US3715615A

US3715615A - Electrode support for arc lamps

Info

Publication number: US3715615A
Application number: US00143377A
Authority: US
Inventors: G Lavering
Original assignee: Varian Associates Inc
Current assignee: Excelitas Technologies Illumination Inc
Priority date: 1971-05-14
Filing date: 1971-05-14
Publication date: 1973-02-06
Anticipated expiration: 1990-02-06
Also published as: NL7206551A; GB1384119A; JPS4849275A; FR2137771A1

Abstract

An electrode support structure is disclosed for use in a short arc lamp. The structure includes a ring having a plurality of flaps formed therein. Struts are attached to the electrode and to the flaps. These struts may be straight and may be abutted to the electrode. The flaps can bend to minimize the stresses created by the contraction and expansion of the struts relative to the ring during the temperature cycling of assembly and operation.

Description

United States Patent 1 Lavering 1 Feb. 6, 1973 [54] ELECTRODE SUPPORT FOR ARC LAMPS [75] Inventor: Gordon R. Lavering, Belmont, Calif. [73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: May 14, I971 [21] Appl. No; 143,377

52 US. Cl ..313 204, 313/285 [Sl] Int. Cl ..H0lj 1/96, H01] 19/50 [58] Field of Search ..3l3/l 13, 285, 205, 204, 226

[56] References Cited UNITED STATES PATENTS l2/l970 Peacher ..3l3/ll3 2/1970 Richter ..3 l3/ll3X 2/1970 Richter ..'.....313/113x 3/1970 Richter ..313/113x Prima ry ExaminerAlfred L. Brody Att0meyStanley Z. Cole 57 ABSTRACT An electrode support structure is disclosed for use in a short arc lamp. The structure includes a ring having a plurality of flaps formed therein. Struts are attached to the electrode and to the flapsl These struts may be straight and may be abutted to the electrode. The

flaps can bend to minimize the stresses created by thecontraction and expansion of the struts relative to the ring during the temperature cycling of assembly and operation.

11 Claims, 5 Drawing Figures ELECTRODE SUPPORT FOR ARC LAMPS BACKGROUND OF THE INVENTION This invention relates to gaseous discharge devices and in particular to an improved support for an electrode in a sealed beam short are lamp. The invention herein described was made in the course of or under a contract with the Department of Defense.

In one common short are lamp design, one electrode is supported adjacent to the lamps window. Such a lamp is described in detail in copending application Ser. No. 143,166 filed May 13, 1971 and assigned to the same assignee as the present application. In order to minimize the amount of light intercepted by the structure supporting this electrode, thin electrode supports are used. Prior art supports are thoroughly described in U.S. Pat. No. 3,495,118 issued Feb. 10, 1970. Two basic types are usedstraight or arcuate. Both types are attached to some sort of ring which is part of the lamp envelope or is attached thereto. As the strut and ring are heated during assembly, the radius of the ring increases more than the length of the strut for typical materials used. For example, with molybdenum struts and a stainless steel ring, the differential expansion is about 1- percent. As a result some means must be provided to relieve the stress caused by this differential expansion. During operation the strut is heated more than the ring so that additional relative expansion or contraction will usually occur.

The prior art straight struts are attached tangentially to the electrodes. As the ring expands relative to the struts during the heating of assembly, the struts slide tangentially across the surface of the electrode or draw partially out of the braze slot in the ring until the bond is formed at a high temperature. Thenas the structure cools down, a compressive force is applied endwise on the struts. A comparable strain must occur in the assembly. The ring is much stronger than the struts, so the ring deforms very little. The compressive stress on the strut is not directly supported at the electrode end. Instead, a bending moment is set up because the strut is attached to the outside of the electrode but the resultant of' forces passes through the center of the electrode. Because of the strong ring, the strut must buckle (which it can do easily if the strut is'arcuate) or the electrode rotates, bending the strut locally at the edge of the brazement between strut and electrode. Bending proceeds until the compressive stress in the strut is equally opposed by the bending moment in the strut. However, stress is not relieved. Instead stress builds up and is limited by the weakest part of the system, namely, the bent part of the strut, which is weakest because it is hot and very ductile when the lamp heats up. Additionally, the stresses in the bonds between members will increase. During operation of the lamp, the electrode again rotates and the struts bend to similarly transform the stresses in the members resulting from the relative contraction and expansion of the struts. The stresses in the bonds also remain, but at some changed value.

