US20230343543A1 - X-ray tube liquid metal bearing structure for reducing trapped gases - Google Patents
X-ray tube liquid metal bearing structure for reducing trapped gases Download PDFInfo
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- US20230343543A1 US20230343543A1 US18/310,023 US202318310023A US2023343543A1 US 20230343543 A1 US20230343543 A1 US 20230343543A1 US 202318310023 A US202318310023 A US 202318310023A US 2023343543 A1 US2023343543 A1 US 2023343543A1
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- liquid metal
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Images
Classifications
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- F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
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- F16C17/102—Sliding-contact bearings for exclusively rotary movement for both radial and axial load with grooves in the bearing surface to generate hydrodynamic pressure
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- H01J35/10—Rotary anodes; Arrangements for rotating anodes; Cooling rotary anodes
- H01J35/101—Arrangements for rotating anodes, e.g. supporting means, means for greasing, means for sealing the axle or means for shielding or protecting the driving
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- F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
- F16C33/00—Parts of bearings; Special methods for making bearings or parts thereof
- F16C33/30—Parts of ball or roller bearings
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- F16C33/6637—Special parts or details in view of lubrication with liquid lubricant
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- F16C33/6692—Liquids other than oil, e.g. water, refrigerants, liquid metal
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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- H01J2235/20—Arrangements for controlling gases within the X-ray tube
Definitions
- the invention relates generally to X-ray tubes, and more particularly to structures and methods of assembly for the bearing utilized in an X-ray tube.
- X-ray systems including computed tomography (CT) imaging systems, may include an X-ray tube, a detector, and a support structure for the X-ray tube and the detector.
- CT computed tomography
- an imaging table on which an object is positioned, may be 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 passes through the object on the imaging table and impinges on the detector.
- internal structures of the object cause spatial variances in the radiation received at the detector.
- the detector then generates data, and the system translates the data into an image, which may be used to evaluate the internal structures of the object.
- 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 an X-ray scanner or CT scanner.
- X-ray tubes include a cathode and an anode located within a high-vacuum environment.
- the anode structure is supported by a liquid metal bearing structure, e.g., a herringbone groove or spiral groove bearing (SGB) structure, also known as a hydrodynamic bearing, formed with a shaft disposed within a sleeve to which the anode target is attached and rotates around the shaft.
- the spiral groove bearing structure also includes grooves, such as spiral or helical grooves, on various surfaces of the shaft and/or the sleeve that serve to take up the radial and axial forces acting on the sleeve as it rotates around the shaft.
- an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into an axle formed at least partially of the sleeve that supports the anode target and a stator 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.
- the 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 target. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at a high rotational speed. This places stringent demands on the bearings and the material forming the anode structure, i.e., the anode target and the shaft supporting the target.
- liquid metal bearings such as spiral groove bearings in X-ray tubes include a high load capability and a high heat transfer capability due to an increased amount of contact area.
- Other advantages include low acoustic noise operation, as is commonly understood in the art.
- Gallium, indium, or tin alloys, among others, are typically used as the liquid metal in the bearing structure, 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.
- the adherence of the liquid metal to the bearing surfaces creates a high-pressure seal, i.e., up to 20 psi, between the liquid metal and the bearing components that often exceeds the pressure of the gas(es) trapped within the bearing, thereby retaining the gas within the bearing.
- the construction of the components of the bearing structure e.g., the grooves, can include textured areas operable to retain the liquid metal in the proper locations for functioning as a bearing fluid, further enhancing the seal and preventing the gas from escaping through the liquid metal.
- the bearing retains significant amounts of trapped gas(es) disposed within the bearing structure, detrimentally affecting the operation of the X-ray tube including the journal bearing, including, but not limited to failure of the X-ray tube.
- a liquid metal or spiral groove bearing structure for an X-ray tube and associated process for manufacturing a bearing structure comprised of a shaft having a journal bearing and a radially protruding thrust bearing encased within a bearing housing or sleeve.
- the sleeve includes a thrust seal that is engaged with the sleeve around the thrust bearing in a manner to maintain coaxiality for the rotating liquid metal seal formed within the sleeve about the journal bearing shaft.
- the engagement of the thrust seal with the sleeve around the thrust bearing on the journal bearing shaft enables the liquid metal to be retained between the journal bearing shaft and the sleeve to enable free rotation of the sleeve around the journal bearing shaft during operation of the X-ray tube.
- the structure of one or both of the journal bearing and the thrust bearing includes at least one vent groove formed therein that improves the ability of the journal bearing structure to enable gases trapped by the liquid metal within the bearing assembly to escape through the vent groove to the exterior of the X-ray tube.
- the pressure level of the gases contained by capillary wetting of the journal bearing shaft and the thrust bearing by the liquid metal can exceed the pressure of the seal in the vent groove, thereby allowing the gas to escape the tube along the vent groove without detrimentally affecting the bearing performance.
- bearing assembly for an X-ray tube including a shaft, a sleeve disposed around the shaft, wherein one of the shaft and the sleeve is rotatable with regard to the other to form a rotating component and a stationary component, a lubricating fluid disposed between the shaft and the sleeve within a gap between the sleeve and the shaft; and at least one vent groove disposed on the stationary component.
- an X-ray tube in another exemplary embodiment of the invention, includes a cathode assembly, and an anode assembly spaced from the cathode assembly, wherein the anode assembly includes a shaft, a sleeve disposed on the shaft, wherein one of the shaft and the sleeve is rotatable with regard to the other to form a rotating component and a stationary component, a lubricating fluid disposed between the shaft and the sleeve within a gap between the sleeve and the shaft, and at least one vent groove disposed on the stationary component, and an anode target operably connected to the sleeve.
- a method for enabling gases trapped in a bearing assembly to be vented from bearing assembly during use in an operating X-ray tube including the steps of providing a bearing assembly having a shaft, a sleeve disposed on the shaft, wherein one of the shaft and the sleeve is rotatable with regard to the other to form a rotating component and a stationary component, a lubricating fluid disposed between the shaft and the sleeve within a gap between the sleeve and the shaft, and at least one vent groove disposed on the stationary component, and operating the X-ray tube to rotate the rotating component relative to the stationary component.
- FIG. 1 is a block diagram of an imaging system incorporating exemplary embodiments of the disclosure.
- FIG. 2 is a cross-sectional view of a portion of an X-ray tube according to an exemplary embodiment of the disclosure and usable with the system illustrated in FIG. 1 .
- FIG. 3 is a cross-sectional side plan view of a bearing structure of an X-ray tube in accordance with an exemplary embodiment of the disclosure.
- FIG. 4 is an isometric view of the bearing structure of FIG. 3 including a vent groove therein formed in accordance with an exemplary embodiment of the disclosure.
- FIG. 5 is a schematic, cross-sectional view of the bearing structure of FIG. 4 formed in accordance with another exemplary embodiment of the disclosure illustrating the location of the vent groove relative to a high pressure region formed between the journal bearing and the sleeve forming the bearing structure.
- FIG. 6 is a front elevation view of the journal bearing of the bearing structure of FIG. 3 including vent grooves in accordance with another exemplary embodiment of the disclosure.
- FIGS. 7 A- 7 C are top plan and cross-sectional schematic views of vent grooves for the bearing structure of FIG. 3 formed in accordance with further exemplary embodiments of the disclosure.
- FIG. 8 is a perspective view of a thrust bearing including vent grooves for the bearing structure of FIG. 3 formed in accordance with further exemplary embodiments of the disclosure.
- FIG. 1 is a block diagram of an embodiment of an imaging system 10 designed both to acquire original image data and to process the image data for display and/or analysis in accordance with embodiments of the invention.
- imaging system 10 designed both to acquire original image data and to process the image data for display and/or analysis in accordance with embodiments of the invention.
