Patent Publication Number: US-6212753-B1

Title: Complaint joint for interfacing dissimilar metals in X-ray tubes

Description:
TECHNICAL FIELD 
     The present invention relates to rotating X-ray tubes and, more particularly, to an interface between dissimilar metals in X-ray tube construction. 
     BACKGROUND ART 
     The X-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical X-ray tubes are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor consisting of a cylindrical rotor built into an axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the X-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is sometimes referenced electrically to ground. The X-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode. 
     In an x-ray tube device with a rotatable anode, the target consists of a disk made of a refractory metal such as tungsten, and the x-rays are generated by making the electron beam collide with this target, while the target is being rotated at high speed. Rotation of the target is achieved by driving the rotor provided on a support shaft extending from the target. Such an arrangement is typical of rotating X-ray tubes and has remained relatively unchanged in concept of operation since its introduction. However, the operating conditions for x-ray tubes have changed considerably in the last two decades. 
     State-of-the-art X-ray tubes utilize large (200 mm diameter, 4.5 kg) cantilever mounted, targets rotating at speeds as high as 10,000 rpm. Extremely large temperature changes occur during the operation of the tube, ranging from room temperature to temperatures as high as 1600° C., produced by the deceleration of fast electrons in the tungsten-rhenium layer of the target track. 
     For the purposes of heat management and safeguarding of components such as bearings, materials with low thermal conductivity are placed in the heat path. In general, such materials have much higher coefficient of thermal expansion than the other materials used in an X-ray tube. However these components must be joined to the others in some fashion (i.e., welding, brazing, bolting, etc.). At these joints, the higher level of growth may cause yielding of the components which grow at a smaller rate. 
     Balance retention at high rotating speeds and high temperatures is extremely crucial. A typical unbalance specification for larger tubes at the time of shipping is 5 g-cm in either the target or rotor planes. Approximately 5% of manufactured tubes with large targets (165 mm diameter, 2.7 kg) are unusable due to high unbalance. A shift of 19 μm of the target center of gravity will produce this amount of unbalance. As anodes become larger and heavier, the amount of shift that will exceed the unbalance specification becomes less. For the latest target size (diameter of approximately 200 mm and mass of approximately 4.5 kg) a shift of 11 μm will exceed the unbalance specification. These small shifts can easily occur because of the large temperature changes, combined with the use of materials that have different coefficients of thermal expansion. Furthermore, the selection of compatible materials for joints is often limited by the operating temperature, material strength and material expansion properties. Additionally, bolted, brazed, and welded joints are a primary source of unbalance. 
     It would be desirable then to have an improved method for joining two or more members of an X-ray tube with dissimilar thermal expansion rates, particularly for high temperature applications. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method to join two or more components of an X-ray tube having dissimilar thermal expansion rates, for high temperature applications. Use of this method involves utilizing an interference fit while increasing the compliance of the joint with geometric modifications. 
     In accordance with one aspect of the present invention, a method for joining components of an X-ray tube is particularly useful for joining components having dissimilar thermal expansion rates. Initially, a first joint is identified, which has an inner component to be received into an outer component. Typically, the inner component has a higher coefficient of expansion than the outer component, so the purpose of the invention is to reduce the physical expansion of that component in the joint. A plurality of slots is introduced along the approximate axial length of the inner component of the joint, to achieve the aforementioned purpose. Alternatively, a coupling member could be provided between two components to be joined, the coupling member having a thermal expansion rate greater than one component and less than the other component. 
     Accordingly, it is an object of the present invention to provide a novel method of joining components, particularly in X-ray tube construction. It is a further object of the present invention to provide an interfacing method that is particularly useful for high temperature applications. It is an advantage of the present invention that the interfacing method has achieved impressive joint integrity and hence retention of balance in X-ray tubes. 
     Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-6 illustrate joining methods for joining components of an X-ray tube, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to X-ray tubes which employ a rotating anode assembly and a cathode assembly. The purpose of this invention is to improve the method of joining components in the X-ray tube construction, particularly components having dissimilar thermal expansion rates. 
