Abstract:
The invention relates to a rotary-anode X-ray tube which includes a sleeve bearing which is composed of an inner and an outer bearing segment, the outer bearing segment including intermediate pieces and a holder on which the intermediate pieces bear so as to transfer the bearing forces. Suitable shaping of the external surfaces of the intermediate pieces and the inner surfaces of the holder which contact these outer surfaces ensures that the intermediate pieces become aligned with the bearing surfaces on the inner bearing segment. This strongly reduces the complexity of manufacture.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a rotary anode X-ray tube which includes a sleeve bearing which serves to journal the rotary-anode and includes an inner bearing segment and an outer bearing segment which is provided with a holder, intermediate pieces being provided between the holder and the inner bearing segment and correspondingly formed bearing surfaces on the inner bearing segment and in the intermediate pieces being arranged to take up radial and axial bearing forces. The invention also relates to a sleeve bearing which is suitable for use in a rotary-anode X-ray tube. 
     2. Description of Related Art 
     A rotary-anode X-ray tube and a sleeve bearing of this kind are known from DE-GM 87 05 478. Therein, the outer surfaces of the intermediate pieces are adapted to the tubular holder whereas their inner surfaces are adapted to the bearing surfaces on the inner bearing segment which have the form of a truncated cone. A bias force, applied to one of the intermediate pieces and produced by a spring, ensures that the bearing clearance remains constant also when the temperature of the sleeve bearing increases; however, the starting moment is thus also increased. 
     The bearing capacity of a sleeve bearing is dependent particularly on the distance between the bearing faces co-operating in the inner and the outer bearing segments. The bearing capacity of the sleeve bearing is greater as this distance is smaller. In practice this distance is of the order of magnitude of 10 μm. Such small distances, which should also remain constant across the bearing surfaces, can be achieved only by costly precision manufacture. In comparison with sleeve bearings which have bearing surfaces extending perpendicularly to the axis of rotation so as to take up the axial bearing forces and at least one cylindrical bearing surface for taking up the radial bearing forces, the known sleeve bearing offers the advantage that only two bearing surfaces (having the shape of a truncated cone) must be formed with the necessary precision on the inner bearing segment and in the intermediate pieces. 
     Moreover, however, the external surfaces of the intermediate pieces which are arranged in the tubular holder must also extend exactly concentrically with the conical bearing surfaces provided therein, because the bearing surfaces will otherwise be tilted. Finally, the outer diameter of the intermediate pieces must be very accurately adapted to the inner diameter of the tubular holder in order to ensure that on the one hand the intermediate pieces can slide in the tubular holder and that on the other hand no play remains between the facing surfaces of the intermediate pieces on the one side and the tubular holder on the other side. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a rotary-anode X-ray tube which includes a sleeve bearing that can be more readily manufactured. On the basis of a rotary-anode X-ray tube of the kind set forth, this object is achieved in that the facing surfaces of the intermediate pieces and the holder are formed in such a manner that the intermediate pieces within the holder are aligned with the bearing surfaces. 
     Because of the shaping of the facing surfaces of the holder and the intermediate pieces according to the invention it is achieved that the bearing surfaces in the intermediate pieces are aligned with the bearing surfaces on the inner bearing segment even when the external surfaces of the intermediate pieces which come into contact with the holder do not extend exactly concentrically with respect to the bearing surfaces provided therein. Thus, in a sleeve bearing of this kind it is only important that the bearing surfaces in the intermediate pieces on the one side and on the inner bearing segment on the other side correspond exactly. 
     The inner surfaces of the holder and the outer surfaces of the intermediate pieces which come into contact therewith must be formed so that the intermediate pieces can be tilted relative to the holder. The maximum tilt angle required is dependent on the precision of manufacture; however, it can be easily kept small, for example less than 1°. To this end, both of said surfaces may be shaped as a segment of sphere as in a spherical bearing. However, the manufacture is easier when said surfaces are formed in conformity with the claims  2  and  3 . 
