US3930178A

US3930178A - Electrical incandescent filament devices

Info

Publication number: US3930178A
Application number: US485778A
Authority: US
Inventors: James Richard Coaton
Original assignee: Thorn Electrical Industries Ltd
Current assignee: Thorn EMI PLC
Priority date: 1973-07-10
Filing date: 1974-07-05
Publication date: 1975-12-30
Anticipated expiration: 1992-12-30
Also published as: GB1449851A; AU7107474A; FR2237314B1; JPS5418868B2; DE2432762A1; ZA744324B; CA1006219A; NL7409134A; IT1017658B; DE2432762B2; JPS5038386A; FR2237314A1; NL161299C; NL161299B

Abstract

An electrical device with an elongated tubular envelope has an incandescent filament supported along the axis of the envelope by ceramic plates mechanically bonded to the filament and fitting closely in the envelope. The plates may be interconnected by a link rod parallel to the filament which is a single-ended lamp serves also as a lead to one end of the filament.

Description

United States Patent 111111111(IIII/flllIlIlI/Tj Coaton Dec. 30, 1975 [54] ELECTRICAL INCANDESCENT FILAMENT 2,813,993 11/1957 Fridrich 313/274 X DEVICES 2,901,658 8/1959 Pero et al. 313 275 x 2,910,605 10/1959 Hodge 313/275 X [75] In n r: Jam ich r a n, L n, 2,980,820 4/1961 Brundige et al. 313 275 x England 3,195,001 7/1965 Hodge 313/274 X [73] Assignee: Thorn Electrical Industries Limited,

London England Primary ExaminerSaxfield Chatmon, Jr. [22] Filed: July 5, 1974 1 Attorney, Agent, or FirmRobert F. OConnell [21] Appl. No.: 485,778

[30] Foreign Application Priority Data [57] ABSTRACT July 10, 1973 United Kingdom 32729/73 An electrical device with an elongated tubular enve- [52] U.S. Cl. 313/275; 313/277; 313/222; lope has an incandescent filament supported along the 313/315 axis of the envelope by ceramic plates mechanically [51] Int. Cl. HOIJ 1/90; l-lOlJ 19/44 bonded to the filament and fitting closely in the enve- [58] Field of Search 313/274, 275, 277, 276, lope. The plates may be interconnected by a link rod 313/279, 222, 315 parallel to the filament which is a single-ended lamp serves also as a lead to one end of the filament. [56] References Cited UNITED STATES PATENTS 5 Claims, 5 Drawing Figures 1,997,975 4/1935 Perrin et al 313/275 X Sheet 1 of 2 3,930,17

US. Patent Dec. 30, 1975 ELECTRICAL INCANDESCENT FILAMENT DEVICES filament lamps. Examples of such devices include linear tungstenhalogen lamps and linear infra-red heater lamps. These lamps have hermetically sealed, narrow bore tubular envelopes containing filaments which extend axially along their envelopes. Commonly, the filaments are coiled.

One of the main problems restricting the successful development of such linear devices has been the difficulty of supporting their filaments adequately within their envelopes. In certain linear lamps, e.g. infra-red lamps, the lamp filament is supported by thin refractory metal discs, e.g. of only several thousandths of an inch thickness, which are spaced at suitable intervals along the lamp. The filament passes through an aperture in the centre of each disc. The disc has a diameter only slightly less than the envelope bore so that this arrangement locates the filament centrally along the axis of the tubular envelope. The support discs are usually made from an active metal, such as tantalum, niobium, titanium etc. which acts as a getter for residual impurities within the lamp envelope. In an alternative construction, spiral wire supports are used. The wire supports each have a few inner turns wrapped around the filament, an outer turn or turns engaging the inner wall of the envelope, and a spiral of wire linking the inner and outer turns. This arrangement is generally used to support the filament of a linear tungsten-halogen lamp, tungsten wire being used to avoid chemical reaction with the halogen and/or halides present in the lamp. Spiral wire supports are also used in conventional linear incandescent lamps, wherein the material used for the support is frequently chosen to act as a getter in the finished lamp.

With both of the above types of filament support it is possible for the support to become tilted or skewed relative to the tube axis. To overcome skewing, the lamp designer has relied upon the coiled filament being sufficiently stiff to prevent the supports from tilting inside the tubular envelope. This effectively sets a lower limit on the filament power rating, and this is particularly restrictive for mains voltage devices. As an example, this limit corresponds approximately to 300W for 240V lamps with 2000b rating.

We have found that by using self-supporting filament supports made from a ceramic material it is possible to avoid undue local stressing of the filaments, which thereby makes practicable linear lamps of lower filament ratings than hitherto available.

