Patent Application
20080095316
In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken as limiting the scope of the invention.
Referring to
A stream of electrons (not shown) are accelerated from the cathode assembly 14 toward the anode assembly 12. The stream of electrons strike a focal spot (not shown) on the anode assembly 12 and produce high frequency electromagnetic waves (not shown), or x-rays, and residual energy. The residual energy is absorbed by the components of the x-ray tube 10 as heat. The x-rays are directed through the vacuum 16 toward an aperture 20 in a thermal storage assembly 22 that is interposed between the cathode assembly 14 and the anode assembly 12. Aperture 20 collimates the x-rays thereby reducing the radiation dosage received by a patient.
The anode assembly 12 includes a generally disc-shaped anode 24 secured to one end of an anode shaft 26. The anode 24 includes a target track or impact zone 28 that is generally fabricated from a refractory metal with a high atomic number such as tungsten or tungsten alloy. Heat is generated in the anode assembly as the electrons from the cathode assembly 14 impact the target track 28. For example, the temperature of the anode focal spot (not shown) can run above 2,000 degrees C. The anode 24 is preferably rotated so that the electron beam (not shown) from the cathode assembly 14 does not focus on the same portion of the target track 28 and thereby cause the accumulation of heat in a localized area. Accordingly, a generally cylindrical stator 30 is provided to selectively rotate the anode shaft 26 and the anode 24 attached thereto. The stator 30 is depicted in
Referring to
Heat from the anode 24 is transferred to the bearing assemblies 34, 36 and can impact bearing performance and durability. A bearing device designed to operate optimally at a first temperature can lock-up and interrupt or impede rotation when exposed to higher temperatures because of the thermal expansion of the bearing components. As an example, if the bearing balls 42 are exposed to heat and thermally expand, they may become compressed between the anode shaft 26 and the outer race 44 such that the individual bearing balls 42 cannot rotate (i.e., are locked-up). If the bearing balls 42 lock-up, the rotational resistance applied to the anode shaft 26 significantly increases such that anode 24 rotation is either interrupted or greatly impeded. The bearing assemblies 34, 36 must therefore be designed with enough radial clearance to accommodate for thermal expansion. Excess radial bearing clearance can, however, reduce bearing life due to its adverse effect on bearing design criteria such as bearing contact angle, internal pre-load, and load distribution. It should therefore be appreciated that bearing performance and durability can be improved by reducing radial bearing clearance as long as there remains enough clearance to accommodate for thermal expansion of the bearing components.
As the front bearing assembly 34 is in closer proximity to the anode 24, it receives more heat and higher loads than the rear bearing assembly 36. Therefore, the first plurality of bearing balls 38 are preferably comprised of silicon nitride (Si3N4) which has a thermal expansion that is appreciably less than that of more conventional ball bearing materials such as steel. The reduction in thermal expansion allows the front bearing assembly 34 to be designed with less radial bearing clearance, and the reduction in radial bearing clearance allows for a more consistently optimal bearing contact angle and a more even load distribution such that the performance and durability of the front bearing assembly 34 are improved. Additional improvements in bearing performance and durability can be realized by the smoother surface finish and increased hardness attainable with ceramic materials. While the composition of the first plurality of bearing balls 38 has been described in accordance with the preferred embodiment as including silicon nitride, it should be appreciated that other ceramic compositions such as silicon carbide, alumina oxide, or zirconia may also be envisioned.
The second plurality of bearing balls 42 are preferably comprised of steel. Because the steel balls are used exclusively in the rear bearing assembly 36 which is exposed to less heat and lighter loads than the front bearing assembly 34, the radial clearance of the rear bearing assembly 36 can also be minimized to improve bearing performance and durability. Advantageously, steel has a relatively high electrical conductivity such that the second plurality of bearing balls 42 may be implemented to transfer electrical current (not shown) from the anode 24. While the composition of the second plurality of bearing balls 42 has been described in accordance with the preferred embodiment as including steel, it should be appreciated that other conductive compositions such as other metallic materials may also be envisioned.
The bearing assemblies 34, 36 are preferably dry meaning that fluid lubricants such as grease are not implemented. This requirement is drawn to the fact that the high temperatures to which the bearing assemblies 34, 36 are exposed could vaporize such lubricants and thereby disrupt the vacuum 16 (shown in
According to an embodiment of the invention, the first outer race 40 and the second outer race 44 each include a ball bearing contact surface having a solid lubricant coating. The core material composition of the first and second outer race 40, 44 is preferably steel, and the solid lubricant coating is preferably comprised of Ag, Au, or any other known solid lubricant material. The solid lubricant coating of the first and second outer race 40, 44 may comprise multiple layers and may be applied, for example, with an ion plating method. By applying a solid lubricant coating to the first and second plurality of bearing balls 38, 42, and to the first and second outer race 40, 44, the coefficient of friction of the bearing assemblies 34, 36 can be minimized such that bearing performance and durability are improved.
While the invention has been described with reference to preferred embodiments, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made to the embodiments without departing from the spirit of the invention. Accordingly, the foregoing description is meant to be exemplary only, and should not limit the scope of the invention as set forth in the following claims.
