US3758801A

US3758801A - Cylindrical target x-ray tube

Info

Publication number: US3758801A
Application number: US00255846A
Authority: US
Inventors: W Merritt
Original assignee: Machlett Laboratories Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1972-05-22
Filing date: 1972-05-22
Publication date: 1973-09-11
Anticipated expiration: 1990-09-11

Abstract

A rotating anode X-ray tube having a shaft supported by two spaced bearings and having a cylindrical anode mounted on the shaft in a manner whereby the weight or load is evenly distributed between the bearings, the anode being shaped as a cylinder, for example, mounted midway of the shaft and having a circumferential X-ray emitting ring thereon, or as a bearingencircling skirt secured to one end of the shaft adjacent one bearing with the skirt being of a size to project substantially beyond the second bearing to provide load distribution, and having a circumferential X-ray emitting ring thereon.

Description

1 1 Sept. 11, 1973 I CYLlNDRlCAL TARGET X-RAY TUBE [75] Inventor: William ll.'Merritt,Ridgefield,

Conn.

[73] Assignee: The Machlett Laboratories,

Incorporated, Springdale, Conn.

[22] Filed: May 22, I972 [211 App]. No.1 255,846

Great Britain 313/60 Primary Examiner-Roy Lake Assistant ExaminerDarwin R. Hostetter Attorney-Harold A. Murphy et al.

[57] ABSTRACT A rotating anode Xray tube having a shaft supported by two spaced bearings and having a cylindrical anode mounted on the shaft in a manner whereby the weight or load is evenly distributed between the bearings, the anode being shaped as a cylinder, for example, mounted midway of the shaft and having a circumferential X-ray emitting ring thereon, or as a bearingencircling skirt secured to one end of the shaft adjacent one bearing with the skirt being of a size to project substantially beyond the second bearing to provide load distribution, and having a circumferential X-ray emitting ring thereon.

12 Claims, 3 Drawing Figures PATENTED SEPI I 1973 SHEET 1 BF 2 1 CYLINDRICAL TARGET X-RAY TUBE BACKGROUND OF THE INVENTION In the manufacture and use of rotating anode X-ray tubes of conventional structure several problems are known to exist. Conventional tubes generally utilize a rotatable shaft which is supported in two spaced bearings and carries an anode target on one end in a cantilever fashion. Obviously rotation of the shaft will cause consequent rotation of the target. Such a cantilever construction creates many problems in the bearings, cau i g unde able n is u n wear, e

Further, anode targets of this type also are subject to early failure because of problems encountered by heat stresses built up within the targets during operation of the tubes, For example, in a conventional disc target stresses are produced within the target by applying heat in the form of high energy electron bombardment of its outer surface.- These stresses cause failures in many forms such as cracking or crazing, separation of X-ray generating layers, or rippling of such layers. 7 Because of the configuration of conventional disctype rotating anodes it is difficult to sufficiently cool the targets to overcome such problems, and cooling is achieved by thermal radiation from the target through the tube envelope to the exterior of the tube.

SUMMARY OF THE INVENTION The above and other disadvantages of conventional rotating anode X-ray tubes are improved upon or overcome by the present invention wherein a novel X-ray tube structure is provided. According to this invention, the tube is provided with a bearing-supported anode structure wherein the load is distributed substantially uniformly to both bearings so as to prevent uneven wear and noise caused by imbalance. This is achieved by employing an anode structure wherein portions of substantially equal weight are located upon opposite sides of a plane extending transversely through the bearing shaft and between the bearings. This is achieved in one embodiment of the invention by a cylindrical anode target which is located on the periphery of a disc mounted substantially midway of the length of a coaxial shaft. The ends of the shaft are positioned in bearings whereby the shaft, disc and target rotate as a unit.

In a second embodiment of the invention a cylindrical target is connected by one end toan end of the shaft and depends therefrom in spaced coaxial relation to the shaft. The shaft is rotatably mounted by spaced bear-. ings on a fixed cylinder so that the target and shaft rtate as a unit. The tar-get is constructed so that portions of substantially equal weight are disposed upon oppo-v site sides of a plane extending-perpendicular to the shaft between the bearings.