According to the present invention there is provided a linear incandescent filament electrical device having an elongated tubular envelope containing a filament extending along the tube parallel to the central axis thereof, the filament being supported, at intervals along the tube, by transversely-directed ceramic plates to which the filament is bonded mechanically, each plate being a snug fit within the envelope. Preferably, a coiled-coil filament is used, and it may be centred upon the central axis of the envelope. Each plate is preferably of such dimensions that it is unable to become skewed relative to the axis of the tube. For the ceramic plates to be stable within the envelope, it is desirable that their edges,-which engage the inside surface of the envelope, be square to the planes of the plates. It is found that a very stable arrangement is obtained if the plates are of the order of 6.6 mm. in diameter and 0.5 mm. in thickness, when the envelope has a nominal inside diameter of 7 mm.

The rigidity of the structure within the envelope, namely, the filament and the ceramic plates can be enhanced by interconnecting the plates by one or more link rods. Conveniently, the link rods are also bonded mechanically to the ceramic plates. The use of such link rod(s) considerably stiffens and stabilises the structure and may be preferred. If desired, the ends of the link rod(s) can be made to engage the envelope to restrict or prevent axial movement of the structure within the envelope.

The link rod can also serve as an electrical lead connected to one end of the filament and extending to the opposite end thereof to give a single-ended lamp in which both filament leads emerge from the same end of the envelope. To prevent arcing the electrically-conductive link rod can be provided with an insulating coating or sheath over at least part of its length.

The mechanical bonding can be regarded as a shrinkfit of the ceramic plates on to the filament obtained during a sintering operation which precedes the installation of the filament and the ceramic plates within the envelope. In some instances, some of the outer turns of the filament may actually become embedded in the ceramic plate. In addition, the ceramic material may become keyed to draw markings" on the surface of the filament wire. Y

The invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a partial side elevation of a linear incandescent lamp embodying the invention and includes a view taken along the line A-A;

FIG. 2 is a view,-similar to FIG. 1, illustrating an intermediate stage in the manufacture of the lamp shown in FIG. 1;

FIG. 3 shows plan views of several filament supports which may be used in the lamp shown in FIG. 1;

FIG. 4 is a side view of a lamp similar to that of FIGS. 1 and 2 but with a link rod which engages the ends of the envelope, and

FIG. 5 is a side view of a single-ended linear incandescent filament lamp in accordance with the invention.

The lamp shown in part in FIG. 1 is a linear incandescent lamp. It can, for example, be a conventional filament lamp such as an infra-red lamp, or a tungstenhalogen lamp. In the latter instance, the lamp may possess a gas filling to 10 atmospheres pressure of a mixture of krypton containing pg of CH]; and I5 pg of PNBr Gas fillings of this type are disclosed in our British Patents Nos. 1,236,174 and 1,318,71 1. Whilst the lamp construction shown in FIG. 1 is suitable for low power, long life lamps operating directly from the normal mains supply, the actual lamp rating is unimportant.

The lamp has an envelope 10, for example of alumino-silicate or high silica glass, which is of elongated tubular shape. A coil filament 11 extends along the length of the envelope 10 parallel to or coincident with the central axis of the envelope 10. The filament has tails 12 which in practice are electrically connected by lead-in conductors to external terminals, and the envelope has hermetic seals, for instance pinch seals at its ends. The nature of the electrical connections and the seals do not form part of this invention and accordingly details thereof are omitted.

Spaced at intervals along the filament l l are filament supports 14 in the form of plates of ceramic material. The ceramic material is preferably an oxide, such as alumina, magnesia, yttria, thoria or common spinel, because these materials can easily be fabricated using well established techniques and remain stable at high lamp operating temperatures. The filament 11 passes through each of the ceramic plates 14 and is bonded mechanically thereto. The bond is effectively a shrinkfit of the plate upon the filament. In some cases, the filament may become partially embedded in the plate.

The thickness and configuration of the ceramic plates 14 are chosen so that they are self-supporting inside the tubular envelope 10, and hence the plates hold the filament 11 in its correct position without relying upon the mechanical strength of the filament to hold the plates 14 in place. Also, the thickness and configuration are chosen so that the operating temperature at the point of contact between the ceramic plate 14 and the filament l 1 is below the melting point of the ceramic. As will be seen, each plate has a substantial thickness and its peripheral edge is normal to the plane of the plate. The plate 14 is a snug fit within the bore of the envelope, and its periphery comprises the major portion of a circle. A minor segment is missing from the circular plate 14, thereby leaving a gap between-the periphery of the plate 14 and the innersurface of the envelope 10. This gap assists thorough exhaustion and gas filling during manufacture.