- CT computed tomography
- RAD digital radiography
- imaging system 10 includes an X-ray tube or X-ray source 12 configured to project a beam of X-rays 14 through an object 16 .
- Object 16 may include a human subject, pieces of baggage, or other objects desired to be scanned.
- X-ray source 12 may be conventional X-ray tubes producing X-rays 14 having a spectrum of energies that range, typically, from thirty (30) keV to two hundred (200) keV.
- the X-rays 14 pass through object 16 and, after being attenuated, impinge upon a detector assembly 18 .
- Each detector module in detector assembly 18 produces electrical signals that represents the intensity of an impinging X-ray beam, and hence the attenuated beam, as it passes through the object 16 .
- detector assembly 18 is a scintillator based detector assembly, however, it is also envisioned that direct-conversion type detectors (e.g., CZT detectors, photon-counting detectors, etc.) may also be implemented.
- a processor 20 receives the signals from the detector 18 and generates an image corresponding to the object 16 being scanned.
- a computer 22 communicates with processor 20 to enable an operator, using operator console 24 , to control the scanning parameters and to view the generated image.
- operator console 24 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 X-ray system 10 and view the reconstructed image or other data from computer 22 on a display unit 26 .
- console 24 allows an operator to store the generated image in a storage device 28 which may include hard drives, floppy discs, compact discs, etc. The operator may also use console 24 to provide commands and instructions to computer 22 for controlling an X-ray source controller 30 that provides power and timing signals to X-ray source 12 .
- FIG. 2 illustrates a cross-sectional view of an X-ray source 12 incorporating embodiments of the invention.
- X-ray source 12 is an X-ray tube 40 that includes an anode assembly 42 and a cathode assembly 44 .
- the anode and cathode assemblies 42 , 44 are supported within an insert or frame 46 , which houses a target or anode 48 , a bearing assembly 50 , and a cathode 52 .
- Frame 46 defines an area of relatively low pressure (e.g., a vacuum) compared to ambient, in which high voltages may be present.
- Frame 46 may be positioned within a casing (not shown) filled with a cooling medium, such as oil, that may also provide high voltage insulation. While the target and anode are described above as being a common component of X-ray tube 40 , the target and anode may be separate components in alternative X-ray tube embodiments.
- an electron beam 54 is produced by cathode assembly 44 .
- cathode 52 receives one or more electrical signals via a plurality of electrical leads 56 .
- the electrical signals may include power and timing/control signals that cause cathode 52 to emit electron beam 54 at one or more energies and at one or more frequencies.
- the electrical signals may also at least partially control the potential between cathode 52 and anode 48 .
- Cathode 52 includes an insulator 58 from which an arm 60 extends. Arm 60 encloses electrical leads 56 , which extend into a cathode cup 62 mounted at the end of arm 60 .
- cathode cup 62 includes focusing elements that focuses electrons emitted from a filament within cathode cup 62 to form electron beam 54 .
- X-rays 64 are produced when high-speed electrons of electron beam 54 from cathode 52 are suddenly decelerated upon impacting a target surface 66 formed on anode target 48 .
- the high-speed electrons forming electron beam 54 are accelerated toward the anode target 48 via a potential difference therebetween of, for example, sixty (60) thousand volts or more in the case of CT applications.
- the X-rays 64 are emitted through a radiation emission window 68 formed in frame 46 that is positioned toward a detector array, such as detector 18 of FIG. 1 .
- Anode assembly 42 includes a rotor 72 and a stator (not shown) located outside X-ray source 40 and partially surrounding rotor 72 for causing rotation of anode target 48 during operation.
- Anode target 48 is supported in rotation by a bearing assembly 50 , which, when rotated, also causes anode target 48 to rotate about a centerline 70 .
- anode target 48 has a generally annular shape, such as a disk, and an annular opening 74 in the center thereof for receiving bearing assembly 50 .
- Target 48 may be manufactured to include a number of metals or composites, such as tungsten, molybdenum, or any material that contributes to Bremsstrahlung (i.e., deceleration radiation) when bombarded with electrons.
- Target surface 66 of anode target 48 may be selected to have a relatively high refractory value so as to withstand the heat generated by electrons impacting target surface 66 .
- the space within insert or frame 46 and between cathode assembly 44 and anode assembly 42 is at vacuum pressure in order to minimize electron collisions with other atoms and to maximize an electric potential.
- rotor 72 rotates target 48 at a high rate of speed (e.g., 90 to 250 Hz) about a centerline 70 .
- a high rate of speed e.g. 90 to 250 Hz
- the X-ray source 40 as a whole is caused to rotate within a gantry (not shown) about an object, such as object 16 of X-ray imaging system 10 in FIG. 1 , at rates of typically 1 Hz or faster.
- Bearing assembly 50 can be formed as necessary, such with a number of suitable ball bearings (not shown), but in the illustrated exemplary embodiment comprises a liquid lubricated or self-acting bearing, such as a liquid metal bearing, having adequate load-bearing capability and acceptable acoustic noise levels for operation within imaging system 10 of FIG. 1 .
- a liquid lubricated or self-acting bearing such as a liquid metal bearing, having adequate load-bearing capability and acceptable acoustic noise levels for operation within imaging system 10 of FIG. 1 .
- self-acting and self-lubricating mean that the bearing lubricating fluid remains distributed on the surfaces of the bearing due to the relative motion of the bearing components and absent an external pump.
- bearing assembly 50 includes a stationary component, such as shaft 76 , and a rotating component, such as sleeve 78 that surrounds the shaft 76 and to which the anode target 48 is attached. While shaft 76 is described with respect to FIG. 2 as the stationary portion of bearing assembly 50 and sleeve 78 is described as the rotating portion of bearing assembly 50 , embodiments of the present invention are also applicable to embodiments wherein the shaft 76 , the rotating component, rotates within a stationary sleeve 78 , a stationary component. In such a configuration, anode target 48 would rotate as shaft 76 rotates.
- Shaft 76 includes a cavity, bore or coolant flow path 80 though which a coolant 82 ( FIG. 3 ), such as oil, flows to cool bearing assembly 50 .
- coolant 82 enables heat generated from anode target 48 of X-ray source 40 ( FIG. 2 ) to be extracted therefrom and transferred external to X-ray source 40 .
- coolant flow path 80 extends along a longitudinal length of shaft 76 .
- bore 80 may extend through only a portion of shaft 76 , such as in configurations where an X-ray source 40 is cantilevered when placed in an X-ray imaging system.
- Bearing assembly 50 includes a shaft 76 positioned within sleeve 78 , which is configured to support an anode target, such as anode target 48 of FIG. 2 .
- a lubricating fluid 84 is positioned in a gap 86 formed between shaft 76 and sleeve 78 .
- lubricating fluid 84 is a metal or metallic alloy that exists in a liquid state at operating temperature of bearing assembly 50 .
- the lubricating fluid 84 flowing between the rotating and stationary components of the bearing assembly 50 may include a variety of individual fluids as well as mixtures of fluids.
- multiple liquid metals and liquid metal alloys may be used as the lubricating fluid, such as an indium gallium alloy.
- fluids with relatively low vapor pressures that are resistant to evaporation in vacuum-level pressures of the X-ray tube may be used.
- low vapor pressures may generally be in the range of 1 ⁇ 10 ⁇ 5 Torr.
- fluids that are stable in vacuums are desirable for use in X-ray tube systems so as to not adversely affect the established vacuum during operation of the system.
- lubricating fluid 84 may be gallium or a gallium alloy as non-limiting examples.
- shaft 76 of bearing assembly 50 is a stationary component and sleeve 78 is a rotatable component constructed to rotate about shaft 76 .
- bearing assembly 50 may instead include a stationary outer component or shell and a rotating shaft comprising an anode target attached thereto.
- bearing assembly 50 may be a “straddle” bearing that is configured to support an anode target between a first and a second liquid metal bearing. In other words, embodiments of this invention may be incorporated into any bearing configuration utilizing a liquid metal bearing to support an anode target.
