Rotary-anode type x-ray tube

Abstract: In a rotary-anode type X-ray tube, a rotary-anode (11) is fixed to a cylindrical rotary structure (12), and a columnar stationary shaft (15) is fitted in the rotary structure (12). A gap is formed between the rotary structure (12) and the stationary shaft (15). The gap is filled with a liquid metal lubricant. Spiral grooves (20) are formed in part of the outer surface of the stationary shaft (15) to form a radial sliding bearing between the stationary shaft (15) and the rotary structure (12). Spiral grooves (21) are formed in the end faces of the stationary shaft (15) to form a thrust sliding bearing between the stationary shaft (15) and the rotary structure (12). A recess is formed in the stationary shaft (15) to communicate with gaps in the radial sliding bearing. A lubricant storage chamber (22) for storing the liquid metal lubricant is formed in the stationary shaft (15) along the center axis. The storage chamber (22) communicates with communicating holes which radially extend to be open to an outer surface region, of the stationary shaft (15), in which no spiral grooves are formed. With this structure, a sufficient amount of liquid metal lubricant required for a long-term operation of the sliding bearings can be stored in the X-ray tube, thereby maintaining a stable dynamic pressure type sliding bearing operation for a long period of time.

Description: The present invention relates to a rotary-anode type X-ray tube and, more particularly to an improvement in the structure of a bearing for supporting a rotary-anode type X-ray tube. As is known, in a rotary-anode type X-ray tube, a disk-like anode target is supported by a rotary structure and a stationary shaft which have a bearing portion therebetween, and an electron beam emitted from a cathode is radiated on the anode target while the anode targel is rotated at a high speed by energizing an electromagnetic coil arranged outside a vacuum envelope, thus irradiating X-rays. The bearing portion is constituted by a roller bearing, such as a ball bearing, or a hydro-dynamic pressure type sliding bearing which has bearing surfaces with spiral grooves and uses a metal lubricant consisting of, e.g., gallium (Ga) or a gallium-, indiumtin (Ga-In-Sn) alloy, which is liquified during an operation. Rotary-anode type X-ray tubes using the latter bearing are disclosed in, e.g., Published Examined Japanese Patent Application No. 60-21463 and Published Unexamined Japanese Patent Application Nos. 60-97536, 60-117531, 62-287555, 2-227947, and 2-227948. In the rotary-anode type X-ray tubes disclosed in the above-mentioned official gazettes, a liquid metal lubrcant consisting of Ga or a Ga-alloy is applied between the bearing surfaces of the sliding bearing. In this arrangement, however when a tube is processed at a high temperature in the process of manufacture an X-ray tube, or the tube is heated to a high temperature due to heat generated during an operation of the X-ray tube mutual penetration may occur between a metal constituting these bearing surfaces and the lubricant resulting in a gradual decrease in the amount of liquid metal lubricant. This may damage the bearing surfaces. As a result the sliding bearing may not be stably operated for a long period of time. Prior art document EP-A-0 378 273 discloses a rotary-anode type X-ray tube having features similar to those of the precharacterizing portion of claim 1. It is an object of the present invention to provide a rotary-anode type X-ray tube which can hold a sufficient amount of liquid metal lubricant for a long-term operation of an X-ray tube, and can maintain a stable bearing operation of a dynamic pressure type sliding bearing for a long period of time. To solve this object the present invention provides a rotary-anode type X-ray tube as specified in claim 1. Embodiments of the rotary-anode type X-ray tube are specified in Claims 2-7. According to the rotary-anode type X-ray tube of the present invention the gaps in the sliding bearings are filled with the liquid metal lubricant and the liquid metal lubricant is stored in the lubricant storage chamber formed in the stationary shaft or the rotary structure arranged on the rotation axis to communicate with the gaps in the bearings thereby ensuring a sufficient amount of lubricant required for a long-term operation. Even if the amount of lubricant is reduced to an insufficient level in a given place, since the lubricant stored in the lubricant storage chamber quickly flows to the place because of its affinity, a proper lubricating function can be maintained. Therefore, a stable operation of the hydrodynamic pressure type sliding bearing can be maintained for a long period of time. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a longitudinal sectional view schematically showing a rotary-anode type X-ray tube according to an embodiment of the present invention; Fig. 2 is an enlarged sectional view showing a part of the rotary-anode type X-ray tube in Fig. 1; Fig. 3 is a top view showing a part of the rotary-anode type X-ray tube in Fig 1; Fig 4 is a cross-sectional view taken along a line 4 - 4 in Fig 2; Fig. 5 is a longitudinal sectional view schematically showing a rotary-anode type X-ray tube according to still another embodiment of the present invention; Fig. 6 is a longitudinal sectional view schematically showing a rotary-anode type X-ray tube according to yet another embodiment of the present invention; and Fig. 7 is a longitudinal sectional view schematically showing a rotary-anode type X-ray tube according to still further embodiment of the present invention. The preferred embodiments of the rotary-anode type X-ray tube of the present invention will be described below with reference to the accompanying drawings Note that the same parts are denoted by the same reference numerals throughout the drawings. A rotary-anode type X-ray tube shown in Figs. 1 to 4 has the following structure As shown in Fig. 1, a disk-like anode target 11 consisting of a heavy metal is integrally fixed to a rotating shaft portion 13 extending from one end of a cylindrical rotary structure 12 with a set screw 14. A columnar stationary shaft 15 is coaxially fitted in the cylindrical rotary structure 12. A ring-like opening sealing member to is fixed to the opening portion of the rotary structure 12. The end portion of the stationary shaft 15 is coupled to an anode support portion 17 which is airtightly fitted in a glass vacuum envelope 18. The fitting portion between the cylindrical rotary structure 12 and the stationary shaft 15 is formed into a hydrodynamic pressure type sliding bearing portion 19 similar to the one disclosed in the above-mentioned official gazettes. That is, spiral grooves 20 and 21 formed as herringbone patterns disclosed in the above-mentioned official gazettes are respectively formed in the outer surface and two end faces, of the stationary shaft 15 which serve as the sliding bearing surface on the stationary shaft side. The sliding bearing surface, on the rotary structure side, which opposes the sliding bearing surface on the stationary shaft side, is formed into a smooth surface or a surface in which spiral grooves are formed as needed. The two bearing surfaces of the rotary structure 12 and the stationary shaft 15 oppose each other and have a gap of 20 µm therebetween to form thrust and radial bearings. A lubricant storage chamber 22 is formed in the stationary shaft 15 on a rotation center axis by boring a hole in the center of the member 15 along the axial direction. In addition, as shown in Figs 1 and 2, the outer surface of a middle portion of the stationary shaft 15 is tapered to form a small-diameter portion 23 having a surface region in which no spiral grooves are formed, and three radial paths 24 extending from the lubricant storage chamber 22 and opened in the small-diameter portion 23 are formed at angular intervals of 120° around the axis of the member 15 to be symmetrical about the axis. The lubricant paths 24 radially extending from the lubricant storage chamber 22 are communicated with a low-pressure space between the cylindrical rotary structure 12 and the small-diameter portion 23. The lubricant in the low-pressure space is maintained at a pressure lower than that of the gaps of the trust and radial bearings. An end opening portion 22a of the lubricant storage chamber 22 is opened in the central region of the end face of the stationary shaft 15, the end opening 22a being surrounded by the spiral grooves 21. The spiral grooves 21 as the thrust bearing are formed in the other region of the end face and the lubricant storage chamber 22 is communicated with the gap in this thrust bearing through the end opening portion 22a. A portion, of the stationary shaft 15, which is located near the opposite end face is cut to form a small-diameter portion so as to form a circumferential recess 26. Spiral grooves 21 formed as circular herringbone patterns are formed in the opposite end face of the stationary shaft 15. Three radial paths 27 extending from the circumferential cavity 26 and communicating with the lubricant storage chamber 22 are formed at angular intervals of 120° around the axis of the chamber 22 to be symmetrical about the axis. With this structure, a communication section 22b the lubricant storage chamber 22 communicates with the gap of the thrust bearing through the radially extending holes 27 and the circumferential cavity 26. Note that the lubricant storage chamber 22 is sealed by a plug 25 consisting of the same material as that for the stationary shaft 15. Spiral grooves 28 having a pumping effect are formed in the inner surface of the sealing member 16 so as to prevent the lubricant from leaking into the space in the tube through the gap between the stationary shaft 15 and the sealing member 16. A liquid metal lubricant (not shown) is filled in the gaps in the sliding bearing portion 19 and the spiral grooves 20 and 21 and stored in the lubricant storage chamber 22 and the radially extending lubricant paths 24. In this rotary-anode type X-ray tube, an electromagnetic coil 40 as a stator is arranged to oppose the rotary structure 12 outside the vacuum envelope 13 and a rotating magnetic field is generated by the electromagnetic coil 40 to rotate the rotary anode 11 at a high speed, as indicated by an arrow P in Fig. 1. The liquid metal lubricant sufficiently fills the sliding bearing portion 19, at least during an operation of