Abstract:
A high energy x-ray tube includes an evacuated chamber ( 12 ) containing a rotor ( 34 ) which rotates an anode ( 10 ) in the path of a stream of electrons (A) to generate an x-ray beam (B) and heat. Heat is carried away from the anode to a bearing shaft ( 54 ) which rotates relative to a stationary rotor ( 42 ) on forward and rear lubricated bearings ( 44 P,  44 R). The heat is directed away from the forward bearings ( 44 F), by a core ( 70 ) of a thermally conductive material, such as copper, disposed in a central cavity ( 60 ) within the shaft. Annular insulating regions ( 74,76 ) are optionally defined between the core and the bearing shaft adjacent the races to increase the thermal path between the anode and the races. The reduction in temperature of the forward bearings results in a decrease in the evaporation rate of the lubricant ( 46 ) and a corresponding increase in the lifetime of the x-ray tube.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the medical diagnostic arts. It finds particular application in connection with reducing the temperature at the forward bearing race of the bearing shaft of an x-ray tube rotor and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to dissipation of heat in other vacuum systems. 
     A high power x-ray tube typically includes a thermionic filament cathode and an anode which are encased in an evacuated envelope. A heating current, commonly on the order of 2-5 amps, is applied through the filament to create a surrounding electron cloud. A high potential, on the order of 100-200 kilovolts, is applied between the filament cathode and the anode to accelerate the electrons from the cloud towards an anode target area. The electron beam impinges on a small area of the anode, or target area, with sufficient energy to generate x-rays. The acceleration of electrons causes a tube or anode current on the order of 5-600 milliamps. Only a small fraction of the energy of the electron beam is converted into x-rays, the majority of the energy being converted to heat which heats the anode white hot. 
     In high energy tubes, the anode rotates at high speeds during x-ray generation to spread the heat energy over a large area and inhibit the target area from overheating. The cathode and the envelope remain stationary. Due to the rotation of the anode, the electron beam does not dwell on the small impingement spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the anode that was heated by the electron beam has substantially cooled before returning to be reheated by the electron beam. 
     The anode is typically rotated by an induction motor. The induction motor includes driving coils, which are placed outside the glass envelope, and a rotor with an armature and a bearing shaft, within the envelope, which is connected to the anode. When the motor is energized, the driving coils induce electric currents and magnetic fields in the armature which cause the armature and other portions of the rotor to rotate. 
     The temperature of the anode can be as high as 1,400° C. Part of the heat is transferred to the rotor, including the armature and the bearing shaft. Heat travels through the bearing shaft to the bearing races and is transferred to the lubricated bearing balls in the races. The lubricants on the bearing balls become hot and tend to evaporate. 
     Because x-ray tubes operate in a vacuum requiring low vapor pressure materials, standard petroleum based lubricating compounds cannot be used. Thus, it is common in the industry to use solid metal lubricants, such as lead, on the bearing races. The evaporation of lead lubricant from a bearing race accelerates rapidly over 350° C. These temperatures can be reached in the bearing, primarily during processing, and also during field life. The evaporation of lubricant leads to a rapid degradation of the bearing surfaces and premature tube failure. In an x-ray tube, the front bearing race is physically closer to the hot target than the rear bearing. Because of this, the front bearing runs about 100  C. hotter than the rear bearing and fails at a much higher rate than the rear bearing. 
     To reduce lubricant evaporation, silver lubrication on the ball bearings is sometimes used in place of lead. Silver has a lower vapor pressure than lead and can be run at least 100  C. hotter than lead. However, silver lubrication has a number of drawbacks. It tends to react with the bearing steel if it becomes too hot and causes grain boundary cracking and premature failure of the bearing. Additionally, silver requires more starting and running torque than lead, due to its lower lubricity. The torque imparts more residual heat into the bearing, through frictional and eddy current induction heating of the bearing and surrounding rotor body components. Silver lubricating material also creates more noise during operation than lead. 