- Such configurations may include a stationary shaft and a rotatable outer sleeve, and vice versa.
- applications need not be limited to X-ray tubes, but may be applied to any structural configuration having a rotating component and a stationary component in a vacuum, the rotating component being supported within a liquid metal bearing assembly.
- the embodiments of the invention disclosed herein are applicable to any bearing configuration having a rotatable component and a stationary component, and a lubricating fluid therebetween, regardless of configuration or application.
- shaft 76 of bearing assembly 50 includes a thrust bearing 88 comprising a radial projection 90 that extends from shaft 76 and is positioned in a radial cavity 92 of sleeve 78 .
- Radial projection 90 of thrust bearing 88 includes a pair of outer bearing surfaces 94 , 96 that face inner bearing surfaces 98 , 100 of radial cavity 92 of sleeve 78 .
- Radial projection 90 limits axial motion of sleeve 78 relative to shaft 76 , and, as illustrated, lubricating fluid 84 is also included between radial projection 90 and sleeve 78 .
- Radial projection 90 need not be limited in axial length, but may be extended in axial length to provide additional mechanical support of components.
- the shaft 76 also includes a journal bearing 102 located adjacent to and extending axially from thrust bearing 88 .
- An outer surface 104 of journal bearing 102 of shaft 76 faces an inner surface 106 of sleeve 78 .
- journal bearing 102 is illustrated on a first side of thrust bearing 88 adjacent outer bearing surface 94 , one skilled in the art will recognize that bearing assembly 50 may include a second journal bearing portion located on a second side of thrust bearing 88 adjacent outer bearing surface 96 .
- Various coatings, textures, and patterns including grooves embedded in the contacting/bearing surfaces of bearing assembly 50 may be applied to alter bearing behavior as the shaft 76 and sleeve 78 rotate relative to each other.
- the sleeve 78 may be formed with a 2-piece construction including a sleeve portion 108 and a thrust seal 110 .
- the sleeve portion 108 is formed of a material that is low cost, with good machinability, good galling/wear characteristics, and good weldability. Further, while these materials do conduct heat, optionally a thermal barrier can be included between the bearing assembly 50 and the anode target 48 to maintain the temperatures in the bearing assembly 50 below the corrosion limits of the materials forming the bearing assembly 50 .
- the material forming the sleeve portion 108 may be a non-refractory metal, such as an iron alloy, including stainless steel, tool carbon steel, such as D2 steel, among others, though refractory materials and metals, such as molybdenum, can also be utilized.
- the sleeve portion 108 may be formed as a single piece of the selected material, with a closed cylindrical cap portion 112 at one end and an open seating portion 114 at the opposite end.
- the seating portion 114 may be optionally integrally formed with the cap portion 112 to form a unitary structure for the sleeve portion 108 within which the shaft 76 and thrust seal 110 may be engaged, such as that disclosed in U.S.
- the sleeve 78 may be formed with a spacer (not shown) disposed between the sleeve portion 108 and the thrust seal 110 , where the spacer is formed of a suitable material, such as a metal including steel, that includes grooves (not shown) and/or suitable anti-wetting coatings (not shown) applied to the surfaces of the spacer disposed immediately adjacent and in contact with the sleeve portion 108 and the thrust seal 110 to form seals therebetween.
- a suitable material such as a metal including steel
- Bearing assembly 50 may be referred to as a spiral groove bearing (SGB) due to the patterning of bearing grooves 200 ( FIG. 4 ) along the various surfaces of the bearing assembly 50 .
- the grooves 200 may be formed in the bearing surfaces, such as on the exterior of the journal bearing 102 , the interior surface of the sleeve portion 108 , and on one or both outer bearing surfaces 94 , 96 and inner bearing surfaces 98 , 100 and can be separated by un-grooved areas 118 .
- the grooves 200 may be formed from a chevron or a logarithmic spiral shape.
- the spiral groove bearing may also be equivalently referred to as a fluid dynamic or hydrodynamic bearing and liquid metal bearing as well.
- ways to contain the lubricating fluids 84 may be categorized in two general methods.
- the first includes providing physical barriers near the ends of the bearing where shaft seals would be placed in other applications. Rubber or other types of shaft seals in the presence of the vacuum inside the X-ray tube may function improperly, degrade quickly, and/or destroy the pressure inside the X-ray tube.
- o-rings, grease, or other conventional means for aiding in rotational lubrication between two components may be undesirable because of the vacuum in the X-ray tube.
- Greases and other lubricating fluids with lower vapor pressure than liquid metals may vaporize and destroy the vacuum.
- physical walls of different shapes and sizes may be placed at different angles to capture the lubricating fluid to reduce leakage through the bearing.
- the second general method includes utilizing the capillary forces of the lubricating fluid, wherein the small gap between two opposing bearing surfaces wets the fluid to retain the fluid within the gap.
- the anti-wetting properties of the surface aids in preventing the lubricating fluid from flowing in between the small gaps.
- the surfaces are coated and/or textured to be more wetted such that the lubricating fluid clings in the small gap to reduce lubricating fluid moving through the gap.
- the surfaces are coated and/or textured to be more anti-wetting such that the lubricating fluid is pushed away from the small gaps near the ends of the bearing assembly.
- the small gap may be in the range of 15-150 microns.
- Operation of a liquid bearing assembly in X-ray tube systems may be at least partially dependent on a tradeoff between load carrying capacity and fluid pumping force.
- the load carrying capacity and fluid pumping force are inversely proportional and directly related to geometry of the bearing grooves 200 .
- deeper grooves 200 may provide a higher pumping force, while the increased clearance between the shaft 76 and sleeve portion 108 can reduce the load carrying ability of the bearing assembly 50 .
- Pumping force may be utilized to contain the lubrication fluid and anti-wetting coatings may be applied to sealing surfaces to further assist in containing the lubrication fluid.
- the lubricating fluid moves in between bearing surfaces as the shaft 76 and sleeve grooves 200 on the respective surfaces that are rotating relative to each other.
- the grooves 200 are separated by ridges 202 to direct the movement of the lubricating fluid along the grooves 200 and consequently provide rotordynamic stability by supplying pressure around the circumference of the bearing assembly 50 .
- the lubricating fluid is moved in a number of ways, including but not limited to, shearing, wedging, and squeezing, thereby creating pressures to lift and separate the shaft 76 and sleeve portion 108 from each other. This effect enables the liquid bearing to function and provide low-friction movement between the shaft 76 and sleeve portion 108 .
- shearing of the lubricating fluid imparts energy into the fluid which cases the fluid to pump, wherein the pumping action into the gap between the shaft 76 and sleeve portion 108 is how the liquid bearing functions.
- Energy transfer from the surfaces to the fluid enables bearing functionality.
- wetting between some bearing surfaces and the lubricating fluid allows shearing to impact energy to the fluid.
- anti-wetting between some bearing surfaces and the lubricating fluid allows friction between the bearing surfaces to be reduced, thereby reducing operating temperatures of the bearing assembly 50 .
- the bearing assembly 50 includes one or more vent grooves 300 formed therein.
- the vent groove 300 can be disposed on the journal bearing 102 on the shaft 76 or on the thrust bearing 88 .
- the vent groove 300 is formed with a size, i.e., a depth and width, to enable the groove 300 to create a pressure seal with the lubricating fluid 84 that readily allows the trapped gas to pass through the vent groove 300 and be vented to the exterior of the bearing assembly 50 .
- the vent groove 300 is formed to create a pressure seal with the lubricating fluid 84 that can resist a pressure less than atmospheric pressure, or less than approximately 14.1 psi. In other exemplary, embodiments, the vent groove 300 is formed to create a pressure seal with the lubricating fluid 84 that resists pressures of between 0 psi to about 1 psi. In another exemplary embodiment, the vent groove 300 can be formed with a width of at least 10 ⁇ m and a depth of at least 10 ⁇ m. In other exemplary embodiments, the vent groove 300 is formed with a width of at least 20 ⁇ m and a depth of at least 20 ⁇ m.
