Patent Application
20110007877
The invention relates generally to x-ray tubes and, more particularly, to a liquid metal bearing in an x-ray tube and a method of assembling same.
X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is typically necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearing assembly, which typically includes tool steel ball bearings and tool steel raceways positioned within the vacuum region, thereby requiring lubrication by a solid lubricant such as silver. Wear of the silver and loss thereof from the bearing contact region increases acoustic noise and slows the rotor during operation.
In addition, the operating conditions of newer generation x-ray tubes have become increasingly aggressive in terms of stresses because of G forces imposed by higher gantry speeds and higher anode run speeds. As a result, there is greater emphasis in finding bearing solutions for improved performance under the more stringent operating conditions.
A liquid metal bearing (i.e. a spiral groove bearing, or SGB) may be employed in lieu of ball bearings. Advantages of liquid metal bearings include a high load capability and a high heat transfer capability due to an increased amount of contact area as compared to a ball bearing. Advantages also include low acoustic noise operation. Gallium, indium, or tin alloys are typically used as the liquid metal, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube.
However, liquid metals typically used in an SGB tend to be highly reactive and corrosive. The liquid metal of an SGB may react with a base metal that it contacts, thus consuming the liquid metal and shortening the life of the SGB. The rate of reaction is a function of temperature, and the temperature of an SGB tends to increase during operation—both because of high temperatures that occur during x-ray generation within the anode, and because of self-heating of the liquid metal. As such, the elevated operating temperature of the liquid metal may increase a loss rate of the liquid metal, leading to early life failure of the x-ray tube.
Therefore, it would be desirable to design an x-ray tube with an SGB having a reduced operating temperature therein.
The invention provides an apparatus for improving an x-ray tube with a SGB bearing, that overcomes the aforementioned drawbacks.
According to one aspect of the invention, an x-ray tube includes a center shaft having an inner surface and an outer surface, the inner surface forming a portion of a cavity therein, a mount having an inner surface, the mount having an x-ray target attached thereto, and a liquid metal positioned between the outer surface of the center shaft and the inner surface of the mount. The x-ray tube further includes a flow diverter positioned in the cavity, the flow diverter having a wall with an inner surface, and a plurality of jets passing through the wall, wherein the plurality of jets are configured such that when a fluid is flowed into the flow diverter and passes along its inner surface, a portion of the fluid passes through the plurality of jets and is directed toward the inner surface of the center shaft.
In accordance with another aspect of the invention, a method of assembling an x-ray tube includes providing a center mount structure having an inner surface and an outer surface, forming a passageway in the center mount structure, the passageway configured to pass a coolant therein, providing a rotatable mount structure having an inner surface, and attaching a target to the rotatable mount structure. The method further includes applying a liquid metal to one of the outer surface of the center mount structure and the inner surface of the rotatable mount structure, coupling the rotatable mount structure to the center mount structure such that the liquid metal is positioned between the outer surface of the center mount structure and the inner surface of the rotatable mount structure, and coupling a porous material to the inner surface of the center mount structure.
Yet another aspect of the invention includes a spiral groove bearing (SGB) that includes a column having an outer diameter and an inner diameter, the inner diameter partially enclosing a hollow, a mount having a flange thereon, the mount having an inner diameter that is larger than the outer diameter of the column, wherein the flange is configured to attach an x-ray target thereto, and a liquid metal positioned between the outer diameter of the column and the inner diameter of the mount. The SGB also includes a porous-meshed heat transfer-enhancement media coupled to the inner diameter of the column.
Various other features and advantages of the invention will be made apparent from the following detailed description and the drawings.
The drawings illustrate preferred embodiments presently contemplated for carrying out the invention.
