Metal working – Method of mechanical manufacture – Assembling or joining
Reexamination Certificate
1997-11-25
2001-04-10
Bryant, David P. (Department: 3726)
Metal working
Method of mechanical manufacture
Assembling or joining
C403S030000, C403S179000, C378S144000, C445S028000
Reexamination Certificate
active
06212753
ABSTRACT:
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 &mgr;m 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 &mgr;m 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.
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Derakhshan Mark O.
Ebben Thomas G.
Bryant David P.
Cabou Christian G.
General Electric Company
Haushalter B. Joan
Price Phyllis Y.
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