Abstract:
The present invention relates to a rotor shaft and rotor body assembly in an x-ray device, and it method of manufacture, that resists the formation of cracks in the braze joint due to the elimination of horizontal shear planes therein. The inventive structure also comprises an enlarged proximal end of the rotor shaft and an inventive assembly method that prevents the rotor shaft from de-coupling from the rotor body should the braze material entirely fail during field use.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to x-ray tubes having rotating anode structures. In particular, embodiments of the present invention relate to structures and assembly methods for a rotor shaft and rotor body assembly of an x-ray tube rotating anode. 
     2. The Relevant Technology 
     X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials testing. 
     The basic operation of typical x-ray producing equipment is similar. In general, x-rays, or x-radiation, are produced when electrons are released, accelerated, and then stopped abruptly. A schematic representation of a typical x-ray tube is shown in FIG.  1 . The illustrated x-ray tube assembly  1  includes three primary elements: a cathode  2 , which is the source of electrons; an anode  3 , which is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode; and a voltage generation element for applying a high voltage potential to accelerate the electrons from the cathode to the anode. 
     The three elements are usually positioned within an evacuated housing  4 . An electrical circuit is connected so that the voltage generation element can apply a high voltage potential (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative). The voltage differential causes the electrons that are emitted from the cathode  6  to form a beam and accelerate towards an x-ray “target” that is positioned on the surface of a anode disk  5 . The target surface (sometimes referred to as the focal track) is comprised of a refractory metal, and when the electrons strike the target at the focal spot, the kinetic energy of the striking electron beam is converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the anode target, and are then collimated through a window  9  for penetration into an object, such as an area of a patient&#39;s body. As is well known, the x-rays that pass through the object can be detected and analyzed so as to be used in any one of a number of applications, such as x-ray medical diagnostic examination or material analysis procedures. 
     In addition to producing x-rays, when the electrons impact the target surface much of the resulting energy is converted to heat. This heat, which can reach extremely high temperatures, is initially concentrated in the anode target and then dissipated to other areas of the x-ray tube. These high operating temperatures can damage the x-ray tube, especially over time. 
     The anode disk  5  (also referred to as the rotary target or the rotary anode) is rotatably mounted on a rotating nose piece or stem and rotating shaft  11 , which is connected to a supporting rotor assembly  7 . The disk  5 , shaft and rotor assembly are rotated by a suitable means, such as a stator motor  8 . The disk is typically rotated at high speeds (often in the range of 10,000 RPM), thereby causing the focal track to rotate into and out of the path of the electron beam. In this way, the electron beam is in contact with specific points along the focal track for only short periods of time, thereby allowing the remaining portion of the track to cool during the time that it takes the portion to rotate back into the path of the electron beam. 
     It will be appreciated that the need to continuously accelerate and rotate the disk at such high speeds in the presence of extremely high temperatures can give rise to a number of problems. For instance, while the rotation of the track helps reduce the amount and duration of heat dissipated in the anode target, the focal track is still exposed to very high temperatures—often temperatures of 2500° C. or higher are encountered at the focal spot of the electron beam. This heat is transferred to other portions of the x-ray tube assembly, including the shaft and rotor assembly, resulting in extreme thermal stresses at the interfaces between the various structures. Moreover, acceleration and deceleration of the relatively heavy anode disk results in severe mechanical stresses being imposed on the rotor assembly. Unfortunately, the structures and assembly methods used for anode disk rotational assemblies have not been entirely satisfactory in addressing the various problems arising from such mechanical and thermal stresses. 
     For example, a rotor shaft and rotor body assembly have typically been interconnected by way of threads formed on an outer portion of the rotor, which is then received within a corresponding threaded bore formed within a portion of the rotor body. In addition, a brazed joint may be applied between the threaded mating surfaces. Also, a screw, pin, or the like may be used to secure the rotor shaft to the rotor, which assures that the rotor shaft does not detach from the rotor body in the event that the threaded engagement/braze joint fails. Finally, the rotor shaft may be further welded to rotor body by use of an electron beam welding method. 
