DUAL SUSPENSION BEARINGS FOR X-RAY TUBE

Abstract: A high energy x-ray tube includes an evacuated chamber (12) containing a rotor (34) which rotates an anode (10) through a stream of electrons (A) in order to generate an x-ray beam (B). The rotor includes an outer cylindrical armature (38) and a bearing assembly (C) having a bearing shaft (52) centrally aligned with a longitudinal axis (Z) of the rotor. An inner bearing member (42) is concentrically spaced from the bearing shaft and defines forward and rearward bearing grooves (46F, 46R). Forward and rearward bearing races (70F, 70R) are positioned between the inner bearing member and the bearing shaft. A plurality of forward and rearward bearings (48F, 48R) are dimensioned to be received between the forward and rearward bearing races and bearing grooves, respectively. Forward and rearward biasing spring washers (74F, 74R) apply a continuous, opposite biasing force to the forward and rearward bearing races, respectively, so that the bearing races urge their respective ball bearings into an optimal operating position.

Description: DUAL SUSPENSION BEARINGS FOR X-RAY TUBE Background of the Invention The present invention relates to the vacuum tube arts. It finds particular application in connection with increasing bearing assembly life and reducing undesirable bearing assembly noise in the bearing assembly of a rotating anode x-ray tube and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable for increasing bearing assembly life and reducing unwanted bearing assembly noise in other vacuum systems. A high power x-ray tube typically includes a rotating anode disposed within a glass envelope. A cathode supplies an electron beam to a target surface of the anode. When the electron beam strikes the rotating anode, a portion of the beam is converted to x-rays which pass out of the glass envelope. An induction motor is typically provided for rotating the anode. The anode is configured to rotate so that the heat energy will be spread over a relatively large area, thereby inhibiting the target area from overheating. The induction motor includes driving coils positioned outside the glass envelope and a rotor within the envelope which is connected to the anode. The rotor includes an outer, cylindrical armature or sleeve and an inner bearing member, which is centrally aligned within the armature . The armature and bearing member are centrally connected to the anode by a neck. A cylindrical bearing shaft is axially aligned with the armature and bearing member and is positioned therebetween. The bearing shaft is connected, at a rearward end, to a housing disposed outside the envelope. When the motor of a typical x-ray tube is energized, the driving coils induce magnetic fields in the armature which cause the armature and bearing member to rotate relative to the stationary bearing shaft. Bearings, such as ball or roller bearings, are positioned between the bearing member and bearing shaft for allowing the bearing member, armature, and anode to rotate smoothly, relative to the bearing shaft. The bearings are positioned between bearing grooves provided in the bearing member and bearing races provided on the stationary bearing shaft . The bearing grooves and bearing races help maintain the proper positioning of the ball bearings. Although the bearing assemblies are manufactured with precision tolerances, there are still minor tolerance stack-ups on the order of one or two thousandths of an inch. These tolerance stack-ups create a clearance between the ball bearings and the bearing races. Thus, under the forces exerted by a relatively heavy anode (on the order of 4.5 kg) , which is rotated at several thousand RPM's (on the order of 10,000 RPM's), the ball bearings experience unwanted movement and play. Such unwanted movement and play is also experienced when the x-ray tube is rotated such as in a CT scanner. When the ball bearings experience this unwanted movement, they tend to rub up against surfaces they are not intended to rub up against. Additionally, the clearance between the ball bearings and the bearing races permits the ball bearings to rattle back and forth. The undesirable rubbing and rattling creates bearing noise in the bearing assembly. Furthermore, rubbing and rattling reduces the life of the ball bearings. In an attempt to overcome some of the foregoing problems, a spring member has been provided at one end of the bearing assembly. However, in such a design, the spring member pushed one set of ball bearings toward their respective bearing race and pushed the other set of ball bearings away from their respective bearing race. The ball bearings being pushed away from their bearing race still experienced unwanted rubbing. Thus, those ball bearings generated noise and had a reduced life. The present invention provides a new and improved bearing assembly and method of operation for an x-ray tube which overcomes the above-referenced problems and others . Summary of the Invention In accordance with one aspect of the present invention, a bearing assembly for a rotor of an x-ray tube is provided. The bearing assembly includes a bearing shaft, centrally aligned with