Patent Application
20080101540
The present invention relates generally to x-ray tubes and, more particularly, to a coating deposited on an x-ray tube bearing assembly.
X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for the purpose of distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearing assembly, which includes tool steel ball bearings and tool steel raceways.
Bearings used in x-ray tubes are required to operate in a vacuum, which precludes lubricating with conventional wet bearing lubricants such as grease or oil. X-ray tube bearing rolling elements are typically coated with a solid layer, or tribological system, of a metal with lubricating properties, such as silver, lead, or lead-tin. Silver, applied by an ion plating or an electroplating process, has been used as a lubricating coating for tool steel bearings in x-ray tube applications where the tubes operate under vacuum and at temperatures in the range of 300-500 degrees Celsius. The performance of the silver coating is optimum at an operating stress level of up to 2.5 GPa and a temperature of 400 to 500 degrees Celsius. Failure of a bearing in an x-ray tube is typically by wear of the plated silver and loss of the silver from the contact region.
Silver is also used because of its electrical characteristics. Tube current flows in the x-ray tube from cathode to anode as an electron beam. The tube electrical circuit requires tube current to flow through the bearing assembly, and as such, the current flows through the rolling contact points of the bearing. The electrical circuit may include the races, the balls, and any lubricant or other material that is deposited on the bearing assembly or its components to enhance the life of the bearing. As such, the tribological system on the balls or races must be sufficiently electrically conductive in order for the x-ray tube to operate.
Silver derives its lubricity from the fact that it is a highly ductile single phase noble metal. This property is dependent on operating at temperatures above the recrystallization temperature of silver, which is 0.4 to 0.5 times the melting point of silver. Therefore, silver is not as effective for bearing lubrication when operating below these temperatures, and other soft metals such as Pb and combinations of Pb and Sn have instead been used to lubricate ball bearings in x-ray applications.
Silver lubricant distributes between the balls and races during initial processing and operation of the x-ray tube to form a thin coating on the rolling contact region. Once the silver coating is worn, wear of the base material commences, which leads to increased noise, failure of the lubricant, and can ultimately lead to catastrophic failure of the bearing.
The operating conditions of newer generation x-ray tubes have become increasingly more aggressive in terms of stresses because of G forces imposed by higher gantry speeds and higher anode runspeeds. As a result there is greater emphasis in finding materials solutions for improved performance and higher reliability of the bearing tribological system under the more stringent operating conditions.
Therefore, it would be desirable to have a method and apparatus to improve reliability of the lubricant in the rolling contact region and to improve the useful life of the x-ray bearing.
The present invention provides a method and apparatus for enhancing x-ray tube bearing lubricants that overcome the aforementioned drawbacks. A coating between the ball and raceway of a bearing assembly includes an augmented lubrication material to improve the lubricant on the base metal of an x-ray tube bearing and to reduce wear of the base metal of an x-ray bearing.
In accordance with one aspect of the invention, a bearing assembly is disclosed that is mounted in an x-ray tube, the bearing assembly includes a bearing race, a bearing ball positioned adjacent to the bearing race, and a combination coating deposited on one of the bearing race and the bearing ball, the combination coating comprising, a lubricant, and a metal matrix deposited on the one of the bearing race and the bearing ball.
In accordance with another aspect of the present invention discloses a method of manufacturing an x-ray tube bearing assembly. The method includes providing a bearing ball, providing a bearing race, and depositing a combination coating on one of the bearing race and the bearing ball, the combination coating comprising, a lubricant, and a metal matrix deposited on the one of the at least one bearing race and the at least one bearing ball.
In accordance with yet another aspect of the present invention, an imaging system includes an x-ray detector, an x-ray tube having a rotatable shaft, and a bearing assembly supporting the rotatable shaft. The bearing assembly includes a bearing race and a bearing ball positioned adjacent to the bearing race. A combination coating is deposited on one of the bearing race and the bearing ball, the combination coating comprising, a lubricant deposited on a first portion of a bare metal of one of the bearing race and the bearing ball, and a metal matrix deposited on a second portion of the bare metal.
Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings.
The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.
In the drawings:
The operating environment of the present invention is described with respect to the use of an x-ray tube as used in a computed tomography (CT) system. However, it will be appreciated by those skilled in the art that the present invention is equally applicable for use in other systems that require the use of an x-ray tube. Such uses include, but are not limited to, x-ray imaging systems (for medical and non-medical use), mammography imaging systems, and RAD systems.
Moreover, the present invention will be described with respect to use in an x-ray tube. However, one skilled in the art will further appreciate that the present invention is equally applicable for other systems that require operation of a bearing in a high vacuum, high temperature, and high contact stress environment, wherein a solid lubricant, such as silver, is plated on the rolling contact components. The present invention will be described with respect to a “third generation” CT medical imaging scanner, but is equally applicable with other CT systems, such as a baggage scanner.
Referring to
Rotation of gantry 12 and the operation of x-ray tube 14 are governed by a control mechanism 26 of CT system 10. Control mechanism 26 includes an x-ray controller 28 that provides power and timing signals to an x-ray tube 14 and a gantry motor controller 30 that controls the rotational speed and position of gantry 12. A data acquisition system (DAS) 32 in control mechanism 26 samples analog data from detectors 20 and converts the data to digital signals for subsequent processing. An image reconstructor 34 receives sampled and digitized x-ray data from DAS 32 and performs high speed reconstruction. The reconstructed image is applied as an input to a computer 36 which stores the image in a mass storage device 38.