- the vent groove 300 on the journal bearing 102 can extend axially along the journal bearing 102 across the grooves 200 formed on the journal bearing 102 or can be a radial vent groove 301 disposed on one of the thrust bearing surfaces 94 , 96 .
- the journal bearing 102 can include one or more circumferential vent grooves 302 disposed on the journal bearing 102 , where the one or more circumferential vent grooves 302 intersect the axial vent groove 300 .
- the axial vent groove 300 can be replaced by the one or more circumferential vent grooves 302 .
- the vent grooves 300 can be formed as deeper sections 304 of one or more of the grooves 200 formed in the journal bearing 102 .
- the deeper sections 304 can extend along the entire length of the groove 200 ( FIG. 7 A ) or can extend over only a portion of the groove 200 ( FIG. 7 B ), with the deeper section 304 being formed as a part of the existing groove 200 ( FIG. 7 C ) on either the inner half bearing or on the outer half bearing.
- gas bubbles are vented into the center reservoir 118 with vent grooves 304 in the inner half and to the outside of the bearing assembly 50 .
- vent groove 300 it is desirable to position the one or more vent grooves 300 at a location on the journal bearing 102 where the vent groove 300 has the least detrimental effect with regard to the load bearing capacity of the bearing assembly 50 , e.g., the ability of the lubricating fluid 84 to support the sleeve portion 108 on the shaft 76 , at a location where the pressure of the fluid 84 within the gap 86 is lowest in all loaded conditions.
- vent groove 300 is disposed on the journal bearing 102 outside of a region of high pressure P in the lubricating fluid 84 at the narrowest point/point of minimum approach h 0 between the sleeve portion 108 and the journal bearing 102 on the shaft 76 .
- This point h 0 is defined as being disposed on the gantry side of the bearing assembly 50 but offset from the center of the alignment of the bearing assembly 50 with the gantry center due to the angular momentum co of the sleeve portion 108 caused by the rotation of the sleeve portion 108 relative to the journal bearing 102 .
- vent groove 300 positioned on the low pressure side of the region of high pressure to minimize the detrimental effects of the vent groove 300 on the load carrying capacity of the lubricating fluid 84 .
- the vent groove 300 on the journal bearing 102 is disposed at a position adjacent the point of minimum approach h 0 but spaced from the point of minimum approach h 0 in the direction of the rotation of moving component of the bearing assembly 50 , or on the low pressure side of the point of minimum approach, or a cavitation region, if cavitation occurs.
- vent groove 300 should be located where most effective for all desired operating conditions, such as in the low pressure side/region for all desired operating conditions, rather than in an optimal position for any particular operating condition of the bearing assembly 50 , but without reducing performance of the bearing assembly 50 at any critical condition.
- the vent grooves 300 are disposed on the thrust bearing 88 , either exclusively or in conjunction with vent grooves 300 located on the journal bearing 102 .
- the vent grooves 300 in the thrust bearing 88 are located on one or both surfaces 94 , 96 of the thrust bearing 88 and can be formed in similar manners and with similar dimensions to the vent grooves 300 formed on the journal bearing 102 .
- the vent grooves 300 on the thrust bearing 88 are disposed in pairs on opposite sides of the surfaces 94 , 96 in order to maintain the balance and/or alignment of the thrust bearing 88 relative to the sleeve portion 108 and thrust seal 110 . Any number of pairs of the vent grooves 300 can be formed on the thrust bearing 88 on one or both surfaces 94 , 96 of the thrust bearing 88 .
- the vent grooves 300 can include suitable wetting and/or anti-wetting coatings (not shown) in order to facilitate the operation of the vent grooves 300 in enabling gases to escape out of the bearing assembly 50 .
- the coatings and optionally surface texturing can be utilized on the surfaces of the thrust bearing 88 and the journal bearing 102 disposed immediately adjacent the vent grooves 300 disposed therein in order to maintain the lubricating fluid 84 within the vent grooves 300 to provide the gas evacuation functionality while maintaining the load carrying pressure required for proper operation of the bearing assembly 50 .
- vent grooves 300 , 304 can be disposed on the various surfaces of the sleeve 108 facing the surfaces of the shaft 76 .
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Abstract
A bearing structure for an X-ray tube is provided that includes a journal bearing shaft with a radially protruding thrust bearing encased within a bearing sleeve, one of which rotates relative to the other. The stationary component, e.g., the journal bearing and/or the thrust bearing includes at least one vent groove formed therein that improves the ability of the journal bearing structure to enable gases trapped by the liquid metal within the bearing assembly to escape through the vent groove to the exterior of the X-ray tube. By adding a strategically located channel or vent groove of sufficient size in at least one of the journal bearing or the thrust bearing, the pressures resisted by the seal created between the liquid metal and the vent groove(s) in the bearing components is significantly reduced, allowing escape of the gases to avoid detrimental effects to the operation of the X-ray tube, while maintaining the load carrying capacity of the bearing assembly.
Description
- This application is a continuation of U.S. application Ser. No. 17/181,303, filed on Feb. 22, 2021, the disclosure of which is incorporated herein by reference in its entirety.
- The invention relates generally to X-ray tubes, and more particularly to structures and methods of assembly for the bearing utilized in an X-ray tube.
- X-ray systems, including computed tomography (CT) imaging systems, may include an X-ray tube, a detector, and a support structure for the X-ray tube and the detector. In operation, an imaging table, on which an object is positioned, may be 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 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 generates data, and the system translates the data into an image, which may be used to evaluate the internal structures of the object. 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 an X-ray scanner or CT scanner.
- X-ray tubes include a cathode and an anode located within a high-vacuum environment. In many configurations, the anode structure is supported by a liquid metal bearing structure, e.g., a herringbone groove or spiral groove bearing (SGB) structure, also known as a hydrodynamic bearing, formed with a shaft disposed within a sleeve to which the anode target is attached and rotates around the shaft. The spiral groove bearing structure also includes grooves, such as spiral or helical grooves, on various surfaces of the shaft and/or the sleeve that serve to take up the radial and axial forces acting on the sleeve as it rotates around the shaft.
- Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into an axle formed at least partially of the sleeve that supports the anode target and a stator 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. The 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 target. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at a high rotational speed. This places stringent demands on the bearings and the material forming the anode structure, i.e., the anode target and the shaft supporting the target.
- Advantages of liquid metal bearings such as spiral groove bearings in X-ray tubes include a high load capability and a high heat transfer capability due to an increased amount of contact area. Other advantages include low acoustic noise operation, as is commonly understood in the art. Gallium, indium, or tin alloys, among others, are typically used as the liquid metal in the bearing structure, 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, during the manufacturing process for the bearings, small amounts of gas(es) can become trapped within the bearing structure. As the interior of the X-ray tube is constructed to create a high vacuum environment therein for the optimal operation of the X-ray tube for generation of X-rays, the presence of even small amounts of gas(es) within the X-ray tube is highly undesirable. While the liquid metals utilized in the construction of the bearings can allow this gas to escape through the volume of the liquid metal between the bearing components, certain aspects of the construction of the liquid metal bearings do not readily allow this to occur. For example, due to the high surface tension of the liquid metal utilized in the bearings, the adherence of the liquid metal to the bearing surfaces creates a high-pressure seal, i.e., up to 20 psi, between the liquid metal and the bearing components that often exceeds the pressure of the gas(es) trapped within the bearing, thereby retaining the gas within the bearing. Further, the construction of the components of the bearing structure, e.g., the grooves, can include textured areas operable to retain the liquid metal in the proper locations for functioning as a bearing fluid, further enhancing the seal and preventing the gas from escaping through the liquid metal. These seals formed by the liquid metal and the bearing structure(s) and their ability to retain gases within the journal bearing structure is further increased by various wetting surfaces and non-wetting coatings that are applied to areas of the surfaces of the journal bearing components to assist in retaining the liquid metal where desired to facilitate the rotation of the journal bearing components relative to one another.
