US3502926A

US3502926A - Rotating anode x-ray tube with magnetic damper

Info

Publication number: US3502926A
Application number: US653824A
Authority: US
Inventors: Shizuo Takano
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1967-03-24
Filing date: 1967-07-17
Publication date: 1970-03-24
Anticipated expiration: 1987-03-24
Also published as: DE1589893A1; GB1192973A

Description

March24, 1970 zuo TAKANO 3,502,926

ROTATING ANODE X-RAY TUBE WITH MAGNETIC DAMPER Filed July 17, 1967 2 Sheets-Sheet l ATTORNEY March 24, 1970 SHIZUO TAKANO 3,502,926

ROTATING ANODE X-RAY TUBE WITH MAGNETIC DAMPER Filed July 1'7, 196'? 2 Sheets-Sheet 2 INVENTOR 5/1/ zua 774/04 N0 BY W a /2 ATTORNEY United States Patent US. Cl. 31360 8 Claims ABSTRACT OF THE DISCLOSURE This specification discloses a rotating anode X-ray tube for diagnosis, which is designed such that the coast rotation of the anode is damped promptly after the completion of radiography, thereby increasing the rotation life of such X-ray tube. For this purpose, a permanent magnet is provided internally of the anode so that the Xray tube per se serves to dampen the coast rotation of the anode with the aid of the electromagnetic force produced by said permanent magnet. Furthermore, use is made of said permanent magnet having a larger outside diameter than that of a bearing and being substantially cylindrical in an integral or divisional form to further increase the rotation life. In this way, there can be easily produced This invention relates to a rotating anode X-ray tube With a magnetic damper, and more particularly it pertains to the structure of the anode portion of such tube.

In general, a rotating anode X-ray tube for diagnosis is designed such that its anode is rotated at a high speed in a highly evacuated seal glass envelope. For this purpose, there is provided ball bearing means (referred to as bearing hereinafter) between the fixed part and the rotatable part of the anode portion. However, since such bearing is situated in a high vacuum and heated to an elevated temperature during the operation of the rotary anode X-ray tube, it requires high-degree techniques for the selection of material, structure and lubrication. In order to ensure a longer rotation life for such bearing, a variety of investigations have heretofore been carried out.

As alternative means for ensuring a longer rotation life, an attempt has been made to dampen the coast rotation of the anode which occurs after the completion of radiography, thereby quickly stopping such rotation. Radiography is generally completed in a shorter period of time than several seconds, while the inertia causes the anode to continue to rotate for several minutes to several tens of minutes even after the radiography has been finished. In the case of a bearing using a good lubricant, the time of such coast rotation becomes longer. Such coast rotation causes the bearing to be wastefully worn down. By quickly stopping such rotation, therefore, it is possible to extend the life of the bearing.

As dampening means for quickly stopping such coast rotation, an attempt is generally made by passing a current through a stator coil provided externally of the rotating anode X-ray tube to produce a magnetic field in the opposite direction to that of the anode rotation. However, this method is disadvantageous in that the electrical circuit for dampening becomes complicated and that the X-ray tube unit is heated to an elevated temperature due to the dampening current flow through the stator coil.

As further dampening means for stopping such coast rotation, a rotating anode X-ray tube with a magnetic 3,502,926 Patented Mar. 24, 1970 damper has already been proposed in which a permanent is incorporated in the anode portion of the rotating anode X-ray tube to dampen the coast rotation of the anode by means of electromagnetic forces produced in the anode. The prior art magnet dampening type X-ray tube, however, has proven less than entirely satisfactory as a means of dampening the nondriven rotation of the anode, owing to its structure, so that a satisfactorily long rotational life with such an arrangement is not attainable.

The present invention is directed to improvements in such magnet dampening type X-ray tubes, the improvements being characterized by a magnet having a larger outside diameter than that of a bearing having a fixed outer ring and a rotatable inner ring, said magnet being substantially in an integrally or divisionally cylindrical form.

The construction of the present invention will now be described in further detail.

