US3699373A - X-ray tube with electrically conductive bearing bypass - Google Patents

Abstract

An X-ray tube of the rotating anode type having an auxiliary contact structure for providing efficient electrical conductivity in bypass relation to the support bearings, and including means for preventing generated wear particles from passing into the support bearings, thus improving support bearing life and permitting use in support bearings of materials having relatively poor electrical conductivity characteristics.

Description

( Oct. 17,1972

References Cited UNlTED STATES PATENTS [54] X-RAY TUBE WITH ELECTRICALLY CONDUCTIVE BEARING BYPASS [72] inventors: William P. Holland, West Redding;

3,634,870 1/1972 Kessler.......................3l3/6O Robert E. Azud, Wilton,,both of Conn. 7 Primary Examiner-Roy Lake D R. The Machlett Laboratories, lncorjfgifgffggf; g tgg zf porated, Springdale, Conn. V July 2, 1971 [22] Filed: ABSTRACT An X-ray tube of the rotating anode type having an auxiliary contact structure for providing efficient electrical conductivity in bypass relation to the support bearings, and including means for preventing generated wear particles from passing into the support bearings, thus improving support bearing life and permitting use in support bearings of materials having relatively poor electrical conductivity characteristics.

900 mm H53 33 0 0 m 6 0" 3 u" 3 u" 5 3 "n" w. u" 9 m 5 mU M l, mua v flu 0 N m 0 w sum A UIF 1] 1 2 00 2 555 it [rt[ 17 Claims, 6 Drawing Figures s. t was 1 X-RAY TUBE WITH ELECTRICALLY CONDUCTIVE BEARING BYPASS BACKGROUND OF THE INVENTION A present state of the art rotating anode X-ray tube embodies ananode structure which comprises essen tially a target which is affixed to or mounted on a shaft supported byv the inner races of ball bearings. Also affixed to the shaft concentrically therewith is a skirtlike rotor which becomes a section of an induction motor cathode causes current to flow through the shaft, the

inner bearing races, the bearing balls, and the outer races to the stationary anode support.

Such a rotating anode structure is operated in a high vacuum environment of the order of Torr andat high temperature which, at the bearings, may reach up to 600C. The particular construction of known conventional rotating anode tubes utilizes bearing structures which carry electrical current between the rotating and the stationary elements of thetube. Since the hearings, in addition to their mechanical requirements, must perform as electrical conductors for passing power, possibly as high as 100 KW, through the anode circuit, this presents serious problems. The environmental conditions do not permit the use of conven tional organic lubricants and solid lubricating techniques must be used, thereby imposing extreme limitationson bearing performance in terms of life and audible noise. Numerous investigators have demonstrated the deleterious effects of electrical current through ball bearings due to arcing phenomena when making and breaking contacts. These effects impose further influences of surface degradation on bearings, leading to shorter life and increased noise.

The requirement of electrical conductivity through the bearings of rotating anode X-ray tubes imposes severe design restrictions on the bearings and lubricants by limitingthe materials selected forthese elements to those which are electrically conductive. Excluded, for example, are nonmetallic materials such as refractory oxides, ceramics, silicates and nitrides, certain of which possess physical requirements which are better suited to X-ray tube bearing requirements from a life and noise design standpoint provided the requirement of electrical conductivity does not exist.

Another undesirable factor in present state of the art bearing design is the presence of an excessive degree of mechanical displacement in the bearings which is manifested in radial play. As bearing wear progresses throughout tube life this displacement will increase. As displacement increases it directly results in increased noise and wear of the bearings which particularly causes adverse effects on fine focus X-ray tubes by causing effective degradation of the focussed spot.

SUMMARY OF THE INVENTION In order to overcome the foregoing and other disadvantages of conventional rotating anode X-ray tubes, the present invention includes an auxiliary contact interposed between the target shaft and the stationary anode support structure of the tube for providing an efficient highly electrically conductive path therebetween so that current need not flow through the conventional bearings. Such a contact is" a springloaded device having a minimum of friction so as to reduce high stress points and subsequent wear at the contact surface. The device furthermore is structured to avoid any responseby the contact device'to eccentric motion of the target shaft or any centrifugal forces acting upon the contact device, either of which might lead to stress points and wear at the contact surfaces. Such undesirable wear produces undesirable wear particle generation and/or seizure when wear particles migrate into the support bearings. Accordingly, to avoid the possibility of this latter undesirable effect, the present invention further includes the utilization of a wear particle trap insert which presents a labyrinth barrier to any wear particles generated by the novel bypass contact device. This barrier confines any such particles to an area preceding the load or support. bearings, thereby preventing migration of the particles and consequent ultimate damage to the bearings or other tube elements.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other advantages of the invention are achieved by the novel structures described hereinafter and shown in the accompanying drawings, wherein:

