided with the glass bulb and pinch-sealing a portion near the glass bulb; a sealing and exhausting process for supplying predetermined filling material such as mercury to the glass bulb, inserting an electrode ashy from the other end side of the glass tube and holding the ashy thereat, supplying inactive gas within the glass bulb and pinch-sealing or tipping off an opening end side of the glass tube to seal within the glass tube; and a secondary pinch seal process for pinch-sealing a portion of the glass tube near the glass bulb,
the shroud glass welding process includes a process for welding the rear end side of the shroud glass tube on the rear end side of the arc tube main body, and a process of welding the front end side of the shroud glass tube on the front end side of the arc tube main body,
in the secondary pinch seal process constituting the arc tube main body fabricating process, a seal expected area near the glass bulb is heated and molten to perform shrink sealing while cooling the glass bulb by using cooling medium, thereafter the glass bulb side of the shrink seal portion is pinch-sealed with a predetermined width to form a shrink seal portion adjacent to the pinch seal portion,
in the shroud glass tube front end side welding process constituting the shroud glass welding process, a pressure within the shroud glass tube which rear end side being welded on the rear end side of the arc tube main body is kept at a negative pressure, a welding expected area on the front end side of the shroud glass tube is heated, molten and softened, and the front end side of the shroud glass tube is shrink-sealed to the shrink seal portion adjacent to the pinch seal portion.
In the secondary pinch seal process of the arc tube main body fabricating process, the seal expected area on the front end side of the glass tube having been heated, molten and softened deforms and shrinks in the diameter reducing direction due to the negative pressure within the glass tube (the negative pressure formed by condensing the filling material such as inactive gas, etc. within the glass bulb) and so the shrink seal portion with the circular cross section is formed. Then, the glass bulb side of the shrink seal portion is pinch-sealed thereby to form the shrink seal portion (shroud glass welded portion) adjacent to the forward portion of the pinch seal portion on the front end side of the arc tube main body.
In the shroud glass tube front end side welding process of the shroud glass welding process, the welding expected area of the shroud glass tube having been heated, molten and softened deforms and shrinks in the diameter reducing direction due to the negative pressure within the glass tube and is molten and welded on the shrink seal portion with the circular cross section (the shroud glass welded portion) on the front end side of the arc tube main body.
In addition, in the method of fabricating an arc tube,
in the arc tube main body fabricating process, a cylindrical non-pinch seal portion is formed in an extended manner at a forward portion of the pinch seal portion on the front end side of the arc tube main body, and
in the shroud glass welding process, the front end side of the shroud glass tube is welded on only the cylindrical non-pinch seal portion on the front end side of the arc tube main body or on a welded portion with a circular cross section including the cylindrical non-pinch seal portion.
Since the front end side of the shroud glass tube is welded on the cylindrical non-pinch seal portion on the front end side of the arc tube main body, the axial length of the welding surface can be made larger.