Patent Publication Number: US-11037752-B2

Title: Spiral groove bearing assembly with minimized deflection

Description:
This Application is a continuation of U.S. patent application Ser. No. 16/030,004, filed Jul. 9, 2018, which application is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     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 that 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. In prior art bearing constructions, a refractory metal such as molybdenum or tungsten can be 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, these materials have a low weldablity and are difficult to machine, such that bearing components of these materials 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. While steel has a lower resistance to corrosion by the liquid metal fluid, it also has the benefits of low cost compared to the refractory metals, good machinability, good galling/wear characteristics, and good weldability. As such, these metals, e.g., steel, can be more easily constructed and joined to form the bearing sleeve. 
     However, one significant drawback to steel is its lower thermal conductance and higher thermal growth, which can limit bearing life by causing deformation in the bearing components formed of the steel, and a consequent alteration in the size of the gap formed between the rotating and stationary components of the bearing assembly, leading to metal to metal contact, i.e., wear, that reduces the useful life of the bearing assembly and associated x-ray tube. 
     In one prior art attempt to address this issue, as disclosed in US Patent Application Publication No. 2011/0280376, in which the stationary component of a bearing assembly is formed with various structures to maintain the gap size between the stationary component (the shaft) and the rotating component (the sleeve). The structures included within the shaft include inserts having different thermal expansion characteristics from the remainder of the material forming the shaft where the inserts can expand to maintain the size of the gap, a mechanical or hydraulic piston operable to expand the shaft to maintain the size of the gap, and structures within the shaft that draw heat toward multiple spots on the sleeve and the shaft to lessen the amount of deformation of the sleeve due to the heating of the sleeve during operation. 
     However, in each of these embodiments of the prior art solution, the structures increase the complexity of the construction of the bearing assembly by including additional components and operating structures within the construction of the shaft and the overall bearing assembly. Further, the additional structures are disposed on the stationary portion of the bearing assembly, i.e., the shaft, and are operable only to adjust the shape of the shaft to accommodate the deformation of the sleeve resulting from the heating of the sleeve during operation of the x-ray tube. 
     As a result, it is desirable to develop a structure and method for the formation of a bearing structure for an x-ray tube that can be formed with a simplified structure using low cost materials in a manner that significantly limits the formation of thermal gradients within the structure, thereby minimizing deformation of the bearing structure. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one exemplary embodiment of the present disclosure, a liquid metal or spiral groove bearing structure for an x-ray tube and associated process for manufacturing the bearing structure is provided in which journal bearing sleeve is formed with a number of structures thereon that function to dissipate heat transmitted to the sleeve during operation of the bearing assembly within the x-ray tube to minimize thermal deformation of the sleeve, thereby minimizing gap size alteration within the bearing assembly. The structures formed within the sleeve lessen the thermal gradients that develop within the sleeve during operation of the x-ray tube, thereby counteracting the thermal conductance properties of the material forming the sleeve. 
     In another exemplary embodiment of the present disclosure, the structures formed within the sleeve are slots disposed within the section of the sleeve in which the highest temperature gradients develop. The slots enable an increase in thermal conductance away from the sleeve while minimizing the stresses created from the deformation of the portion(s) of the sleeve between the slots. 
     In one exemplary embodiment of the disclosure, a bearing assembly adapted for use with an x-ray tube includes a shaft, a sleeve rotatably disposed around the shaft, the sleeve including a seating portion forming an open end of the sleeve, wherein the seating portion includes at least one slot formed therein and a thrust seal seated at least partially within the seating portion, the thrust seal having a central aperture through which the shaft extends. 
     In another exemplary embodiment of the disclosure, a sleeve adapted for use within an x-ray tube bearing assembly includes a cap portion forming a closed end of the sleeve and a seating portion engaged with the cap portion opposite the closed end and forming an open end of the sleeve, wherein the seating portion includes at least one slot formed therein. 
     In an exemplary embodiment of the method of the disclosure, the method includes the steps of providing a sleeve formed of a non-refractory material, the sleeve having a seating portion forming an open end of the sleeve, the seating portion having a number of slots formed therein, placing an amount of a liquid metal bearing fluid into the sleeve, inserting a shaft into the seating portion of the sleeve and securing a thrust seal in the seating portion of the sleeve around the shaft. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an imaging system incorporating exemplary embodiments of the invention. 