     The bearing surfaces must be shaped so that they are capable of taking up radial forces on the one hand and axial bearing forces on the other hand, the axial bearing forces to be taken up by the two bearings being oppositely directed. The normals to the rotationally symmetrical bearing surfaces, therefore, may extend neither parallel with nor at right angles to the axis of rotation, at least not in a given region. A shape of the bearing surfaces that can be comparatively easily realized is disclosed in claim  4 . 
     Claim  5  describes an embodiment which facilitates the assembly of the sleeve bearing. In conformity with one embodiment, the adjustment of the two parts, and hence the adjustment of the bearing play, can be performed by screwing one part more or less far into the other part. The bearing play governing the operation of the sleeve bearing, therefore, is not imposed by high-precision manufacture, as in rotary-anode X-ray tubes with separate bearing surfaces for the axial and radial bearing forces, but by means of a simple adjusting operation. 
     Generally speaking, the friction between the intermediate pieces and the holder suffices to prevent rotation of the intermediate pieces relative to the holder during rotation of the sleeve bearing. However, if necessary, the possibility of such rotation can be completely precluded by means of the embodiment disclosed in claim  7 . 
     The claims  8  and  9  disclose possibilities for connecting the bearing segments to the envelope of the X-ray tube. The alternative disclosed in claim  8  offers the advantage that heat can be discharged from the bearing via an inner bore which extends inside the inner bearing segment in the direction of the axis of rotation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described in detail hereinafter with reference to the drawings. Therein: 
     FIG. 1 shows an X-ray tube according to the invention, 
     FIG. 2 shows the sleeve bearing of this X-ray tube, 
     FIG. 3 shows a further embodiment of an X-ray tube, 
     FIG. 4 shows the associated sleeve bearing, and 
     FIG. 5 shows a further embodiment of the sleeve bearing. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows diagrammatically a rotary-anode X-ray tube whose vacuum envelope, for example made of glass, is denoted by the reference numeral  101 . A cathode  102  and a rotary-anode  103  are connected to the vacuum envelope  101 . The rotary-anode includes an anode disc  104  which is connected to a holder  105 . A rotor  106  is connected to the holder, said rotor co-operating with a stator (not shown) outside the tube envelope  101  in order to drive the anode disc  104 . 
     The anode disc  104  is journaled in a sleeve bearing which consists of an outer (in this case rotatable) bearing segment and an inner (stationary) bearing segment which is enclosed thereby. FIG. 2 shows the sleeve bearing at a larger scale than FIG.  1 . The Figure shows a shaft  109  which is associated with the inner bearing segment and is rigidly connected to the tube envelope, the upper end of said shaft being provided with a bearing surface  110  which is tapered upwards as a truncated cone and is provided with a spiral groove pattern. At the lower area of the shaft  109  there is provided a similar bearing surface  111  which, however, is tapered downwards; the shaft  109  extends further downwards with a correspondingly reduced diameter. 
     In conjunction with the inner surfaces of two intermediate pieces  112  and  113 , also shaped as truncated cones, the bearing surfaces  110  and  111  constitute a respective spiral groove bearing which is capable of taking up axial and radial bearing forces at the same time. The outer surfaces of the intermediate pieces  112 ,  113  are preferably shaped as a segment of sphere. The outer surface of the intermediate piece  112  bears on an inner surface  114  of a holder portion  105 ; this inner surface  114  is symmetrical with respect to the axis of rotation and is shaped as a truncated cone. Similarly, the outer surface of the lower intermediate piece  113  bears on the inner surface  115 , again shaped as a truncated cone, of a further holder portion  116  which is also symmetrical with respect to the axis of rotation and is connected to the holder portion  105 , via a thread, so as to be adjustable in the axial direction. The two holder portions  105  and  116  together constitute a holder  108  which supports the intermediate pieces  112  and  113  in the radial direction and in the axial direction. A liquid lubricant having a low vapor pressure, for example a gallium alloy, is present in the gap between the bearing surface  110  and the intermediate piece  112  as well as in the gap between the bearing surface  111  and the intermediate piece  113  (these gaps are not visible in the drawing). 