     The present invention provides a new and improved x-ray tube and rotor and method of operation which overcome the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a high energy x-ray tube for providing a beam of x-rays is provided. The tube includes an envelope which defines an evacuated chamber. A cathode is disposed within the chamber for providing a source of electrons. An anode is disposed within the chamber and is struck by the electrons to generate x-rays. A rotor is provided for rotating the anode relative to the cathode. The rotor includes a rotor core having a high thermal conductivity such that heat is conducted by the core away from the anode, and a forward bearing race of a lower conductivity than the core, such that the core conducts heat past the forward bearing race. Lubricated bearings are received in the forward bearing race. 
     In accordance with another aspect of the present invention, a rotor for an x-ray tube is provided. The rotor includes a bearing member including a hollow cylindrical shaft formed from a material of a first thermal conductivity, the cylindrical shaft defining forward and rear bearing races on an exterior surface thereof to receive lubricated bearings therein. A neck is connected with the bearing member, adjacent the forward bearing races, to connect the rotor to an anode of the x-ray tube. An insert, received within the hollow shaft, and formed from a material of a second thermal conductivity, which is higher than the first thermal conductivity, transports heat away from the forward bearing race and reduces the temperature of the forward bearing race during operation of the x-ray tube. 
     In accordance with another aspect of the present invention, a method of reducing evaporation of a bearing lubricant in an x-ray tube having an anode and a rotor assembly connected therewith is provided. The rotor assembly includes a forward bearing race and a rear bearing race, the forward bearing race being closer to the anode than the rear bearing race. The method includes conducting heat around and past the forward bearing race toward the rear bearing race. 
     One advantage of the present invention resides in a reduction in operating temperature of the forward bearing of an x-ray tube bearing shaft. 
     Another advantage of the present invention is that the evaporation rate of the lubricant for the bearing balls is reduced. 
     Another advantage of the present invention is an increased life of the bearings and the tube. 
     Another advantage of the present invention is that it enables the use of lead as a bearing ball lubricant. 
     Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment, and are not to be construed as limiting the invention. 
     FIG. 1 is a schematic view of a rotating anode tube according to the present invention; 
     FIG. 2 is an enlarged sectional perspective view of a bearing member and neck of the x-ray tube of FIG. 1; 
     FIG. 3 is an enlarged sectional view of the bearing shaft and core of FIG. 2; 
     FIG. 4 is an enlarged side sectional view of an alternative embodiment of a bearing shaft, core, and neck according to the present invention; 
     FIG. 5 is a second alternative embodiment of a bearing shaft, core, and neck according to the present invention; 
     FIG. 6 is a plot of hours to entirely evaporate bearing lubricant from a bearing ball versus the temperature of the lubricant calculated for lead and silver lubricants; 
     FIG. 7 is a plot of evaporation rate of lubricant versus temperature calculated for lead and silver lubricants; 
     FIG. 8A is a thermal profile of the neck and bearing member of a conventional x-ray tube when heated to 1200° C. at the anode end of the neck; 
     FIG. 8B is a thermal profile of the neck and bearing member of a tube with a thermally conductive core according to the present invention when heated to 1200° C. at the anode end of the neck; and 
     FIG. 9 is a side sectional view of another alternative embodiment of a bearing shaft and neck for an x-ray tube according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a rotating anode x-ray tube of the type used in medical diagnostic systems for providing a focused beam of x-ray radiation is shown. The tube includes a rotating anode  10  which is operated in an evacuated chamber  12  defined typically by a glass envelope  14 . The anode is disc-shaped and beveled adjacent its annular peripheral edge to define an anode surface or target area  16 . A cathode assembly  18  supplies and focuses an electron beam A which strikes the anode surface  16 . 
     Filament leads  20  lead in through the glass envelope to the cathode assembly to supply an electrical current to the assembly. When the electron beam strikes the rotating anode, a portion of the beam is converted to x-rays B which are emitted from the anode surface and a beam of the x-rays passes out of the tube through the envelope  14  and a window of a surrounding cooling oil enclosure or housing  22 . 
