Stress relieved filament support assembly

The structure is an improvement to prevent thermal cycle damage to a braze joint in a magnetron filament assembly. The filament is welded to a molybdenum filament weld ring which, in turn, is brazed to a solid iron filament support cylinder. This braze joint is sometimes broken because of the different thermal expansion coefficients of molybdenum and iron, even though slots are formed in the molybdenum cylinder to reduce the stress. The improvement is the addition of a thin yieldable cylinder along the top outer edge of the iron support cylinder to which the molybdenum cylinder is brazed. This thin cylinder can be constructed by cutting an annular groove adjacent to the top outer edge of the iron support cylinder. The groove then forms the cylinder to which the weld ring is attached, and the thin iron cylinder yields with thermal stress and therefore relieves the stress on the adjacent braze joint.

BACKGROUND OF THE INVENTION
 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 degree s 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 90 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 molybdenum 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
 cylindrical 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 molybdenum weld
 ring is itself attached to, supported by, and receives the electrical
 power for the filament through an iron filament support cylinder around
 which the lower lip of the cylindrical skirt 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 iron filament support
 cylinder attached to the bottom lip of its cylindrical portion, thereby
 subjecting the filament support cylinder to heat conducted through the
 weld ring. It is not uncommon, especially in tubes with high power
 ratings, for the braze between the filament weld ring and the filament
 support cylinder to crack because of the differential thermal expansion
 between the filament weld ring and the filament support cylinder to which
 it is attached.
 This problem is 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 because of the required magnetic properties. 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 of the bond between
 the parts. The iron filament support cylinder expands when heated and
 contracts when cooled much faster than the molybdenum filament weld ring
 does, and the braze at their junction tends to crack under the stress of
 the differential expansion and contraction.
 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 heating causes the support cylinder to expand, and prevents the
 outer cylinder of the weld ring from resisting the expansion.
 However, for the higher power tubes now being built, the stress relief
 afforded by the slotted construction has not been completely effective.
 The braze between the filament weld ring and the filament support cylinder
 continues to crack, and the addition of more slots in the skirt of the
 weld ring is limited by the requirement of the weld ring to conduct large
 filament currents, thus requiring a large cross sectional area for the
 conductive path. Additional slots would reduce this cross sectional area.
 SUMMARY OF THE INVENTION
 The present invention solves the problem of excessive stress on the braze
 between the filament weld ring and the filament support cylinder by
 changing the structure of the filament support cylinder, not the filament
 weld ring.
 The structural change is a very simple one. The structure of the prior art
 filament support cylinder is essentially a solid iron cylinder with a
 central hole. Such a structure is much stronger than the thin sleeve-like
 skirt of the filament weld ring which is attached to the outer surface of
 the filament support cylinder, and the filament support cylinder therefore
 does not yield even slightly when the differential expansion occurs.
 The invention is the addition of a thin yieldable cylinder at the top of
 the filament support cylinder formed by cutting an annular groove slightly
 radially inward from the cylindrical surface to which the filament weld
 ring is attached. This unsupported, short, thin cylinder at the outer edge
 of the iron filament support cylinder then is the part to which the
 molybdenum filament weld ring is brazed. The thin iron cylinder is
 flexible enough, so that it yields to absorb the stress of the
 differential thermal expansion between it and the weld ring. Thus, the
 braze at the weld ring is not subjected to as high a level of stress as is
 present in the tubes without the thin cylinder at the top of the filament
 support cylinder, and the braze does not crack.
 The formation of the thin cylinder by cutting an annular groove in the
 support cylinder is a convenient construction method, particularly because
 it maintains the previous magnetic circuit required for operation of the
 magnetron tube, but the essential structure is the thin, yieldable
 cylinder to which the filament weld ring is attached. This simple and
 inexpensive structure can therefore save a complex and very expensive tube
 from destruction.

DETAILED DESCRIPTION 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 16, and upper filament
 support 18.
 Filament 12 is a conventional helical filament which is supported
 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 and also in the present invention, as
 shown in FIG. 1, weld ring 14 is attached to solid support cylinder 16,
 which is essentially an iron cylinder with central hole 34. Cylindrical
 skirt 28 of weld ring 14 surrounds and is attached to the upper outer
 cylindrical surface 36 of support cylinder 16. However, such construction
 sometimes causes the braze attaching cylindrical skirt 28 and support
 cylinder 16 to crack.
 In the present invention, such problems are prevented by forming thin
 cylinder 42 extending from lowered top surface 21 of support cylinder 16,
 and attaching skirt 28 of filament weld ring 14 to thin cylinder 42 as
 shown in FIG. 1. As shown in FIG. 2, cylinder 42 is thin enough and
 flexible enough to yield with the stress of the differential thermal
 expansion between iron support cylinder 16 and molybdenum skirt 28 of
 filament weld ring 14. It is the fact that cylinder 42 yields with
 differential thermal expansion that prevents the braze between skirt 28
 and support cylinder 16 from cracking.
 One simple method of making thin cylinder 42 is by forming annular groove
 20 into original top surface 23 of support cylinder 16. Annular groove 20
 is a minor modification to prior art support cylinder 16. As shown in
 FIGS. 2 and 3, support cylinder 16 has a top section 36 which is slightly
 smaller in diameter than bottom section 38. This difference in diameter
 forms shelf 40 which permits a continuous smooth exterior surface of the
 two cylinders when skirt 28 and support cylinder 16 are assembled.
 In the embodiment of the invention shown in FIGS. 1 and 3, annular groove
 20 is located radially inward from outer surface 36 of support cylinder 16
 to which skirt 28 is attached. When annular groove 20 is formed into
 original top surface 23 of support cylinder 16, annular groove 20 extends
 down into support cylinder 16 to approximately the same depth as shelf 40,
 and therefore is essentially behind the region of support cylinder 16 to
 which skirt 28 attaches.
 In the embodiment shown in FIG. 3, in a 90 KW magnetron with a filament
 current of 120 amps, the diameter of original top surface 23 of support
 cylinder 16 is 0.92 inch, the width of annular groove 20 is 0.050 inch,
 the wall thickness of thin cylinder 42 is 0.035 inch, and the depth of
 annular groove 20 is 0.120 inch. Groove 20 thereby forms thin radially
 unsupported cylinder 42 attached to support cylinder 16.
 The wall thickness of thin cylinder 42 is typically chosen to be
 approximately the same thickness as the lower portion of skirt 28 of
 molybdenum weld ring 14. It should be appreciated that, although the basis
 of the stress relief of the invention is the yieldable structure of thin
 cylinder 42, the additional iron material available within the body of
 support cylinder 16 because of the use of groove 20 and original top
 surface 23 instead of lowered top surface 21 is desirable in order to
 maintain the magnetic characteristics of the magnetron tube into which the
 invention is installed.
 Thus, the simple modification of forming an annular groove in filament
 support cylinder 16 behind the surface to which filament weld ring 14 is
 attached, solves the recurrent problem of cracking the braze which
 attaches the parts.
 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 size 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 spirit and scope of the invention
 as defined in the following claims.