     It will be appreciated these types of manufacturing steps are time consuming, expensive, and can result in an assembly with multiple points of potential failure. For instance, the formation of a threaded rotor shaft and corresponding mating rotor body, along with the placement of a screw or the like, entails intensive machining and assembly. Additionally, the placement of a screw or similar fastening means may itself be an operation that is subject to occasional defects. Also, electron beam welding can cause brittleness at the weld that may lead to structural failure, which is made even more likely due the extreme temperature fluctuations that are encountered during operation of the x-ray tube. Finally, each of these techniques entail expensive and time consuming manufacturing steps, which increase the overall production cost of the x-ray tube device. 
     The types of materials that are typically used in the construction of a rotor shaft and rotor body can also give rise to problems. For instance, to restrict the flow of heat by conduction into the rotor shaft and rotor body assembly from the rotating anode target disk, the rotor shaft is often provided with a minimum cross-sectional size and is generally made of a relatively poor heat conductive material, such as a molybdenum alloy called TZM. TZM comprises about 99% molybdenum with the balance making up various proportions of titanium and zirconium. While the TZM material exhibits superior structural strength, it can have a different linear coefficient of thermal expansion than the material making up rotor body  14 . For instance, the rotor body is often made of an iron alloy such as Incoloy 909 sold by Inco Alloys International Inc. of Huntington, W. Va., which has a linear thermal expansion coefficient that is slightly different from that of TZM. This can give rise to significant structure-weakening events during operation, due to the varying rate of thermal expansions of the two materials. 
     Also, where the rotor body is constructed of iron or an iron alloy material, the extreme temperature fluctuations can cause such an iron-based alloy to experience allotropic transformation from body centered cubic (bcc) to face centered cubic (fcc). For instance, when rising through about 912° C., iron transforms from bce to fcc and consequently shrinks in volume. Therefore, in addition to disparate linear thermal expansion coefficients, allotropic transformations cause additional stress upon a braze joint at the interface between rotor shaft and rotor body. 
     Many of these problems can be manifested during repeated operation of the x-ray tube. During operation, the rotor shaft begins to heat up and mechanical stresses from high rotational speeds are imposed. For instance, when the rotor shaft is connected to the rotor assembly with a threaded interface and a braze joint, a horizontal thermal shear plane is often produced at the threaded interface between the shaft and the rotor body within the braze joint. This thermal shear stress can be transferred through the braze material. Moreover, the condition is exacerbated if rotor body  14  is made of iron or an iron alloy, and is taken through the allotropic transformation temperature threshold of about 912° C., as noted. Over time, this continuous cycle of expansion and contraction can result in a cracks or other failure points in the joint. Once a crack has nucleated, propagation of the crack typically results, ultimately resulting in failure of the x-ray tube. 
     Other problems can also result when traditional methods are used to interconnect the rotor assembly. For instance, the braze joint is often comprised of a braze material that will readily flow along and between the threaded surfaces of the rotor shaft and the rotor body. In the event that the braze material has a melting temperature above 1150° C., the molybdenum component of the TZM material forming the rotor shaft forms a eutectic with the metal component of the brazing material, that in turn produces an intermetallic compound. This compound can be brittle in comparison to most metals at room temperature, and can become more ductile as the temperature increases, where conventional metals may tend to allotropically transform and fail or even reach liquidus temperatures. Alternatively, if the braze material has a melting temperature below about 900° C., the braze joint may soften during operation of the x-ray tube and fail to withstand the resulting mechanical stresses. 
     Thus, what is needed is a rotor shaft and rotor body assembly that overcomes the problems of the prior art. In particular, it would be advantageous to have a rotor shaft and rotor body assembly that are interconnected in a manner so as to better withstand the extremely high temperatures and mechanical stresses imposed during operation of the x-ray tube. Additionally, it would be advantageous to provide a rotor shaft and rotor body assembly that are interconnected in a manner so as to resist cracking within the braze joint. Also, it would be advantageous to provide a interconnection scheme that is easy and low in cost to implement and manufacture. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is therefore a primary object of the invention to provide a rotor shaft and rotor body assembly that overcomes the problems of the prior art, namely, to provide an assembly that is better able to withstand the mechanical and thermal stresses generated in an operating x-ray tube. It is also an object of embodiments of the invention to provide a rotor shaft and rotor body assembly that substantially eliminates the presence of horizontal thermal shear planes in the braze between a rotor shaft and rotor body assembly. Still another object of embodiments of the present invention is to provide a rotor shaft and rotor body assembly having a rotor shaft that is implemented so as to resist decoupling from the rotor body, even in the presence of high temperatures, high operational speeds, and repeated and prolonged operation. It is also an object of the present invention to provide a method of assembling a rotor shaft and rotor body that is simplified, and that uses fewer complex and time consuming assembly steps. 