Arcuate struts can be abutted to the electrode and are under stress when placed in the brazing fixture. As the ring expands relative to the struts during the heating ofassembly, the struts unbend to relieve the stresses described above. On cooling down the relative expansion of the struts causes them to return to their approxi mate original shape in which they are again under some stress, as are the bonds. During operation similar bending and unbending occur to minimize the stresses in the members caused by temperature cycling, although stresses in the bonds remain. The arcuate struts can also be tangentially attached to the electrode with structural stresses relieved by a combination of bending and rotation as described above.

In addition to the inadequate stress relief provided by the prior art electrode supports, they are difficult to as- SUMMARY OF THE INVENTION The present invention is an improved support structure for an electrode in a short are lamp. The support structure comprises thin struts attached to the electrode, a ring forming part of or attached to the lamp envelope, and flap portions in the ring, with each strut attached to a separate flap portion. In a preferred embodiment, the flap portions are formed from the ring in a rectangular shape, the struts are abutted to the surface of the electrode and the free end of the flap portions are attached to the struts at one end thereof. The struts can be abutted to the flaps or the flaps can be abutted to the struts.

The primary object of this invention is to provide means for minimizing the residual stress in a straight strut in an electrode support structure which allows the electrode to remain stationary during thermal cycling, and which results in deformation in a desirable location. This is accomplished by attaching the outer ends of the struts to flap portions in a ring. As the struts contract and expand relative to the ring during the temperature cycling of assembly and operation, the flaps bend as required to minimize stress in the struts. The flaps can be designed to support whatever bending stress is desired.

BRIEF DESCRIPTION OF THE DRAWINGS form of attachment of the strut to the flap portion of.

the ring.

FIG. 5 is a cross-sectional view, partially schematic,

of a short are lamp employing the embodiment of FIG. 1.

DESCRIPTION'OF THE PREFERRED EMBODIMENTS FIGS. 1-3 illustrate a preferred embodiment of the electrode support structure of the present invention. Electrode l is supported by struts 3, usually three in number to provide stability. Struts 3 are thin straight members. They are thin to minimize the interference with the light being projected past them. They' are straight because this is the most convenient and economical form in which to produce them. As men-. tioned earlier, ease of assembly makes it desirable to abut struts 3 to the electrode 1 rather than attach them tangentially as in the prior art. This abutment can be done directly but use of a slotted sleeve 2 attached to electrode 1 makes it easy to position struts 3 during assembly.

The other endsof struts 3 are to be attached to a ring 7. This ring 7 either forms part of the lamp envelope or is attached to it. In the prior art the struts were typically abutted to a nonflexible ring. In this embodiment of the present invention a flap 6 is cut in the lower portion 5 of ring 7. As detailed in FIG. 3, the open end of flap 6 is abutted to the side of strut 3 adjacent to the end of strut Ring 7 also has an upper portion 4. This upper por- 7 tion provides surfaces for attachment to other portions of the lamp, depending on the lamp design. It is thick enough to provide stability during temperature cycling. The lower portion 5 of ring 7 is thin enough to permit the flaps 6 cut therein to bend during temperature cycling, as described more fully below. The relative dimensions of the ring portions can vary depending on lamp design including such factors as strut dimensions, operating pressure of the lamp and which members are attached to the ring and. where they are attached. As an example, in one ring an inner diameter of two inches the thickness of lower portion 5 was about 0.05 inch, the height of lower portion 5 was about 0.21 inch, the thickness of upper portion 4 (along a radius) was about gitudinal stress in the struts, caused by this relative contraction, is balanced by the stress set up by the bending outward of flaps 6. Therefore at temperatures less than brazing temperature, flaps 6 and struts 3, as well as the bonds between the various members, are under stress.

