GB2100506A

GB2100506A - An incandescent lamp coil

Info

Publication number: GB2100506A
Application number: GB8216183A
Authority: GB
Original assignee: Patent Treuhand Gesellschaft fuer Elektrische Gluehlampen mbH
Current assignee: Osram GmbH
Priority date: 1982-06-03
Filing date: 1982-06-03
Publication date: 1982-12-22
Also published as: GB2100506B

Abstract

Each turn of the wire 1 of an incandescent lamp coil has straight sections and curved sections, each curved section being offset relative to the corresponding section of the adjacent turns by an angle, alpha , lying between 10 DEG and 110 DEG about the axis of the coil. The coil is produced by winding the wire on a mandrel with an elongate cross-section, annealing the wire to partially relieve the stresses caused by winding, and then dissolving the mandrel so that the residual stresses cause the curved sections to open out to a predetermined extent. Wire 1 may be surrounded by a covering coil 3. Coil 3 is initially wound round both wire 1 and a round mandrel (2) the round mandrel subsequently being dissolved. <IMAGE>

Description

SPECIFICATION An incandescent lamp coil The present invention relates to an incandescent coil for an electric lamp in which the cross-sectional area of a turn is defined by one or more approximately straight sections and one or more circular sections.

Coils for incandescent lamps are usually wound about a flat mandrel of which the cross-sectional length is greater than its width. It has proved advantageous in practice to wind the coil wire about two equally large and parallel wires of, for instance, molybdenum or iron. In this case, the ratio of the length L to the width B of the cross section of the flat mandrel is 2. Depending on the application for which the finished coil is intended, different manufacturing steps precede the winding of the coil wire.

It is an aim in the manufacture of low pressure discharge lamps, especially of fluorescent lamps, to provide the coil with as much emitter paste as possible. A further aim is that the fluorescent lamp coil should become hot during current flow and thus emit light as rapidly as possible. In fluorescent lamps, a flat mandrel coil is frequently used for this purpose. The coil wire is provided during manufacture with a covering coil which encompasses, as well as the coil wire, a round mandrel of molybdenum or iron extending parallel to the coil wire. After winding, these coils are annealed to relieve stress at about 1 1000C to about 1400"C. In a further working step, the molybdenum or iron mandrels, respectively, are chemically dissolved.Later on, the emitter material is pasted on the cut coils after they have been assembled in the mount of the lamp stem. The emitter quantity accommodated by the coil has a substantial influence on the life of the lamp, since, especially when the lamp is switched on, emitter spatters off the coil, thus reducing the life of the lamp.

The present invention seeks to provide a coil which is easy to paste with emitter, has a greater emitter volume in the interior of the coil without reducing the power of the coil for the ignition operation, and/or which renders spattering away of the emitter more difficult.

According to a first aspect of the present invention there is provided a incandescent coil for an electric lamp in which the cross-sectional area of a turn is formed by one or more substantially straight sections and one or more curved sections wherein each curved section is off-set relative to the respective section of the adjacent turns by an angle a of from 10 to 1 10" about the axis of the coil.

According to a second aspect of the present invention there is provided a method of manufacturing an incandescent coil for an electric lamp comprising the steps of: a) winding the coil wire about a mandrel of elongate cross-section; b) annealing the coil wire wound about the mandrel at a temperature at which a part of the stresses in the coil wire, which have been generated by the winding process, is maintained; c) dissolving the mandrel chemically, with the circular sections springing backfrom the bent bending radius by a predetermined angle owing to the residual stresses, and each curved section of the coil turning relative to the respective section of the adjacent turns by an angle a of from 100 to 1100 about the coil axis.

The angle a by which each circular section is off-set relative to the respective sections of the adjacent turns is advantageously in the range of from about 60 to about 80". Preferably, every second turn of the coil is designed as a contraction.

At least every second circular section, preferably, however, each circular section, describes substantially the same length of a circular arc and has the same bending radius.

Outside the above indicated angular range, the coil cross-section would approach an oval shape if the angle a became smaller, and would approach the shape of a rounded-of triangle if a became as large as 1200. The individual circular sections of each turn would then no longer be sufficiently offset relative to the respective sections of the adjacent turns, and the appearance of the coil with its contractions, which is typical of the invention, would not occur.

The manufacture of the coil may be effected in that the coil wire is wound about a generally flat mandrel of molybdenum or of iron, with the length L of the cross-section of the flat mandrel being larger than its width B. Different coil shapes are obtained depending on the shape of the flat mandrel that is used. As a rule, the coil wire is wound about a flat mandrel whose ratio of the length L to the width B of the cross-section is equal to 2. The individual crosssections of each turn in this case initially extend identically and in a non-overlapping manner along the longitudinal coil axis. The straight section of the coil wire, in contrast to the finished coil, run parallel to one another, and the circular sections describe a semicircle.