     An induction motor  30  rotates the anode  10 . The induction motor includes a stator having driving coils  32 , which are positioned outside the glass envelope, and a rotor  34 , within the envelope, which is connected to the anode  10 . The rotor includes an outer, cylindrical armature or sleeve portion  36  and an inner bearing member  38 , which is centrally aligned within the armature. The armature  36  and bearing member  38  are connected to the anode by a neck  40  of molybdenum or other suitable material. The armature  36  is formed from a thermally and electrically conductive material, such as copper. When the motor is energized, the driving coils  32  induce magnetic fields in the armature which cause the armature and bearing member to rotate relative to a stationary, cylindrical rotor support  42 , which is axially aligned with the armature and bearing member and is positioned therebetween. The rotor support is connected, at a rearward end, by a mounting assembly  43  with the housing  22 , which extends outside the envelope  14  for rigidly supporting the rotor bearing. 
     With reference also to FIG. 2, bearings  44 , such as ball or roller bearings, positioned between the bearing member  38  and the rotor support  42 , allow the bearing member, armature  36 , and anode  10  to rotate smoothly, relative to the rotor support  42 . The bearing balls are coated with a lubricant  46  (not to scale), such as lead or silver at a thickness of about 1000-3000 Å. Preferably the x-ray tube includes both forward and rear bearing balls  44 F and  44 R, respectively. 
     As used herein, the terms “forward,” “rear,” and the like, are used to define relative positions of components along an axis Z passing through the bearing member  38  and anode  10 . Components which are described as forward are closer to the anode, while components described as rearward are further from the anode. 
     The neck  40  includes a stem portion  50  and a flange  52  which extends radially from a rearward end of the stem portion for connecting the neck with the bearing member and the armature. The bearing member  38  includes a generally cylindrical shaft  54  and an annular hub  56 , which extends radially from the shaft at the neck end of the bearing member and includes suitably positioned apertures  58 . 
     Fixing members (not shown), such as screws or bolts, connect the bearing member hub  56  and armature  36  with the neck flange  52  via the apertures. 
     The stem portion  50  of the neck defines a hollow, interior cavity  59  with an opening adjacent the flange  52  such that heat traveling through the neck from the anode enters the bearing primarily through the neck flange  52  and bearing hub  56 , as shown by the heat flow arrows H in FIG.  2 . Heat also flows into armature  36  (not shown in FIG.  2 ). The stem cavity  59  reduces the cross sectional area through which heat flows as compared with a solid stem. The rate of heat flow is dependent on the cross sectional area, and thus a reduction in the cross sectional area reduces the rate of heat flow to the bearing shaft. 
     The x-ray tube bearing shaft  54  defines an interior, cylindrical cavity or bore  60 , which extends axially into the shaft from the hub end, at least part-way along the shaft, with an opening  62 , adjacent the hub or forward end of the shaft. The bearing shaft cavity  60  is preferably formed by boring out up to approximately 70% of the shaft internal diameter. If too much material is taken out, however, the bearing shaft will not be sufficiently rigid to withstand the loads placed on the bearing member. This could lead to premature bearing member failure. Other methods of forming the cavity are also contemplated, including molding the shaft with a cavity. 
     Forward and rear bearing races  64  and  66 , respectively, are defined in an outer cylindrical surface  68  of the shaft  54 . The forward bearing race is closest to the anode  10 . The bearing races comprise annular recesses or grooves in the bearing shaft with a semicircular cross section for receiving the forward and rear bearing balls  44 F and  44 R, respectively. 
     A generally cylindrical core or insert  70 , is received within the bearing cavity  60  and makes thermal contact with at least a portion of an interior, cylindrical surface  72  of the bearing shaft  54 . The core  70  is made from a material with a high thermal conductivity (i.e., a conductivity higher than that of the bearing shaft material). The bearing shaft is formed from a material which has a relatively low thermal conductivity, such as tool steel. Rex20™ is a commonly used tool steel for bearing shafts and may be used for the hollow bearing shaft  54 . 
     Heat, therefore, flows preferentially through the high conductivity core  70 . As shown in FIG. 2, heat enters the core adjacent the bearing hub  56  and flows along the core toward the rearward end of the shaft  54 , adjacent the rear bearing race  66 . This has the effect of transferring heat from the front race  64  to the rear race, reducing the temperature at the front race, while increasing the temperature of the rear race somewhat. Since the temperature of the rear race is still at a lower temperature than the forward race, the temperature of the forward race governs the x-ray tube lifetime (time to x-ray tube failure due to complete evaporation of lubricant from the forward bearing balls). Accordingly, an increase in the rear bearing race temperature does not have a significant effect on tube lifetime. 