In the drawings:
As shown in
A processor 12 receives the signals from the detector 10 and generates an image corresponding to the object 8 being scanned. A computer 14 communicates with processor 12 to enable an operator, using operator console 16, to control the scanning parameters and to view the generated image. That is, operator console 16 includes some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus that allows an operator to control the imaging system 2 and view the reconstructed image or other data from computer 14 on a display unit 18. Additionally, operator console 16 allows an operator to store the generated image in a storage device 20 which may include hard drives, flash memory, compact discs, etc. The operator may also use operator console 16 to provide commands and instructions to computer 14 for controlling a source controller 22 that provides power and timing signals to x-ray source 4.
Liquid metal 58 serves to support first sleeve 44, second sleeve 46, and target 32. Liquid metal 58 thereby functions as a lubricant between rotating and stationary components. In the embodiment illustrated, center shaft 38 is caused to be stationary with respect to frame 24, and target 32, first sleeve 44, and second sleeve 46 are caused to rotate about an axis of rotation 64 of x-ray tube 4. Thus, x-rays 6 are produced when high-speed electrons are suddenly decelerated when directed from the cathode 36 to the anode 32 via a potential difference therebetween of, for example, 60 thousand volts or more in the case of CT applications. The x-rays 6 are emitted through radiation emission passage 28 toward a detector array, such as detector 10 of
Heating within liquid metal 58 may be non-uniform because of various features of SGB 34. For instance, some locations or surfaces within SGB 34 may have a higher relative motion than other surfaces. As an example, radial projection 42 has a radial diameter 66 that is greater than at other surfaces within SGB 34, such as at diameter 68. Thus, because of the increased radial diameter of radial projection 42, a higher relative surface velocity occurs at diameter 66 than at diameter 68. As such, radial projection 42 may cause localized heating within SGB 34 and may cause liquid metal 58 at diameter 66 to have an increased temperature above liquid metal 58 at other locations, such as at diameter 68.
SGBs typically include angled grooves for containing liquid metal therein and preventing loss of liquid metal from gaps such as gap 50 of SGB 34, as is commonly understood in the art. For instance, grooves may be positioned on outer surface 52 of center shaft 38, on inner surface 54 of first sleeve 44, on inner surfaces 56 of second sleeve 46, and on combinations thereof. Thus, though the grooves function to contain liquid metal 58 within gap 50, they do so at the expense of increased frictional heating within SGB 34 of liquid metal 58. As such, locations that include grooves may experience an increased temperature relative to locations within gap 50 that do not include angled grooves.
Thus, localized heating may occur within SGB 34 for at least the two reasons outlined above. As such, because a rate of corrosion or reaction is typically temperature dependent and increases with increasing temperature, hot spots may form within SGB 34 that may precipitate early life failure of x-ray tube 4. Accordingly, cavity 65 of SGB 34 includes a flow diverter or flow separator 70 at end 72 according to embodiments of the invention. Flow separator 70 is positioned therein having a fluid inlet 74 and an annular fluid exit 76. Flow separator 70 includes an axial endcap 78 that prevents axial flow of fluid from passing unimpeded by end 72 of flow separator 70.
Flow separator 70 includes a plurality of nozzles, jets, or passageways 80 positioned therein, according to an embodiment of the invention. Jets 80 are configured to direct fluid toward inner surface 67 of cavity 65, and in embodiments of the invention, jets 80 are selectively positioned to direct fluid toward specific locations of inner surface 67 that otherwise would have increased temperatures for reasons as stated above. In embodiments of the invention, axial endcap may 78 include one or more nozzles 82 that pass fluid toward a surface 84 of cavity 65.
In another embodiment of the invention, SGB 34 includes one or more porous or heat transfer-enhancement media 83, 85 coupled to inner surfaces 67, 84, respectively, of cavity 65. According to embodiments, media 83, 85 include foam comprised of graphite, copper, aluminum, and the like that may be coupled to surfaces 67, 84 by an interference fit, by brazing, or other mechanical attachments. Thus, because of an increased surface area within media 83, 85 as compared to surfaces 67, 84, and because media 83, 85 tend to increase turbulence of fluid passing therethrough, heat transfer tends to be dramatically increased as compared to natural or forced convection occurring over a surface such as surfaces 67, 84. And, as illustrated, media 83, 85 may be embedded within surfaces 67, 84 and between surfaces 67, 84 and flow separator 70. In one embodiment media 83, 85 is not embedded within surfaces 67, 84. Further, although media 83, 85 are positioned intermittently along surfaces 67, 84, media 83 may extend over an entire axial length of surface 67, and media 85 may extend over an entire area of surface 84. Media 83, 85 may provide structural support for the flow separator 70.
