Description:
BACKGROUND OF THE INVENTION
This invention relates to the manufacture of photoflash lamps and, more particularly, to flashlamps containing filament lead-in wires and means for preventing post-ignition short circuits therebetween.
Photoflash lamps generate an actinic light output by the burning of an energetic fuel, such as finely shredded zirconium, hafnium or aluminum metal foil, in a combustion supporting atmosphere, such as oxygen. In some of the tubular electrically ignitable photoflash lamps presently manufactured, the ignition means comprises a pair of lead-in wires sealed through one end of the tubular glass envelope and supported in a spaced side-by-side relation by a glass bead fused about the wires. A tungsten filament is mounted across the inner ends of the two lead-in wires with the ends of the wires at their junctions with the filament being coated with a primer material, such as a powdered zirconium mixture. When battery power is applied to the external projecting portions of the two lead-in wires, the filament glows to incandescence, causing the primer material to ignite, which in turn ignites the finely shredded metallic combustible in the lamp to produce a predetermined quantity of light output.
Certain other lamp types additionally employ a glass sleeve disposed about a portion of one of the lead-in wires as an insulating shield extending from the glass bead toward the filament for preventing post-ignition short circuits across the lead-in wires. Such a feature is required for the proper operation of certain flash sequencing circuitry for controlling linear arrays of flash lamps. For example, in one presently marketed photoflash array application, if a short circuit occurs between the melted lead-in wires in the first (or subsequent) lamp of the array to be flashed by the sequencing circuitry, the entire array of lamps is rendered useless.
Although reliably providing the desired lead-wire isolating function, however, the use of a glass insulating sleeve significantly increases both the manufacturing and materials cost of the lamp unit. For example, according to a present manufacturing method for processing the ignition mount structure, the lead wires are initially provided in the form of a hairpin with the closed end facing downward. The glass bead is then melted and fused about the lead wires to retain the spacing therebetween; this is a conventional beading operation. Next, the closed end of the hairpin is trimed off, in the normal manner. The mount structure is then rotated 180°, and a glass tube is placed over one of the lead wires so that it rests on the bead. The end of the tube in contact with the bead is then heated just enough to fuse it to the bead but without distorting its upper portion. The mount structure must then again be rotated by 180° to make it ready for further processing.
It will be noted that this method requires turning the mount structure head 180° at two separate locations. It also requires a tube feeder and loader, a device to locate one of the wires so that the glass tube can be fed over it, and a great deal of skill on the machine attendant's part.
SUMMARY OF THE INVENTION
In view of the foregoing, one of the principal objects of this invention is to provide an electrically ignitable photoflash lamp having improved means for preventing post-ignition short circuits across the lead-in wires.
A further object is to provide an improved and more economical method of manufacturing an ignition mount structure for a photoflash lamp which maintains a suitable isolation between the lead-in wires to prevent post-ignition short circuits.
These and other objects, advantages and features are attained, in accordance with the principles of this invention, by fusing an insulating bead to the lead wires, as at present, but then while the bead is still in a plastic state, stretching the midportion of the bead. In this manner, the insulating bead of the finished mount structure is shaped between the lead-in wires to protrude toward the filament, thereby providing at minimum cost a reliable means for preventing post-ignition short circuits between the lead-in wires.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention will be more fully described hereinafter in conjunction with the accompanying enlarged scale drawings, in which:
FIG. 1 is an elevational view, partly in section, of a photoflash lamp having an ignition mount structure in accordance with the invention;
FIG. 2 is an enlarged diagram illustrating the shaped bead employed in the mount structure of FIG. 1 and the manner in which the bead prevents post-igintion short circuits; and
FIGS. 3-5 are sequential representations of the steps of a method in accordance with the invention which may be employed for making the mount structure of FIG. 1.