FIG. 1 is an elevational view partly in axial section of a rotating anode tube having inner race rotation and embodying a preferred form of this invention;

FIG. 2 is an enlarged sectional view of the bypass contact structure shown in the tube of FIG. 1;

FIGS. 3-5 are sectional views of modified forms of bypass contact devices which may be used with the tube of FIG. 1; and

FIG. 6 is an axial sectional view of a rotating anode tube having outer race rotation and including a form of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The rotating anode X-ray tube shown in FIG. 1 comprises an evacuated envelope 10 containing within one end portion thereof a cathode structure 12 which is suitably mounted upon supporting structure 14 sealed to the end of the envelope and which is adapted to be supplied with filament power by means of externally extending leads 16 as is well known.

The opposite end of the envelope has an axially extending hollow neck portion 18 with a reentrant end 20 to the termination of which is sealed one end of a Kovar collar 22. The opposite end of the collar 22 is sealed to the circumference of a tubular-shaped bearing housing 24 which has a solid reduced-diameter end portion 26 extending exteriorly of the envelope within reentrant portion 20 for cooling purposes and for providing means for applying anode potential to the tube in a well-known manner.

Bearing housing 24 is provided on its inner surface with a circumferential shoulder 28 on which rests one side of an annular outer bearing race 30 of a first bearend of atubular spacer or cylinder 34 which is fitted within the housing and which is engaged at its other end by an outer race 36 of a second bearing 38. These bearings and spacer form an assembly which is held in place at one end by set screws 40 which extend through the walls of housing 24 adjacent the exposed end of race 36.

Balls 42 and an inner race 44 complete the bearing unit 38, the race 44 being mounted in a circumferentially grooved area of a shaft 46, preferably of steel, which extends axially of the tube within housing 24 and spaced therefrom as shown. The lower end of shaft 46 has a portion 47 of reduced diameter on which a nut 45 is threaded into engagement with the inner race 43 of bearing 32 for holding the bearing-spacer assembly in place. The inner end of shaft 46 is threadably or otherwise connected to a head 48 which is bolted or otherwise fixed to the inner end of a hollow skirtlike rotor 50. Rotor 50 encircles the bearing support housing 24 in spaced relation thereto within the envelope portion 18.

A disclike anode target 52 is mounted on one end of a short hub or shaft 54 which has its opposite end fixed to the adjacent end of rotor 50. Thus, when a stator (not shown) surrounding envelope portion 18 is electrically activated, this will induce a changing field in the rotor causing it and the attached anode target 52 to rotate within the stationary housing 24. At the same time, in accordance with well-known procedures, the cathode may be electrically activated to dispense a flow of electrons onto the adjacent target surface. This will cause generation of X-rays which will pass from the target out through the envelope 10.

The interior of the envelope is evacuated to the order of 10' Torr. During X-ray generation the target 52 becomes extremely hot, which heat is transmitted through hub 54 into head 48 and shaft 46 to the bearings which sometimes assume temperatures as high as 600C. Therefore, in conventional X-ray tubes of this type the environmental conditions do not permit the use of conventional organic lubricants for the bearings. Solid lubricants must be utilized, thereby imposing extreme limitations on the bearing performance in terms oflife and noise.

Additionally, the bearings of conventional tubes must also perform as electrical conductors between the rotating and stationary members of the tube so that power may be conducted throughout the anode circuit, which power may peak as high as 100 KW. Silver, therefore, has conveniently been commonly used as a bearing lubricant.

Numerous investigations have demonstrated the deleterious effects of electrical current through ball bearings due to arcing phenomena when making and breaking contacts. These effects impose further influences of surface degradation on bearings, leading to shorter life and increased noise. Furthermore, the requirement of electrical conductivity through the bearings impose severe design restrictions on bearings and lubricants by limiting the selection of materials for these elements to those that are electrically conductive.

In accordance with this invention, additional or auxiliary electrical contact means is provided, as will be described, so that nonconductive materials may be used for the bearings and the lubricant. As examples, the bearingsmay be made of a refractory oxide, ceramic, silicate or nitride or the like, certain of which possess physical requirements which are better suited to X- ray tube bearing requirements from a life and noise viewpoint when the requirement of electrical conductivity does not exist. An example of an especially convenient nonconductive lubricant is molydisulphide.