a longitudinal axis of the rotor, and a bearing member, concentrically spaced from the bearing shaft. A first bearing race is positioned between the bearing member and the bearing shaft. A first, plurality of bearings is dimensioned to be received between the bearing shaft and the first bearing race. A first biasing means applies a force to the first bearing race so that the first bearing race urges the first plurality of bearings into an optimal operating position. In accordance with another aspect of the present invention, a method of reducing noise and increasing bearing life in an x-ray tube having an anode rotatably connected to a rotor is provided. The rotor has a first bearing race positioned between a bearing member and a bearing shaft and a first plurality of bearings positioned between a seat of the first bearing race and a bearing groove defined by the bearing member. The method includes biasing the first bearing race so that the first bearing race urges the bearings into an optimal operating position. One advantage of the present invention is that it reduces operating noise. Another advantage of the present invention is that it increases bearing life. Another advantage of the present invention resides in the optimal, individual positioning of the ball bearings with respect to the bearing races and bearing grooves . Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon a reading and understanding of the following detailed description of the preferred embodiment . Brief Description of the Drawings The invention may take form in various components and arrangements of components and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment, and are not to be construed as limiting the invention. FIGURE 1 is a schematic view of an x-ray tube in accordance with the present invention; FIGURE 2 is a sectional view of the rotor, neck and anode of the x-ray tube of FIGURE 1 ; FIGURE 3 is a sectional view of the bearing assembly of the x-ray tube of FIGURE 1; and FIGURE 4 is a sectional view of the rotor of FIGURE 1 before being fully assembled. Detailed Description of the Preferred Embodiment With reference to FIGURE 1, a rotating anode x- ray tube of the type used in medical diagnostic systems for providing a focused beam of x-ray radiation includes a rotating anode 10 which is operated in an evacuated chamber 12 defined preferably by a glass envelope 14. The anode is disc-shaped and beveled adjacent its annular peripheral edge to define an anode surface or target area 16. A cathode assembly 18 supplies and focuses an electron beam A which strikes the anode surface 16. Leads 20 enter through the glass envelope and are connected to the cathode assembly for supplying an electrical current to the assembly and for providing a high voltage differential between the anode and the cathode. When the electron beam strikes the rotating anode, a portion of the beam is converted into x-rays B which are emitted from the anode surface and through the envelope 14. Upon exiting the envelope, the x-rays leave through a window 22 of a surrounding cooling oil enclosure or housing 24. With reference also to FIGURE 2, an induction motor 30 is provided for rotating the anode 10. The induction motor includes a stator having driving coils 32 which are positioned outside the glass envelope 14. The induction motor further includes a rotor 34, having a substantially cylindrical configuration, disposed within the envelope. The rotor includes a neck 36, which interconnects the rotor 34 with the anode 10, and an outer, cylindrical armature or sleeve portion 38 formed from a thermally and electrically conductive material, such as copper. An intermediate cylindrical member 40, preferably fabricated from steel, circumferentially engages the inside diameter of the armature . The cylindrical member 40 provides rigidity and strength to the rotor. With reference also to FIGURE 3, the rotor further includes a bearing assembly C. The bearing assembly includes an inner bearing member 42 centrally aligned with the armature and circumferentially engaging the inside diameter of the cylindrical member 40. An inner surface of the bearing member defines a rotor cavity 44. The inner bearing member has forward and rearward bearing grooves 46F, 46R configured to receive forward and rearward bearings 48F, 48R, respectively, such as ball or roller bearings. Each of the ball bearings is coated with a lubricant 50 (not to scale) , such as lead or silver, at a thickness of about 1000-2000 A. As used herein, the terms "forward," and "rear," and the like, are used to define relative positions of components along a central longitudinal axis Z of the rotor. Components which are described as forward are closer to the anode, while components described as rearward are further from the anode . The bearing assembly further includes a substantially cylindrical bearing shaft 52 centrally aligned with the rotor and configured to be inserted into the rotor cavity 44. The rearward end of the bearing shaft extends outside the glass envelope 14 and is rigidly connected to the housing 24 (see FIGURE 1) . The bearing shaft 52 includes an interior annular wall 54 having