the X-ray tube, to allow a smooth dynamic pressure bearing operation. The spiral grooves formed as the herringbone patterns serve to concentrate this liquid metal lubricant toward their, central portions to increase the pressures thereat so that the lubricant flows to maintain a predetermined gap between the bearing surfaces, thus contributing to a stable dynamic pressure bearing effect The lubricant stored in the lubricant storage chamber 22 is supplied into gaps in bearing surface portions, when an amount of the lubricant is decreased in the gaps of the bearing, thereby ensuring a stable operation of the dynamic pressure type sliding bearing portion. Note that an electron beam emitted from a cathode (not shown) is inpinged on the anode target 11 to irradiate X-rays. Most of the heat generated by this target is dissipated by radiation, while part of the heat is transferred from the rotary structure 12 to the liquid metal lubricant in the bearing portion 19 and is dissipated through the stationary shaft 15. In the embodiment shown in Fig. 5, a hole is bored in the center of a stationary shaft 15 along the axial direction to extend halfway in the member 15, thus forming a lubricant storage chamber 22. In addition, three radial paths 24 extending from the lubricant storage chamber 22 are formed at angular intervals of 120° around the axis of the stationary shaft 15 to be symmetrical about the axis. These paths 24 are opened in an intermediate portion in which two sets of spiral grooves of a radial bearing are not formed. According to the embodiment shown in Fig. 6, since each lubricant storage chamber has no path communicating with the recess 26 formed near the opening of the rotary structure 12, the lubricant, which fills the lubricant storage chamber and the gaps in the bearing surfaces, does not easily leak into the space in the tube through the gaps between the bearing surfaces, the stationary shaft, and the sealing member, thereby maintaining a stable operation of the dynamic pressure type sliding bearing for a long period of time. In the embodiment shown in Fig. 6, three each of inclined paths 31 and 32 are formed at angular intervals of 120° around the axis of a stationary shaft 15 to be symmetrical about the axis. These paths 31 and 32 are respectively opened in corner portions 29 and 30, of the stationary shaft 15, corresponding to the boundaries between spiral grooves 20 constituting a radial sliding bearing and spiral grooves 21 constituting a thrust sliding bearing. With this structure, a lubricant in a lubricant storage chamber is supplied to low-pressure portions between the respective spiral grooves through the respective paths during an operation of the X-ray tube, thus ensuring a more stable dynamic pressure bearing operation. In an embodiment shown in Fig. 7, the large diameter disk section 15c is provided at the anode side on the stationary shaft 15. The spiral grooves 21 are formed on the outer surfaces of the disk section 15c to constitute the thrust bearing. The lubricant storage chamber 22 have an opening 22a in the gap S1 and is communicated with the gap of the radial bearing. The paths 24 extending in the radial direction of the shaft 15 is opened in the gap S2 between a small diameter section 23 of the shaft 15 and the inner surface of the rotary structure 12 and is communicated with the gaps of the radial bearings. In the embodiment shown in Figs 5 to 7, the lubricant storage chamber 22 and the paths 24, 31 32 are designed to have a total volume which is sufficiently larger than that of the gaps and the spiral grooves of the thrust and radial bearings. In the above described embodiment, the lubricant storage chamber 22 is formed along the center axis of the rotary structure or stationary shaft. It is not limited to a single lubricant storage chamber 22 but a plurality of lubricant storage chambers 22 may be formed. The chamber 22 may not be formed in a straight hole but in a bent hole. A lubricant essentially consisting of of Ga such as a Ga. Ga-In or Ga-In-Sn lubricant, may be used. However the present invention is not limited to this. For example a lubricant consisting of an alloy containing a relatively large amount of bismuth (Bi) e g a Bi-In-Pb-Sn alloy or a lubricant consisting of an alloy containing a relatively large amount of In, e .g., an In-Bi or In-Bi-Sn alloy, may be used Since these materials have melting points higher than the room temperature it is preferable that a metal lubricant consisting of such a material be preheated to a temperature higher than its melting point before an anode target is rotated. As has been described above, according to the present invention, a lubricant storage chamber for storing part of a lubricant is formed in a stationary shaft or a rotary structure on the rotation center axis to communicate with the bearing surfaces of a sliding bearing portion. With this structure a sufficient amount of lubricant required for a long-term operation can be stored, and the lubricant evenly flows in the sliding bearing portion during an operation, thus obtaining a proper lubricating function. That is, a rotary-anode type X-ray tube capable of performing a stable bearing operation for a long period of time can be obtained.