In a further working step, the coil wound about the flat mandrel is annealed at a temperature below 11 00 C, for instance at 900"C. Thus, a portion of the mechanical stresses generated by the winding process is maintained in the coil wire. When the flat mandrel is dissolved in an acid bath, the bent portion of the coil wire springs back from the bent shape by a certain angle as a result of the residual stresses that have remained in it.The circular sections of the coil wire now describe less than a half and more than a third circle and are offset relative to the respective sections of the adjacent turns by an angle a which lies in the range between 10 and 110 . How high the annealing temperature is determines by what angle the coil twists from the original winding shape to the final shape having the characteristic contractions. In such a coil, each circular section describes substantially the same length of a circular arc and each circular section has the same bending radius.

In a further embodiment of the invention, the ratio of the length Lto the width B of the cross-section of the mandrel about which the coil wire is wound is smaller than 2. This is the case when, for instance, the coil wire is wound about two wires of unequal thickness which form the mandrel and wherein the thicker one of the two wires constitutes the measure for the width of the mandrel. In such a coil, every second one of two circular sections describes substantially the same length of a circular arc and every second circular section has the same bending radius.

It is particularly advantageous when the ratio of the length Lto the width B of the cross-section of the mandrel is selected greater than 2; this makes it possible to obtain a coil with even more strongly emphasized contractions than can be obtained with the previously described mandrels. The emitter accommodating capacity, and the retention of the emitter is thus further enhanced.

In the above described coil for fluorescent lamps, the coil wire is provided with a covering coil, which has a smaller wire diameter, prior to its being wound about the mandrel. The covering coil encompasses, apart from the coil wire, a round mandrel of molybdenum or iron which extends parallel to the coil wire and is dissolved together with the flat mandrel.

The invention is not limited solely to flat or elongate mandrel coils for low pressure discharge lamps. Other coils such as coiled-coils for low pressure discharge lamps or for incandescent lamps may be manufactured substantially in accordance with the same manufacturing method.

The surface of coils in accordance with the present invention appears wavy and rough from the outside, and the coil comprises in this region a greater quantity of emitter which is better retained thereon.

This gives a markedly longer life of the lamps provided with these coils.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings of which: Figure 1 illustrates the winding of a coil in accordance with a first embodiment of the present invention; Figure 2 shows a schematic sectional view of the coil of Figure 1; Figures 3 and 4 show schematic sectional view of coils in accordance with second and third embodiments of the invention respectively; Figure shows a schematic view of a coil in accordance with the present invention; and Figure 6 shows a schematic view of a portion of a coil in accordance with the present invention.

Figures 1 and 2 show the way in which the coil is wound. A coil wire 1 of tungsten and a mandrel wire 2 of iron which extends parallel thereto are encompassed by a covering coil 3. The diameters of the said wires are: coil wire 1 about 55cm, mandrel wire 2 about 120cm, and cover wire 3 about 22 Rm. This so-called first continuous wound coil is then wound about the flat mandrel which is formed by two equal mandrel wires 4 extending parallel to one another.

The mandrel wires 4 of iron each have a diameter of about 400 um. The ratio of the length L to the width B of the cross-section of the whole of the flat mandrel, about which the covering coil 3 is wound, is here about equal to 2. The cross-section of the coil is formed by approximately straight sections 5 and circular sections 6. The straight sections 5 initially extend parallel to one another, and the circular sections 6 describe approximately a semi-circle. The individual turns of the coil wound about the flat mandrel are identical and non-overlapping.

In Figure 3, the covering coil 3 is wound about two mandrel wires 4 and 7 of different diameter, with the mandrel wire 7 having a smaller diameter than the mandrel wire 4. Here, the ratio of the length Loo the width B of the cross-section of the whole of the mandrel, about which the covering coil is wound, is smaller than 2. The cross-section of the coil is formed by approximatelky straight sections 8 and circular sections 9 and 10. Owing to the different diameters of the mandrel wires 4 and 7, the circular section 9 describes more, and the circular section 10 describes less than a semicircle. The individual turns of the coil wound about the elongate mandrel are identical and non-overlapping.

In the embodiment of Figure 4, the flat mandrel 11 is formed by a flat-rolled iron wire whose crosssectional length L is 0.8 mm and whose width B is 0.25 mm. The ratio of the length Lto the width B of the cross-section of the flat mandrel 11 is, in this case 3.2, that is, greater than 2. The cross-section of the coil is formed by two straight sections 12 which extend parallel to one another and by two circular sections 13 which describe a semicircle. The crosssectional shape of the flat mandrel 11 may be adapted to each individual intended application. By incorporating for instance, grooves in the long sides, the effective flat mandrel cross-section may be reduced and the surface thus enlarged, thus permitting a substantial reduction of the time required for the subsequent dissolving process.