In both embodiments, the dielectric envelope is pro.- vided with wall portions which lie relatively closely spaced from the heat-generating target so that the thermal radiation paths are considerably shortened between the target and the exterior of the tube. In many cases the tube is immersed in a cooling fluid and, therefore, such an envelope structure allows the fluid to enter areas much closer to. the source of heat than is possible with conventional tubes.

BRIEF DESCRIPTION OF THE DRAWINGS anode 2-2 of FIG. 1 looking in the direction of arrows; and

FIG. 3 is an axial sectional view similar to FIG. 1 showing another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more particularly to the drawings wherein like characters. of reference designate like parts throughout the views, one embodiment of the invention, is shown in FIGS. 1 and 2, comprises an axially disposed shaft located within a central portion of a dielectric envelope 12 and rotatably mounted therein on a pair of bearings 14 and 16. The bearings each have an inner race 14a and 16a respectively which is disposed upon an end portion of the shaft and held thereon between threaded

nuts

18 and 20. At the upper end of the shaft the outer race 14b of the bearing 14 is mounted on the inner surface of a cap 22, which cap has a ledge 24 formed on its inner circumference against which race 14b is seated. Cap 22 is sealed by a kovar ring 26 to a tubular portion 28 of the envelope 12, which tubular portion encloses the shaft 10 in spaced coaxial relation with it.

At the other end of the shaft 10 of the outer race 16b of bearing 16 is located against a ledge 24a formed on the inner circumference of a bore 30 formed in the end of a support 32. The support may take any suitable form and may provide a means by which the tube is supported in position of use.

The tubes anode structure includes a disc 34 which is fixed coaxially upon the shaft 10 about midway thereof and which extends perpendicular to the shaft, as shown,-for rotation therewith about the axis of the shaft. Disc 34 may take a form such as a solid member or may be a spoked wheel or spider, if desired. The anode target 36 is a hollow cylinder which is fixed around the outer periphery of the disc 34 and which extends parallel to and coaxially of the shaft. The target cylinder 36 is attached to the. disc 34 about midway of its length and is constructed as shown so that its center of gravity is located in a plane G-G which extends through or adjacent the disc 34. However, the gravital plane, which is depicted in FIG. 1 by line GG, may be located anywhere between thebearings l4 and 16 whereby the weight or load of the target is evenly distributed between the bearings. Thus, when the target is rotated, less noise and uneven bearing wear occurs than occurs in conventional cantilevertype anode structures.

In further accordance with this invention the target 36 comprises a cylinder 38 of molybdenum which extends substantially the entirelength of the cylinder and carries on its outer circumference a narrow band 40 of X-ray emitting material. The band 40 is preferably a thin layer of tungsten or rhenium-tung sten or other high atomic number material which efficiently generates large quantities of X-rays when subjected to electron bornbardment. The band 40 may be applied as a securely adhered ring of the selected material or may be plated onto the cylinder 38'by any of the known processes.

A cathode 42 is carried by a reentrant portion 44 of the envelope and is located so that electrons from a filament (not shown) therein will pass directly onto the target ring 40 as a controlled electrom beam 46,'as is well known in the art. The X-rays' generated in response to such electron bombardment are emitted in all directions from the target ring 40, and a portion thereof pass outwardly of the tube through a window in the envelope as an'X ray beam 48 to be utilized as the useful X-radiation for radiographic, diagnostic, therapeutic or other purposes. The cathode 42 and filament therein are, of course, adapted to be connected to external sources of electrical potential by leads 50, and suitable potentials may be applied to the anode through the support 32.