Heat transfer may be further enhanced by combining nozzles and heat transfer media within SGB 34. Thus, although embodiments described above may include only nozzles 80, 82, it is to be understood that embodiments include both nozzles 80, 82 and media 83, 85 in a single embodiment. Further, in such an embodiment, nozzles 80, 82 and media 83, 85 may be selectively placed within SGB 34 at hot spots therein, or may include nozzles 80, 82 and media 83, 85 positioned therein along and throughout the entire surfaces 67, 84 of SGB 34.
Thus, in operation, target 32 is caused to rotate about axis of rotation 64 via rotor 60, which is mechanically coupled thereto via first and second sleeves 44, 46. Cooling fluid, which may include a liquid such as dielectric oil, ethylene glycol, propylene glycol, and the like, or which may include a gas such as air, nitrogen, argon, and the like, is pressurized and caused to flow into flow separator 70 at inlet 74. Fluid thus flows along an inner surface 86 of flow separator 70 and passes through jets 80, 82 and is caused to impinge upon surfaces 67, 84 of cavity 65. Accordingly, because fluid velocity is typically increased as it passes through jets 80, 82, heat transfer from surfaces 67, 84 is thereby enhanced because of an increased convection coefficient. Further, in an embodiment that includes heat transfer-enhancement media 83, 85, heat transfer is further enhanced as fluid passes through jets 80, 82 and impinges on media 83, 85. As such, such embodiments enhance heat transfer within SGB 34 and cause liquid metal 58 to decrease in temperature.
Further, because of the increased capability to transfer heat, such an embodiment may increase an amount of heat transferred from target 32 into SGB 34. And, although x-ray tube designs typically include materials having a high thermal resistance between the target and the shaft on which it is mounted in order to reduce heat transfer to the shaft, in the embodiments illustrated herein, such steps may be unnecessary. Thus, because of the enhancements to heat transfer within SGB 34 as disclosed herein, target 32 may operate at a cooler temperature than would otherwise be experienced without such enhancements.
According to this embodiment, x-ray tube 4 includes a center shaft, column, or center mount structure 100 that is configured to be attached to frame 24 at attachment points 102, 104. SGB 34 includes gap 50 formed between outer surface 52 of center shaft 100 and inner surface 54 of first sleeve 44. Similarly, gap 50 is formed between outer surface 52 of center shaft 100 and inner surfaces 56 of second sleeve 46. Liquid metal 58 is positioned within gap 50, and in embodiments of the invention, liquid metal 58 comprises gallium, tin, indium, and alloys thereof, as examples.
Center shaft 100 includes a hollow or cavity 106 formed by an inner surface 108 of center shaft 100 for passage of liquid coolant therein, and center shaft 100 includes an inlet 110 and an outlet 112. Thus, fluid may be passed from inlet 110 to outlet 112 and as a consequence, heat energy may be drawn from SGB 34 during operation thereof. According to one embodiment, cavity 106 of SGB 34 includes a flow diverter or flow separator 114. This embodiment may include an annular obstruction 130 attached or coupled to flow separator 114 and positioned to prevent flow from passing from inlet 110 and then flowing back toward inlet 110 once it passes through passageways 122 positioned therein. Flow separator 114 includes an axial endcap 118 that prevents axial flow of fluid from passing unimpeded by end 120 of flow separator 114.