Such a magnet dampening type X-ray tube should be designed such that its magnet produces a remarkable rotation-dampening eifect without adversely affecting the speed of anode rotation during the operation for radiography, thereby ceasing the coast rotation as promptly as possible. For this purpose, it is preferred that the magnet has so large an outside diameter as to increase the total flux, thus increasing the dampening torque produced in the rotating portion. In this case, it is required that the magnet be formed of a heat resisting material which can be readily subjected to outgassing. This means that a limitation is imposed upon an attempt to increase the total flux from the standpoint of material selection. Therefore, it is preferable that the magnet is made to have as large an outside diameter as possible.

In general, on the assumtion that bearings have the same shape and size, it will be apparent that the revolutions per minute of the balls and cages thereof are lower in the case where they are provided with a fixed outer ring and a rotatable inner ring than in the case where they are provided with a rotatable outer ring and a fixed inner ring. (The movement of the bearing balls includes two components, namely, revolution and rotation. In this case, revolution is referred to, and likewise hereinafter.) In the former case, the wear of the bearings can be minimized.

In the case of a bearing provided in an elevated temperature environment in a high vacuum. as in a rotating anode X-ray tube, it is effective to use a rotatable inner ring.

Furthermore, it will be apparent that the speeds of the balls and the cage of a smaller-diameter bearing on its circle of rotation are lower than those of a larger-diameter bearing on its circle of rotation since their circles of rotation have different diameters, even though the balls and the cages of these bearings have equal revolutions per minute.

In the case Where a bearing having an outerring 26 mm. in outer diameter and an inside ring 10 mm. in inside diameter, for example, is used in such a manner that the outer ring is rotated and the inner ring is fixed, the balls and the cage make about 0.63 of one revolution respectively for one rotation of the outer ring, while in the case where a bearing having an outer ring 19 :mm. in outer diameter and an inner ring 6 mm. in inner diameter is employed in such a manner that the outer ring is fixed and the inner ring is rotated (this is the case with the structure according to this invention, which will be described hereinafter), the balls and the cages make about 0.38 of one revolution respectively for one revolution of the inner ring. The rotating speeds of the balls and the cage on the rotation circle are further reduced in the latter case since the diameter of the revolution circle of the balls and the cage is smaller in the latter case than in the former case. The inventors know from experience that in a rotating anode X-ray tube the rotation life of its bearing can be made longer by making the rotating speeds of the balls and the cage of the bearing as low as possible.

From the foregoing, it will be readily appreciated that in the case of a rotating anode X-ray tube the rotation life can effectively be made longer by using a bearing having an outer ring of a relatively reduced diameter in such a manner that the outer ring is fixed and an inner ring is rotated within the range of allowable load for the bearing. Although there have already been proposed such rotating anode X-ray tubes using a bearing having an outer ring of a relatively reduced diameter in such a manner that the outer ring is fixed and an inner ring is rotated, it is an object of this invention to provide a magnet dampening type X-ray tube utilizing the advantages of the aforementioned rotating anode X-ray tube and adapted for maximization of the magnet-dampening effect.

This and other objects of this invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a longitudinal sectional view showing the anode portion of the rotating anode X-ray tube with a magnetic damper according to a first embodiment of this invention;

FIGURE 2 is a cross-sectional view taken along line A-A of FIGURE 1;

FIGURE 3 is a longitudinal sectional view showing the anode portion of the rotating anode X-ray tube with a magnetic damper according to a second embodiment of this invention; and

FIGURE 4 is a cross-sectional view taken along line B-B of FIGURE 3.