- As a result, while the structure of the journal bearing components and the liquid metal disposed between the components operates to effectively enable the sleeve to rotate with respect to the shaft, the bearing retains significant amounts of trapped gas(es) disposed within the bearing structure, detrimentally affecting the operation of the X-ray tube including the journal bearing, including, but not limited to failure of the X-ray tube.
- Therefore, it is desirable to develop a structure and method for the formation and operation of a liquid metal bearing structure for an X-ray tube to significantly improve the transfer or escape of gas(es) trapped in the assembled bearing out of the liquid metal bearing structure to minimize the detrimental effects of the gas(es) on the operation of the X-ray tube.
- In the present disclosure a liquid metal or spiral groove bearing structure for an X-ray tube and associated process for manufacturing a bearing structure comprised of a shaft having a journal bearing and a radially protruding thrust bearing encased within a bearing housing or sleeve. The sleeve includes a thrust seal that is engaged with the sleeve around the thrust bearing in a manner to maintain coaxiality for the rotating liquid metal seal formed within the sleeve about the journal bearing shaft. The engagement of the thrust seal with the sleeve around the thrust bearing on the journal bearing shaft enables the liquid metal to be retained between the journal bearing shaft and the sleeve to enable free rotation of the sleeve around the journal bearing shaft during operation of the X-ray tube.
- The structure of one or both of the journal bearing and the thrust bearing includes at least one vent groove formed therein that improves the ability of the journal bearing structure to enable gases trapped by the liquid metal within the bearing assembly to escape through the vent groove to the exterior of the X-ray tube. By adding a strategically located channel or vent groove of sufficient size in at least one of the journal bearings or the thrust bearings, the pressures resisted by the seal created between the liquid metal and the vent groove(s) in the bearing components is significantly reduced, while maintaining the load carrying capacity of the bearing assembly. As a result, the pressure level of the gases contained by capillary wetting of the journal bearing shaft and the thrust bearing by the liquid metal can exceed the pressure of the seal in the vent groove, thereby allowing the gas to escape the tube along the vent groove without detrimentally affecting the bearing performance.
- In one exemplary embodiment of the invention, bearing assembly for an X-ray tube including a shaft, a sleeve disposed around the shaft, wherein one of the shaft and the sleeve is rotatable with regard to the other to form a rotating component and a stationary component, a lubricating fluid disposed between the shaft and the sleeve within a gap between the sleeve and the shaft; and at least one vent groove disposed on the stationary component.
- In another exemplary embodiment of the invention, an X-ray tube includes a cathode assembly, and an anode assembly spaced from the cathode assembly, wherein the anode assembly includes a shaft, a sleeve disposed on the shaft, wherein one of the shaft and the sleeve is rotatable with regard to the other to form a rotating component and a stationary component, a lubricating fluid disposed between the shaft and the sleeve within a gap between the sleeve and the shaft, and at least one vent groove disposed on the stationary component, and an anode target operably connected to the sleeve.
- In still another exemplary embodiment of the method of the invention, a method for enabling gases trapped in a bearing assembly to be vented from bearing assembly during use in an operating X-ray tube including the steps of providing a bearing assembly having a shaft, a sleeve disposed on the shaft, wherein one of the shaft and the sleeve is rotatable with regard to the other to form a rotating component and a stationary component, a lubricating fluid disposed between the shaft and the sleeve within a gap between the sleeve and the shaft, and at least one vent groove disposed on the stationary component, and operating the X-ray tube to rotate the rotating component relative to the stationary component.
- It should be understood that the brief description above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.
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FIG. 1 is a block diagram of an imaging system incorporating exemplary embodiments of the disclosure. -
FIG. 2 is a cross-sectional view of a portion of an X-ray tube according to an exemplary embodiment of the disclosure and usable with the system illustrated inFIG. 1 . -
FIG. 3 is a cross-sectional side plan view of a bearing structure of an X-ray tube in accordance with an exemplary embodiment of the disclosure. -
FIG. 4 is an isometric view of the bearing structure ofFIG. 3 including a vent groove therein formed in accordance with an exemplary embodiment of the disclosure. -
FIG. 5 is a schematic, cross-sectional view of the bearing structure ofFIG. 4 formed in accordance with another exemplary embodiment of the disclosure illustrating the location of the vent groove relative to a high pressure region formed between the journal bearing and the sleeve forming the bearing structure. -
FIG. 6 is a front elevation view of the journal bearing of the bearing structure ofFIG. 3 including vent grooves in accordance with another exemplary embodiment of the disclosure. -
FIGS. 7A-7C are top plan and cross-sectional schematic views of vent grooves for the bearing structure ofFIG. 3 formed in accordance with further exemplary embodiments of the disclosure. -
FIG. 8 is a perspective view of a thrust bearing including vent grooves for the bearing structure ofFIG. 3 formed in accordance with further exemplary embodiments of the disclosure. -
FIG. 1 is a block diagram of an embodiment of animaging system 10 designed both to acquire original image data and to process the image data for display and/or analysis in accordance with embodiments of the invention. It will be appreciated by those skilled in the art that various embodiments of the invention are applicable to numerous medical imaging systems implementing an X-ray tube, such as X-ray imaging systems or fluoroscopic imaging systems. Other imaging systems such as computed tomography (CT) imaging systems and digital radiography (RAD) imaging systems, which acquire image three-dimensional data for a volume, also benefit from the invention. The following discussion ofX-ray imaging system 10 is merely an example of one such implementation and is not intended to be limiting in terms of modality. - As shown in
FIG. 1 ,imaging system 10 includes an X-ray tube orX-ray source 12 configured to project a beam ofX-rays 14 through anobject 16.Object 16 may include a human subject, pieces of baggage, or other objects desired to be scanned.X-ray source 12 may be conventional X-raytubes producing X-rays 14 having a spectrum of energies that range, typically, from thirty (30) keV to two hundred (200) keV. TheX-rays 14 pass throughobject 16 and, after being attenuated, impinge upon adetector assembly 18. Each detector module indetector assembly 18 produces electrical signals that represents the intensity of an impinging X-ray beam, and hence the attenuated beam, as it passes through theobject 16. In one embodiment,detector assembly 18 is a scintillator based detector assembly, however, it is also envisioned that direct-conversion type detectors (e.g., CZT detectors, photon-counting detectors, etc.) may also be implemented. - A
processor 20 receives the signals from thedetector 18 and generates an image corresponding to theobject 16 being scanned. Acomputer 22 communicates withprocessor 20 to enable an operator, usingoperator console 24, to control the scanning parameters and to view the generated image. That is,operator console 24 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 theX-ray system 10 and view the reconstructed image or other data fromcomputer 22 on adisplay unit 26. Additionally,console 24 allows an operator to store the generated image in astorage device 28 which may include hard drives, floppy discs, compact discs, etc. The operator may also useconsole 24 to provide commands and instructions tocomputer 22 for controlling anX-ray source controller 30 that provides power and timing signals to X-raysource 12. -