The auxiliary electrical connection mentioned above may take one of several forms but will preferably be structured as shown in FIGS. 1 and 2. The lower end portion of bearing support 24 is solid and thus seals the lower end of the cavity 56 within which the shaft 46 projects. Such a solid area of support 24 also seals off the vacuumized envelope. Conductively affixed to the support 24 within the lower end of the cavity 56 is a ring or disc 58 to which is fixed one end of a leaf spring 60. The free end of the leaf spring 60 projects toward the adjacent end of shaft 46 and carries a contact button 62 which is disposed in engagement with an insert 64 fixed in the extreme end of shaft portion 47. The insert 64 is of any suitable conductive metal having a bearing surface of high hardness rhenium, tungsten, rhodium or the like. The contact button 62 is preferably of silver, silver-copper eutectic, molydisulphide or silver-tungsten-moly alloy, or the like, which have self-lubricating characteristics. However, in some cases it has been found satisfactory to omit the insert 64 and to position the contact button 62 directly upon the end of the shaft if the end is polished or is provided with the hard coating mentioned above.

It will be apparent that the leaf springs inherent resiliency and the thickness of the ring or disc 58 will control the contact pressure between the button 62 and insert 64, and that the button 62 will constantly remain in sliding contact with the insert 64 while the shaft 46 is rotating. Such continuous contact provides efficient electrical current flow from shaft 46 to the support 24 in bypass relation to the load bearings, as is desired. The major wear will occur on the silver contact button 62 and, therefore, a preferential and mechanically restraining wear track will not form on the shaft due to the high polish, the high hardness, or the wear resistance provided by the coating.

Furthermore, the pressure of the contact spring upon the shaft will function to take up some of the undesirable play in the bearings, reducing noise. Additionally, the contact device provides a more efficient path for exit of heat from the shaft toward the bearing support and skirt than is possible through the bearings.

A further significant feature is the inclusion of a wear particle trap 66 which presents a labyrinth barrier to the wear particles generated by the button-insert contact. The trap 66 comprises a short hollow cylinder or collar 68 which extends axially within cavity 56 from the ring or disc 58 to a point or level beyond the button-insert contact, at which level the end of the cylinder has an inwardly turned flange portion 70 the inner periphery of which relatively closely encircles the nut as shown clearly in FIG. 2. This barrier confines the wear particles to an area preceding the load bearings, thereby preventing migration to the bearings and ultimate damage.

In FIGS. 3 and 4 there are shown alternative rolling friction element bypass contacts which are believed to increase wear-life and lower the friction in high speed rotating devices as opposed to sliding friction. However, these rolling contact structures, when used as electrical contacts, tend to generate more electrical noise than do sliding elements. In numerous applications of X-ray tubes, electrical noise is unimportant in which case the structures in FIGS. 3 and 4 could be employed satisfactorily.

In FIG. 3, the inner end of the cavity 56 in support 24 contains a retaining ring 72 on which is seated a ball bearing assembly 74 including a first race 76 and a second race 78with balls or rollers therebetween. The adjacent reduced diameter end portion 47 of shaft 46 and nut 45 thereon are slightly spaced from race 78 but are electrically connected to it by a coiled contact pressure spring 80, the lower end of which rests in a spring retainer 82 carried by race 78, with its upper end engaging a flanged portion of nut 45.

The retaining ring 72 on race 76 may be of selected thickness to control pressure of the spring 80 upon the nut and shaft, and also is provided with an inwardly directed flange portion 84 which encircles the second race 78 to trap wear particles from this bearing in a localized area.

The rolling contact device of FIG. 4 differs from that of FIG. 3 by the elimination of the coiled spring and the inclusion of a flat annular contact spring 86 between the retaining ring 72 and adjacent race 76. The other race78 is fixed directly to the end portion 47 of shaft 46.

A modified sliding contact device is illustrated in FIG. 5 and comprises a flat-faced silver insert 88 in the extreme end of portion 47 of shaft 46 which is used as the contact of the rotating shaft 46. The extreme inner end of the cavity 56 is provided with an axial bore 90 within which is slidably mounted a contact pin 92 which has its inner end engaged by a coiled spring 94 which is confined within the bore and urges the pin 92 into engagement with the insert 88. In a structure of this type limited axial constraint is imposed on the pin 92 and permits it to form its own wear path.

A wear particle trap 94 is located in the base of the cavity 56 and has a flanged portion closely encircling the nut 45 to prevent wear particles from migrating into the support bearings.

Referring now to FIG. 6, there is shown a rotating anode tube wherein the anode is supported in a somewhat different manner than in the structure of FIG. 1. In the structure of FIG. 6 the anode 100 has a hub 102 fixed to one end of skirt 104. Within the cavity 106 in skirt 104 is a coaxial cylindrical bearing support 108 connected by relatively small annular areas to the skirt to reduce the amount of heat which is transmitted by the skirt into the support 108.