     With X-ray tubes, in particular, the components that the X-ray tube manufacturer selects and uses are chosen for their X-ray performance. Specifically, X-ray tube components are selected for their advantages in heat distribution, heat generation, x-ray generation, high-speed rotation capabilities, and particularly for heat management and safeguarding of components. Necessarily selecting the materials for their X-ray tube performance results in materials having differing thermal expansion properties, an undesirable side effect of the selection criteria. Nonetheless, these dissimilar metals have to be joined. Hence, the X-ray tube manufacturer selects materials for their X-ray tube application, but finds that these materials have dissimilar thermal expansion properties, so then must address the problems (imbalance) created by the use of these different materials. 
     In a typical X-ray tube assembly, the target, rotor assembly, and bearing assembly, for example, are assembled using bolted, brazed and/or welded joints. The present invention provides for a significant improvement in the fit between joined members of the X-ray tube, particularly members having dissimilar thermal expansion rates. 
     Referring to FIGS. 1-6, the present invention proposes using interference fit assembly in the X-ray tube assembly, to improve joint retention and balance integrity of components at the joints. Compliance of the joint is increased by incorporating geometric modifications, preferably modifications which reduce the hoop and radial stiffness and stresses of the joint that occur due to a difference in thermal expansion of the joint materials. The method of the present invention achieves this while maintaining interference fit throughout the operating temperature of the joint. 
     Referring now to FIG. 1, the present invention provides for a significant improvement in the interface between components with dissimilar thermal expansion rates. In FIG. 1, component  10  is to be bolted to component  12 . In this example, component  10  is the component with the higher coefficient of expansion, and is inserted into component  12 . The compliance of the joint is achieved in accordance with the present invention by introducing geometric modifications into the X-ray tube structure. Specifically, slots  14  are provided, preferably terminating with an aperture  24  at each end to reduce the stress concentration at the ends of each slot  14 . Such slotting produces compliant captured fingers  16  which are used to mechanically join the member to other members of the joint. 
     The inclusion of slots  14  in member  10  has the advantage of reducing the physical expansion of that component  10  in the joint. Without the joining method of the present invention, higher coefficient component  10  would tend to expand faster than lower coefficient component  12 , adversely affecting balance retention of the final structure. With the joining method of the present invention, rather than being a single less compliant member, slotted component  10  becomes multiple more compliant members. Specifically, in FIGS. 1 and 2, with six slots  14 , component  10  now comprises six compliant portions  16 , rather than a single (non-slotted) stiff component. 
     In accordance with the present invention, the slotted component may comprise the component with the higher coefficient of expansion, so the slots have the effect of reducing the amount of physical growth of that component within the joint to produce lower stresses, particularly when that component is to be inserted into a component of lower coefficient of expansion and, thereby, slower growth. Of course, if stress analysis and configuration of the joint so dictate, the component of lower coefficient of expansion could be slotted. 
     In an alternative embodiment of the invention, a separate slotted member or coupling member  18 , as illustrated in FIGS. 3-6, could be introduced between two components  20  and  22  having dissimilar thermal expansion rates, to join the two components  20  and  22 . In this case, the coupling member  18  would typically have a coefficient of thermal expansion which is less than that of one member  20  or  22 , and greater than that of the other member  22  or  20 . 
     In the drawings, the slots  14  are preferably along the axial length of the slotted component  10  or  18 , to hold the circular integrity of the structure. Furthermore, the slots are typically spaced symmetrically for uniform stresses, although an anti-symmetric slotting can be selected to produce certain stress situations, as desired. The length and number of slots are the control factors for controlling compliance. For example, longer slots will necessarily provide more compliance, while shorter slots will decrease the amount of compliance achieved. Likewise, increasing the number of slots causes less expansion or growth, while decreasing the number of slots allows for more expansion than a greater number of slots but still less expansion than a non-slotted component. Consequently, the number of slots, length and distribution can be varied to achieve various realizations. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that modifications and variations can be effected within the spirit and scope of the invention.