FIG. 2 illustrates a cross-sectional view of anX-ray source 12 incorporating embodiments of the invention. In the illustrated embodiment,X-ray source 12 is anX-ray tube 40 that includes ananode assembly 42 and acathode assembly 44. The anode andcathode assemblies frame 46, which houses a target oranode 48, a bearingassembly 50, and a cathode 52.Frame 46 defines an area of relatively low pressure (e.g., a vacuum) compared to ambient, in which high voltages may be present.Frame 46 may be positioned within a casing (not shown) filled with a cooling medium, such as oil, that may also provide high voltage insulation. While the target and anode are described above as being a common component ofX-ray tube 40, the target and anode may be separate components in alternative X-ray tube embodiments. - In operation, an
electron beam 54 is produced bycathode assembly 44. In particular, cathode 52 receives one or more electrical signals via a plurality of electrical leads 56. The electrical signals may include power and timing/control signals that cause cathode 52 to emitelectron beam 54 at one or more energies and at one or more frequencies. The electrical signals may also at least partially control the potential between cathode 52 andanode 48. Cathode 52 includes aninsulator 58 from which anarm 60 extends.Arm 60 encloses electrical leads 56, which extend into acathode cup 62 mounted at the end ofarm 60. In some embodiments,cathode cup 62 includes focusing elements that focuses electrons emitted from a filament withincathode cup 62 to formelectron beam 54. -
X-rays 64 are produced when high-speed electrons ofelectron beam 54 from cathode 52 are suddenly decelerated upon impacting atarget surface 66 formed onanode target 48. The high-speed electrons formingelectron beam 54 are accelerated toward theanode target 48 via a potential difference therebetween of, for example, sixty (60) thousand volts or more in the case of CT applications. TheX-rays 64 are emitted through aradiation emission window 68 formed inframe 46 that is positioned toward a detector array, such asdetector 18 ofFIG. 1 . -
Anode assembly 42 includes arotor 72 and a stator (not shown) located outsideX-ray source 40 and partially surroundingrotor 72 for causing rotation ofanode target 48 during operation.Anode target 48 is supported in rotation by a bearingassembly 50, which, when rotated, also causesanode target 48 to rotate about acenterline 70. As shown,anode target 48 has a generally annular shape, such as a disk, and anannular opening 74 in the center thereof for receivingbearing assembly 50. -
Target 48 may be manufactured to include a number of metals or composites, such as tungsten, molybdenum, or any material that contributes to Bremsstrahlung (i.e., deceleration radiation) when bombarded with electrons.Target surface 66 ofanode target 48 may be selected to have a relatively high refractory value so as to withstand the heat generated by electrons impactingtarget surface 66. Further, the space within insert orframe 46 and betweencathode assembly 44 andanode assembly 42 is at vacuum pressure in order to minimize electron collisions with other atoms and to maximize an electric potential. - To avoid overheating of the
target 48 when bombarded by the electrons,rotor 72 rotates target 48 at a high rate of speed (e.g., 90 to 250 Hz) about acenterline 70. In addition to the rotation ofanode target 48 withinX-ray tube frame 46, in a CT application, theX-ray source 40 as a whole is caused to rotate within a gantry (not shown) about an object, such asobject 16 ofX-ray imaging system 10 inFIG. 1 , at rates of typically 1 Hz or faster. - Bearing
assembly 50 can be formed as necessary, such with a number of suitable ball bearings (not shown), but in the illustrated exemplary embodiment comprises a liquid lubricated or self-acting bearing, such as a liquid metal bearing, having adequate load-bearing capability and acceptable acoustic noise levels for operation withinimaging system 10 ofFIG. 1 . As used herein, the terms “self-acting” and “self-lubricating” mean that the bearing lubricating fluid remains distributed on the surfaces of the bearing due to the relative motion of the bearing components and absent an external pump. - In general, bearing
assembly 50 includes a stationary component, such asshaft 76, and a rotating component, such assleeve 78 that surrounds theshaft 76 and to which theanode target 48 is attached. Whileshaft 76 is described with respect toFIG. 2 as the stationary portion of bearingassembly 50 andsleeve 78 is described as the rotating portion of bearingassembly 50, embodiments of the present invention are also applicable to embodiments wherein theshaft 76, the rotating component, rotates within astationary sleeve 78, a stationary component. In such a configuration,anode target 48 would rotate asshaft 76 rotates. -
Shaft 76 includes a cavity, bore orcoolant flow path 80 though which a coolant 82 (FIG. 3 ), such as oil, flows to cool bearingassembly 50. As such, coolant 82 enables heat generated fromanode target 48 of X-ray source 40 (FIG. 2 ) to be extracted therefrom and transferred external to X-raysource 40. In straddle mounted bearing assembly configurations,coolant flow path 80 extends along a longitudinal length ofshaft 76. In alternative embodiments, bore 80 may extend through only a portion ofshaft 76, such as in configurations where anX-ray source 40 is cantilevered when placed in an X-ray imaging system. - Referring now to
FIG. 3 , a cross-sectional view of a portion of bearing assembly orstructure 50 is shown according to an embodiment of the invention. Bearingassembly 50 includes ashaft 76 positioned withinsleeve 78, which is configured to support an anode target, such asanode target 48 ofFIG. 2 . A lubricatingfluid 84 is positioned in agap 86 formed betweenshaft 76 andsleeve 78. In embodiments of the invention, lubricatingfluid 84 is a metal or metallic alloy that exists in a liquid state at operating temperature of bearingassembly 50. - The lubricating
fluid 84 flowing between the rotating and stationary components of the bearingassembly 50 may include a variety of individual fluids as well as mixtures of fluids. For example, multiple liquid metals and liquid metal alloys may be used as the lubricating fluid, such as an indium gallium alloy. More generally, fluids with relatively low vapor pressures that are resistant to evaporation in vacuum-level pressures of the X-ray tube may be used. In the present context, low vapor pressures may generally be in the range of 1×10−5 Torr. In other words, fluids that are stable in vacuums are desirable for use in X-ray tube systems so as to not adversely affect the established vacuum during operation of the system. In the present disclosure, lubricatingfluid 84 may be gallium or a gallium alloy as non-limiting examples. - In the embodiment illustrated in
FIG. 3 ,shaft 76 of bearingassembly 50 is a stationary component andsleeve 78 is a rotatable component constructed to rotate aboutshaft 76. However, one skilled in the art will recognize the inventive concepts described herein are applicable to alternative bearing configurations. As one example, bearingassembly 50 may instead include a stationary outer component or shell and a rotating shaft comprising an anode target attached thereto. As another example, bearingassembly 50 may be a “straddle” bearing that is configured to support an anode target between a first and a second liquid metal bearing. In other words, embodiments of this invention may be incorporated into any bearing configuration utilizing a liquid metal bearing to support an anode target. Such configurations may include a stationary shaft and a rotatable outer sleeve, and vice versa. Further, one skilled in the art will recognize that such applications need not be limited to X-ray tubes, but may be applied to any structural configuration having a rotating component and a stationary component in a vacuum, the rotating component being supported within a liquid metal bearing assembly. Thus, the embodiments of the invention disclosed herein are applicable to any bearing configuration having a rotatable component and a stationary component, and a lubricating fluid therebetween, regardless of configuration or application. - As illustrated in
FIG. 3 ,shaft 76 of bearingassembly 50 includes athrust bearing 88 comprising aradial projection 90 that extends fromshaft 76 and is positioned in aradial cavity 92 ofsleeve 78.Radial projection 90 of thrust bearing 88 includes a pair of outer bearing surfaces 94, 96 that face inner bearing surfaces 98, 100 ofradial cavity 92 ofsleeve 78.Radial projection 90 limits axial motion ofsleeve 78 relative toshaft 76, and, as illustrated, lubricatingfluid 84 is also included betweenradial projection 90 andsleeve 78.Radial projection 90 need not be limited in axial length, but may be extended in axial length to provide additional mechanical support of components. - The
