X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2002-01-10
2004-08-17
Glick, Edward J. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S142000
Reexamination Certificate
active
06778635
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to x-ray tubes that use a rotating anode target supported by a bearing assembly. More particularly, embodiments of the present invention relate to systems and devices concerned with improving the rate of heat transfer from the x-ray tube bearing assembly and related components so as to facilitate a relative increase in the life of the bearing assembly, and thus the x-ray device as a whole.
2. The Relevant Technology
X-ray producing devices are valuable tools that are used in a wide variety of industrial, medical, and other applications. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis and testing. While they are used in various applications, the different x-ray devices share the same underlying operational principles. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated, and then impinged upon a material of a particular composition.
Typically, these processes are carried out within a vacuum enclosure. Disposed within the vacuum enclosure is an electron source, or cathode, and an anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes a stream of electrons to be emitted by the process of thermionic emission. A high voltage potential applied across the anode and the cathode causes the electrons emitted from the cathode to rapidly accelerate towards a target surface, or focal track, positioned on the anode.
The accelerating electrons in the stream strike the target surface, typically a refractory metal having a high atomic number, at a high velocity and a portion of the kinetic energy of the striking electron stream is converted to electromagnetic waves of very high frequency, or x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as the body of a patient. As is well known, the x-rays can be used for therapeutic treatment, or for x-ray medical diagnostic examination or material analysis procedures.
In addition to stimulating the production of x-rays, the kinetic energy of the striking electron stream also causes a significant amount of heat to be produced in the anode. As a result, the anode typically experiences extremely high operating temperatures. However, the anode is not the only element of the x-ray tube subjected to such extreme operating temperatures.
In particular, a percentage of the electrons that strike the target surface do not generate x-rays, and instead simply rebound from the surface and then impact another “non-target” surfaces and structures within the x-ray tube evacuated enclosure. These are often referred to as “secondary” electrons. These secondary electrons retain a large percentage of their kinetic energy after rebounding, and when they impact non-target surfaces, a significant amount of heat is generated that is conducted to various other elements, such as the bearing assembly, of the x-ray device.
The heat produced by secondary electrons, in conjunction with the high temperatures present at the target anode, often reaches levels high enough to damage portions of the x-ray tube structure. In particular, such extreme temperature operating conditions can shorten the operational life of the x-ray device, affect its efficiency and performance, and/or render it inoperable. Such high heat levels present special problems in the context of rotating anode type x-ray tubes.
In a typical rotating anode type x-ray tube, the anode is mounted to a shaft that is rotatably supported by a bearing assembly contained in a bearing housing. A stator serves to rotate the shaft, and the anode accordingly rotates as well. As the anode rotates, each point on the focal track is rotated into and out of the path of the electron beam generated by the cathode. In this way, the electron beam is in contact with a given point on the focal track for only short periods of time, thereby allowing the remaining portion of the focal track to cool during the time that it takes such given portion to rotate back into the path of the electron beam.
The rotating anode x-ray tube of this sort is used in a variety of applications, some of which require that the anode be rotated at relatively high speeds so as to maintain an acceptable heat distribution along the focal track. For instance, x-ray tubes used in mammography equipment have typically been operated with anode rotation speeds around 3500 revolutions per minute (rpm). However, the demands of the industry have continued to change and high-speed machines for mammography and other applications are now being produced that operate at anode rotation speeds of around 10,000 rpm and higher.
High rotational speeds coupled with extreme operating temperatures place tremendous stress and strain on the bearing assembly and related components of the rotating anode x-ray tube, resulting in a variety of undesirable consequences. For example, high rotational speeds and operating temperatures may cause increased vibration and noise in the bearing assembly. This increase in noise and vibration is undesirable, because it can be unsettling to a patient, particularly in applications such as mammography where the patient is in intimate contact with the x-ray machine. Moreover, noise and vibration can be distracting to the x-ray machine operator. Also, unchecked vibration can shorten the operating life of the x-ray tube. Finally, the quality of the images produced by the x-ray device are at least partly a function of the stability of the focal spot on the target surface. Thus, vibration may compromise the quality of the x-ray image by causing undesirable movement of the focal spot.
There are various mechanisms by which high rotational speeds and extreme operating temperatures may cause increased vibration and noise in the bearing assembly. For example, excessively high temperatures can melt the thin film metal lubricant that is typically present on the ball bearings of the bearing assembly. When the bearings cool, the metal lubricant may clump and then create rough spots in the bearing races. Upon subsequent start-up of the x-ray device, the balls travel at high speeds over the rough spots in the races, thereby creating vibration and noise. Moreover, repeated exposure to high temperatures can degrade the bearings, thereby reducing their useful life as well as that of the x-ray tube.
Another mechanism by which high rotational speeds and extreme operating temperatures generate vibration and noise relates to the physical arrangement of the components in the bearing assembly and bearing housing, and the materials from which those components are constructed. In particular, in some known designs, heat generated at the anode and as a result of secondary electron impacts is conducted directly to the bearing assembly by way of solid metal parts that collectively form a heat path between the anode and the bearing assembly. Thus, operational heat is readily transmitted from the anode to the bearing assembly and related components. Additional heat is also generated in the bearing assembly as a result of bearing friction, which increases as operating speeds increase.
As a result of physical arrangements such as that just described, excessive heat coupled with high rotational speeds often causes the physical connections or interfaces in the shaft and bearing assembly to loosen and vibrate. Loosening can occur when the shaft and the bearing assembly are constructed of different metals that have different thermal expansion rates. In such a case, the various parts will each expand and contract at different respective rates when heated and cooled.
By way of example, the bearing housing is typically constructed of copper, or an alloy thereof. The bearings, which are generally constructed of a steel alloy are captured in the cavity formed
Glick Edward J.
Kao Chih-Cheng Glen
Varian Medical Systems Inc.
Workman Nydegger
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