A coil wound in accordance with the Figures 1 and 2 does not involve any working steps which would differ from those required for making a conventional flat mandrel coil such as is normally used for fluorescent lamps. The final coil shape as shown in Fiures 5 and 6 which differs from the conventional shape is only obtained by the subsequent annealing at a reduced temperature of about 900"C. By this lower-temperature-annealing, a residual stress remains in the thicker coil wire 1 and, when the iron mandrels 2 and 4 are dissolved in an acid bath, this residual stress causes the circular sections 6 to spring back slightly. In order to maintain the clarity of the illustration, Figure 5 only shows the envelope of the covering coil 3 which encompasses the coil wire 1.It can be seen that the coil of the invention has distinct contractions 14 and broadened portions 15, one cm of coil length comprising nine contractions 14.

The highly simplified illustration of Figure 6 shows only a few turns of the coil wire 1 which has sprung back, and the latter is in turn encompassed by only a few turns of the covering coil 3. The circular sections 13 now describe less than a semicircle and are offset relative to the respective sections of the adjacent turns by an angle a of about 90". The dimension of the angle" is determined by the annealing temperature and may be readily reproduced on a commercial scale. The difference in height between one contraction of constriction 14 and one enlargement 15 is a measure for the roughness R of the coil. It is particularly advantageous when the contraction turn 14 and the enlargement turn 15 are adjacent.The roughness R of a coil increases when the ratio of the length Lto the width B of the cross-section of the mandrel 11 is larger than 2 (see Figure 4) and the annealing temperature of the wound coil from which the mandrel has not yet been dissolved is lowerthan 1100 C, particularly, however, when it is about 900 C.

Further processing of the above coils shows that they accommodate about 10% more emitter than conventional flat mandrel coils. The emitter consumption speed of, e.g., 40W fluorescent lamps in accordance with the present invention is about 20% lower than in the conventional lamps comprising flat mandrel coils. Lamps using coils in accordance with the present invention have about 30% longer life than conventional fluorescent lamps.

The above described coils may be manufactured on existing coil wining ng machines with identical primary materials. The finished coils may be processed conventionally on existing machines. No additional investment costs have to be incurred. The pasting of the coils on the finished lamp mounts can be more easily effected than in the case of the conventional flat mandrel coils. Especially when the ratio of the length Lto the width B of the crosssection of the flat or elongate mandrel is more than 2, the mandrel may have a smaller cross-sectional area than the mandrel of conventional flat mandrel coils with the emitter capacity of the finished coil remaining the same; the dissolving time, however, is reduced. In total, the material and manufacturing costs are reduced. Due to the lower annealing temperature after winding the coils, heating energy is saved.

Claims

1. An incandescent coil for an electric lamp in which the cross-sectional area of a turn is formed by one or more substantially straight sections and one or more curved sections wherein each curved section is off-set relative to the respective section of the adjacent turns by an angle cr. a of from 100 to 1 10" about the axis of the coil.

2. A coil according to claim 1 wherein there are at least five contractions per cm of coil length in the envelope defined by the surface of the coil.

3. A coil according to claim 1 or 2 wherein said angle a lies within the range 60 to 800.

4. A coil as claimed in any preceding claim wherein every second turn of the coil forms a contraction in the envelope defined by the surface of the coil.

5. A lamp according to any preceding claim wherein said curved sections are circular.

6. A coil as claimed in claim 5 wherein each circular section describes substantially the same length of a circular arc and has the same bending radius.

7. A coil as claimed in any preceding claim wherein at least every second curved section describes substantially the same length of a circular arc and has the same bending radius.

8. A method of manufacturing an incandescent coil for an electric lamp comprising the steps of; a) winding the coil wire about a mandrel of elongate cross-section; b) annealing the coil wire wound about the mandrel at a temperature at which a part of the stresses in the coil wire, which have been generated by the winding process, is maintained; c) dissolving the mandrel chemically, with the circular sections springing back from the bent bending radius by a predetermined angle owing to the residual stresses, and each curved section of the coil turning relative to the respective section of the adjacent turns by an angle " of from 10 to 1 10" about the coil axis.

9. A method as claimed in claim 8 wherein the ratio of the length L to the width B of the crosssection of the mandrel about which the coil wire is wound is greater than 2.

10. A method as claimed in claim 8 or 9 wherein the mandrel is of molybdenum or iron.

11. A method as claimed in claims 8 or 9 wherein the coil wire is provided prior to its being wound about the mandrel with a covering coil.

12. A method according to claim 11 wherein the covering coil encompasses a round mandrel or wall as the coil wire, the round mandrel extending parallel to the coil wire.

13. A method according to claim 12 wherein the round mandrel is of molybdenum or iron.

14. An incandescent coil for an electric lamp substantially as herein described with reference to Figures 1 and 2, Figure 3, Figure 4, Figure 5 or Figure 6 of the accompanying drawings.

15. A method of manufacturing an incandescent coil for an electric lamp substantially as herein described with reference to Figures 1 and 2, Figure 3, Figure 4, Figure 5 or Figure 6 of the accompanying drawings.