shaft 76 also includes a journal bearing 102 located adjacent to and extending axially fromthrust bearing 88. Anouter surface 104 of journal bearing 102 ofshaft 76 faces aninner surface 106 ofsleeve 78. While journal bearing 102 is illustrated on a first side of thrust bearing 88 adjacentouter bearing surface 94, one skilled in the art will recognize that bearingassembly 50 may include a second journal bearing portion located on a second side of thrust bearing 88 adjacentouter bearing surface 96. Various coatings, textures, and patterns including grooves embedded in the contacting/bearing surfaces of bearingassembly 50 may be applied to alter bearing behavior as theshaft 76 andsleeve 78 rotate relative to each other. - In the exemplary embodiment illustrated in
FIG. 3 thesleeve 78 may be formed with a 2-piece construction including asleeve portion 108 and athrust seal 110. Thesleeve portion 108 is formed of a material that is low cost, with good machinability, good galling/wear characteristics, and good weldability. Further, while these materials do conduct heat, optionally a thermal barrier can be included between the bearingassembly 50 and theanode target 48 to maintain the temperatures in the bearingassembly 50 below the corrosion limits of the materials forming the bearingassembly 50. In an exemplary embodiment of the invention, the material forming thesleeve portion 108 may be a non-refractory metal, such as an iron alloy, including stainless steel, tool carbon steel, such as D2 steel, among others, though refractory materials and metals, such as molybdenum, can also be utilized. Thesleeve portion 108 may be formed as a single piece of the selected material, with a closedcylindrical cap portion 112 at one end and anopen seating portion 114 at the opposite end. In the illustrated exemplary embodiment, theseating portion 114 may be optionally integrally formed with thecap portion 112 to form a unitary structure for thesleeve portion 108 within which theshaft 76 and thrustseal 110 may be engaged, such as that disclosed in U.S. Patent Application Publication No. US2016/0133431, entitled Welded Spiral Groove Bearing Assembly, the entirety of which is expressly incorporated herein by reference. In an alternative exemplary embodiment of the bearingassembly 50, thesleeve 78 may be formed with a spacer (not shown) disposed between thesleeve portion 108 and thethrust seal 110, where the spacer is formed of a suitable material, such as a metal including steel, that includes grooves (not shown) and/or suitable anti-wetting coatings (not shown) applied to the surfaces of the spacer disposed immediately adjacent and in contact with thesleeve portion 108 and thethrust seal 110 to form seals therebetween. - Bearing
assembly 50 may be referred to as a spiral groove bearing (SGB) due to the patterning of bearing grooves 200 (FIG. 4 ) along the various surfaces of the bearingassembly 50. Thegrooves 200 may be formed in the bearing surfaces, such as on the exterior of the journal bearing 102, the interior surface of thesleeve portion 108, and on one or both outer bearing surfaces 94, 96 and inner bearing surfaces 98, 100 and can be separated byun-grooved areas 118. In some examples, thegrooves 200 may be formed from a chevron or a logarithmic spiral shape. The spiral groove bearing may also be equivalently referred to as a fluid dynamic or hydrodynamic bearing and liquid metal bearing as well. In such spiral groove bearings, ways to contain thelubricating fluids 84 may be categorized in two general methods. The first includes providing physical barriers near the ends of the bearing where shaft seals would be placed in other applications. Rubber or other types of shaft seals in the presence of the vacuum inside the X-ray tube may function improperly, degrade quickly, and/or destroy the pressure inside the X-ray tube. For similar reasons, o-rings, grease, or other conventional means for aiding in rotational lubrication between two components may be undesirable because of the vacuum in the X-ray tube. Greases and other lubricating fluids with lower vapor pressure than liquid metals may vaporize and destroy the vacuum. In some examples, physical walls of different shapes and sizes may be placed at different angles to capture the lubricating fluid to reduce leakage through the bearing. - The second general method includes utilizing the capillary forces of the lubricating fluid, wherein the small gap between two opposing bearing surfaces wets the fluid to retain the fluid within the gap. In other cases, the anti-wetting properties of the surface (via texturing, coating, or both) aids in preventing the lubricating fluid from flowing in between the small gaps. In some examples, the surfaces are coated and/or textured to be more wetted such that the lubricating fluid clings in the small gap to reduce lubricating fluid moving through the gap. In other examples, the surfaces are coated and/or textured to be more anti-wetting such that the lubricating fluid is pushed away from the small gaps near the ends of the bearing assembly. In this context, the small gap may be in the range of 15-150 microns.
- Operation of a liquid bearing assembly in X-ray tube systems, such as bearing
assembly 50 ofFIGS. 2 and 3 , may be at least partially dependent on a tradeoff between load carrying capacity and fluid pumping force. In some examples, the load carrying capacity and fluid pumping force are inversely proportional and directly related to geometry of the bearinggrooves 200. For example, given a substantially constant rotational speed of the liquid bearing assembly,deeper grooves 200 may provide a higher pumping force, while the increased clearance between theshaft 76 andsleeve portion 108 can reduce the load carrying ability of the bearingassembly 50. Pumping force may be utilized to contain the lubrication fluid and anti-wetting coatings may be applied to sealing surfaces to further assist in containing the lubrication fluid. - The lubricating fluid moves in between bearing surfaces as the
shaft 76 andsleeve grooves 200 on the respective surfaces that are rotating relative to each other. Thegrooves 200 are separated byridges 202 to direct the movement of the lubricating fluid along thegrooves 200 and consequently provide rotordynamic stability by supplying pressure around the circumference of the bearingassembly 50. As such, the lubricating fluid is moved in a number of ways, including but not limited to, shearing, wedging, and squeezing, thereby creating pressures to lift and separate theshaft 76 andsleeve portion 108 from each other. This effect enables the liquid bearing to function and provide low-friction movement between theshaft 76 andsleeve portion 108. In other words, shearing of the lubricating fluid imparts energy into the fluid which cases the fluid to pump, wherein the pumping action into the gap between theshaft 76 andsleeve portion 108 is how the liquid bearing functions. Energy transfer from the surfaces to the fluid enables bearing functionality. In application, in the context of the X-ray tube, wetting between some bearing surfaces and the lubricating fluid allows shearing to impact energy to the fluid. However, anti-wetting between some bearing surfaces and the lubricating fluid allows friction between the bearing surfaces to be reduced, thereby reducing operating temperatures of the bearingassembly 50. - Referring now to
FIG. 4 , in order to enable gas, gas bubbles, etc., trapped in the lubricatingfluid 84 to escape the bearingassembly 50, the bearingassembly 50 includes one ormore vent grooves 300 formed therein. In the illustrated exemplary embodiment ofFIG. 4 , thevent groove 300 can be disposed on the journal bearing 102 on theshaft 76 or on thethrust bearing 88. Thevent groove 300 is formed with a size, i.e., a depth and width, to enable thegroove 300 to create a pressure seal with the lubricatingfluid 84 that readily allows the trapped gas to pass through thevent groove 300 and be vented to the exterior of the bearingassembly 50. In one exemplary embodiment, thevent groove 300 is formed to create a pressure seal with the lubricatingfluid 84 that can resist a pressure less than atmospheric pressure, or less than approximately 14.1 psi. In other exemplary, embodiments, thevent groove 300 is formed to create a pressure seal with the lubricatingfluid 84 that resists pressures of between 0 psi to about 1 psi. In another exemplary embodiment, thevent groove 300 can be formed with a width of at least 10 μm and a depth of at least 10 μm. In other exemplary embodiments, thevent groove 300 is formed with a width of at least 20 μm and a depth of at least 20 μm. - As illustrated in the exemplary embodiment of