an inner surface 56 which defines a central bore or passageway 58 (see FIGURE 3) . Annular wall 54 has an outer surface 60, which with an inner surface of the bearing shaft, defines an annular chamber 62. An opening 64 is provided at the forward end of wall 54, thus providing access from passageway 58 to chamber 62. In operation, a cooling fluid, such as oil, is fed into passageway 58, through opening 64, and into chamber 62. After cooling the bearing assembly, the cooling fluid exits through channels 66 provided at the rearward end of the bearing shaft. The bearing shaft is held stationary with respect to the rotating armature so that the cooling fluid may be continuously fed therethrough. A first forward bearing race 7OF and a second rearward bearing race 70R are circumferentially disposed around forward and rearward portions of the bearing shaft, respectively. Each bearing race has a seat 72F, 72R formed therein of a generally semicircular shape, which is dimensioned to receive its respective plurality of ball bearings 48F, 48R. The bearing races are configured to urge the ball bearings 48F, 48R into an optimal position within their respective bearing grooves 46F, 46R. In order to urge the ball bearings into an optimal position, a forward and rearward biasing system 74F, 74R is provided behind the forward and rearward bearing races 70F, 70R, respectively. In a preferred embodiment, the biasing systems include a spring, such as a wave spring. However, it must be understood that the biasing systems can include any suitable urging member, such as an elastic material, a hydraulic system, coil springs, bellview washers, and the like. In the illustrated embodiment, the springs 74F, 74R preferably have a spring rating of 4.5 - 9 kg. In order to hold the races and springs in place, retaining rings 76F, 76R are connected to the free ends of the springs 74F and 74R, respectively. Retaining rings 76F and 76R are dimensioned to be snapped into snap ring grooves 80F and 80R, respectively, which are disposed at predetermined locations along the bearing shaft. The springs and retaining rings are positioned relative to each other to ensure that there will always be a spring force on the bearing races. Thus, the bearing parts, particularly the ball bearings, will stay in their proper location, thereby reducing unwanted movement, noise and wear. In operation, the induction motor 30 is energized causing the driving coils 32 to induce magnetic fields in the armature 38. The magnetic fields cause the rotor 34 to rotate relative to the stationary bearing shaft 52. As the rotor begins to rotate, the anode 10, which is connected to the rotor via the neck 36, also begins to rotate. In addition, the ball bearings 48F, 48R begin to roll between the bearing grooves 46F, 46R and the bearing races 70F, 70R, becoming extremely hot. A cooling fluid is fed into passageway 58, through opening 62, and into annular chamber 62 where it cools the ball bearings. The cooling fluid ultimately exits through channels 66 . Although the bearing assemblies are manufactured with precision tolerances, there are still minor tolerance stack-ups on the order of one or two thousandths of an inch. These tolerance stack-ups create a clearance between the ball bearings and the bearing races. Thus, under the forces exerted by the relatively heavy anode (on the order of 4.5 kg), which is rotating at several thousand RPM's (on the order of 10,000 RPM's), the ball bearings have a tendency to move and experience unwanted play. If the ball bearings experience this unwanted movement, they will rub up against surfaces they are not intended to rub up against, thus causing undesirable noise and reducing bearing life. In order to overcome these problems, the biasing systems 74F, 74R continuously urge the bearing races 70F, 70R toward the ball bearings 48F, 48R. Such a continuous spring force dynamically optimizes the seating of the ball bearings. Therefore, unwanted noise is minimized and bearing life is increased. Without the biasing system, the bearing piece parts would be free to move under forces generated during normal x-ray tube operation, such as those generated from rotation of the heavy anode. The biasing systems of the present invention overcome these forces and dynamically maintain each bearing piece in its optimal positions. With reference to FIGURE 4, the method of assembling the rotor 34 and bearing assembly includes placing a magnetic member 82 circumferentially around the perimeter of the rotor and in the same plane as the front bearing groove 46F. The forward ball bearings 48F are dropped into the rotor cavity 44. The magnetic force of the magnetic member pulls the ball bearings into the front bearing groove 46F. The rotor shaft, with the biasing systems 74F, 74R snapped therein, is inserted into the rotor cavity until the front bearing race 70F has adequately secured the front plurality of ball bearings 48F into the front bearing groove. The magnetic member is then removed. Finally, the rear ball bearings 48R are placed into the rotor cavity where they are secured between the rear bearing race 7OR and the rear bearing groove 46R (see FIGURE 3) .