Referring now to FIGURES 1 and 2, the reference numeral 1 represents a target plate secured to a rotor 2, and the reference numeral 3 denotes a center shaft fixed in the rotor 2 by suitable means such as a screw 4. A bearing housing 5 formed of a non-magnetic material is interposed between the rotor 2 and the center shaft 3, and it is provided integrally with an anode terminal 7 which is air-tightly sealed to one end of an envelope 6. The reference numeral 8 represents a bearing which comprises an outer ring 8a, an inner ring 8b, balls 8c and cage 8d. The outer and

inner rings

8a and 8b are attached to the inner surface of the housing 5 and the outer periphery of the center shaft 3, respectively, thereby holding the center shaft 3 and the housing 5. A member 9 is formed of a ferromagnetic substance, which is attached to the inner periphery of the rotor 2 by means of a screw 10 so as to form a portion of the rotor 2 and rotate with the latter. The reference numeral 11 denotes a cylindrical magnet of which the outer diameter is made larger than that of said bearing 8. The magnet 11 is mounted to the outside of the housing 5 by means of a key 12, a hold cylinder 13 and a screw 14. The polarities of this magnet are as indicated by symbols N and S in FIGURE 2. The ferromagnetic member 9 and the magnet 11 are arranged so that a small gap 15 is defined therebetween.

If it is difiicult to provide a sufiiciently large magnet since the spacing between the inner surface of the ferromagnetic member 9 and the outer surface of the housing 5 is too small, the magnet may be embedded in the housing. This constitutes a second embodiment of this invention as illustrated in FIGURES 3 and 4.

Referring to FIGURES 3 and 4, the reference numeral 21 represents a target plate fixed to a rotor 22, and 23 a center shaft which is fixed in the rotor 22 by suitable means such as a screw 24. 25 is a bearing housing formed of a non-magnetic material which is integral with an anode terminal 27 which is air-tightly sealed to one end of an envelope 26 provided between the rotor 22 and the center shaft. The reference numeral 28 denotes a bearing which comprises an outer ring 28a, an inner ring 28b, balls 28c and a cage 28:1. The outer and inner rings 28a and 2812 are attached to the inner surface of the housing 25 and the outer periphery of the center shaft 23, respectively, thereby holding the shaft 23 and the housing 25. A ferromagnetic member 29 is attached to the rotor 22 by means of a screw 30 to form a portion of the rotor. 31 is a magnet comprising a plurality of

pieces

31a, 31b, 31c and 31d. This magnet is inserted in a hole 32 formed in the housing 25, and it is mounted to the housing 25 by means of a cylinder 33 formed of a ferromagnetic material, a

hold cylinders

34 and 35 formed of a non-magnetic material and screws 36. The outer diameter of the magnet 31 is made larger than that of the housing 25 so that the magnet 31 projects beyond the housing 25, and also the magnet 31 is arranged so as to define a small gap 37 between it and the magnetic member 29. FIGURE 4 shows an example of such magnet 31, which comprises two pairs of magnet pieces of which the directions of magnetization differ, as indicated by symbols N and S. However, it is to be noted that the number of the magnet piece pairs may be increased as desired. In the example as shown in FIGURES 3 a d 4, said plurality of magnet pieces produce the same effect as a single cylindrical magnet since provision is made of the cylinder 33 formed of a magnetic material. Therefore, it is to be noted that this arrangement is effective in the case where the gap between the magnetic member and the housing is small, as described hereinabove.

While this invention is constructed as described above, detailed description will now be made of the operational effect resulting from the first embodiment of this invention as illustrated in FIGURES 1 and 2. Magnetic fluxes emanating from the N-poles of the magnet 11 enter the ferromagnetic member 9 through the small gap 15 and therefrom go to the S-poles of the magnet 11 through the small gap 15. In the case where the rotor 2 having a portion thereof formed of the ferromagnetic member 9 is rotated, the ferromagnetic member 9 cuts the magnetic fluxes emanating from the magnet 11, with the result that an eddy current flows through the ferromagnetic member 9. Thus, the resulting eddy current produces an electromatic force in such a direction as to dampen the rotation of the ferromagnetic member 9, thereby dampening the rotation of the rotor 2 during coast rotation so that the rotor is quickly stopped. Furthermore, a magnetic force produced between the magnetic poles of the magnet 11 and the forromagnetic member 9 also serve to dampen the coast rotation. The inventors have already obtained a rotating anode X-ray tube in which the anode is enabled to make coast rotation as long as several tens of minutes if no dampening device is provided, by using an excellent solid lubricant in the bearing. By providing the magnet as illustrated in FIGURES 1 and 2 on such a rotating anode X-ray tube, it is possible to reduce the time of coast rotation down to about 30 seconds to 3 minutes. Thus, the rotation life of the bearing of a rotating anode X-ray tube can be greatly increased when it is applied to radiography.