FIG. 4 , thevent groove 300 on the journal bearing 102 can extend axially along the journal bearing 102 across thegrooves 200 formed on the journal bearing 102 or can be aradial vent groove 301 disposed on one of the thrust bearing surfaces 94,96. In alternative exemplary embodiments, as shown inFIG. 6 , the journal bearing 102 can include one or morecircumferential vent grooves 302 disposed on the journal bearing 102, where the one or morecircumferential vent grooves 302 intersect theaxial vent groove 300. Alternatively, theaxial vent groove 300 can be replaced by the one or morecircumferential vent grooves 302. - In still other alternative embodiments, as shown in
FIGS. 7A-7C , thevent grooves 300 can be formed asdeeper sections 304 of one or more of thegrooves 200 formed in thejournal bearing 102. Thedeeper sections 304 can extend along the entire length of the groove 200 (FIG. 7A ) or can extend over only a portion of the groove 200 (FIG. 7B ), with thedeeper section 304 being formed as a part of the existing groove 200 (FIG. 7C ) on either the inner half bearing or on the outer half bearing. In these embodiments for thevent groove 304, gas bubbles are vented into thecenter reservoir 118 withvent grooves 304 in the inner half and to the outside of the bearingassembly 50. - Looking now at
FIG. 5 , with regard to any of the prior embodiments, in other exemplary embodiments it is desirable to position the one ormore vent grooves 300 at a location on the journal bearing 102 where thevent groove 300 has the least detrimental effect with regard to the load bearing capacity of the bearingassembly 50, e.g., the ability of the lubricatingfluid 84 to support thesleeve portion 108 on theshaft 76, at a location where the pressure of the fluid 84 within thegap 86 is lowest in all loaded conditions. To achieve this, thevent groove 300 is disposed on the journal bearing 102 outside of a region of high pressure P in the lubricatingfluid 84 at the narrowest point/point of minimum approach h0 between thesleeve portion 108 and the journal bearing 102 on theshaft 76. This point h0 is defined as being disposed on the gantry side of the bearingassembly 50 but offset from the center of the alignment of the bearingassembly 50 with the gantry center due to the angular momentum co of thesleeve portion 108 caused by the rotation of thesleeve portion 108 relative to thejournal bearing 102. It is desirable to have thevent groove 300 positioned on the low pressure side of the region of high pressure to minimize the detrimental effects of thevent groove 300 on the load carrying capacity of the lubricatingfluid 84. As such, in an exemplary embodiment thevent groove 300 on the journal bearing 102 is disposed at a position adjacent the point of minimum approach h0 but spaced from the point of minimum approach h0 in the direction of the rotation of moving component of the bearingassembly 50, or on the low pressure side of the point of minimum approach, or a cavitation region, if cavitation occurs. Further, as the location of the point of minimum approach h0 varies based on the operating condition of the bearingassembly 50, in certain embodiments thevent groove 300 should be located where most effective for all desired operating conditions, such as in the low pressure side/region for all desired operating conditions, rather than in an optimal position for any particular operating condition of the bearingassembly 50, but without reducing performance of the bearingassembly 50 at any critical condition. - Referring now to
FIGS. 4 and 8 , in an exemplary embodiment thevent grooves 300 are disposed on thethrust bearing 88, either exclusively or in conjunction withvent grooves 300 located on thejournal bearing 102. Thevent grooves 300 in thethrust bearing 88 are located on one or bothsurfaces thrust bearing 88 and can be formed in similar manners and with similar dimensions to thevent grooves 300 formed on thejournal bearing 102. Thevent grooves 300 on thethrust bearing 88 are disposed in pairs on opposite sides of thesurfaces sleeve portion 108 and thrustseal 110. Any number of pairs of thevent grooves 300 can be formed on the thrust bearing 88 on one or bothsurfaces thrust bearing 88. - In other exemplary embodiments, the
vent grooves 300 can include suitable wetting and/or anti-wetting coatings (not shown) in order to facilitate the operation of thevent grooves 300 in enabling gases to escape out of the bearingassembly 50. Further, the coatings and optionally surface texturing (not shown) can be utilized on the surfaces of thethrust bearing 88 and the journal bearing 102 disposed immediately adjacent thevent grooves 300 disposed therein in order to maintain the lubricatingfluid 84 within thevent grooves 300 to provide the gas evacuation functionality while maintaining the load carrying pressure required for proper operation of the bearingassembly 50. In addition, as some constructions for the bearingassembly 50 employ astationary sleeve 108 and arotating shaft 76 disposed within thesleeve 108, in alternative embodiments thevent grooves sleeve 108 facing the surfaces of theshaft 76. - The written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
Claims (1)
1. A bearing assembly for an X-ray tube, the bearing assembly comprising:
a shaft;
a sleeve disposed around the shaft, wherein one of the shaft and the sleeve is rotatable with regard to the other to form a rotating component and a stationary component;
a lubricating fluid disposed between the shaft and the sleeve within a gap between the sleeve and the shaft; and
at least one vent groove disposed on the stationary component.
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US18/310,023 US20230343543A1 (en) | 2021-02-22 | 2023-05-01 | X-ray tube liquid metal bearing structure for reducing trapped gases |
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US17/181,303 US11676791B2 (en) | 2021-02-22 | 2021-02-22 | X-ray tube liquid metal bearing structure for reducing trapped gases |
US18/310,023 US20230343543A1 (en) | 2021-02-22 | 2023-05-01 | X-ray tube liquid metal bearing structure for reducing trapped gases |
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US17/181,303 Continuation US11676791B2 (en) | 2021-02-22 | 2021-02-22 | X-ray tube liquid metal bearing structure for reducing trapped gases |
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US18/310,023 Pending US20230343543A1 (en) | 2021-02-22 | 2023-05-01 | X-ray tube liquid metal bearing structure for reducing trapped gases |
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US11920630B2 (en) * | 2018-09-20 | 2024-03-05 | Koninklijke Philips N.V. | Self-lubricated sliding bearing |
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CN1039561C (en) * | 1992-04-08 | 1998-08-19 | 株式会社东芝 | Rotating positive X-ray tube |
US5483570A (en) * | 1994-06-24 | 1996-01-09 | General Electric Company | Bearings for x-ray tubes |
JP2001325908A (en) * | 2000-03-09 | 2001-11-22 | Toshiba Corp | Rotating anode type x-ray tube |
JP2005172223A (en) | 2003-11-20 | 2005-06-30 | Matsushita Electric Ind Co Ltd | Thrust dynamic pressure bearing, and spindle motor and information recording and reproducing device using it |
JP2007016884A (en) * | 2005-07-07 | 2007-01-25 | Ge Medical Systems Global Technology Co Llc | Bearing mechanism, and x-ray tube |
JP6091930B2 (en) * | 2013-03-04 | 2017-03-08 | 東芝電子管デバイス株式会社 | Rotating anode X-ray tube |
US10094425B2 (en) * | 2014-05-21 | 2018-10-09 | Koninklijke Philips N.V. | Hydrodynamic bearing, and X-ray tube, and X-ray system, and a method for manufacturing a hydrodynamic bearing, |
CN106104025B (en) * | 2014-06-04 | 2018-04-24 | 皇家飞利浦有限公司 | Hydrodynamic bearing |
US9500226B2 (en) * | 2014-08-13 | 2016-11-22 | General Electric Company | Method and systems for texturing liquid bearing surfaces in X-ray tubes |
US9972472B2 (en) | 2014-11-10 | 2018-05-15 | General Electric Company | Welded spiral groove bearing assembly |
US10438767B2 (en) * | 2017-11-30 | 2019-10-08 | General Electric Company | Thrust flange for x-ray tube with internal cooling channels |
JP7066568B2 (en) | 2018-08-06 | 2022-05-13 | キヤノン電子管デバイス株式会社 | Rotating anode X-ray tube and rotating anode X-ray tube device |
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- 2021-02-22 US US17/181,303 patent/US11676791B2/en active Active
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US5583907A (en) * | 1994-10-13 | 1996-12-10 | Kabushiki Kaisha Toshiba | Rotary anode type x-ray tube and method of manufacturing the same |
US5602898A (en) * | 1995-01-25 | 1997-02-11 | U.S. Philips Corporation | Rotary-anode X-ray tube comprising a sleeve bearing |
US11920630B2 (en) * | 2018-09-20 | 2024-03-05 | Koninklijke Philips N.V. | Self-lubricated sliding bearing |
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US11676791B2 (en) | 2023-06-13 |
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