- In a particular example of a lamp constructed as shown in FIG. 1, the envelope had a nominal internal diameter of 7 mm, .the filament coil had an outside 1 diameter of 0.76 mm and the ceramic plate had a diameter of 6.6 mm. Its thickness was approximately 0.5 mm. It will thus be seen that the plate 14 is somewhat smaller than thenominal bore of the envelope 10 so as to allow for the dimensional'tolerance on the bore. Nevertheless, it is found that the plates 14 will stand within the envelope 10, as shown, without tending to tilt or skew relative to the central axisof the envelope 10. The difference of size does mean that the plates 14 can roll to and fro slightly within the envelope 14, but such movement only shifts the filament slightly in a transverse direction, without stressing the filament significantly. Undue stressing of the filament 11 in the region of each plate 14 is thus avoided.

The rigidity of the internal structurewithin the envelope.l0 can optionally be improved by means of a rod 15 linking the ceramic plates 14. The rod 15, like the filament 11, is bonded mechanically-to the plates 14. The rod 15 can be metallic or ceramic, and significantly strengthens the internal structure. Whilst only one'rod 15 is shown, additional stiffening rods parallel to the filament 11 could be employed. The rod or rods 15 can be made from high purity densealumina, 1 mm.

in diameter. l

The method of making a linear lamp' embodying the invention, which has an elongated tubular envelope containing a filament extending lengthwise along the envelope, comprises the steps of fabricating apertured support plates from a ceramic material such that in the unsintered state the plates are oversize relative to the envelope bore, threading the filament through a plurality of the support plates and spacing them at intervals along the filament, and firing or sintering the assembled plates and filament so as to cause the plates to contract about the filament and to contract sufficiently to fit snugly within the envelope, the assembly thereafter being installed in the envelope. Usually, a refractory metal mandrel is threaded through the filament to prevent sag and distortion during the ceramic sintering process, and thisis removed after sintering. In addition, a ceramic or metallic linking rod is desirably threaded through, or otherwise attached, to the plates prior to firing or sintering, so as to become mechanically bonded to the plates to rigidify the structure.

FIG. 2 shows the assembly of plates 14, filament 11 and link rod 15 for the lamp shown in FIG. 1, prior to firing. It will be seen that the plates 14' are of similar shape to the plates 14 of the finished lamp, having peripheral edges square to the planes of the plates and being larger than the finished plates 14. At this pre-firing stage, the diameter of the plates 14' is 8.2 mm compared with the 6.6 mm diameter of the plates 14 after firing. Each plate has two apertures 16, 17 to receive the filament and link rodyIt is found that the apertures 16, 17 should be approximately 5% larger in diameter than the-

parts

11, 15 they are to accommodate. The actual diameter is not critical provided that it is greater than the outside diameter of the coiled filament 11 before sintering but less than that after sintering. Satisfactory results have been obtained when the platescontract by approximately 20%.

The ceramic plates 14' are fabricated in the usual way familiar to those versed in the art, so as tohave a thickness of 0.63 mm. The plates are made from A1 0 powder to which 0.05% MgO and 0.O5Y O have been added to improve sinterability. Three of these plates are threaded on to a 240V W coiled coil filament having an outside diameter of 0.76 mm and containing a molybdenum mandrel 20 to keep the filament straight. At this stage the filament wire is in a fibrous condition.

A number of these assemblies are held in .a suitable jig fabricated from molybdenum, which isthen loaded into a hydrogen tube furnace for approximately 2h at 1750C. During this time the ceramic material forms a dense polycrystalline structure, shrinks onto the filament 11 and the filament wire partially recrystallises giving a more stable structure. The final diameter of each sintered plate 14 is 6.6 mm and its thickness is approximately 0.5 mm. Finally, the molybdenum mandrel is withdrawn or dissolved in nitro-sulphuric acid and the assembly is washed and dried ready for installation in the lamp envelope tube. Appropriate lead-in conductors are connected to the filament tails l2 and the envelope 10 is then evacuated and gas filled in the usual way, using our gas fillings exemplified hereinbefore in the case'of a tungsten halogen lamp.

If desired, getters may be applied by attaching an active metal to the support arrangement or by applying a getter in suspension or solution to the filament or support assembly, prior to installation in the envelope tube. For a conventional non-regenerative lamp, the getter can be tantalum.

FIG. 3 of the drawings illustrates the profiles of various alternative support plates. The plate 30 is wholly circular, and has two apertures. One aperture is to accommodate the filament and the other to assist through exhaustion and gas filling. A third aperture may be provided if several such plates are to be connected by a link rod. The remaining plate profiles, as shown in FIG. 3, are not intended necessarily to be connected by link rods. Nevertheless, one or more apertures can be provided in these alternative plates to accommodate a corresponding number of link rods, if required.