Generation of dense quantities of X-rays by electron bombardment requires driving the tube at high power levels. Consequently, the target cylinder 38 will acquire considerable amounts of heat which must be removed to prevent damage tothe target. One means employed to aid in the dissipation of heat from the cylinder 38 is to dispose the target cylinder on a second'cylinder 52 which is made of a low atomic number material such as carbon (graphite). The carbon cylinder 52 is disposed in efficient heat-conductive relation to the molybdenum cylinder 38 so that heat may be efficiently transeasily determined and the structure designed so that ferred by conduction directly from the cylinder 38 into cylinder 52 which has much higher thermal storage capacity per unit weight than cylinder 38. Therefore, the carbon cylinder 52 will store large amounts of the heat which is initially generated by the electron bombardment of the target ring 40; I

X-ray tubes are often operated by making a series of even when the target is rotated at speeds of 10,000

20,000 revolutions per minute to constantly present different surfaces of the focal ring 40 to the electron beam 46 instead of constantly bombarding a single immovable focal spot area.

To further aid in the dissipation, of heat in the presently described tube, the envelope is provided with recessed

annular portions

54 and 56 which

present walls

56 and 56a and 58 and 58a which lie relatively close to the target 36. Thus, the thermal radiation path for heat passing from the target to the exterior of the tube is relatively short. The tube, furthermore, is often operated in a bath of coolant fluid, such as oil. Therefore, cooling is greatly facilitated in a tube of the type described.

Rotation of the tube is effected by suitable inductive means 60 which is located in encircling relation to an end portion of the tube as shown in FIG. 1. To aid in creating effective rotary movement of the target, a ring 62 of magnetic material such as steel, for example, may be fastened around theadjacent end of the target cylinder 38 adjacent the device 60. Thus, rotation is achieved in the known manner.

It will be apparent from the above that not only is improved cooling achieved, but also improved rotation is accomplished since it will be readily understood in the light of the foregoing description that the weights of the respective parts of the anode target structure can be loads will be located substantially equally on either side of the gravital plane G-G and consequently loads will be applied substantially equally to the bearings.

In FIG. 3 there is shown a modification of this invention wherein there is provided an evacuated dielectric envelope of generally cup-shaped configuration and having a transverse portion 72 with a communicating cylindrical portion 74 extending from the periphery thereof. The anode target structure 76 is disposed within the envelope 70 and has a similar configuration, including a cup-shaped target having a disclike base 78 within envelope portion 72 and a skirtlike portion 80 extending downwardly from the periphery of portion 78 within envelope portion 74.'The target is mounted onone end of a shaft 82 by its base portion 78, being bolted or otherwise fixed to the shaft in a manner whereby it will rotate with the shaft upon operation of the tube. The shaft 82 is mounted in the inner races of a pair of spaced

ball bearings

84 and 86 the outer races of which are secured to the inner surface of a bore 88 within a support 90. Support 90 is hermetically sealed to a central depending tubular portion 92 of the envelope by a kovar ring seal 94 as shown. Thus, the tube can be supported in position of use as by a post 96 or the like attached to the support 90.

In accordance with this invention, the parts of the anode structure 76 are so designed and constructed that the weight thereof is substantially equally distributed on either side of a plane G-G which extends perpendicular to the shaft 82 between the

bearings

84 and 86. Since the plane G-G represents the center of gravity of the anode structure, it will be understood in accordance with this invention that the loads on the bearings will be substantially uniform.

The target 76 and particularly portion 80 may be made of molybdenum and may be backed by a layer 98 of carbon for increased heat dissipation as discussed in connection with the target 36 in FIG. 1. Target 80 also will be provided with a circumferential ringlike layer 100 of X-ray emitting material such as tungsten or rhenium-tungsten alloy.

To generate X-radiation from the focal track ring 100, a cathode 102 is mounted in a transversely projecting portion 104 of the envelope side wall similarly to the cathode-envelope structure in FIG. 1. Therefore, when electrical potential is applied between the cathode and anode, and'filament potential is also supplied to the cathode, electrons from the cathode will bombard the target focal ring 100 and cause resultant gen eration of X-rays therefrom.

The anode will simultaneously be rotated by the induction device 106 acting upon a ring 108 of magnetic material'carried by the target cylinder 80. In this described structure a cooling fluid is enabled to approach relatively closely to the target by virtue of the fact that the envelope walls lie in relatively close-spaced relation to the heated portions of the target. Therefore, thermal radiation paths between the target and extension of the tube are comparatively short.

