GB2259402A - Thermally graded filament assembly - Google Patents

Abstract

A filament support structure, e.g. for a magnetron, includes a thermal stress relief adapter (20), attached between the filament weld ring (14) and the filament support cylinder (16) to which the weld ring is usually attached, in order to prevent fracture of the weld ring. The thermal stress relief adapter (20) is selected to have a coefficient thermal of expansion intermediate between those of the weld ring (14) and the support cylinder (16) so that the lower thermal expansion of the adapter does subject the weld ring to stress sufficient to cause damage. <IMAGE>

Description

THERMALLY GRADED FTLAMENT ASSEMBLY This invention deals generally with electric lamp and discharge devices and more specifically with the support structure for the filament of an electron tube.

A typical power tube filament operates at a temperature of approximately 2200 degrees centigrade, and this can lead to severe structural problems. Not only is it necessary to support such filaments against structural movement when they are at such high temperatures, but it must be kept in mind that the filaments are not always at that temperature. Since the tubes must be turned on and off for various reasons, the filament will actually vary in temperature from near room temperature up to and including its operating temperature. Moreover, operational considerations require that the tubes must turn on rather quickly, thus causing the temperature of a filament to change at a very rapid rate.

This extreme temperature and dramatic temperature change places severe thermal stress, not only on the filament -itself, but on the entire support structure of the filament. This occurs because the support structure generally is subjected to filament temperatures at one of its extremities and, therefore, temperatures throughout the support structure, even remote from the filament, are also very hot.

In a typical high power tube, such as a 75 KW continuous wave magnetron, in which the high power exaggerates the problems, this thermal stress can cause fracture of the typical filament support structure. In that particular type tube, it has been standard practice to use a helically wound tungsten filament. This configuration is supported at its lower end by a weld ring which is essentially an inverted cup comprising a planar portion to which is attached a cylindrical side portion.

The top of the inverted cup, the planar portion, has a central hole with a lip around the circumference of the hole, and the bottom of the helical filament is welded to this lip. Once assembled, the weld ring looks very much like a skirt attached at the bottom of the helical filament.

The top end of the filament is welded to a disc-like fixture which has a circular protruding lip to which the filament is attached. This top disc is attached to and supported by a conductive rod which passes through the centers of the helical filament, the lower weld ring, and the rest of the filament support structure in order to both support the remote upper end of the filament and to act as an electrical connector for that end.

The lower filament weld ring also acts as the electrical connector at the lower end of the filament to which it is attached. The weld ring is itself attached to, supported by and receives the electrical power for the filament through a filament support cylinder around which the lower lip of the cylindrical side of the filament weld ring fits.

It is the filament weld ring which is most affected by the thermal stress to which the entire assembly is subjected. The relatively short filament weld ring has the extremely hot temperature of the filament attached to the central lip of its planar portion and the filament support cylinder attached to the outer lip of its cylindrical portion, thereby subjecting the filament support cylinder to heat conducted through the weld ring. It is not uncolmnon, especially in tubes with high power ratings, for the filament weld ring to crack because of the thermal expansion of the support cylinder to which it is attached.

This problem has been aggravated by the materials required to be used for the various parts. The filament weld ring is typically constructed of molybdenum so that it may be welded to the tungsten filament and also withstand the high temperature, while the filament support cylinder, which is also attached to the filament weld ring, is typically constructed of iron. Since iron has a dramatic increase in its coefficient of thermal expansion when it rises above 900 degrees C., the increased temperature within the higher power tubes is at least part of the problem for the cracking. The iron filament support cylinder essentially expands much faster than the molybdenum filament weld ring and tends to burst the weld ring.

Until now, the only means by which this problem has been alleviated has been to provide the cylindrical side portion of the filament weld ring with slots to relieve the.mechanical stress caused by the expanding support cylinder. Such slots permit the fingers formed between them to deflect as the filament temperature causes the support cylinder to expand, and prevents the outer lip of the weld ring from being cracked by the expansion. In effect, this technique designs the cracks into the weld ring.

However, for the higher power tubes now being built, the stress relief afforded by the slotted construction has not been completely effective. Filament weld rings continue to crack, and the addition of more slots is limited by the requirement of the weld ring to conduct large filament currents, thus requiring greater cross sectional area for the conductive path.

Additional slots reduce this cross sectional area.

The present invention solves the problem of excessive stress on the filament weld ring by adding a single simple part to the assembly and thereby reducing the thermal stress to which the filament weld ring is subjected. This additional part designated adapter means and preferably in the form of a ring, is inserted between the filament weld ring and the filament support cylinder. Its preferred basic configuration is that of a short cylinder, one flat surface of which is brazed to the upper flat surface of the filament support cylinder. The filament weld ring is then attached to the outside cylindrical surface of the adapter rng.