     These and other objectives are addressed by the present invention, which relates to a rotor shaft and rotor body assembly that maintains structural integrity through extreme temperature fluctuations, and in the presence of severe mechanical stresses. As noted, in a rotating anode x-ray tube, a rotor shaft and rotor body assembly experiences temperature changes between room temperature and 1,000° C. and higher during routine usage. Moreover, the assembly is subjected to dramatic mechanical stresses resulting from the high rotational speeds. In one preferred embodiment of the present invention, these problems are addressed with a rotor shaft and rotor body assembly that eliminates the occurrence of horizontal thermal shear planes that are otherwise present at connection points between the shaft and rotor body in the prior art. The assembly also eliminates catastrophic decoupling of the rotor shaft and rotor body, without the use of a screw or the like. 
     A first embodiment of the present invention includes a rotor shaft having an end that has an enlarged convex profile such as a chamfer or a flange. Formed within a corresponding end of the rotor body is an inner bore or recess that has an enlarged concave profile that is complimentary in size and shape to the rotor shaft enlarged convex profile. When assembled, the shaft chamfer or flange on the shaft is matingly received within the recess of the rotor body. In the preferred embodiment, a braze joint is then formed between the mated rotor shaft end and the recess of the rotor body. Moreover, the orientation of the shaft and rotor ensure that the braze joint is predominantly axially disposed between the shaft and the rotor body. Also, the braze joint can be formed to be substantially vertical, thereby eliminating any horizontal thermal shear planes between the shaft and the rotor joint. Various other embodiments vary the shape, size and/or configuration of the rotor shaft end and the corresponding mating surface within the rotor body. These various configurations provide different attachment characteristics, and allow for different types of braze joint configurations. 
     Embodiments of the present invention also include a method of assembling a rotor body and a rotor shaft system. For instance, one assembly method comprises the insertion of a distal end of the rotor shaft entirely through a bore formed within the rotor body until the enlarged convex profile of the rotor shaft seats against the complimentary contour of the recess found within the rotor body. The two ends can then be affixed with the application of a braze joint. 
     These and other objects, features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the above-recited and other advantages and objects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to a specific embodiment thereof which is illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a cut-away perspective view of a conventional x-ray tube assembly; 
     FIG. 2 is an exploded perspective view of one presently preferred embodiment of a rotor shaft and rotor body assembly, wherein a cut-away view of the rotor body reveals a rotor body inner bore and a rotor body outer bore that are separated by a rotor shaft chamfer seat; 
     FIG. 3 is a cut-away perspective view of the rotor shaft and rotor body assembly of FIG. 2, wherein it can be seen that the rotor shaft chamfer is disposed against the rotor body chamfer seat in preparation for brazing by melting of a braze ring; 
     FIG. 4 is a detail section taken from FIG. 3, wherein the assembly is illustrated at the braze joint; 
     FIG. 5 is a detail section taken from FIG. 3, wherein an alternative embodiment is illustrated at the braze joint; 
     FIG. 6 is a detail section taken from FIG. 3, wherein yet another alternative embodiment is illustrated at the braze joint; 
     FIG. 7 is a detail section taken from a structure similar to the location depicted in FIG. 3, wherein an alternative embodiment is illustrated to demonstrate a vertical braze joint with no horizontal or diagonal braze joints present; 
     FIG. 8 is a detail section taken from a structure similar to that seen in FIG. 3, wherein an alternative embodiment depicts a vertical braze joint and a close contact between a chamfer and chamfer seat that restrict the flow of braze material therebetween; 
     FIG. 9 is a detail section taken from a structure similar to that seen in FIG. 3, wherein an alternative embodiment illustrates a rotor body v-notch that acts as a stop or braze material well in order to achieve a vertical braze joint with no horizontal shear structures; 
     FIG. 10 is a detail section taken from a structure similar to that depicted in FIG. 3, wherein an alternative embodiment illustrates a flange at the proximal end of a rotor shaft and a v-notch cut into the rotor body that acts as a stop or well for braze material during the brazing of the shaft to the body; and 
     FIG. 11 is a detail section taken from a structure similar to that depicted in FIG. 3, wherein an alternative embodiment depicts a rotor shaft v-notch cut above the chamfer that acts as a braze material stop or well in order to achieve a vertical braze joint according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to a rotor shaft and rotor body assembly suitable for use in a x-ray device having a rotating anode. In particular, presently preferred embodiments significantly reduce or eliminate thermal expansion and contraction shear stresses in the brazed interface between the shaft and the rotor body. In addition, embodiments of the present invention also provide an improved interconnection between the rotor shaft and the rotor body assembly that resists decoupling in the event of a catastrophic failure of the braze material between the two components. 
