US2716714A - Incandescent electric lamp - Google Patents

Description

Aug. 30, 1955 E. T. ADAMS ET AL f INCANDESCENT ELECTRIC LAMP Filed Aug. 20, 1951 INVENTORS E. VTHDH/V .Z1-S'. 6500.9/1/608/A %BY 917% l A'I"TORNEY nited States Patent Ofic 2,7 l 5,7 ld Patented Aug. 3f), 1955 INCANDESCENT ELECTRIC LAMP Edward T. Adams, Long Island City, N. Y., and Isaac S. Goodman, Belleville, N. J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 20, 1951, Serial No. 242,606

1 Claim. (Cl. 313--27i) This invention relates to lamps, and more particularly, to leading in and supporting conductors for lamps, such at those employed as automobile head lamps.

In general, leading-in and supporting conductors for the filaments of incandescent lamps are multiple piece leads as, for example, a three-piece lead comprising an external base contact portion, a sealing portion and an inner filament supporting portion. This external portion for making electrical contact with the base is usually a copper Wire. The sealing portion having a coefficient of expansion close to that of the glass in the stem, is sealed through the press of said stem. The inner portions welded at their upper extremity to the legs of a refractory metal filament, usually coiled tungsten wire, in present practice are ferrous metal, such as, for example, nickel or iron.

In most resistance welding as normally employed to form the leading-in conductors, the inner portions of such leading-in conductors, melting at a lower temperature than the refractory filament leg, become molten at the point of the weld to a depth about one half the thickness of the leg. The filament leg is forced into the molten metal which iiows up the sides of the leg, thereby forming a fillet and locking the leg to the conductor. A thin intermediate layer of alloy composed of the metals in the leg and inner portions of the conductor is formed between said leg and said conductor and bonds them together.

in some lamps, such as automobile headlight lamps, where repeated intermittent burning is normal operation, it has been found that the lamps lose focus and eventually fail prematurely due to a separation of the filament. leg from the inner portion of the leading-in and supporting conductor. This separation occurs not in the alloy bond between the filament leg and the conductor, but in the conductor adjacent to the bond.

lt is believed that the considerable differential in the coefficient of thermal expansion between the tungsten filament and the nickel inner portion of the conductor in the Zone of expansion created by intermittent operation is the primary cause of this separation or tearing of the conductor within itself adjacent to the bond area. When the current is initially passed through the leadingin conductor and the filament, a high temperature, approximately 1300", is created in the bond area, within a relatively short time and is accompanied by expansion of the metals involved.

The faster expanding nickel in the inner portion of the leading-in conductor is impeded by the slower expanding tungsten of the filament, thereby creating stresses in the nickel. Conversely, while cooling, the faster contracting nickel tends to pull away from the slower contracting tungsten, thus creating additional stresses in the nickel. A series of such expansions and contractions in a short time results in the tearing of the nickel adjacent the bond due 'to the work hardening of the nickel which has exceeded its elastic limit in the zone of expansion and contraction.

The immediate result of any slight tear within the conductor adjacent the bond area is a shift in the exactly mounted position of the filament, which results, in the case of a headlight, in a loss of focus and defective operation of the lamp. If the tear enlarges and the separation becomes complete, premature failure of the lamp occurs.

Hence it has been found advantageous according to our invention to employ metals having comparable coefiicients of thermal expansion in the zone of expansion or heating created by the intermittent starting and stopping of the lamp. Metals for the inner portion of the conductor are chosen which are compatible in coeflicient of thermal expansion with that of the refractory metal filament. Further, the bond between the extremity of the new metal inner portion and the former nickel or iron inner portion of the conductor is moved out of the expansion zone to a cooler portion of the lamp.

ln its general aspect the present invention has the object of overcoming the aforementioned disadvantages of the prior art relating to leading-in and supporting conductors for incandescent lamps.

Another and specific object is a leading-in and supporting conductor having an inner portion which will not tear within itself adjacent the bond area in the weld between said conductor and a refractory filament.

An additional object is a leading-in and supporting conductor for an incandescent lamp which will eliminate shifting of the filament mounted thereon during operation anda resultant loss of focus.

Another object is a leading-in and supporting conductor for an incandescent lamp which will not tear within itself adjacent the bond area of the weld between said conductor and said filament, thereby eliminating premature lamp failures.

A further object is a multiple section leading-in and supporting conductor having an inner extremity in the Zone of expansion whose coefiicient of thermal expansion is comparable to that of the refractory metal filament welded thereon.

Other objects and advantages of the invention will appear to those skilled in the art to which is appertains as the description thereof proceeds both by direct recitation thereof and by implication from the context.

Referring to the drawings in which like numerals of reference indicate similar parts throughout the several views:

Fig. l is a perspective view of an automobile headlight lamp embodying our invention;

Fig. 2 is an axial sectional view of a mount for the lamp of Fig. 1;

Fig. 3 is an axial sectional view similar to Fig. 2 showing an alternative embodiment of our invention;

Fig. 4 is a fragmentary sectional view of a molybdenum leading-in conductor and a tungsten filament leg along line IV-lV of Fig. 2 in the direction of the arrows;

Fig. 5 is a view similar to Fig. 4, showing the tearing of a prior art nickel leading-in conductor within itself adjacent a tungsten filament leg.