The ceramic support plates are considerably cheaper than conventional metallic supports. It would be wholly impractical to make refractory metal supports of comparably massive thickness, and not only on the ground of expense. A major failing of a thick metal support would lie in the amount of heat which it would conduct away from the filament. This would, inter alia, cause severe local filament cooling, unduly heat the envelope with the attendant risk of envelope failure, and reduce the light output, hence efficacy, of the lamp. The ceramic support plates are quite poor heat conductors, and by appropriate choice of their profiles, their cooling effects upon the filament can be largely controlled. The ceramic plates are further of advantage in that, unlike refractory metal supports, they can be accurately pressed, and require no further machining or deforming prior to attachment to the filament.

FIG. 4 of the drawings shows a modification of the lamp of FIGS. 1 and 2 in which corresponding parts have been given the same reference numerals. In this construction the link rod is extended so that its ends 21 engage the end walls 22 of the tubular envelope 10. The link rod 15 thus not only improves the stability of the structure and maintains the correct spacing between the ceramic suppo'rts but also prevents axial movement of the structure so that the filament and support assembly is mechanically stable at every burning angle.

In the single-ended lamp of FIG. 5 a filament 31 is supported along the axis of a tubular envelope 32 by ceramic'plates 33. The composition and method of assembly of these parts and the lamp fill may be as described with reference to FIGS. land 2. The plates 33 are joined by a link rod 34 which in this case is an electrically-conductive metal rod. One 'end of the link rod 34 is bent over to form an arm 35 to which a tail 36 of the filament 31 is attached. The other end of the link rod 34 forms an electrical lead 37 which passes through the-end of the envelope 32 remote from the filament tail 36. The filament 31 has a second tail 38 which is 5 joining the

leads

37 and 39. In this way the filament,

together with its leads and supports, forms a single rigid assembly secured together by the link rod 34. The manufacture of the lamp is thus greatly simplified. After the assembly has been constructed and sintered it can be inserted in the envelope, which is then sealed and exhausted in a conventional manner.

In some cases it is necessary, in order to avoid arcing, to isulate at least part of the link rod 34 which carries current to the filament 31. This can be achieved by the use of a glass or ceramic coating or, as shown in FIG. 5, by threading insulating tubes 41 over the rod 34 between the support plates 33.

I claim:

'1. In a linear incandescent filament electrical device having an elongated tubular envelope containing a filament extending along the tube parallel to the central axis thereof and supported at intervals by supports which engage the wall of the envelope, the improvement wherein the supports are plates of electrically- 'insulating, ceramic material heat-shrunk onto the filament to form amechanical bond therewith, each plate having edges which fit closely against the inside surface of the envelope and are of sufficient width to prevent skewing of the plate relative to the axis of the envelope.

2. In an electrical device as claimed in claim 1, wherein the improvement further includes at least one link rod extending parallel to the axis of the envelope and interconnecting the plates.

3. An electrical device as claimed in claim 2, in which each end of the link rod engages with the ends of the envelope to prevent axial movement of the supports.

4. An electrical device as claimed in claim 2, in which the link rod forms part of an electrical lead which is connected to one end of the filament and extends by way of the link rod to the opposite end of the filament, and-passes through the same end of the envelope as a lead connected to the said opposite end.

5. An electrical device as claimed in claim 4, in which the electrically-conductive link rod is provided with an Disclaimer 3,930,178.James Richard Coaton, London, England. ELECTRICAL INCAN- DESCENT FILAMENT DEVICES. Patent dated Dec. -30, 1975. Disclaimer filed Feb. 8, 1985, by the assignee, Thorn EMIpIc.

Hereby enters this disclaimer to all claims of said patent.

[Official Gazette April 8, 1986.]

Claims

1. In a linear incandescent filament electrical device having an elongated tubular envelope containing a filament extending along the tube parallel to the central axis thereof and supported at intervals by supports which engage the wall of the envelope, the improvement wherein the supports are plates of electricallyinsulating, ceramic material heat-shrunk onto the filament to form a mechanical bond therewith, each plate having edges which fit closely against the inside surface of the envelope and are of sufficient width to prevent skewing of the plate relative to the axis of the envelope.

4. An electrical device as claimed in claim 2, in which the link rod forms part of an electrical lead which is connected to one end of the filament and extends by way of the link rod to the opposite end of the filament, and passes through the same end of the envelope as a lead connected to the said opposite end.

5. An electrical device as claimed in claim 4, in which the electrically-conductive link rod is provided with an insulating coating or sheath.