     Reference will now be made to the drawings wherein like structures will be provided with like reference designations. It is to be understood that the drawings are diagrammatic and schematic representations of presently preferred embodiments of the present invention, and are not necessarily drawn to scale. 
     FIG. 2 is a perspective view of a presently preferred embodiment of a disassembled rotor shaft and rotor body assembly  110  constructed in accordance with teachings of the present invention. The rotor shaft and the rotor body assembly  110  comprise a cylindrical rotor shaft  112  having a reduced diameter towards the distal end  140 , and an enlarged diameter towards the proximal end  154 . Formed at the distal end  154  is a rotor shaft chamfer section  156 . Formed within rotor shaft  112  is bore  130 , as is designated via phantom lines. Also shown is a rotor body  114 , which is also cylindrical in shape. A cut-away view of rotor body  114  reveals a rotor body inner bore  136  and a rotor body outer bore  138 , axially disposed within rotor body  114 , and separated by a rotor shaft chamfer seat  164 . 
     Assembly of the rotor shaft  112  and the rotor body  114  of FIG. 2 requires the rotor shaft  112  to pass through rotor body  114  with the distal end  140  first, from below the rotor body  114 . 
     The rotor shaft chamfer  156  has a diameter that exceeds the diameter of rotor body inner-bore  136 . Consequently, when the rotor shaft  112  distal end  140  is passed through rotor body outer bore  138 , past rotor shaft chamfer seat  164 , the rotor shaft chamfer  156  seats against rotor shaft chamfer seal  164 . For dimensional analysis purposes, the dimensions depicted in FIG. 4 are in arbitrary units, but they may be considered to be in inches by way of non-limiting example. 
     In a preferred embodiment, rotor shaft  112  is made of molybdenum or a molybdenum alloy called TZM or another refractory or alloy. TZM comprises about 99% molybdenum with variable fractional percentages of titanium and zirconium. The TZM material exhibits superior structural strength to pure molybdenum material, it is easier to machine, and it withstands the centrifugal stresses imposed upon it during rotation and cycling through a thermal change from approximately room temperature to about 900° C. and above, returning to room temperature. 
     FIG. 3 illustrates the rotor shaft and rotor body assembly  110  in an elevational cut-away cross section immediately prior to brazing the rotor shaft  112  to the rotor body  114 . It can be seen that the interface between the rotor shaft  112  and the rotor body  114  is entirely devoid of any horizontal thermal shear planes. In this embodiment, the lack of any horizontal thermal shear planes is made possible by the interface of the right-cylinder shape of the rotor shaft main section  142  within the rotor body inner bore  136  and the diagonal, frusto-conical interface between the rotor shaft chamfer seat  164  and the rotor shaft chamfer  156 . A braze ring  168  is depicted as sitting against the rotor shaft main section  142  and simultaneously sitting upon proximal surface  148  of the rotor body  114  adjacent the rotor shaft main section  112 . 