         FIG. 2  is a cross-sectional view of a portion of an x-ray tube according to an exemplary embodiment of the invention 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 invention. 
         FIG. 4  is an isometric view of a prior art journal bearing sleeve. 
         FIG. 4A  is a partially broken away, cross-sectional view along line  4 A- 4 A of  FIG. 4 . 
         FIG. 5  is an isometric view of a journal bearing sleeve in accordance with one exemplary embodiment of the invention. 
         FIG. 5A  is a partially broken away, cross-sectional view along line  5 A- 5 A of  FIG. 5 . 
         FIG. 6  is an isometric view of a journal bearing sleeve in accordance with another exemplary embodiment of the invention. 
         FIG. 6A  is a partially broken away, cross-sectional view along line  6 A- 6 A of  FIG. 6 . 
         FIG. 7  is a graph of the gap deformation detected in the journal bearing sleeves of  FIGS. 4-6A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       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. 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 or mammography systems. Other imaging systems such as computed tomography (CT) systems and digital radiography (RAD) systems, which acquire image three dimensional data for a volume, also benefit from the invention. The following discussion of x-ray 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 or 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 an analog electrical signal that represents the intensity of an impinging x-ray beam, and hence the attenuated beam, as it passes through the object  16 . In one embodiment, detector assembly  18  is a scintillation based detector assembly, however, it is also envisioned that direct-conversion type detectors (e.g., CZT 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. 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 . 
       FIG. 2  illustrates a cross-sectional view of an x-ray source  12  incorporating embodiments of the invention. In the illustrated embodiment, x-ray source  12  is formed of an x-ray tube  40  that includes an anode assembly  42  and a cathode assembly  44 . X-ray tube  40  is supported by the anode and cathode assemblies  42 ,  44  within an envelope 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. 
     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  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  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, copper, 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  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 having adequate load-bearing capability and acceptable acoustic noise levels for operation within imaging system  10  of  FIG. 1 . As used herein, the terms “self-acting” and “self-lubricating” mean that the bearing lubricant 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 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  FIG. 2  as the stationary portion of bearing assembly  50  and shell  78  is described as the rotating portion of bearing assembly  50 , embodiments of the present invention are also applicable to embodiments wherein center shaft  76  is a rotary shaft and shell  78  is a stationary component. In such a configuration, target  48  would rotate as center shaft  76  rotates. 
     Center shaft  76  may optionally include a cavity or coolant flow path  80  though which a coolant (not shown), such as oil, may flow to cool bearing assembly  50 . As such, coolant enables heat generated from target  48  of x-ray source  40  to be extracted therefrom and transferred external to x-ray source  40 . In straddle mounted x-ray tube configurations, coolant flow path  80  extends along a longitudinal length of x-ray source  40 . In alternative embodiments, coolant flow path  80  may extend through only a portion of x-ray source  40 , such as in configurations where x-ray source  40  is cantilevered when placed in an imaging system. 
     Referring now to  FIG. 3 , a cross-sectional view of a portion of bearing assembly or structure  50  is shown according to an embodiment of the invention. Bearing assembly  50  includes a center shaft  76  positioned within shell  78 , which is configured to support an anode (not shown), such as target  48  of  FIG. 2 . A lubricant  84  is positioned in a gap  86  formed between center shaft  76  and shell  78 . In embodiments of the invention, lubricant  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 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  FIG. 3 , center shaft  76  of bearing assembly  50  is a stationary component and shell  78  is a rotatable component constructed to rotate about center shaft  76 . However, one skilled in the art will recognize the inventive concepts described herein are applicable to alternative bearing configurations. As one example, bearing assembly  50  may instead include a stationary outer component and a rotating center shaft having a target attached thereto. As another example, bearing assembly  50  may be a “straddle” bearing that is configured to support a 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 lubricated bearing to support an anode or target. Such configurations may include a stationary center shaft and a rotatable outer shaft, 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 configuration having a rotating component in a vacuum, the rotating component being supported by a liquid lubricated bearing. Thus, this invention is applicable to any bearing configuration having a rotatable component and a stationary component, and a liquid lubricant therebetween, regardless of configuration or application. 
     As illustrated in  FIG. 3 , center shaft  76  of bearing assembly  50  includes a thrust bearing portion  88  comprising a radial projection  90  that extends from center shaft  76  and is positioned in a radial cavity  92  of shell  78 . Radial projection  90  limits axial motion of sleeve  78  relative to center shaft  76 , and, as illustrated, lubricant  84  is also included between radial projection  90  and shell  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. 
