Patent Application
20190103244
The invention relates generally to x-ray tubes, and more particularly to structures and methods of assembly for the spiral groove bearing (SGB) utilized in an x-ray tube.
X-ray 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 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. 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 computed tomography (CT) package 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 spiral groove bearing (SGB) structure, formed with a support shaft disposed within a sleeve or shell to which the anode is attached and that rotates around the support shaft. The spiral groove bearing structure also includes spiral or helical grooves on various surfaces of the sleeve or shell that serve to take up the radial and axial forces acting on the sleeve as it rotates around the support 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 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. The x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode 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 necessary to rotate the anode assembly at 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 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, liquid metals tend to be highly reactive and corrosive. Thus, a base metal that is resistant to such corrosion is desirable for the components that come into contact with the liquid metal bearing, such as the shaft of the anode assembly and is rotated for the purpose of distributing the heat generated at a focal spot.
As a result, the structure of the sleeve to which the anode is connected and the support shaft must be capable of withstanding the high temperatures and mechanical stresses created within the x-ray tube, as well as be able to withstand the corrosive effects of the liquid metal bearing. As such, a refractory metal such as molybdenum or tungsten is typically used as the base material for the construction of the sleeve or shell as well as for the other bearing components. Not only are such materials resistant to corrosion and high temperatures, but they tend to be vacuum-compatible and thus lend themselves to an x-ray tube application. In addition, cooling of the bearing structure can be effected by flowing a cooling fluid into the center of the support shaft to thermally contact the heat taken from the anode by the sleeve and liquid metal bearing fluid.
However, as the refractory materials are difficult to machine, these surfaces are hard to manufacture without surface imperfections that enable leaks to occur in the seals. Also, due to the low galling/wear properties of the refractory materials, these surface imperfections, even if not present after machining, can occur during normal use of the tube resulting in the formation of fluid leaks, thereby shortening the useful life of the tube.
In an alternative construction for a liquid metal/spiral groove bearing structure, other metals, such as steel, can be utilized in place of the refractory metals for the construction of the sleeve and support shaft, such as disclosed in U.S. Pat. No. 6,477,236. While these other metals have a lower thermal conductivity, they have the benefits of low cost compared to the refractory metals, good machinability, good galling/wear characteristics, and good weldability. In particular, steel is a potential journal bearing material in x-ray tubes as it has better wear resistance compared to molybdenum. As such, these metals can be more easily constructed and joined to form the bearing sleeve.
However, one drawback to steel is that it has a much lower thermal conductance and a higher coefficient of thermal expansion compared to molybdenum. Therefore the steel is more prone to thermal gradients and resulting non-uniform bearing deflections, which can lower bearing load carrying capability. As a result of the decreased thermal conductivity, in prior art bearing structures, an oil cooling tube is disposed within the central bore of the shaft on order to direct a flow of cooling oil into contact with the bearing shaft. The contact of the cooling oil with the shaft allows for heat exchange between the shaft and the oil, consequently reducing the heat affecting the bearing structure.
However, in prior art bearing structures, the cooling tube is formed as a constant diameter tube positioned within the bore of the bearing shaft and having a single discharge aperture for the cooling oil located within the bore. In this configuration, the amount of heat exchange that occurs between the cooling oil and the shaft is insufficient to counteract the lower thermal conductivity of steel, such that refractory metals are required for the construction of the bearing shaft in order to withstand the temperatures created when the bearing structure is in operation.
Therefore, it is desirable to develop a structure and method for the formation and operation of a bearing structure for an x-ray tube with an improved cooling structure to enable the use of low cost materials for the shaft to significantly improves heat transfer out of the bearing structure to minimize the thermal gradients and resulting non-uniform bearing deflections in the structure.