The flap is so dimensioned that the bending moment will not permit buildup of the compressive stress in the strut to the point where buckling will occur. It is the object of the invention that the flap be designed to prevent the critical buckling stress in the strut from being reached. Maximum fiber stresses in the flap may reach the yield point safely. If they do the flap will deform by plastic deformation in addition to elastic deformation. In fact in a preferred embodiment the yield strength of the stainless steel ring is exceeded 10- cally in the bent region, resulting in a small amountof permanent deformation. The fact that the strut is permitted to yield and that space is provided for it to yield means that there is a maximum compressive stress on the strut that can be balanced by the stress in the bending fla'p. This maximum compressive stressis limited by the maximum bending moment that can be achieved in the flap, which is in turn, limited by the yield strength of the flap material. During operation of the lamp, struts 3 are hottest adjacent to electrode 1. Relative expansion or contraction is difficult to predict because the temperature gradient is not known but excessive buildup of stress in struts 3 is prevented as flaps 6 bend as required.

As shown in FIG. 4, the end of struts 3 can also be abutted to the inside of flaps 6. Because ring 7 will expand relative to struts 3 during the heating of assembly,

' when the structure is first placed in the brazing fixture,

0.14 inch, the height of upper portion 4 at its outer diameter was about 0.12 inch with the height at its inner diameter about 0.06 inch, the width of flap 6 was about 0.15 inch, and the length of flap 6 was about 0.5 inch.

In assembling the electrode support structure, the structure is heated to a temperature sufficient to form the bonds between struts 3 and electrode 1 and between struts 3 and flaps 6. Typically this is done by brazing. As the structure is heated, in addition to other directions electrode 1 expands radially, struts3 expand longitudinally and ring 7 expands radially. However, the radial expansion of ring 7 is more than the sum of the longitudinal expansion of struts 3 and the radial ex pansion of electrode 1 for typical materials-brazingly compatible with the tungsten electrode required by the extreme heat generated at the electrode tip, such as molybdenum struts and a stainless steel ring. As a result flaps 6,move outward relative to the ends of struts 3, until thebondsbetw'een flaps 6 and struts 3 form. Struts 3 must be sufficiently longer than the distance from the electrode 1 to the'outer surface of flap 6 to accommodate this relative expansion of ring 7: As the strucflaps 6 are forced outward 'by the extra length of struts 3. Then as the structure is heated flaps 6 tend to return to their original position but on cooling down they are The embodiment in FIG. 3 allows for loose tolerancein the lengths, widths and thicknesses of struts 3 but some means is required during brazing to insure that the end of flaps 6 are in contact with struts 3. On the other hand, the embodiment in FIG. 4 requires close tolerance in the lengths of struts 3 but assembly is easi- FIG. 5 illustrates a short are lamp showing the electrode support structure of FIG. 1 in place. Details of construction of the lamp are not shown, aside from those in the electrode support structure. Such a lamp is described in more detail in copending U.S. Patent application Ser. No. 143,166 filed May 13, 1971, and assigned to the same assignee as the present invention. The lamp has acylindrical member 52 which, together with window 54 and base 53, forms a sealed envelope filled with gas under high pressure. A reflector 55 is attached to ring 7. A second electrode 51 is supported by appropriate means from base 53.

The shape of ring 7 can vary as required by the particular design of the lamp into which the electrode support structure is to be put. Some portion of the ring must be thick enough to provide structural stability and must have the proper surface for attachment to the rest of the lamp. In the embodiment just described, these functions were served by upper portion 4. Some portion of ring 7 must be thin enough to accommodate the flap portions 6 and to allow space for the required bending. In the embodiment just described, this func- .long, the flap can vibrate when the lamp is jarred even in normal use, thus causing the electrode to move relative to the reflector focal point, causing beam swing. A change in the arc gap can cause the arc to be extinguished. A length to width ratio for the flap of 3 to 4 has been found effective and convenient. The position of the flap relative to an edge of the ring is also a matter of convenience, with positions possible other than ones which have a-coterminous edge.