     One suitable core  70  is made from copper, although other thermally conductive materials are also contemplated. Copper provides several advantages over many other thermally conductive materials. First, it has a high thermal expansion coefficient relative to the tool steel commonly used in the manufacture of bearing shafts. This difference in expansion coefficient causes the copper core to expand within the bearing cavity  60 , when heated during operation of the x-ray tube, ensuring good contact, and thus low thermal contact resistance, between the bearing shaft  54  and the core  70 . This enhances heat transfer between the bearing and the core. Copper also has a low vapor pressure, making it suitable for use in the high vacuum conditions of the x-ray tube. Other suitable materials for forming the core include silver and beryllium. The core  70  preferably has a very finely finished outer surface to decrease the contact resistance between the core and the bearing shaft interior surface  72 . 
     With reference also to FIG. 3, in a preferred embodiment, the core  70  includes one or more relief cuts or grooves  74 ,  76 . The grooves  74 ,  76  may be formed by machining portions of the outer surface of the core to create a cylindrical indent. The relief grooves  74 ,  76  each define a annular space  78 ,  80 , respectively, between the core and the interior surface  72  of the shaft. The space is evacuated during the initial evacuation of the x-ray tube-envelope, creating an insulation space of low thermal conductivity which resists the transfer of heat between the core and the bearing shaft. The length and position of the grooves  74 ,  76  define the location and dimensions of annular, thermal contact areas or pads  84 ,  86  where the core and the bearing shaft are in thermal contact. The conductive heat transfer between the bearing shaft and the core primarily occurs at these contact areas. By selecting the position and length of the grooves, the heat may be transferred along the core for a selected distance before the heat is able to return to the bearing shaft. In this way the temperature difference between the front and the rear bearing races is reduced and, preferably, balanced. 
     FIG. 3 shows two grooves  74  and  76 , although fewer or ore grooves could also be employed. The forward groove  74  extends rearwardly along the core  70  from slightly forward of the front race  64 . This creates a forward contact area  84 , adjacent the hub  56  and forward of the front race  64 . The heat flows rearwardly from the neck  40  into the bearing shaft  54  and a significant portion of the heat flows into the core  70  via the forward contact area  84 . This flow of heat into the core reduces the amount of heat flowing through a forward portion  85  of the bearing shaft, between the hub and the front race, directly to the front race  64 . A significant proportion of the heat thus flows through the core towards the rear contact area  86 , which is located at the end of the forward groove, and considerably rearward of the front race. Some of the heat entering the shaft via the second contact area  86  travels forward along the bearing shaft to the first race  64 , and some travels rearward along the bearing shaft to the rear race  66 . 
     As can be seen from FIG. 3, the core  70  increases the path length for a portion of the heat to travel through the shaft  54  to the reach the front bearing race  64 , and, at the same time, reduces the path length for the heat to travel through the shaft to the rear bearing race. Thus, the heat is distributed more evenly between the two races. In a conventional bearing shaft, the heat entering the bearing shaft has a relatively short distance x to travel to reach the front race. In the bearing member of FIG. 3, at least a portion of the heat entering the shaft from the stem travels along the core and reenters the bearing shaft a distance y from the front bearing race, which is longer than distance x. Since the temperature of a point along the bearing shaft is dependent on the cross sectional area and the distance traveled by the heat, the added distance 2y-x results in a decrease in temperature at the front bearing race. In contrast, with a conventional solid core of tool steel, rather than the thermally conductive core of the present invention, the heat travels progressively through the shaft, heating the front race to a much higher temperature than the rear race, because of the closer proximity, and shorter path length, from the anode  10  to the front race  64 . 
     In an alternative embodiment, the grooves are filled with a material having low thermal conductivity (i.e., lower than that of the bearing shaft), such as ceramic. 