Computer 36 also receives commands and scanning parameters from an operator via console 40 that has a keyboard. An associated cathode ray tube display 42 allows the operator to observe the reconstructed image and other data from computer 36. The operator supplied commands and parameters are used by computer 36 to provide control signals and information to DAS 32, x-ray controller 28 and gantry motor controller 30. In addition, computer 36 operates a table motor controller 44 which controls a motorized table 46 to position patient 22 and gantry 12. Particularly, table 46 moves portions of patient 22 through a gantry opening 48.
The bearing assembly 58 includes a center shaft 66 attached to the rotor 62 at first end 68 and attached to the anode 56 at second end 70. A front inner race 72 and a rear inner race 74 of center shaft 66 rollingly engage a plurality of front balls 76 and a plurality of rear balls 78, respectively. Bearing assembly 58 also includes a front outer race 80 and a rear outer race 82 configured to rollingly engage and position, respectively, the plurality of front balls 76 and the plurality of rear balls 78. Bearing assembly 58 includes a stem 84 which is supported by the x-ray tube 14. Stator 86 drives rotor 62, which rotationally drives anode 56.
In addition to rotation of the anode 56 within x-ray tube 14, the x-ray tube 14 as a whole is caused to rotate about gantry 12 at rates of, typically, 1 Hz or faster. The rotational effects of both the x-ray tube 14 about the gantry 12 and the anode 56 within the x-ray tube 14 cause the anode 56 weight to be compounded significantly, hence leading to operating contact stresses in the races 72, 74, 80, 82 and balls 76, 78 of up to 2.5 GPa. Additionally, heat generated from operation of the cathode 60, the resulting deceleration of electrons in anode 56, and heat generated from frictional self-heating of the races 72, 74, 80, 82 and balls 76, 78 to operate typically above 400 degrees Celsius. Operation at such high temperatures and operation at high rotational speeds require a lubricant to be applied between races 72, 74, 80, 82 and balls 76, 78 in order to reduce friction therebetween.
Silver is typically used as the lubricant when operating temperatures of the x-ray tube 14 exceed 400 degrees Celsius. Silver may be applied to the races 72, 74, 80, 82 or balls 76, 78 or to both in x-ray tube applications. When applied to balls 76, 78 silver is usually applied by, for instance, ion plating or electroplating. Silver minimizes formation of adhesive junctions between the base materials of the 72, 74, 80, 82 and balls 76, 78. Being a relatively soft coating, silver is able to transfer from, for example, the lubricated balls 76, 78 to races 72, 74, 80, and 82 and maintain low friction therebetween. Optimal operating stresses of an x-ray tube typically range from 1-2.5 GPa with optimal temperatures typically ranging from 400-500 degrees Celsius.
Silver is a face-centered cubic (FCC) alloy which minimally work hardens above 400 degrees Celsius. Additionally, silver plastically flows easily to form a transfer film that prevents tool steel to tool steel adhesive wear processes between bearing balls 76, 78 and races 72, 74, 80, and 82. As such, silver is a preferred lubricant when the operating temperature is above 400 degrees Celsius. However, the ability of silver to plastically flow is not retained at lower temperatures (e.g. <400 degrees Celsius). To improve the lubricity and enhance the performance of silver over a wider temperature range, other solid lubricants may be added thereto.
Referring to
Referring to
In one embodiment, the hard coating 122 includes a monolithic nitride coating deposited by PVD, chemical vapor deposition (CVD) or deposited through ion nitriding. Nitride coatings can be doped with Cl ions by injecting traces of additional TiCl4 during processing. The nitrides can include TiN or other metallic alloyed nitrides. An advantage of the CVD process is that it can be integrated with the tool steel heat treatment cycle, then air quenched and tempered.
In another embodiment, hard coating 122 includes multiple layers of nitride such as TiNZrN. Nitrides enhance overall adhesion between the base material 88 and the lubricant 124. The thickness of each layer is preferably 100 nm or lower, while the thickness of the combined layers is preferably not greater than 10 microns.
In yet another embodiment, hard coating 122 includes carbide and oxide coatings with lubricating phases. A CerMet (ceramic and metal) coating such as WC—Co(Cr) or Metal matrix/alumina is co-deposited with a moderate temperature lubricant phase capable of operating in vacuum, such as MoS2, WS2, CaF2, CaF2BaF2 eutectics. These coatings can be deposited by a High Velocity Oxygen Fuel (HVOF) process, to produce a dense adherent coating.
According to one embodiment of the invention, a bearing assembly is disclosed that is mounted in an x-ray tube, the bearing assembly includes a bearing race, a bearing ball positioned adjacent to the bearing race, and a combination coating deposited on one of the bearing race and the bearing ball, the combination coating comprising, a lubricant, and a metal matrix deposited on the one of the bearing race and the bearing ball.
In accordance with another embodiment of the present invention discloses a method of manufacturing an x-ray tube bearing assembly. The method includes providing a bearing ball, providing a bearing race, and depositing a combination coating on one of the bearing race and the bearing ball, the combination coating comprising, a lubricant, and a metal matrix deposited on the one of the at least one bearing race and the at least one bearing ball.
In accordance with yet another embodiment of the present invention, an imaging system includes an x-ray detector, an x-ray tube having a rotatable shaft, and a bearing assembly supporting the rotatable shaft. The bearing assembly includes a bearing race and a bearing ball positioned adjacent to the bearing race. A combination coating is deposited on one of the bearing race and the bearing ball, the combination coating comprising, a lubricant deposited on a first portion of a bare metal of one of the bearing race and the bearing ball, and a metal matrix deposited on a second portion of the bare metal.
The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.