The invention is illustrated in Fig. l as being embodied in a miniature lamp such as an automobile headlight lamp 1i), comprising a vitreous envelope, a base 14, a stem 15 having a press 16 and an exhaust tubulation 17 (Fig. 2) and a major filament 18 and a minor filament 20.

The major filament 18 is carried by two multi-piece leading-in conductors, namely, the major leading-in conductors 22 and the common leading-in conductor 24. The minor filament 20 is carried by a multi-piece minor filament leading-in conductor 26 similar to conductors 22 and 24 and by an L-shaped metallic support 28, suit- 3 ably molybdenum or tungsten. Support 28 is affixed as by welding to the common leading-in conductor 24.

The major filament 18 is tungsten wire and has a helically wound body portion 30 with legs 31 and 32 depending from said body in parallel vertical planes. Prior to mounting, legs 31 and 32 are bent outwardly from and parallel to said body 30.

The minor filament 20 is similar to the major filament 18 and has a body portion 34 with legs 35 and 36.

A spacing between said filaments 1S and 20 in lamps of this type is critical such, for example, that for suitable operation the center line of minor filament 20 is mounted five thousandthsiten thousandths above the center line of major filament 1S. The spacing between filament 18 and filament 20 is 100 thousandthsiten thousandths. One end of filament 1S is offset 10 thousandthsilO thousandths from the coaxis of major filament 18 and stem 16.

According to our invention, as shown in Fig. 2, the four piece leading-in conductors 22, 24 and 26 each have an inner portion or piece 38 of a refractory metal, such as molybdenum or tungsten for joining to the fila ment legs 31 and 32 of major filament 18 and the leg 35 of minor filament 20. An intermediate portion 40 suitably nickel, a seal portion 42, suitably Dumet for joining to the press 16 of stem 15, and an external portion 44 handily copper wire for afixng to the contacts of base 14 are provided. The inner portions 38 may also be Kovar" which has the same approximate coefficient of expansion as tungsten. Kovar is the trade name for a metal alloy comprising primarly nickel, cobalt and iron as described in U. S. Patent No. 2,062,335. Dumet is the trade name for a wire consisting of a nickel iron core, enclosed in a copper sheath having a total thickness approximately of the finished wire diameter.

In Fig. 3 an alternative embodiment shows each of the legs 31 and 32 of the major filament 18 depending vertically downward from the body and joined as by welding, to a pad 46, suitably a metallic wire having a coefiicient of thermal expansion intermediate between that of nickel and tungsten. The pad 46 has been previously joined, as by welding, to the inner portion 38 of conductors 22 and 24. As an example, pad 46 may be sealmet No. 4 which is a nickel, iron alloy having the following approximate composition:

Metal: Percentage Chromium 5.5 Nickel 42.0 Iron 52.5

A comparison of the approximate thermal coefficients of expansion of the metals referred to in the foregoing is tabulated below:

Coefiicient of Expansion (10*6 cm. per cm.

In Fig. 5 a prior art leading-in conductor 48 is shown having a nickel inner portion 49 which has torn within itself adjacent the alloy bond between itself and a tungsten filament leg 50. A failure of this type causes shifting of the major and minor filaments and subsequent loss of `focus prior to the eventual failure due to the separation of the filament leg from the inner portion 49 of the leading-in conductor 48.

Thus it will be seen from the foregoing that the present invention has overcome the aforementioned disadvantages of the prior art leading-in and supporting conductors for miniature incandescent lamps such as the auto headlight lamp 10. Our invention employs a leading-in and supporting conductor such as 22, 24 and 26, which will not tear within themselves adjacent the bond area in the weld between said conductors and refractory filament leg such as 31 and 32 of major filament 18 and 35 of minor filament 20. Use of such a leading-in and supporting conductors eliminates shifting of the major and minor filaments mounted thereon during lamp operation and resultant loss of focus. By employing in the zone of expansion created by the intermittent starting and stopping of the lamp 10 leading-in and supporting conductors having an inner extremity, whose coefficient of thermal expansion is comparable to that of the refractory filament leg welded thereon, defective welds and premature lamp failures are eliminated.

Whereas a preferred embodiment of our invention has becn disclosed, it will be understood that modifications may be made Within the spirit and scope of the appended claim.

We claim:

A filament structure for a lamp comprising a nickel leading-in conductor, an alloy member secured to said conductor in the zone of expansion of said structure, said member having about 5.5% chromium, 42.0% nickel and the balance iron and a tungsten filament secured to said member.

References Cited in the file of this patent UNITED STATES PATENTS 2,341,716' Herdman Feb. l5, 1944 2,396,674 Braunsdorff et al Mar. 19, 1946 2,434,478 Allen Jan. 13, 1948

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