Claims: A rotary-anode type X-ray tube comprising: an anode target (11); a rotary structure (12) which has a rotation center axis and to which said anode target (11) is fixed; a stationary structure (15) having a columnar shape, coaxially fitted in said rotary structure (12), for rotatably holding said rotary structure (12); and a hydrodynamic bearing (19) formed between said rotary structure (12) and said stationary structure (15), having spiral grooves formed as herringbone patterns and a bearing gap between bearing surfaces thereof in which a metal lubricant is applied, the lubricant being in liquid state during rotation of said rotary structure (12); wherein said hydrodynamic bearing (19) includes first and second radial bearing sections arranged along the rotation center axis, and a thrust bearing section (21) ; the first and second radial bearing sections having first high pressure centre regions, respectively, which maintain the lubricant at a high pressure during the rotation of the rotary structure, a first low pressure region being defined between the high pressure regions, which maintains the lubricant at a lower pressure than the high pressure in said first high pressure center regions during the rotation of the rotary structure (12), the thrust bearing section (21) having spiral grooves formed as herringbone patterns which have a second high pressure region and a bearing gap between an end face of the stationary structure (15) and an inner bottom face of the rotary structure (12), characterized in that the spiral grooves of the thrust herringbone patterns are arranged around a center region of said end face of the stationary structure (15), which corresponds to a second low pressure region, the lubricant on the second high pressure region being maintained at a high pressure and the lubricant on the second low pressure region being maintained at a low pressure lower than the high pressure during the rotation of the rotary structure, a lubricant storage chamber (22) for storing part of the lubricant, which is formed in said stationary structure (15), is extended along the rotation center axis and opened in the center region of the end face, and said stationary structure (15) has at least one radial communication path (24) which is opened in the lubricant storage chamber (22) and is opened in the first low pressure region, the lubricant stored in said lubricant storage chamber (22) being supplied to the thrust bearing through the opening in the end face of the stationary structure and also to the first and second radial bearing sections through the radial communication path (24). An X-ray tube according to claim 1, characterized in that said lubricant storage chamber has a volume which is larger than that of the gap of said bearing. An X-ray tube according to claim 1, characterized in that said lubricant storage chamber (22) includes a second path (31) opened in a low pressure region between said thrust bearing (21) and said first radial bearing section which is the radial bearing section closer to the thrust bearing (21), the lubricant stored in said lubricant storage chamber (22) being supplied to the thrust and radial bearing through the second path. An X-ray tube according to claim 1, characterized in that said hydrodynamic bearing (19) further includes a thrust bearing (21) opposite to said thrust bearing at said inner bottom surface of the rotary structure, having a gap and said lubricant storage chamber (22) includes a third path (32) opened in low pressure regions between said thrust bearing (21) opposite to said thrust bearing at said inner bottom surface of the rotary structure and said second radial bearing section which is the radial bearing section more distant to said thrust bearing at said inner bottom surface, the lubricant stored in said lubricant storage chamber (22) being supplied to the thrust and radial bearing through the third path. An X-ray tube according to claim 1, further comprising sealing means (16) for sealing said hydrodynamic bearing (19). An X-ray tube according to claim 5, characterized in that said sealing means (16) includes a cavity (26) defined by said rotary and stationary structures (12, 15) and communicated with said gap of said hydrodynamic bearing (19) and said lubricant storage chamber (22). An X-ray tube according to claim 1, characterized in that said stationary structure (15) has an outer surface, said rotary structure (12) has an inner surface and said hydrodynamic bearing (19) includes spiral grooves formed on the outer surface of said stationary structure (15) or on the outer surface of said stationary structure (15) and the inner surface of said rotary structure (12).

Inventors: ANNO HIDERO, KITAMI TAKAYUKI, ONO KATSUHIRO, SUGIURA HIROYUKI, TAZAWA HIROAKI, ANNO, HIDERO, KITAMI, TAKAYUKI, ONO, KATSUHIRO, SUGIURA, HIROYUKI, TAZAWA, HIROAKI, ANNO, HIDERO, C/O INTELLECTUAL PROPERTY DIVISION, KITAMI, TAKAYUKI, C/O INTELLECTUAL PROPERTY DIV., ONO, KATSUHIRO, C/O INTELLECTUAL PROPERTY DIVISION, SUGIURA, HIROYUKI, C/O INTELLECTUAL PROPERTY DIV., TAZAWA, HIROAKI, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H01J35
IPC codes: H01J35