Flat Rotary-Anode X-Ray Tube

The invention relates to an X-ray tube which comprises a metal housing in which a rotary anode which is mounted on a shaft is rotatably journalled by means of two bearings, one bearing being connected on the one side to the metal housing and on the other side, by means of an electrical insulator, to the shaft, while the other bearing is connected on the one side to the shaft and on the other side, by means of an electrical insulator, to the housing.

An X-ray tube of this kind is disclosed in U.S. Pat. No. 4,024,424. Therein, bearings in the form of ball bearings are used. Only a comparatively small heat flow can be dissipated by way of these bearings. Therefore, the electrical power which can be applied to the anode disc, and hence the radiation power to be generated, is limited. Furthermore, the ball bearings must be situated at a comparatively large distance from the anode disc in order to prevent overheating during operation. Consequently, the length of such an X-ray tube in the direction of the drive shaft is comparatively large. Moreover, the loadability of the above X-ray tube is limited because increased loadability would necessitate a substantially increased diameter of the anode disc; this would mean a substantial increase of the moment of inertia and a substantially heavier drive motor would be required.

The invention has for its object to provide a rotary-anode X-ray tube of the described kind which has a small height in the axial direction and improved dissipation of heat via the bearings. To this end, an X-ray tube in accordance with the invention is characterized in that at least one of the bearings is a sleeve bearing comprising dish-shaped, mutually cooperating metal bearing faces directed transversely of the shaft and separated from each other by a lubricant layer. The lubricant is a liquid metal or a liquid metal alloy. This electrical insulator connected to the bearing is a flat disc directed transversely of the shaft.

In conjunction with the electrical insulator connected thereto, the bearing which itself exhibits a low thermal resistance forms, a flat construction whose thermal resistance is low in the direction of the shaft, so that heat developed in the rotary anode can be suitably dissipated.

In a preferred embodiment of an X-ray tube in accordance with the invention the electrical insulator connected to the metal-lubricated sleeve bearing is a flat cone having a diameter which decreases in the direction of the rotary anode.

The distance between the rotary anode and the insulator may be small, because damaging of the insulator due to discharges possibly occurring due to this short distance is counteracted. Because the diameter of the insulator decreases in the direction of the rotary anode, any electron which reaches the insulator will be exposed to an electrical field which is directed from the insulator to the rotary anode, so that an electron which reaches the insulator (for example, originating from the metal housing) is accelerated towards the anode and will not travel along the insulator surface. Therefore, such an electron will not release other electrons which themselves would release other electrons, so that no electron avalanche will be produced in the direction of the insulator surface. Failures, such as gas eruptions and breakdown of the insulator, are thus avoided.

It is to be noted that sleeve bearings of the kind used in accordance with the invention are already described in U.S. Pat. No. 4,210,371. Sleeve bearings of this kind have a much longer service life than ball bearings. Therefore, unlike X-ray tubes having ball bearings, it is not necessary to accelerate the anode disc to the nominal speed prior to the start of the exposure, after which it has to be brought to a standstill again in order to avoid bearing wear which influences the service life of the rotary-anode X-ray tube. At the beginning of a complete X-ray examination, the drive for the anode disc can be switched on and it can be switched off after termination of this examination, so that the anode disc has already reached the nominal speed during the X-ray exposures performed during the examination and need not be accelerated from standstill. Thus, the anode disc need not be accelerated to the nominal speed in a short period of time, so that a drive power suffices which is substantially smaller than for customary rotary-anode X-ray tubes.

An embodiment in accordance with the invention will now be described with reference to the single drawing FIGURE which shows the embodiment in cross-section;

The drawing shows an X-ray tube which comprises a rotary anode 6 and a metal housing 1. The housing 1 has a rotation-symmetrical construction, except for its part which is situated in the vicinity of a radiation exit window 2. A rotor 3 which is arranged inside the metal housing 1 can be driven in known manner by a stator 18 which is arranged outside the metal housing 1. The rotor 3 is rigidly connected to an insulator 4, which itself is connected to a preferably hollow shaft 5 which supports the anode 6 having a diameter of approximately 300 mm.

A cathode 7 is provided on an insulator 8 on the outer circumference of the metal housing 1 in the plane of the anode disc 6. Control electrodes 19 and 20 which are arranged between the cathode 7 and the anode 6 do not form the subject of the present invention. They are described in German Patent Application P No. 28 07 735. Therefore, they will not be elaborated herein. The radiation emitted by a focal path 21 provided on the outer circumference of the anode disc 6 emerges via the window 2 at the side of the housing 1 which is remote from the insulator 4, as denoted by the broken lines 9. However, the radiation can alternatively emerge at the side of the housing 1 which faces the insulator 4 if the anode disc 6 is rotated through 180.degree. with respect to the plane of the drawing around an axis extending perpendicularly to its axis of rotation.

The insulator 4 is arranged in the immediate vicinity of the anode disc and is directly connected to a circular disc 10a which is arranged at a small distance from the anode disc 6 and which prevents heating of the insulator by radiation from the central area of the anode disc. A corresponding disc 10b is arranged at the opposite side of the anode disc 6.

The insulator 4, preferably being made of aluminium oxide ceramic, is comparatively flat, so that its thermal resistance in the direction of the shaft 5 is low. The insulator is tapered in the direction of the anode 6 or the disc 10a, so that discharge phenomena which could damage the insulator surface are counteracted. The shaft 5 is journalled in the metal housing by means of a sleeve bearing 22 which is connected to the insulator 4. This sleeve bearing 22 is comprised of a first dish-shaped bearing face 11 which is connected to the insulator 4 and which widens in the direction of the anode disc 6. The metal housing comprises a similarly shaped bearing face 12, a narrow gap being maintained between the two bearing faces 11 and 12. One of the bearing faces, the bearing face 11 in the embodiment, is provided with a spiral groove pattern consisting of two groups of spiral grooves which extend at an angle with respect to the axis of rotation and which form a herring bone pattern. Between the two bearings faces 11 and 12, preferably made of W or Mo, there is provided a layer of metal which is liquid at room temperature or a liquid metal alloy, preferably a eutectic alloy of gallium on the one hand and indium and/or tin on the other hand. These alloys are characterized by a low melting point, a low vapour pressure and a high surface stress, so that even in the case of standstill the metal alloy cannot escape from the bearing and the two bearing faces are separated in the operating condition. The spiral grooves force the metal or the metal alloy into the bearing, so that the latter has an extra high dynamic stability. A sleeve bearing of this kind not only has a long service life, but also a low thermal resistance and a low electrical resistance.

The rotor can in principle also be used as a bearing supporting face. This rotor is then preferably provided with a groove pattern. Because copper is not a suitable material for the bearing faces, but is very well suitable for the rotor jacket, the copper jacket of the rotor is then preferably covered with a thin layer of a suitable metal which is not attacked by the metal alloy used as the lubricant, for example, tungsten or molybdenum. In this metal coating there are provided the grooves of the sleeve bearing or the grooves are provided on the bearing face provided on the metal housing.

On the other side of the anode 6 there is provided a sleeve bearing 13 with a liquid metal lubricant. One bearing half is mounted on the hollow shaft 5 and the other bearing half is mounted on an insulator 14 which is connected to the metal housing 1 in a vacuumtight manner. Via the hollow shaft 5, the bearing 13 and a high voltage connector 15, high voltage is applied to the anode disc 6. The high voltage connector 15 is slid onto the insulator 14 via an intermediate rubber seal 16.