Support 108 carries a pair of spaced bearing races 110 and 112 which are fixed thereto and support ball bearings ll4-l16 respectively which rotate about the circumference ofa fixed shaft 118.

The upper end of shaft 118 is spaced from the ad jacent inner end. surface of the skirt 104 while its lower end is provided with a reduced diameter end portion 120 which is threaded into a blocklike terminal 122 extending out the end of the tube envelope and sealed thereto as by collar 124.

At the inner end of the shaft 118 the cavity 106 becomes a compartment 126 within which the selected bypass contact structure is mounted. For example, the

contact structure may take the form illustrated in FIG. 2 wherein a leaf spring device 128 is mounted in the base of the compartment and'carries a silver contact button 130 which engages the polished or hard-coated end of the shaft 118, the exposed surface of an insert 132 therein, or a hardening coating on said surface.

It is to be understood, however, that other types of bypass contact structures may be employed with the anode structure of FIG. 6 such as the structures shown in FIGS. 35, in accordance with this invention. A particle trap 134 also may be employed,as shown.

Thus, there is provided in accordance with this invention a novel bearing bypass for electrical current I flow in a rotating anode X-ray tube whereby the support bearings may be conveniently made of nonconductive material, as well as the lubricant therefor. Such a bearing bypass as described will provide improved electrical current flow through the tubes anode. circuit and permit reduction in overall tube noise.

It will be understood that modifications and changes 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 set forth in the preceding description and in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

We claim:

1. An X-ray tube of the rotating anode type comprising an envelope containing an anode structure and a cathode structure in spaced cooperative relation, the anode structure including an X-ray generating target and a member supported for rotation with said target, a fixed support member mounted on said envelope, bearings between said rotatable member and said support member, and an electrical contact device electrically interconnecting said rotatable member and said support member for providing electrical conduction therebetween independent of and in bypass relation to' said bearings.

2. An X-ray tube as set forth in claim 1 wherein said contact device comprises resilient conductive means for providing an electrical path between said members and for simultaneously applying pressure upon said rotatable member to reduce slack in said bearings.

3. An X-ray tube as set forth in claim 1 wherein said contact device comprises a leaf spring located in firm abutting relation upon one of said members and having a contact thereon which is disposed in engagement with the other of said members.

4. An X-ray tube as set forth in claim 3 wherein said contact is comprised of a self-lubricatin g metal.

5. An X-ray tube as set forth in claim 3 wherein said other member is provided in the area of engagement by said contact with wear-reducing characteristics.

6. An X-ray tube as set forth in claim 2 wherein trap means is located in encircling relation to the contact device for reducing the opportunity for wear particles from the contact to migrate to said bearings.

7. An X-ray tube as set forth in claim 2 wherein said contact device comprises a rotating bearing located with a first race thereof in firm abutting relation upon 7 one of said members and with a second race spaced from the other of said members, and spring means is located between said second race and said other of said members for resiliently electrically connecting same.

8. An X-ray tube comprising an envelope containing an anode structure and a cathode structure in spaced cooperative relation, the anode structure including an X-ray generating target, a shaft connected at one end to the target, a cylindrical support fixed to said envelope and projecting coaxially in encircling relation to said shaft, bearings rotatably connecting said shaft to said support, and an electrical contact device electrically interconnecting said shaft and said support for providing electrical conduction therebetween independent of and in bypass relation to said bearings.

9. An X-ray tube as set forth in claim 8 wherein said contact device comprises resilient conductive means for providing an electrical path between said shaft and member and for simultaneously applying pressure upon said shaft to reduce slack in said bearings.

10. An X-ray tube as set forth in claim 8 wherein said contact device comprises a leaf spring located in firm abutting relation upon said support and having a contact thereon which is disposed in engagement with said shaft.

11. An X-ray tube as set forth in claim 10 wherein said contact is comprised of a self-lubricating metal.

12. An X-ray tube as set forth in claim 10 wherein said shaft is provided in the area of engagement by said contact with wear-reducing characteristics.

13. An X-ray tube as set forth in claim 10 wherein trap means is located in encircling relation to the contact device for reducing the opportunity for wear particles from the contact to migrate to said bearings.

14. An X-ray tube as set forth in claim 10 wherein said contact device comprises a rotating bearing I located with a first race thereof in firm abutting relation upon said support and with a second race spaced from said shaft, and spring means is located between said second race and said shaft for resiliently electrically connecting same.

15. An X-ray tube as set forth in claim 10 wherein said contact device comprises a rotating bearing located with a first race thereof in firm abutting relation against said shaft and with a second race spaced from said support, and spring means is located between said second race and said support for resiliently electrically connecting same.