     In order to achieve a reliable braze joint between the rotor body  114  and the rotor shaft  112 , it is preferable to configure respective diametric sizes that provide an interposed gap when the rotor shaft  112  is fully inserted upwardly through the rotor body  114  until the rotor shaft chamfer  156  seats against the rotor shaft chamfer seat  164 . In a presently preferred embodiment, the gap that forms the interface between the rotor shaft  112  and the rotor body  114  may have a dimension in the range of about 100 mils to about 1,000 mils in order to provide spacing that will facilitate capillary action wetting as braze ring  168  liquefies and fills into the gap to form the braze. Various suitable braze materials are well known in the art. Examples of preferred brazing materials may be found in U.S. Pat. Nos. 4,736,400 and 4,969,172, the disclosures of which are incorporated herein by specific reference. Preferably, the brazing material has a melting temperature so that it doesn&#39;t melt under ordinary operating temperatures of the x-ray tube. The brazing material may also be a composition that forms an intermetallic with rotor shaft  112  and/or rotor body  114 . At room temperature, an intermetallic composition is brittle relative to traditional metals, but at elevated temperatures where traditional metals begin to soften and/or melt, an intermetallic begins to behave as a traditional metal with favorable ductility, tensile, and compressive qualities at operating temperatures in the range from about 700° C. to about 1,200° C. and higher. 
     In an alternative embodiment of the present invention, the rotor shaft and the rotor body assembly  110 , depicted in FIG. 3, is assembled entirely without braze material. Tolerances are chosen between the convex right-cylinder interface of the rotor shaft main section  142  and the concave right-cylinder shape of the rotor body inner bore  136  such that the rotor shaft and rotor body assembly  110  can be assembled only by applying force to push the rotor shaft  112  into the rotor body inner bore  136 , and thereby provide a tight and frictionally secure fit between the two. 
     Another preferred method of making the rotor shaft and rotor body assembly  110  without the presence of a braze material is to heat the rotor body  114  to a temperature sufficiently high such that thermal expansion allows for rotor shaft  112  to pass substantially through rotor body inner bore  136  until the chamfer  156  abuts against rotor shaft chamfer seat  164 . As the rotor body  114  cools, the interface between the rotor shaft  112  and the rotor body  114  become increasingly tight due to the thermal contraction of rotor body  114 . Once the rotor shaft and rotor body assembly  110  have substantially cooled to room temperature following assembly, field use thereof will not substantially diminish the tightness of the fit of the rotor shaft  112  within the rotor body because both the rotor shaft  112  and the rotor body will be heated and cooled substantially as a unit. In this embodiment, a failure of rotor shaft and rotor body assembly  110  would require either the rotor body  114  to crack under tensile stress or the rotor shaft  112  to crack under compressive stress. Preferred temperature differentials between the rotor body  114  and the rotor shaft  112  for this type of assembly process are in a range from about 0° C. to 900° C., and in a preferred embodiment are between about 200° C. to about 350° C. The coefficient of static friction between the rotor shaft  112  and the rotor body  114  is sufficient to hold assembly  110  together, similar to the use of the braze material. As an alternative embodiment, brazing may be done in addition to the tight fit. 
     FIG. 4 is a detail section taken along the dashed line  4 — 4  from FIG. 3, in which it can be seen that a vertical braze joint  170  and a diagonal braze joint  172  form a continuous braze interface between the rotor shaft  112  and the rotor body  114  beginning at the proximal surface  148  where braze ring  168  (see FIG. 3) was located, and ending approximately at rotor shaft proximal end  154 . 
     In comparison to the type of braze joints utilized in the prior art and discussed above, no horizontal thermal shear plane is present between the shaft  112  and the rotor body  114 . Additionally, as the rotor shaft  112  heats by conduction from the rotating anode target disk, thermal expansion of the rotor shaft  112  exerts only a compressive stress upon the braze at vertical braze joint  170 . Similarly, during temperature escalation of the rotor shaft and rotor body assembly  110 , and where the rotor body  114  experiences an allotropic phase transformation from bcc to fcc, additional non-shear stresses upon the vertical braze joint  170  may be experienced. 
     It can be seen that a diagonal braze joint  172  completes the braze that connects the rotor shaft  112  with the rotor body  114 . The diagonal braze joint  172  may carry a horizontal thermal shear component that is proportional to the compressive stress in the vertical brazed joint  170  multiplied by the cosine of the angle α. The total amount of horizontal thermal shear experienced between the rotor shaft chamfer  156  and the rotor shaft chamfer seat  164  is minimal and substantially nondestructive compared to stresses existing in structures of the prior art. One possible reason for this is that the heating of the rotor body  114  begins substantially at the proximal surface  148  across vertical braze joint  170 , and then continues downward in both the rotor shaft  112  and the rotor body  114 . This heat conduction pattern ensures that the thermal gradients within the diagonal braze joint  172  cause substantially only compressive stresses to occur. 