Claims: Having thus described the preferred embodiment, the invention is now claimed to be: 1. A bearing assembly (C) for a rotor (34) of an x-ray tube, the bearing assembly comprising: a bearing shaft (52) centrally aligned with a longitudinal axis (Z) of the rotor; a bearing member (42) concentrically spaced from the bearing shaft; a first bearing race (70F, 70R) positioned between the bearing member and the bearing shaft ; a first plurality of bearings (48F, 48R) dimensioned to be received between the bearing shaft and the first bearing race; and a first biasing means (74F, 74R) for applying a force to the first bearing race so that the first bearing race urges the first plurality of bearings into an optimal operating position. 2. The bearing assembly of claim 1, further characterized by: the first biasing means (74F, 74R) including a spring loaded member. 3. The bearing assembly of claim 2, further characterized by: a retainer (76F, 76R) engaging a side of the spring loaded member, opposite to the first bearing race, the retainer being anchored at a predetermined location along the bearing shaft . 4. The bearing assembly of either one of preceding claims 2 and 3, further characterized by: - li the spring loaded member including a wave spring having a spring rating of between 4.5 and 9 kg. 5. The bearing assembly of any one of preceding claims 1-4, further characterized by: a second bearing race (70R) spaced from the first bearing race (7OF) and disposed between the bearing member (42) and the bearing shaft (52) , the second bearing race having a seat (72R) for receiving a second plurality of bearings (48R) ; and a second biasing means (74R) for applying a force to the second bearing race (70R) so that the second bearing race urges the second plurality of bearings into an optimal operating position. 6. The bearing assembly of claim 5, further characterized by: a retaining means (76F, 76R, 80F, 80R) disposed between the bearing races, the retaining means having oppositely disposed faces for engaging the first and second biasing means, the first and second biasing means being disposed between the retaining means and the bearing races such that the bearing races are biased apart . 7. The bearing assembly of claim 6 , further characterized by: the retaining means including two snap rings (76F, 76R) that are received in grooves (80F, 80R) in the inner bearing member. 8. The bearing assembly of any one of preceding claims 5-7, further characterized by: the first and second biasing means including spring loaded washers . 9. The bearing assembly of any one of preceding claims 5-8, further characterized by: the first and second bearing races being biased in opposite directions. 10. The bearing assembly of any one of preceding claims 5-9, further characterized by: the first and second bearing races being independently biased into optimal operating positions. 11. The bearing assembly of any one of preceding claims 1-10, further characterized by: a surface of the bearing member defining a bearing groove (46F, 46R) for receiving the bearings. 12. The bearing assembly of any one of preceding claims 1-11, further characterized by: each of the first plurality of bearings being ball bearings coated with a lubricant (50) . 13. An x-ray tube for providing a beam of x- rays, the tube comprising: an envelope (14) which defines an evacuated chamber (12) ; a cathode (18) disposed within the chamber for providing a source of electrons; an anode (10) disposed within the chamber positioned to be struck by the electrons and generate x- rays ; a rotor (34) operatively connected to the anode for rotating the anode relative to the cathode, the rotor including the bearing assembly (C) of any one of preceding claims 1-12. 14. The x-ray tube of claim 13, further characterized by: the bearing shaft remaining stationary relative to the anode and rotor during operation of the x-ray tube. 15. The x-ray tube of either one of preceding claims 13 and 14, further characterized by: the rotor of the x-ray tube further including: a rigid member (40) axially aligned with and circumferentially engaging the inner bearing member for providing rigidity to the rotor, an armature (34) axially aligned with and circumferentially engaging the rigid member, and a neck (36) interconnecting the rotor to the anode . 16. A method of manufacturing the bearing assembly of any one of preceding claims 1-12, characterized by the steps of : positioning the first plurality of bearings (48F)in a first bearing groove (46F) of the inner bearing member; after positioning the bearings, inserting the bearing shaft into a cavity (44) defined by the inner bearing member so that the seat of the first bearing race (70F) urges the first plurality of bearings into the first bearing groove; and positioning a second plurality of bearings, (48R) between a second bearing groove (46R) of the inner bearing member and a second bearing race (7OR) disposed on the bearing shaft . 17. The method of claim 16, further characterized by: the step of positioning the first plurality of bearings includes placing a magnetic member (82) adjacent an outer surface of the rotor in aligned relation with the first bearing groove in order to hold the first plurality of bearings in place during assembly. 18. A method of reducing noise and increasing bearing life in an x-ray tube having an anode (10) rotatably connected to a rotor (34) , the rotor having a first bearing race (70F, 70R) positioned between a bearing member (42) and a bearing shaft (52) and a first plurality of bearings (48F, 48R) positioned between a seat (72F, 72R) of the first bearing race and a bearing groove (46F, 46R) defined by the bearing member, the method comprising: biasing the first bearing race so that the first bearing race urges the bearings into an optimal operating position. 19. The method of claim 18, further characterized by: the rotor having a second bearing race (70F, 70R) and a second bearing groove (46F, 46R) and further including: resiliently biasing the first and second bearing races in opposite directions.

Inventors: PANASIK CHERYL L, RANCHERO NICANOR C, PANASIK, CHERYL, L., RANCHERO, NICANOR, C.
Applicants: MARCONI MEDICAL SYS INC, MARCONI MEDICAL SYSTEMS, INC.
Application year: 2001
Main classification: H01J35
IPC codes: F16C19, F16C25, F16C33, F16C35, F16C37, F16C43, H01J35