DESCRIPTION OF PREFERRED EMBODIMENT
Referring to FIG. 1 an electrically ignitable photoflash lamp is shown comprising an hermetically sealed, light-transmitting envelope 10 of glass tubing having a press 12 defining one end thereof and an exhaust tip 14 defining the other end thereof. A quantity of filamentary combustible material 16, such as shredded zirconium or hafnium foil, is located within the lamp envelope. The envelope is also provided with a filling of combustion-supporting gas, such as oxygen, at a pressure of several atmospheres.
The ignition mount structure comprises a pair of lead-in wires 18 and 20 extending through and sealed into the press 12. A filament 22 spans the inner ends of the lead-in wires, and beads of primer 24 and 26 are located on the inner ends of the lead-in wires 18 and 20, respectively, at their junctions with the filament. The lead-in wires are supported in a spaced side-by-side relation by a glass bead 28 fused about the wires. In accordance with the invention, the glass bead is uniquely shaped by a distortion of its midportion 30 between the lead-in wires to protrude toward the filament 22.
When battery current is applied to the external projecting portions of the two lead-in wires the filament 22 glows to incandescence, causing the primer material 24, 26 to ignite, which in turn ignites the finely shredded metallic combustible material 16 in the lamp to produce the desired flash of light output. The intense heat of this combustion process causes the top portion of the lead-in wires 18 and 20 to melt away down to the top surface of the glass bead 28. As illustrated by the diagram of FIG. 2, however, the upwardly protruding portion 30 of the glass bead serves to isolate the molten portions 32 of the two lead-in wires so that a short circuit conductive path is not inadverently effected by a chance fusion of the two melting wires subsequent to flashing.
FIGS. 3-5 illustrate a method of making the mount structure of FIG. 1, in accordance with the invention, which has been found to result in a highly reliable flashlamp structure with protection against post-ignition shorting, while at the same time being particularly suited to automated assembly and providing a significantly more convenient and economical mode of manufacture. Basically, the method comprises: providing a pair of lead-in wires 18 and 20 closed at one end 21 in the form of a hairpin; next, placing a glass bead 28' between the lead-in wires and fusing the bead to the wires to retain a spacing therebetween, as illustrated in FIG. 3; then, while the bead is still in a plastic state from the fusing step, a blade 34 is inserted between the pair of wires from the open end and pushed downwardly against the midpoint of the fused bead to stretch and displace the midportion 30 so that it protrudes toward the closed end 21 of the "hairpin," as illustrated in FIG. 4. Thereafter, the closed end 21 of the lead-in wires 18 and 20 is trimmed off, as illustrated by FIG. 5, and the filament 22 is attached across the wires near the trimmed ends thereof, whereby the stretched midportion 30 of the bead 28 protrudes toward the filament (as in FIG. 1).
It will be noted that the process steps illustrated by FIGS. 3-5 all take place with the mount structure held in an inverted position. No 180° rotations of the mount are required. The cost of the glass tube, previously employed on mount structures with post-ignition short protection, has been eliminated. The process also eliminates the previously required bead feeder, bead loader, wire gatherer, two turn-over devices, extra burner to melt the glass tube, tube detectors and mount ejector.
In one specific embodiment of the mount structure of FIGS. 1-5, the unstretched bead 28' had an initial height dimension of about 0.080 inch, and the stretching operation to produce the protruding midportion 30 extended the height to about 0.165 inch. Hence, the bead was stretched to approximately double its initial height. In other words, the height of the stretched bead is preferably at least about twice the height of the bead portions fused about the lead-in wires. Such a stretched bead ignition structure has proved to be extremely reliable in preventing post-ignition short circuits, as several thousand lamps of this type have been test flashed without a post-ignition short failure.
Although the invention has been described with respect to specific embodiments, it will be appreciated that modifications and changes may be made by those skilled in the art without departing from the true spirit and scope of the invention. For example, bead 28 may be formed of other meltable insulating materials in lieu of glass, and the original and stretched dimensions of the bead may vary for different lamp parameters.