     In another alternative embodiment, the contact pads  84 ,  86  are formed by encircling the core with washers or other suitable spacers, rather than by cutting out grooves in the core. The washers are preferably welded to the core to maintain good thermal contact with the core. The washers are preferably thermally conductive, although low conductivity materials, such as tool steel may also be used, which are, nevertheless, more conductive than the vacuum which develops in the adjacent groove spaces  78 ,  80 . Alternatively, the contact pads and grooves may be formed by machining the bearing shaft inner surface  72 , or by welding washers or spacers to the shaft inner surface. 
     The second, rear groove  76  is optionally positioned adjacent the rear race  66 . The rear groove causes a portion of the heat flowing through the core  70  to travel rearward of the rear groove and enter the bearing shaft  54  through a third contact area  90  defined rearwardly of the rear race. This has the effect of causing some of the heat flowing through the core to heat a rear portion  92  of the bearing shaft  54 , rearward of the rear race, and thus reduces the amount of heat reaching the rear race. 
     The width and length of the grooves  74 ,  76  and the length of the contact areas  84 ,  86 ,  90  can be adjusted to optimize the transfer of heat from the front race to the rear race and reduce the temperature difference between the front and rear race. Fewer, or more grooves and contact areas may be provided to optimize the heat distribution along the bearing shaft  54 . In this way, the bearing temperature is adjustable to customize the performance characteristics of the x-ray tube. 
     To maintain the core  70  firmly positioned within the bearing shaft cavity, the core is preferably welded to the shaft  54 . As shown in FIG. 3, the core includes an annular, rearwardly extending welding rim  93  at its rearward end. The bearing shaft has a corresponding welding rim  94 . With the core in position in the bearing shaft cavity, the two rims  93 ,  94  are welded together to create a weld joint. Although this is a convenient method for welding the core to the bearing shaft, other methods are also contemplated. 
     In an alternative embodiment, the core  70  is cast directly into the bearing shaft cavity  60 . In this embodiment, the bearing shaft  54  is bored out and filled with a molten metal for forming the core, such as molten copper. The molten copper is allowed to cool under pressure, forming the core. This method results in good thermal contact between the core and the bearing shaft, but does not readily facilitate formation of grooves. 
     With reference once more to FIG. 2, the bearing hub  56  optionally includes several bores  96  therethrough to provide a spoked hub. The bores preferably result in removal of approximately 10-30% of the mass of the hub. The decrease in cross sectional area of the hub resulting from the removal of material increases the thermal resistance through the hub and adjacent forward portion  85  of the bearing shaft, without appreciably reducing the strength of the hub. The spoked bearing hub, in combination with a copper core, results in a reduction in the temperature of the forward bearing race by about 40° C. This can reduce bearing lubricant evaporation by a factor of four during processing. 
     With reference also to FIG. 4, in another alternative embodiment, the core  70  is formed from a material which is liquid at the operating temperatures of the x-ray tube or at ambient temperature. Suitable liquid cores include mercury, gallium, and gallium-indium-tin mixed alloy compositions. The liquid material for the core is poured or otherwise introduced to the bearing cavity  60 . The forward end of the cavity is then sealed with an end cap or plug  100  to prevent leakage of the liquid metal core. Any leakage of the metal during operation of the x-ray tube could result in evaporation of the metal into the x-ray tube vacuum and shortened operational life. Thus, the plug is preferably tightly sealed through welding. A rear end of the bearing cavity is plugged with a similar end cap or plug  102  prior to introduction of the liquid metal. Alternatively, the bearing shaft is not fully bored out along its length, leaving a portion at the rear end which acts as the plug. 
     While liquid metals, such as mercury, tend to have lower thermal conductivity than copper, silver, and some other solid metals, they can ensure very good contact between surfaces. Thus, a good thermal contact between the bearing shaft  54  and the liquid core  70  compensates, to some extent, for the reduction in thermal conductivity of the core. 
     Optionally, surface treatment of the interior surface  72  of the shaft, such as plating with a corrosion-resistant material, is used to inhibit corrosion by corrosive, liquid metals. FIG. 4 shows the inner surface of the bearing shaft coated with a thin layer of corrosion resistant material  104  (not to scale). The material  104  should also be thermally conductive. Nickel is a suitable corrosion-resistant material. 