     In the exemplary embodiment of the invention illustrated in  FIG. 3  the shell  78  is formed with a 2-piece construction including a sleeve  108  and a thrust seal  110 . In the exemplary construction of the sleeve shown in  FIGS. 3, 6 and 7 , the sleeve  108  is formed of a material that is low cost, with good machinability, good galling/wear characteristics, and good weldability. In an exemplary embodiment of the invention, the material forming the sleeve  108  is a non-refractory metal, such as an iron alloy, including stainless steel, tool carbon steel, such as D2 steel, among others. The sleeve  108  is 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. In the illustrated exemplary embodiment, the seating portion  114  is optionally integrally formed with the cap portion  112  to form a unitary structure for the sleeve  108  within which the shaft  76  and thrust seal  110  can be engaged, such as that disclosed in US Patent Application Publication No. US2016/0133431, entitled Welded Spiral Groove Bearing Assembly, the entirety of which is expressly incorporated herein by reference. 
     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 15-50 microns. 
     Operation of liquid bearings in x-ray tube systems, such as bearing assembly  50  of  FIGS. 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 bearing grooves. For example, given a substantially constant rotational speed of the liquid bearing, deeper grooves may provide a higher pumping force, while the increased clearance between the shaft and sleeve can reduce the load carrying ability of the bearing. 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 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. 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 bearing assembly. 
     Looking now at  FIG. 4 , in a prior art journal bearing sleeve construction the sleeve  108  includes a bolted joint flange  120  formed as part of the seating portion  114  and spaced from the remainder of the seating portion  114  by a peripheral/circumferential notch  122 . The target  48  is secured to the flange  120  using suitable bolts (not shown) or other fasteners, such that during operation of the x-ray tube  12  heat is transferred from the target  48  through the sleeve bolted joint flange  120  and the seating portion  114  of the sleeve  108 . This heat is removed by convection of the coolant within the coolant flow path  80  within the shaft  76  by thermally contacting the coolant with the seating portion  114  and bolted joint flange  120 . Due to the properties of the material forming the sleeve  108 , such as D2 steel, a thermal gradient is developed within the sleeve  108  near the bolted joint flange  120 . This thermal gradient heats and causes deformation in the structure of the sleeve  108 , resulting in a change in the bearing gap  86  between the sleeve  108  and shaft  76  as shown in  FIG. 7 . This maximum change in the width of the gap  86  is 10 μm, with the size of the gap  86  being between 30 μm-120 μm. The increase in the width of the gap  86  at a point located about 65 mm along the axis of the sleeve  108  measured from the closed end  112  essentially shortens the bearing sleeve  108 , causing metal to metal contact between the shaft  76  and the sleeve  108  resulting in premature wear and failure of the bearing assembly  50 . The mechanism that produces the change in the width of the gap  86  is predominately thermally induced hoop deformation, where a higher temperature outer diameter  126  of the sleeve  108  is opening/deflecting up a lower temperature inner diameter  128  of the sleeve  108 . 
     Referring now to the illustrated exemplary embodiment of  FIG. 5 , to address this problem the sleeve  108  is modified by the formation of a number of equally spaced slots  130 . In this exemplary embodiment the slots  130  are machined within an already formed sleeve  108  to have a width of between approximately 3 mm-5 mm, and are spaced equidistant from one another around the circumference of the sleeve  108 . The particular location of the slots  130  may be varied, but in the exemplary embodiment of  FIGS. 5-5A , the slots  130  are formed to extend radially inwardly into the flange  120 , and optionally partially into the cap portion  112 , to form the flange  120  into a number of petals or sections  132 . The sections  132  each include a bolt aperture  134  for securing the target  48  to the respective section  132 . The slots  130  are oriented axially with regard to the sleeve  108  and reduce the thermal gradient-induced hoop strain by allowing the thermal gradient that develops within the flange  120  to deform the individual sections  132  in the flange  120 , rather than the cap portion  112  of the sleeve  108 . The deflection of the individual sections  132  absorbs some of the strain energy and reduces the amount the “hot” outer diameter  126  pulls open the “cool” inner diameter  128 . The geometry of the slots  130  is sized/shaped to minimize stresses created in the flange  120  as a result of the deformation of the sections  132 , while also minimizing the loss of the overall thermal conduction area of the flange  120 . 
     Testing to determine the improvement provided by the presence of the slots  130  was performed by measuring the speed at which a cold, e.g., at start up, and a hot, e.g., running at the maximum capable steady state thermal conditions of the test setup, bearing sleeve  108  lands on the stationary shaft  76  during gantry rotation. As illustrated below in Table 1, the delta/difference in the speed of landing a cold versus a hot bearing sleeve  108  on the shaft  76  was 57 Hz for the sleeve  108  without the slots  130 , and was 12 Hz for the sleeve  108  including the slots  130 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Gantry Landing Speed Test Results 
               