When this adapter ring is constructed of a material whose coefficient of thermal expansion is lntermediate to the coefficients of thermal expansion of the materials of the filament weld ring and the filament support cylinder, the weld ring is not subjected to as high a level of stress as is present in the tubes without the adapter ring, and the weld ring does not crack.This simple and inexpensive part can therefore save a complex and very expensive tube from destruction An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: FIG. 1 is a cross section drawing of a filament assembly, taken through the axls of the filament assembly, for a magnetron tube which includes the invention FIG. 2 is a perspective view of the adapter ring of the preferred embodiment of the invention.

FIG. 1 is a cross section drawing of the preferred embodiment of the invention in which filament assembly 10 is comprised of filament 12, filament weld ring 14, filament support cylinder 10, upper filament support is and adapter ring 20.

Filament 12 is a conventional helical filament which is supported essentially vertically between upper filament support 18 and filament weld ring 14. Upper filament support 18 is essentially a disk with weld lip 22 protruding from its lower surface. Filament 12 is welded to weld lip 22. Upper filament support 18 is attached to central support 24 which functions as one electrical connector for filament 12.

At its lower end, filament 12 is attached to filament weld ring 14 by welding. Weld ring 14 is constructed with a planar surface 26 and a cylindrical skirt 28 to essentially form an inverted cup. Planar surface 26 of weld ring 14 includes central hole 30 through which central support 24 passes, and weld lip 32 is formed to protrude up from planar surface 26 adjacent to central hole 30. Filament 12 is welded to weld ring 14 at weld lip 32.

In conventional tube construction, weld ring 14 would be attached directly to support cylinder 16, which is essentially a cylinder with a central hole, by cylindrical skirt 28 of weld ring 14 surrounding and being attached to the outer cylindrical surface of support cylinder 16. However, as described previously, such construction sometimes causes cylindrical skirt 28 of weld ring 14 to crack. In the present invention, such problems are eliminated by using adapter ring 20.

Adapter ring 20 is a simple part inserted between weld ring 14 and support cylinder 16. As shown in FIG. 2 the configuration of adapter ring 20 is essentially a short cylinder with hole 34 along its axis and shoulder 36 located along its bottom. Shoulder 36 is constructed so that the thickness of skirt 28 of weld ring 14 will match the width of shoulder 36, and therefore furnish a smooth exterior surface when skirt 28 and adapter ring 20 are assembled.

The most important characteristic of adapter ring 20 is that the material from which it is constructed must have a coefficient of thermal expansion which is less than the coefficient of thermal expansion of the material from which support cylinder 16 is made and a coefficient of thermal expansion which is greater than that of the material from which weld ring 14 is made.

As shown in FIG. 1, in the preferred embodiment, adapter ring 20 is attached to the top of support cylinder 16, and weld ring 14 is attached to the outer surface 38 of adapter ring 20.

Neither the method of attachment of these parts nor the exact location of the attachment is critical for the benefits of the invention. It is the fact that adapter ring 20 reduces the thermal expansion to which weld ring 14 is subjected that prevents skirt 28 of weld ring 14 from cracking. Thus, adapter ring 20 may be either welded or brazed onto support cylinder 16, and it can be attached to the top planar surface of support cylinder 16 or to the outer cylindrical surface.

Similarly, the exact construction of central hole 34 in adapter ring 20 is not critical. In the preferred embodiment shown in FIG. 1, central hole 34 is dimensioned to fit around central extension 40 of support cylinder 16.

The proper selection of the coefficient of thermal expansion of adapter ring 20 can, however, eliminate all damage to filament weld ring 14, and thereby prevent a troublesome failure mode in tube filament assemblies.

It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the scope of the invention as defined in the following claims.

Claims (5)

CLAIMS:

1. A tube filament assembly comprising: a filament structure; weld means to which the filament structure is attached, the weld means comprising a first surface with a skirt attached to the outer perimeter of the first surface and extending away from the first surface, with the filament structure attached to the first surface and extending away from the first surface in a direction opposite from the skirt; adapter means, with the adapter means attached to the skirt of the weld means at a location remote from the first surface of the weld means, with the adapter means fitting within the skirt of the weld means; and filament support means attached to the adapter means at a location on the adapter means remote from the location of the attachment of the adapter means to the skirt of the weld means; wherein the coefficient of thermal expansion of material from which the adapter means is constructed is intermediate between the coefficient of thermal expansion of material from which the filament support means is constructed and the coefficient of thermal expansion of material from which the weld means is constructed.

2. The filament assembly of claim 1 wherein the first surface of the weld means is a disk, and the skirt of the weld means forms a cylinder.

3. The filament assembly of claim 1 or 2 wherein the adapter means is a cylinder and the cylindrical surface of the adapter means is attached to the inside surface of the skirt of the weld means.

4. The filament assembly of claim 1,2 or 3 wherein the adapter means is constructed with a shoulder at one end with the dimension of the width of the shoulder matching the thickness of the skirt of the weld means.

5. A filament assembly substantially as hereinbefore described with reference to the accompanying drawings.