In the present invention a liquid metal or spiral groove bearing structure for an x-ray tube and associated process for manufacturing the bearing structure is provided that includes a bearing shaft rotatably disposed in a bearing housing or shell. The shell includes a thrust seal engaged with a sleeve to maintain coaxiality for the rotating liquid metal seal formed in the shell about the shaft. The shaft has a bore for the introduction of a cooling fluid into the bearing assembly in which is disposed a cooling tube. The cooling tube includes turbulence-inducing features to increase the turbulence of the cooling fluid flowing through the cooling tube, consequently enhancing the heat exchange between the cooling fluid and the shaft. This maximizes the heat transfer from the shaft to the oil, allowing materials with lower thermal conductivities, such as non-refractory materials, to be used to form the bearing shaft and shell.
In one exemplary embodiment of the invention, a bearing assembly includes a shell, a shaft defining a bore therein and rotatably disposed within the shell; and a cooling tube disposed within the bore of the shaft, the cooling tube including at least one turbulence-inducing feature.
In another exemplary embodiment of the invention, An x-ray tube includes a frame defining an enclosure, a cathode assembly disposed in the enclosure and an anode assembly disposed in the enclosure spaced from the cathode assembly, wherein the anode assembly includes a sleeve, a shaft rotatably disposed within the sleeve and defining a bore therein, a cooling tube coaxially disposed within the bore in the shaft, the cooling tube including at least one turbulence-inducing feature thereon and an anode target operably connected to the sleeve.
In an exemplary embodiment of the method of the invention, a method for forming a bearing assembly for use in an x-ray tube, the method includes the steps of providing a cooling tube including at least one turbulence-inducing feature thereon, positioning the cooling tube coaxially within a defined within a shaft and securing the shaft within a shell.
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.
As shown in
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. 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 the x-ray system 10 and view the reconstructed image or other data from computer 22 on a display unit 26. Additionally, 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 a source controller 30 that provides power and timing signals to x-ray source 12.
In operation, an electron beam 54 is produced by cathode assembly 44. In particular, cathode 52 receives one or more electrical signals via a series of electrical leads 56. The electrical signals may be 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 a central insulating shell 58 from which a mask 60 extends. Mask 60 encloses electrical leads 56, which extend to a cathode cup 62 mounted at the end of mask 60. In some embodiments, cathode cup 62 serves as an electrostatic lens that focuses electrons emitted from a thermionic filament within cathode cup 62 to form electron beam 54.
X-rays 64 are produced when high-speed electrons of electron beam 54 are suddenly decelerated when directed from the cathode 52 to a target or focal surface 66 formed on 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 passage 68 formed in frame 46 toward a detector array, such as detector 18 of
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 48 during operation. Target 48 is supported in rotation by a bearing assembly 50, which, when rotated, also causes target 48 to rotate about the centerline 70. As shown, 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 Bermsstrahlung (i.e., deceleration radiation) when bombarded with electrodes. Target or focal surface 66 of target 48 may be selected to have a relatively high refractory value so as to withstand the heat generated by electrons impacting target 48. Further, the space between cathode assembly 44 and target 48 may be evacuated 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 a centerline 70. In addition to the rotation of target 48 within x-ray tube volume 46, in a CT application, the x-ray source 40 as a whole is caused to rotate about an object, such as object 16 of imaging system 10 in
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 having adequate load-bearing capability and acceptable acoustic noise levels for operation within imaging system 10 of
In general, bearing assembly 50 includes a stationary portion, such as center shaft 76, and a rotating portion, such as shell 78 to which the target 48 is attached. While center shaft 76 is described with respect to
Center shaft 76 can be formed of a refractory metal or a non-refractory metal, such as an iron alloy, and includes a cavity, bore or coolant flow path 80 though which a coolant/cooling fluid 82 (
Referring now to
The lubricating fluid 84 flowing between the rotating and stationary components of the bearing assembly or structure 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, lubricant 84 may be gallium or a gallium alloy as non-limiting examples.
In the embodiment illustrated in
As illustrated in
Bearing assembly or structure 50 may be referred to as a spiral groove bearing (SGB) due to the patterning of grooves along the various surfaces of the bearing. In some examples, the spiral groove may be formed from a logarithmic spiral shape. The spiral groove bearing may also be equivalently referred to as a fluid dynamic bearing and liquid bearing as well. In such spiral groove bearings, ways to contain the liquid lubricant 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 lube. Greases and other lubricants 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 lubricant to reduce leakage through the bearing.