            
           
           
               
               
               
            
               
                   
                 Gantry Landing (Hz) 
                   
               
               
                   
                 at 3.5 kW SS 
               
            
           
           
               
               
               
               
            
               
                   
                 Cold 
                 Hot 
                 Delta 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 (No slots) 
                 28 
                 85 
                 57 
               
               
                   
                 (Machined 
                 26 
                 37 
                 12 
               
               
                   
                 Slots) 
               
               
                   
                   
               
            
           
         
       
     
     These results clearly illustrate that the deformation of the sleeve  108  during operation is significantly reduced by the presence of the slots  130  due to the greatly reduced speed at which the rotating sleeve  108  lands on the shaft  76 , providing evidence of the lessened deformation of the sleeve  108 . In addition, in looking at the results for the location and amount of the maximum change in width of the gap  86  illustrated in  FIG. 7 , the presence of the slots  130  reduces the maximum gap change by 30% compared to the maximum gap change of a sleeve  108  without the slots  130 , i.e., reduced to 7 μm from 10 μm. 
     To even further reduce sleeve deformation, in the illustrated exemplary embodiment of  FIGS. 6-6A , the sleeve  108  is formed with slots  140  having a geometry that can be created by additive manufacturing of the sleeve  108 . The additive manufacturing process enables the slots  140  to be formed with minimal width between adjacent sections  132 , such as a width of between 1 mm-3 mm, to reduce the amount material removed from the flange  120  to form the slot  140 , thereby maintaining maximum thermal conductance through the flange  120 /sections  132 . In addition, the additive manufacturing of the sleeve  108  enables the slots  140  to be formed with an optimized shape/profile to further reduce the maximum gap change by ˜50%, i.e., to approximately 4.8 μm from 10 μm, with respect to the prior art sleeve  108  formed without the slots  140 . The slots  140  formed in the sleeve  108  can also include a radius  142 , such as, for example a radius  142  on the order of a couple of millimeters, within the slot  140  that is not possible to form by machining and that lower stresses created in the flange  120  as a result of the deformation of the sections  132  by approximately 30% as compared to the sleeve  108  illustrated in  FIG. 5  including the machined slots  130  as calculated using structural FEA models. 
     With regard to the illustrated exemplary embodiments and other embodiments of the disclosure, the sleeve  108  formed with the slots  130 , 140  and the bearing assembly  50  incorporating the sleeve  108  provides the benefits of reducing bearing deformation in x-ray tube bearings formed of non-refractory metals, such as D2 steel, among other materials by minimizing sleeve deformation. The reduction in deformation of the sleeve  108  and the bearing  50  consequently increases the useful life of the sleeve  108  and the bearing  50  by reducing premature wear in the bearing  50 , whether formed in a cantilever or straddle-type bearing construction. Further, the construction of sleeve  108  with the slots  130 , 140  negates any need for construction of a larger bearing assembly  50  to accommodate for the deformation and increased wear, which will increase tube power density and lower friction within the bearing assembly  50 . The advantages provide significant cost reduction for the construction of bearings  50  and sleeves  108  using non-refractory metals compared to more expensive refractory materials, along with cost avoidance of constructing larger, more expensive bearings to address the deformation issue. 
     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.