     In an alternative embodiment, shown in FIG. 5, a core  70 ′ is integral with the x-ray tube mounting  43 ′. Specifically, the core extends forwardly from the mounting and is received by a hollow bearing shaft  54 ′. The core may be welded or otherwise attached at its rearward end to the mounting. In this embodiment, the core  70 ′ remains stationary, with the rotor support  42 ′ and mounting, while the bearing shaft  54 ′ rotates relative to the core. 
     The core is of a slightly smaller diameter than the internal diameter of the bearing shaft cavity  60 ′ so that a narrow vacuum gap  110  (not shown to scale) is defined between the core and the bearing shaft during operation. The core  70 ′ may be formed with grooves, as for the core  70  of FIGS. 1-4, the grooves being wider than the vacuum gap  110 . During operation of the x-ray tube, heat is transferred to the core  70 ′ from the hollow bearing shaft  54 ′, through the narrow vacuum gap  110 . The heat is transferred from the core to the mounting  43 ′ and housing and thus will be passed outside the envelope, to a cooling oil, which fills the housing. By cooling the oil, the core is also cooled. Optionally, a welded end cap  112 , positioned at the rearward end of the stem cavity  59 ′, or at the forward end of the bearing shaft cavity, keeps the forward end of the core from moving out of the bearing shaft cavity and into the stem cavity. The core is thereby positioned to maintain an annular gap  114  between the rearward end of the bearing shaft  54 ′ and the mounting  43 ′. 
     With reference to FIG. 6 a plot of life of bearing lubricant with operating temperature is shown for two bearing lubricants, lead and silver. As shown in FIG. 6, there is an exponential decline in lubricant life (expressed as hours to entirely evaporate the lubricant) with increasing temperature. Thus, it can be seen that significant improvements in lubrication lifetime may be achieved by reducing the temperature of the front bearing race by only a few degrees, even if the rear bearing temperature is increased by an equivalent amount. For example, for a lead bearing, a temperature reduction from 350° C. to 320° C. (i.e., a 30° C. drop in temperature) increases bearing lubrication life from less than 150 hours to 750 hours and a 40° C. drop to 310° C. increases the lifetime to 1000 hours. 
     FIG. 6 was derived from calculations of the evaporation rates (in grams/square centimeter/second), shown in FIG.  7 . The hours H to evaporate the lubricant (silver or lead) at temperature T i  (degrees K) were calculated by multiplying the evaporation rate measured by the lubricant available, as follows:              H   =     M       E   i     ×   F   ×   3600               (   1   )                                
     where 
     F is the area of lubricant surface exposed in cm 2    
     E i  is the evaporation rate of the lubricant at 
     temperature T i  in gm/cm 2 /sec 
     M is the mass of the lubricant in gm 
     The evaporation rates were calculated as follows: 
     
       
         Surface area  F  of a bearing ball of radius  r =4 πr   2   
       
     
     For a ball of radius 0.125 inches (0.3175 cm), the surface area F is 1.267 cm 2 . 
     The initial weight M 0  of lubricant applied to each ball is approximately: 
     
       
           M   0   =F×t×ρ   (2) 
       
     
     where 
     t is the thickness of lubricant applied in cm 
     ρ is the density of the lubricant in gm/cm 3    
     If the thickness t of the lubricant applied is 1500 Å (lead) or 1000 Å (silver), and the density ρ of lead and silver are 11.34 gm/cm 3  and 10.500 gm/cm 3 , respectively. 
     Then, for lead, M 0 =2.155×10 −4  g 
     and for silver, M 0 =1.33×10 −4  g 
     Evaporation rates E i  for lead and silver in gm/cm 2 /sec where calculated using the following equation: 
     
       
           E   i =10 exp[( H −(0.5 log  T   i ))−( G/T   i )]  (3) 
       
     
     Values for G and H were obtained from Duschman,  Scientific Foundations of Vacuum Technique,  Table 10.2, p. 700, as follows: 
     For lead, G=9710, H=7.69 
     For silver, G=14270, and H=8.63 
     Using values of E i  calculated from Equation 3, the hours to evaporation are estimated from Equation 1. 