The second general method includes utilizing the capillary forces of the lubricant, wherein the small gap between two opposing bearing surfaces wets the fluid to retain the fluid within the gap. In other words, the anti-wetting properties of the surface (via texturing, coating, or both) aids in preventing the lubricant from flowing in between the small gaps. In some examples, the surfaces are coated and/or textured to be more wetted such that the lubricant clings in the small gap to reduce lubricant moving through the gap. In other examples, the surfaces are coated and/or textured to be more anti-wetting such that the lubricant 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 30-120 microns.
Operation of liquid bearings in x-ray tube systems, such as bearing assembly 50 of
The lubricating fluid in between bearing surfaces such as the shaft and sleeve are rotating relative to each other. 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 the shaft and sleeve from each other. This effect enables the liquid bearing to function and provide low-friction movement between the shaft and sleeve. 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 the shaft and sleeve 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.
In the exemplary embodiment of the invention illustrated in
Looking now at
The cooling tube 200 functions to direct the coolant 82 into the bore 80 of the shaft 76 through the channel 202. Upon exiting the channel 202 adjacent the closed end 210 of the bore 80, the coolant 82 comes into contact with the internal diameter of the bore 80 of the shaft 76. Heat from the shaft 76 is exchanged into the cooling fluid 82 upon contact of the coolant 82 with the shaft 76 and the heated cooling fluid 82 is withdrawn from the bore 80 around the channel 202 via exit apertures 205 formed in the retaining ring 204 adjacent the channel 202.
In order to maximize the thermal contact of the coolant 82 with the surfaces of the bore 80, the channel 202 of the cooling tube 200 is formed with a number of turbulence-inducing features that can be utilized individually or in combination with one another to maximize heat transfer coefficient for a given surface area in steel spiral groove bearings used in x-ray tubes.
In the illustrated exemplary embodiment of
In addition, the channel 202 also includes one or more protrusions or vanes 214 disposed on the exterior surface 216 of the channel 202 adjacent or spaced from the open end 212. The vanes 214 can have any desired configuration that extends radially outwardly from the exterior surface 216 of the channel 202 to create turbulence in the flow of the cooling fluid 82 passing between the channel 202 and the bore 80. In the illustrated exemplary embodiment of
To further assist in increasing the thermal contact of the cooling fluid 82 with the surfaces of the bore 80, the channel 202 can additionally include a number of spray jet openings 220 formed in the channel 202. The openings 220 can be disposed at any location along the channel 202 and in the exemplary embodiment of
Looking now at the exemplary embodiment of
With a cooling tube 200 having one or more of these features, the tube 200 minimizes the thermal gradients and non-uniform bearing growth by maximizing heat transfer coefficient through the impingement jets/openings 220 and turbulence from the vanes 214/216, while minimizing overall flow rate and pressure drop of the cooling fluid 82. As a result, steel can be used as a material for the bearing shaft 76 and shell 78 without having to increase the bearing size, or increase the size of any heat exchanger (not shown) to remove additional heat from the bearing assembly 50. This, in turn, minimizes bearing friction and heat exchanger pump power consumption. Further, the bearing shaft 76 and shell 78 formed of steel are a fraction of the cost of component formed from refractory metals, such as molybdenum, due to both labor and material costs, and have an increased bearing life.
Further, as the cooling tube 200 itself can be formed in any suitable manufacturing process and of any suitable material, such as a metal, including steel. In one exemplary embodiment, the tube 200 is formed of a suitable material, including, but not limited to, stainless steel, carbon steel, aluminum, plastic, or carbon fiber, among others, in an additive manufacturing process in order to closely control the size, position and materials utilized in the formation of each of the features 210, 214, 218, 220, 222 to maximize heat transfer coefficient for a given surface area in spiral groove bearings used in x-ray tubes, whether the bearings include steel components and/or components formed of other materials.
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.