     The angle α designated in FIG. 4 defines the contour of the rotor shaft chamfer  156  in relation to the axial configuration of the rotor shaft main section  142 . The angle may be varied to minimize a horizontal thermal shear component within diagonal braze joint  172 . For instance, as the angle α becomes larger and approaches 90°, any horizontal thermal shear component within diagonal braze joint  172  approaches zero. In presently preferred embodiments, the value for angle α is in a range from about 30° to about 80°, and in one embodiment is in a range from about 60° to about 70°. 
     A primary purpose for the rotor shaft chamfer  156  is to retain the rotor shaft  112  within the rotor body  114 , even in the event that the braze  170  or  172  fails due to a crack. As such, the angle α need only be any angle less than 90° that will facilitate retention of rotor shaft  112  within rotor body  114  under the operating conditions of the particular x-ray device. Rotor shaft chamfer  156 , with the above-discussed configurations of angle α, is one example of a means for retaining the rotor shaft in the rotor body. 
     FIG. 5 illustrates another embodiment of the present invention, which illustrates how the size and shape of the rotor shaft chamfer may be varied in its vertical height, v, and in its horizontal extension, h, in relation to the rest of the rotor shaft. In FIG. 5, the rotor shaft  212  has a rotor shaft chamfer  256  that originates substantially at the same height as the proximal surface  248  of the rotor body  214 , and that terminates at the rotor shaft proximal end  254 . The vertical height, v, of the rotor shaft chamfer  256  corresponds to the distance between the rotor shaft proximal end  254 , and the rotor body proximal end  258 , which is also at the same height as the proximal surface  248 . A diagonal braze joint  272  comprises the entire braze that attaches the rotor shaft  212  to the rotor body  214 . Again, the angle α determines the amount of a horizontal thermal shear component that may be experienced within the diagonal brazed joint  272 . Where the horizontal extension, designated as h, is sufficiently small such that angle α approaches 90°, any horizontal thermal shear component experienced within the diagonal braze joint  272  approaches zero. Where the vertical height v of rotor shaft chamfer  256  begins at rotor shaft proximal end  254  and terminates at the level of the rotor body proximal end  258 , the angle α may be small. For example, in this illustrated embodiment angle α may be in a range from about 30° to about 89°, and preferably is from about 60° to about 89°. 
     FIG. 6 is a detail section taken from a structure at a location similar to that taken from FIG. 3, and illustrates another embodiment of the present invention. Here, the vertical height v of the diagonal braze joint  372  depicted between the rotor shaft chamfer  356  and the rotor body chamfer seat  364  is minimized due to the relatively larger height of a vertical braze joint  370 . Diagonal braze joint  372  is therefore present as a minor portion of the braze. In this embodiment, vertical height, v of the rotor shaft chamfer  356  is minimized and angle α is maximized to approach 90°. While the structure depicted in FIG. 6 may not have the same capability to retain rotor shaft  312  upon catastrophic failure of the braze, it does minimize the extent of diagonal braze joint  372  and therefore minimizes any horizontal thermal shear component that may occur therewithin. In one preferred embodiment, vertical height v has a value of approximately 0.022 inches, and angle α has a value in a range from about 45° to about 89°, and preferably is between about 75° to about 89°. 
     The rotor shaft chamfer in connection with the rotor body may be implemented with other structures. FIG. 7 is a detail section illustrating one such embodiment. In FIG. 7, an amount of a braze material is provided to form a vertical braze joint  470 , which stops at or before the braze material makes contact with rotor shaft chamfer seat  164 . To do so, the cross-sectional area of the braze ring  168  (seen in FIG. 3) must be substantially equal to the cross-sectional area of the vertical braze joint  470  seen in FIG.  7 . As such, the rotor shaft chamfer seat  164  is in contact with little or no braze material. One of ordinary skill in the art may calculate the amount of braze material needed by determining the cross-sectional area of the gap that forms the interface between the rotor shaft  112 , and the rotor body  114 , a representative portion of which is indicated in the hatched section of FIG.  7 . 