In accordance with this invention, as described above, the bearing having a smaller diameter than that of the magnet is used in such a manner that the inner ring is rotated and the outer ring is fixed, thereby increasing the rotation life of the bearing per se without deteriorating the dampening effect produced by the magnet. Thus there can be provided a magnet-dampening type X-ray tube having a much longer rotation life than conventional ones. The magnet which is illustrated in FIGURES l and 2 as being provided in the anode applies a slight dampening force to the anode when the rotation of the anode is to be started. Practically, however, this has no adverse effect on the starting operation in the case where the starting time (time required for the anode to reach a predetermined normal speed) is about 1 second.

It is to be understood that the operational effect as described above can also be produced by the second embodiment of this invention as illustrated in FIGURES 3 and 4.

What is claimed is:

1. A rotating anode X-ray tube with a magnetic damper, comprising a substantially cylindrical permanent magnet mounted to a fixed portion connected with an envelope, and a ferromagnetic member attached to a rotary portion provided in opposing relationship with said permanent magnet, said permanent magnet and said'ferromagnetic member being adapted for acting to dampen the coast rotating of an anode, a ball bearing having a smaller outside diameter than that of said permanent magnet, an outer ring of said ball bearing being attached to a bearing housing fixedly connected with said envelope, and an inner ring of said ball bearing being attached to a center shaft adapted for rotating integrally with a target plate.

2. A rotating anode X-ray tube with a magnetic damper as set forth in claim 1, wherein said substantially cylindrical permanent magnet is divided into at least two magnet pieces, said permanent magnet producing substantially the same effect as a single substantially cylindrical magnet.

3. A rotating anode X-ray tube as defined in claim 1 wherein said substantially cylindrical permanent magnet provides circumferentially spaced alternating poles in a plane perpendicular to the axis thereof so that the flux from said permanent magnet flows through said ferromagnetic member along paths substantially parallel to the plane of rotation of said anode.

4. A rotating anode X-ray tube as defined in claim 2 wherein said magnet pieces each have respective poles positioned radially of one another with respect to the axis of rotation of said anode.

5. A rotating anode X-ray tube as defined in claim 4 wherein the poles of one magnet piece are oriented in an opposite sense to the poles of the magnet piece adjacent thereto.

6. A rotary anode X-ray tube as defined in claim 5 wherein said permanent magnet comprises four magnet pieces disposed at respective orthogonal coordinates about the axis of rotation of said anode.

7. A rotating anode X-ray tube with a magnetic damper, comprising a ferromagnetic member mounted to a fixed portion connected with an envelope, and a substantially cylindrical permanent magnet attached to a rotary portion provided in opposing relationship with said ferromagnetic member, said permanent magnet and said ferromagnetic member being adapted for acting to dampen the coast rotating of an anode, a ball bearing having a smaller outside diameter than that of said permanent magnet, an outer ring of said ball bearing being attached to a bearing housing fixedly connected with said envelope, and an inner ring of said ball bearing being attached to a center shaft adapted for rotating integrally with a target plate.

8. A rotating anode X-ray tube as defined in claim 7 wherein said substantially cylindrical permanent magnet provides circumferentially spaced alternating poles in a plane perpendicular to the axis thereof so that the flux from said permanent magnet flows through said ferromagnetic member along paths substantially parallel to the plane of rotation of said anode.

References Cited UNITED STATES PATENTS 2,141,924 12/1938 Middel 313 2,311,724 2/1943 Atlee 313-60 2,311,725 2/1943 Atlee 3136O 2,332,044 10/1943 Bell 313 6O JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner US. Cl. KR, 442