From the foregoing it will be apparent that all of the objectives of this invention have been achieved by the structures shown and described wherein the constructions provide substantially uniform distribution of loads on bearing structures, reducing uneven wear and noise, and provides efficient and improved cooling of targets in high power X-ray tubes of the rotating anode type.

It is to be understood, however, that various changes and modifications in the structures shown and described may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claims. Therefore, all matter shown and described is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. An X-ray tube comprising an evacuated envelope, an anode structure and a cathode structure spaced apart within the envelope, said anode structure including a pair of spaced bearings, a shaft rotatably mounted in said bearings for rotation about its longitudinal axis, and a target structure connected to said shaft for rotation therewith, said target structure including an axially extending elongate hollow cylindrical target member encircling said shaft and substantially coaxial and parallel therewith, means connecting the inner circumference of the target member to the shaft, and a focal track located opposite the cathode and extending circumferentially around the target member.

2. An X-ray tube as set forth in claim 1 wherein said means is a disc which extends radially from the shaft and which is thin in comparison to the axial length of the target member.

3. An X-ray tube as set forth in claim 2 wherein said I cylinder is connected at one end to said disc.

4. An X-ray tube as set forth in claim 2 wherein the inner circumference of said cylinder is connected to said shaft at a location between said bearings.

5. An X-ray tube as set forth in claim 2 wherein an inner circumferential portion of said target member is fixed to the periphery of the disc.

6. An X-ray tube comprising an evacuated envelope containing spaced anode and cathode structures therein, said anode structure comprising an elongate hollow cylindrical target member disposed axially of the envelope, a shaft rotatably mounted coaxially within the target cylinder and having spaced portions connected to said envelope, and means connecting the inner circumference of the target cylinder to the shaft,

said envelope having cylindrical wall portions located in parallel close spaced relation to the inner and outer surfaces of said target cylinder whereby short heat radiation paths are provided from the target cylinder to the exterior of the envelope.

7. An X-ray tube as set forth in claim 6 wherein said means is a disc which extends radially from the shaft and which is thin in comparison to the axial length of the target cylinder.

8. An X-ray tube as set forth in claim 7 wherein the disc is disposed in a predetermined plane and the peripheral edge of said disc is affixed to the inner circumference of the target cylinder with said plane being disposed between said spaced portions of the shaft.

9. An X-ray tube as set forth in claim 6 wherein said means comprises a disc, said envelope has an outer axially extending cylindrical wall portion located in parallel closespaced relation to the outer surface of the target cylinder, and has recessed portions located in parallel close-spaced relation to the inner surface of the target cylinder and said disc.

10. An X-ray tube comprising an evacuated envelope, an anode structure and a cathode structure spaced apart within the envelope, said anode structure including a hollow elongated support, a pair of spaced bearings within the support, a shaft rotatably mounted in said bearings for rotation about its longitudinal axis, and a target structure connected to said shaft for rotation therewith, said target structure including an axially elongate hollow cylindrical target member encircling said shaft and extending substantially coaxially parallel and in spaced relation with said support, means connecting one end of the target member to one end of the shaft at a location remote from and outside the space between said bearings, and a focal track located opposite the cathode and extending circumferentially around the target member.

11. An X-ray tube as set forth in claim 10 wherein said target structure has a weight distribution which is substantially equal on each side of a plane extending through it and between said bearings perpendicular to the axis of the shaft.

12. An X-ray tube as set forth in claim 10 wherein said target is provided on its outer surface with a ring of magnetic material, and an induction device encircles the envelope opposite the magnetic ring.

t IF

Claims

3. An X-ray tube as set forth in claim 2 wherein said cylinder is connected at one end to said disc.

6. An X-ray tube comprising an evacuated envelope containing spaced anode and cathode structures therein, said anode structure comprising an elongate hollow cylindrical target member disposed axially of the envelope, a shaft rotatably mounted coaxially within the target cylinder and having spaced portions connected to said envelope, and means connecting the inner circumference of the target cylinder to the shaft, said envelope having cylindrical wall portions located in parallel close spaced relation to the inner and outer surfaces of said target cylinder whereby short heat radiation paths are provided from the target cylinder to the exterior of the envelope.