Passageways 122 may include a nozzles, jets, and the like that direct and accelerate fluid passing therethrough. Passageways 122 are configured to direct fluid toward surface 124 of cavity 106, and in embodiments of the invention, passageways 122 are selectively positioned to direct fluid toward specific locations of surface 124 that otherwise would have increased temperatures for reasons as stated above. In embodiments of the invention, axial endcap may 118 include one or more nozzles or passageways 126 that pass fluid therethrough, which may function to regulate passage of fluid therein. According to one embodiment, heat transfer-enhancement media 127 are included in SGB 34 and are positioned either on surface 124 of SGB 34, or embedded therein. Further, although illustrated as being intermittently positioned along surface 124, media 127 may be positioned along selective portions or an entire axial length of surface 124.
In one embodiment, a porous media 129 having an annular shape, or one or more disks of media positioned within cavity 106, may be attached to inner surface 108 of center shaft 100. Thus, in this embodiment, porous media 129 may further enhance heat transfer of fluid passing through flow diverter 114 and passing through passageways 122 before exiting cavity 106 at fluid exit 112. In yet another embodiment of the invention, no flow separator 114 is provided and in this embodiment porous media 129 may be positioned anywhere within cavity 106 at one or multiple locations, or along much or all of the length of cavity 106. This embodiment may or may not include porous media 127, depending on desired heat transfer characteristics within cavity 106. Thus, in this embodiment, fluid may pass from inlet 110, into cavity 106, and through porous media 129 before passing through fluid exit 112.
Thus, in operation, target 32 is caused to rotate about axis of rotation 64 via rotor 60, which is mechanically coupled thereto via first and second sleeves 44, 46. Cooling fluid, which may include a liquid such as dielectric oil, ethylene glycol, propylene glycol, and the like, or which may include a gas such as air, nitrogen, argon, and the like, is pressurized and caused to flow into flow separator 114 at fluid inlet 116. Fluid thus flows along an inner surface 128 of flow separator 114 and passes through jets 122 and passageways 126 and is caused to impinge upon surface 124 of cavity 106, or upon heat transfer-enhancement media 127. Accordingly, because fluid velocity is typically increased as it passes through jets 122, heat transfer from surface 124 is thereby enhanced because of an increased convection coefficient. Likewise, in alternate embodiments, a porous media 129 may be included and fluid passing therethrough may enhance convection therein. Such embodiments may be used alone and without a flow separator 114, or may be used in conjunction therewith, and in any and all combinations thereof to enhance convection within cavity 106.
Therefore, according to one embodiment of the invention, an x-ray tube includes a center shaft having an inner surface and an outer surface, the inner surface forming a portion of a cavity therein, a mount having an inner surface, the mount having an x-ray target attached thereto, and a liquid metal positioned between the outer surface of the center shaft and the inner surface of the mount. The x-ray tube further includes a flow diverter positioned in the cavity, the flow diverter having a wall with an inner surface, and a plurality of jets passing through the wall, wherein the plurality of jets are configured such that when a fluid is flowed into the flow diverter and passes along its inner surface, a portion of the fluid passes through the plurality of jets and is directed toward the inner surface of the center shaft.
In accordance with another embodiment of the invention, a method of assembling an x-ray tube includes providing a center mount structure having an inner surface and an outer surface, forming a passageway in the center mount structure, the passageway configured to pass a coolant therein, providing a rotatable mount structure having an inner surface, and attaching a target to the rotatable mount structure. The method further includes applying a liquid metal to one of the outer surface of the center mount structure and the inner surface of the rotatable mount structure, coupling the rotatable mount structure to the center mount structure such that the liquid metal is positioned between the outer surface of the center mount structure and the inner surface of the rotatable mount structure, and coupling a porous material to the inner surface of the center mount structure.
Yet another embodiment of the invention includes a spiral groove bearing (SGB) that includes a column having an outer diameter and an inner diameter, the inner diameter partially enclosing a hollow, a mount having a flange thereon, the mount having an inner diameter that is larger than the outer diameter of the column, wherein the flange is configured to attach an x-ray target thereto, and a liquid metal positioned between the outer diameter of the column and the inner diameter of the mount. The SGB also includes a porous-meshed heat transfer-enhancement media coupled to the inner diameter of the column.
The invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.