     The fact that a given braze material will tend to show a greater affinity for either the rotor shaft  112  or rotor body  114  may be used as an advantage. For example, in one instance the particular braze material may be selected to have an affinity for, and tend to wet rotor body  114 . When the braze material is applied to form vertical braze joint  470 , the rotor shaft and rotor body assembly  110  may be inverted and a capillary action and wetting of the rotor body  114  by the braze material may be balanced against the force of gravity. Moreover, temperature control may be used to adjust the brazing process in order to achieve a vertical braze joint  470  that does not wet chamfer  156  and/or chamfer seat  164 . This method of providing an amount of braze material so as to only form a vertical braze joint  470  and at the same time avoid the formation of any diagonal braze joint is one example of a step for resisting the formation of a braze joint with horizontal thermal shear. 
     Reference is next made to FIG. 8, which illustrates yet another embodiment. Here, a vertical braze joint  570 , in the form of a cylindrical shell, is formed between the rotor shaft  512  and the rotor body  114 . The vertical braze joint  570  has filled the space between the rotor shaft  512  and the rotor body  114  from the proximal surface  148  down to about the level of vertical height v of the rotor shaft chamfer  556 . Also, the spacing between rotor shaft main section  542  and rotor body  114  is relatively larger than the spacing between rotor shaft chamfer  556  and rotor shaft chamfer scat  564 . In the illustrated embodiment, the space or interface between the rotor shaft chamfer  556  and rotor shaft chamfer seat  564  is in the form of a frusto-cone shell. 
     The reduced spacing between rotor shaft chamfer  556  and rotor shaft chamfer seat  564  as compared to that between rotor shaft main section  542  and rotor body  114  reduces the amount of braze material needed between chamfer  556  and chamfer seat  564 . Preferably, the spacing between chamfer  556  and seat  564  is less than 100 mils, and in a most preferred embodiment is less than about 10 mils. The first spacing (between  542  and  114 ) facilitates the flow of braze material, and the second spacing stops (or reduces) the flow of braze material. Preferably, the braze material between rotor shaft  512  and rotor body  114  comprises the entire vertical braze joint  570 . This embodiment may also be fabricated by selecting an amount of braze material that will be equivalent to the area between rotor shaft  512  and rotor body  114  above the level of rotor shaft chamfer  556  and rotor shaft chamfer seat  564 . 
     In the embodiment of FIG. 8, the interface between chamfer  556  and chamfer seat  564  involves two vertical heights v and v′. In this embodiment, v′ is less than v. The process of selecting a braze material under sufficient flow conditions to form a braze joint and to braze such that substantially no braze material fills between rotor shaft chamfer  564  and rotor shaft chamfer seat  556  is another example of a step for resisting the formation of a braze joint with horizontal thermal shear. 
     FIG. 9 illustrates yet another embodiment of the present invention. A vertical braze joint  670  is depicted as being between a rotor shaft  612  and a rotor body  614 . Because capillary action of braze material under flow conditions may cause wetting to extend downwardly beyond the occurrence of the rotor shaft chamfer  656  and the rotor shaft chamfer seat  664 , a rotor body depression such as a rotor body v-notch  676  and optionally a rotor shaft v-notch  674  may be provided. Either or both of these v-notches act as a braze material stop or well that will accumulate braze material and that will stop the downward flow of the braze material during the formation of vertical braze joint  670 . Thus, a rotor shaft and rotor body assembly  610  comprises rotor shaft  612 , rotor body  614 , rotor shaft v-notch  674 , and rotor body v-notch  676  into which vertical braze joint  670  has filled and has substantially stopped the downward flow of braze material during formation of the assembly. 
     Rotor shaft v-notch  674  or rotor body v-notch  676  may be configured at a level at or above vertical height v according to the specific application. Additionally, either v-notch can have an angular shape, or any other geometric configuration that may receive the excess braze material to a sufficient volume. In a preferred embodiment, the rotor shaft v-notch  674  and rotor body v-notch  676  may each have an angle in a range from about 90° to 30°, and most preferably about 60°. The configuration of rotor shaft v-notch  674  to act as a stop or braze material well is an example of a means for resisting the formation of a braze joint with horizontal thermal shear. 