     The temperature reductions achievable with the core  70 ,  70 ′ of the present invention make it favorable to use lead, rather than silver, as a bearing lubrication, providing several advantages in x-ray tube operation, such as reduced torque requirements and lowered noise levels in operation, as compared with a silver lubricant. Preferably, the core  70 ,  70 ′ reduces the temperature of the forward race  64  by at least 20° C., more preferably, by a temperature of up to about 40° C., or more, over a conventional x-ray tube. X-ray tubes formed with the core of the present invention will show extended tube life due to the increase in the time to failure of the front bearings. 
     To form the x-ray tube of FIGS. 1 to  4 , the core  70  is inserted into the bearing shaft cavity  60  and the weld rims  93 ,  94  are welded together, or other means employed, such as an end cap, for retaining the core within the shaft cavity. The hub  56  of the shaft is connected to the neck flange with screws, or other suitable fixing members, and the anode  10  is connected with the neck. The bearing balls  44  are positioned between the forward and rear bearing races  64  and  66  and corresponding races on the rotor support  42 . Once the x-ray tube has been assembled, it is evacuated to a low pressure and the interior parts, including the anode, and hence the bearing shaft, are heated. Materials evaporating from the parts under the temperature and pressure conditions are withdrawn from the x-ray tube. Much less of the lubricant  46  will evaporate from the bearing balls  44  during this processing step due to the reduced temperature at the front bearing race  64 . 
     With reference now to FIG. 9, in an alternative embodiment, a first, generally cylindrical bearing shaft portion  120  is welded to, or otherwise firmly connected with, an outer cylindrical surface  121  of a thermally conductive core  122  adjacent a forward end  124  thereof. A neck  126  connects the core with the anode of an x-ray tube (not shown) via the forward end  124 . A second, generally cylindrical bearing shaft portion  128  is welded to, or otherwise firmly connected with, the conductive core  122  adjacent a rearward end  130  thereof. The first and second bearing shaft portions  120 ,  128  are formed from a material of low thermal conductivity, such as tool steel. The bearing shaft portions  120 ,  128  define bearing races  134 ,  136 , respectively, for receiving lubricated bearing balls (not shown). Optionally, the bearing shaft portions each include a groove  138 ,  140 , respectively, which separates the region of the bearing shaft portion which carries the bearing race from the adjacent region of the core. Preferably, the groove is accessible to the x-ray tube chamber  12 , so that it is evacuated during evacuation of the chamber. For this purpose a passageway  144  is optionally formed in each of the bearing shaft portions. 
     Optionally, a support member  150 , such as a shaft, extends axially along the core  122 , for providing rigidity to the core. The shaft  150  is preferably made from tool steel or other rigid material. The shaft is connected with the anode directly, or with the neck  126 . 
     Without intending to limit the scope of the invention, the following examples show the improvements which may be achieved in bearing race temperature distribution using the cores according to the present invention. 
     EXAMPLES 
     Example 1 
     Comparison of Copper Core with Conventional Solid Shaft 
     The effect of a copper core on the bearing race temperatures was determined by comparing the temperature profile of a conventional solid tool steel shaft and the temperature profile of a hollow tool steel shaft  54  and a copper core  70 , formed according to the present invention. Each of the shafts was coupled to a neck, as shown in FIG.  8 B. The temperatures of the two shafts were determined by computer modeling techniques, using Finite Element Analysis. The forward end of the neck of each was heated to 1200° C. and the temperature profile of the stem and shaft modeled. 
     With reference to FIGS. 8A and 8B, the temperature profiles of the bearing shafts operated under these conditions show that the front bearing race of the conventional shaft (FIG. 8A) reached 195° C., while the temperature of the rear bearing race reached 101° C. In comparison, the front bearing race of the shaft according to the present invention FIG. 8B) reached 155° C., 40 degrees less than that of the conventional shaft. The temperature of the rear bearing race reached 135° C. which, although higher than that of the conventional rear bearing race, was less than the temperature of the front bearing race. Accordingly, it can be expected that the x-ray tube of the present invention may be run for a longer time than a conventional x-ray tube, before the lubricant evaporates from the bearing races. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations in so far as they come within the scope of the appended claims or the equivalents thereof.