What is claimed is:

1. An electrode support structure in a short are lamp comprising an electrode, a ring coaxial with and spaced from said electrode, said ring having a plurality of flap portions thereon, and a plurality of electrode support struts, each of said struts being attached to a different one of said flap portions and to said electrode whereby said flap portions can more relative to the remaining portions of said ring to accommodate relative movement between said remaining portions of the ring and said struts.

2. An electrode support structure as claimed in claim 3 wherein said struts are of molybdenum and said ring is of stainless steel.

5. An electrode support structure as claimed in claim 1 wherein said flap portions are rectangular shaped.

6. An electrode support structure as claimed in claim 5 wherein said rectangular-shaped flap portion is coterminous with said ring at one sidealong a line substantially parallel to the axis of said ring.

7. An electrode support structure as claimed in claim 5 wherein the ratio of length to'width of said flap portion is in the range 3.0 to 4.0. v

8. An electrode support structure as claimed in claim 1 wherein said flap portions are abutted to the sides of said struts.

9. An electrode support structure as claimed in claim 1 wherein said flap portions each have a groove formed therein and said struts are abutted to said flap portions in said grooves.

10. An arc lamp comprising:

a sealed envelope including an optical window at one end thereof, two electrodes supported within said envelope and spaced apart to define a short are gap therebetween, with the means of support of at least one of said electrodes comprising a ring coaxial with and spaced from said electrode, said ring having a plurality of flap portions formed therein, and a plurality of electrode support struts, each of said struts being attached to a different one of said flap portions and to said electrode,

an ionizable gas at least filling said are gap, and a reflector supported within said envelope whereby said flap portions can more relative to the remaining'portions of said ring to accommodate relative movement between said remaining portions of the ring and said struts.

11. An electrode support structure as claimed in claim 1 wherein each of said flap portions comprises an elongated spring member having one end thereof attached to said remaining portion and having the longitudinal axis thereof oriented substantially circumferentially of said ring.

Claims

1. An electrode support structure in a short arc lamp comprising an electrode, a ring coaxial with and spaced from said electrode, said ring having a plurality of flap portions thereon, and a plurality of electrode support struts, each of said struts being attached to a different one of said flap portions and to said electrode whereby said flap portions can more relative to the remaining portions of said ring to accommodate relative movement between said remaining poRtions of the ring and said struts.

2. An electrode support structure as claimed in claim 1 wherein said struts are straight and one end of each of said struts is abutted to said electrode.

3. An electrode support structure as claimed in claim 2 wherein a slotted sleeve is attached to said electrode and said struts are abutted to said electrode in the slots of said sleeve.

4. An electrode support structure as claimed in claim 3 wherein said struts are of molybdenum and said ring is of stainless steel.

6. An electrode support structure as claimed in claim 5 wherein said rectangular-shaped flap portion is coterminous with said ring at one side along a line substantially parallel to the axis of said ring.

7. An electrode support structure as claimed in claim 5 wherein the ratio of length to width of said flap portion is in the range 3.0 to 4.0.

10. An arc lamp comprising: a sealed envelope including an optical window at one end thereof, two electrodes supported within said envelope and spaced apart to define a short arc gap therebetween, with the means of support of at least one of said electrodes comprising a ring coaxial with and spaced from said electrode, said ring having a plurality of flap portions formed therein, and a plurality of electrode support struts, each of said struts being attached to a different one of said flap portions and to said electrode, an ionizable gas at least filling said arc gap, and a reflector supported within said envelope whereby said flap portions can more relative to the remaining portions of said ring to accommodate relative movement between said remaining portions of the ring and said struts.