     FIG. 10 is a detail section taken from a structure at a location similar to that taken from FIG. 3 along the line  4 — 4  that illustrates yet another embodiment of the present invention. In place of a rotor shaft chamfer, a rotor shaft  712  may have an enlarged portion near the rotor shaft proximal end  754 . In this embodiment, the enlarged portion is depicted as a flange  757 . A vertical braze joint  770  is depicted as having filled against rotor shaft main section  742  beginning at proximal surface  748  and as having terminated at a rotor body v-notch  774 . The rotor body  714  has a flange seat  765  that abuts against rotor shaft flange  757 . 
     An alternative embodiment of the invention depicted in FIG. 10 is eliminates the rotor shaft v-notch  774 . In this embodiment, an amount of braze material is selected so as to only form vertical braze joint  770 , for example as is set forth for the embodiment depicted in FIG.  7 . Additionally and/or alternatively, the spacing between rotor shaft main section  742  and rotor body  714  and rotor shaft flange  757  and flange seat  765  can be adjusted such that braze material flows to form vertical braze joint  770 , but is prevented from forming any horizontal thermal shear joint between flange  757  and rotor body  714 . In preferred embodiments, spacing between flange  757  and flange seat may be less than 100 mils, and most preferably less than 10 mils. Either or both of rotor body v-notch  774  and spacing between rotor shaft flange  757  and the abutting portion of rotor body  714  is another example of a means for resisting the formation of a braze joint with a horizontal thermal shear. 
     FIG. 11 is a detail section taken from a structure at a location similar to that taken from FIG. 3 along the line  4 — 4  that illustrates yet another embodiment of the present invention. In FIG. 11, it can be seen that rotor body  114  is coupled with a rotor shaft  812  that contains a depression such as a rotor shaft v-notch  875  that acts as a stop or well for braze material as it flows from proximal surface  148  downwardly in the direction of the rotor shaft chamfer  856  and rotor shaft chamfer seat  164 . As with other embodiments previously set forth, spacing between rotor shaft main section  842  and rotor body  114  may be larger than spacing between rotor shaft chamfer  856  and rotor shaft chamfer seat  164  to control the flow of braze material. Where the braze material that is used to form vertical braze joint  870  has a wetting affinity for rotor body  114  greater than rotor shaft  812 , greater care may be required to form vertical braze joint  870  without filling braze material into the space between rotor shaft chamfer  856  and rotor shaft chamfer seat  164 . The presence of rotor shaft v-notch  875  as well as the optional close proximity between rotor shaft chamfer  856  and rotor shaft chamfer seat  164 , that resists the flow of a selected amount of braze material beyond the occurrence of rotor shaft v-notch  875  is another example of a means for resisting the formation of a braze joint with horizontal thermal shear. 
     A depression such as a v-notch or another shape may be cut into either the rotor shaft or the rotor body, or both, in order to facilitate the formation of a vertical braze joint and avoid horizontal thermal shear planes. Additionally, other notch profiles may be formed such as a notch with a curvilinear cross-sectional profile as opposed to a notch with a rectilinear cross-sectional profile of a v-notch Other “notch” configurations that control the flow of braze material could also be used. 
     Presently preferred embodiments of the present invention utilize a PALCO® braze material under braze temperatures known in the prior art. Other materials could also be used. 
     To summarize, embodiments of the present invention have distinct advantages over that of the prior art. One advantage is that the parts are more easily machined because there is no thread-cutting operation, either for the rotor shaft where external threads were previously required, or for the rotor body where internal threads were previously required. As a result of the absence of threads, the parts are more easily machined and also easier to assemble. 
     Another distinct advantage is that no special welding or bonding techniques are required such as electron beam welding often required in the prior art. The absence of any special welding or bonding techniques also eliminates destructive embrittlement of the interface between the rotor shaft and rotor body. Another distinct advantage of embodiments of the present invention is that they eliminate substantially all thermal sheer stresses in the rotor braze joint. This greatly increases the operational life of the assembly. 
     Another distinct advantage of embodiments of the present invention is that the rotor shaft and rotor body assembly allows the x-ray tube to be operated at higher temperatures. Substantially no thermal sheer is experienced to compromise the integrity of the braze joint. Moreover, even if the braze joint is compromised, the rotor shaft and rotor body assembly will not de-couple because of the chamfer or flange feature that holds the assembly together regardless of the presence or absence of the braze joint. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrated and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.