16. An X-ray tube comprising an envelope containing an anode structure and a cathode structure in spaced cooperative relation, the anode structure including an X-ray generating target, a hollow cylindrical member connected at one end to the target, a shaft fixed to the envelope and extending coaxially within said member, bearings between said shaft and member whereby said member and target are rotatably supported by said shaft, and an electrical contact device electrically interconnecting said member and said shaft for providing electrical conduction therebetween independent of and in bypass relation to said bearings.

17. An X-ray tube as set forth in claim 16 wherein said contact device comprises resilient conductive means for providing an electrical path between said shaft and member and for simultaneously applying pressure upon said member to reduce slack in said bearings.

Claims (17)

1. An X-ray tube of the rotating anode type comprising an envelope containing an anode structure and a cathode structure in spaced cooperative relation, the anode structure including an Xray generating target and a member supported for rotation with said target, a fixed support member mounted on said envelope, bearings between said rotatable member and said support member, and an electrical contact device electrically interconnecting said rotatable member and said support member for providing electrical conduction therebetween independent of and in bypass relation to said bearings.

2. An X-ray tube as set forth in claim 1 wherein said contact device comprises resilient conductive means for providing an electrical path between said members and for simultaneously applying pressure upon said rotatable member to reduce slack in said bearings.

3. An X-ray tube as set forth in claim 1 wherein said contact device comprises a leaf spring located in firm abutting relation upon one of said members and having a contact thereon which is disposed in engagement with the other of said members.

4. An X-ray tube as set forth in claim 3 wherein said contact is comprised of a self-lubricating metal.

5. An X-ray tube as set forth in claim 3 wherein said other member is provided in the area of engagement by said contact with wear-reducing characteristics.

6. An X-ray tube as set forth in claim 2 wherein trap means is located in encircling relation to the contact device for reducing the opportunity for wear particles from the contact to migrate to said bearings.

7. An X-ray tube as set forth in claim 2 wherein said contact device comprises a rotating bearing located with a first race thereof in firm abutting relation upon one of said members and with a second race spaced from the other of said members, and spring means is located between said second race and said other of said members for resiliently electrically connecting same.

8. An X-ray tube comprising an envelope containing an anode structure and a cathode structure in spaced cooperative relation, the anode structure including an X-ray generating target, a shaft connected at one end to the target, a cylindrical support fixed to said envelope and projecting coaxially in encircling relation to said shaft, bearings rotatably connecting said shaft to said support, and an electrical contact device electrically interconnecting said shaft and said support for providing electrical conduction therebetween independent of and in bypass relation to said bearings.

9. An X-ray tube as set forth in claim 8 wherein said contact device comprises resilient conductive means for providing an electrical path between said shaft and member and for simultaneously applying pressure upon said shaft to reduce slack in said bearings.

10. An X-ray tube as set forth in claim 8 wherein said contact device comprises a leaf spring located in firm abutting relation upon said support and having a contact thereon which is disposed in engagement with said shaft.

11. An X-ray tube as set forth in claim 10 wherein said contact is comprised of a self-lubricating metal.

12. An X-ray tube as set forth in claim 10 wherein said shaft is provided in the area of engagement by said contact with wear-reducing characteristics.

13. An X-ray tube as set forth in claim 10 wherein trap means is located in encircling relation to the contact device for reducing the opportunity for wear particles from the contact to migrate to said bearings.

14. An X-ray tube as set forth in claim 10 wherein said contact device comprises a rotating bearing located with a first race thereof in firm abutting relation upon said support and with a second race spaced from said shaft, and spring means is located between said second race and said shaft for resiliently electrically connecting same.

15. An X-ray tube as set forth in claim 10 wherein said contact device comprises a rotating bearing located with a first race thereof in firm abutting relation against said shaft and with a second race spaced from said support, and spring means is located between said second race and said support for resiliently electrically connecting same.

16. An X-ray tube comprising an envelope containing an anode structure and a cathode structure in spaced cooperative relation, the anode structure including an X-ray generating target, a hollow cylindrical member connected at one end to the target, a shaft fixed to the envelope and extending coaxially within said member, bearings between said shaft and member whereby said member and target are rotatably supported by said shaft, and an electrical contact device electrically interconnecting said member and said shaft for providing electrical conduction therebetween independent of and in bypass relation to said bearings.

17. An X-ray tube as set forth in claim 16 wherein said contact device comprises resilient conductive means for providing an electrical path between said shaft and member and for simultaneously applying pressure upon said member to reduce slack in said bearings.

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