     Upon assembly of the sleeve bearing, the intermediate piece  112  is arranged in the holder while the holder portion  116  is still screwed out, after which the shaft  109  is inserted into the holder and the intermediate piece  113  is slid onto the lower bearing surface. Subsequently, the holder portion  116  is screwed into the holder portion  105 , the external surfaces of the intermediate pieces  112  and  113  then being aligned in such a manner that the bearing surfaces provided in the interior thereof are pressed against the bearing surfaces  110  and  111  over their entire surface. Subsequently, the holder portion  116  is loosened in a defined manner so as to adjust the desired bearing play. 
     The shaft  9  there may be provided with an interior cooling bore which extends in the axial direction and via which the heat which reaches the shaft  109  via the holder, the intermediate pieces and the lubricant can be dissipated. During rotation of the anode disc  104 , the intermediate pieces  112 ,  113  rotate in synchronism therewith. Their areas of contact with the holder portions  105  and  116 , respectively, is small but generally they suffice to take along the intermediate pieces  112  and  113 . 
     Rotation between the intermediate pieces  112 ,  113  and the holder portions  105 ,  116  can be avoided by providing the holder portion (as is shown in a plan view in FIG. 5 for the holder portion  116 ) with a radially projecting rib  121  (or a pin) which can engage a groove (not shown) in the intermediate piece. This groove should be deeper than the rib  121 , so that only the convex outer surface of the intermediate piece can bear on the conical inner surface  114  or  115 . Moreover, the grooves should be wider than the ribs so that the intermediate pieces can be tilted relative to the inner surfaces. 
     In the FIGS. 1 and 2 an embodiment is concerned in which the inner segment of the sleeve bearing is rigidly connected to the tube envelope and the outer segment is connected to the anode disc  104 . The FIGS. 3 and 4 show an embodiment in which the outer bearing segment of the sleeve bearing is connected to the tube envelope and the inner bearing segment is connected to the anode disc  104 . The rotor  106  is connected to a shaft  129 . 
     As is shown in FIG. 4, the anode disc is supported by the shaft  129  which widens downwards as a truncated cone at the area of a bearing surface  110  and changes over into a portion of a larger diameter; at the end of this portion there is provided the bearing surface  111  which is tapered downwards as a truncated cone. In conjunction with the correspondingly shaped bearing surfaces in the intermediate pieces  112 ,  113 , the bearing surfaces  110 ,  111  again form sleeve bearings for taking up radial and axial bearing forces. The holder, being stationary while the anode disc rotates, includes a portion  125  which is open towards the anode, is connected to the tube envelope and has inner surfaces comprise an area  115  at their lower end which is shaped as a truncated cone; it also includes a holder portion  126  which is adjustable in the axial direction therein by way of an external thread and whose surfaces  114 , extending downwards as a truncated cone, fix the upper intermediate piece  112 . Dissipation of heat via a bore extending in the axial direction is not simply possible in this case. 
     The ring-shaped intermediate pieces at the upper end of the sleeve bearing shown in FIG.  2  and at the lower end of the sleeve bearing shown in FIG. 4 may also be shaped as a cap, so that they seal the upper and the lower bearing from the environment and no lubricant can escape at these areas. 
     The inner surfaces of the holder in the embodiment have a conical shape and the contacting external surfaces of the intermediate pieces have a convex, rotationally symmetrical shape. These surfaces may also have a different shape, for example the shape of a segment of sphere, or the inner surfaces may be concave and the external surfaces may have a convex shape. It is essential for all of said possibilities that the intermediate pieces can be tilted relative to the holder.