US3855492A

US3855492A - Vibration reduced x-ray anode

Info

Publication number: US3855492A
Application number: US00417464A
Authority: US
Inventors: H Langer; V Sollima
Original assignee: Machlett Laboratories Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1973-11-19
Filing date: 1973-11-19
Publication date: 1974-12-17
Anticipated expiration: 1991-12-17
Also published as: CA1004279A; DE2454823C2; DE2454823A1; GB1461008A

Abstract

An x-ray tube of the rotating anode type comprising a rotor and bearing assembly including a rotatable rotor shaft which is connected to and supports an anode assembly for rotation therewith, the interconnected parts of the rotor and anode assemblies being substantially matched as to thermal conductivity, expansion, and yield strength at elevated temperatures and low vapor pressures, thus providing improved resistance to system imbalance which is incurred during processing and operation of the tube.

Description

Unite States Patent Langer et al.

[ Dec. 17, 1974 VIBRATION REDUCED X-RAY ANODE Inventors: Helmut Langer, Springdale; Vincent Sollima, Stamford, both of Conn.

Assignee: The Machlett Laboratories,

Incorporated, Springdale, Conn.

Filed: Nov. 19, 1973 App]. No.: 417,464

3,710,162 l/l973 Bougle 313/60 Primary Examiner-John Kominski Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or FirmI-Iarold A. Murphy; Joseph D. Pannone; John T. Meaney [57] ABSTRACT An x-ray tube of the rotating anode type comprising a rotor and bearing assembly including a rotatable rotor shaft which is connected to and supports an anode assembly for rotation therewith, the interconnected parts of the rotor and anode assemblies being substantially matched as to thermal conductivity, expansion, and yield strength at elevated temperatures and low vapor pressures, thus providing improved resistance to system imbalance which is incurred during processing and operation of the tube.

5 Claims, 2 Drawing Figures I if ease s YIII, III

VIBRATION REDUCED X-RAY ANODE BACKGROUND OF THE INVENTION In an x-ray tube of the rotating anode type, imbalance of components is undesirable and detrimental. Operation of an imbalanced rotor-anode structure in such a tube leads to many undesirable factors, in particular excessive noise and vibration and production of poorly defined focal spots, and often results in premature failure of the x-ray tube. In addition, some of these factors, such as noise and vibration, often cause discomfort in patients.

Although in the manufacture of x-ray tubes the rotating anode and rotor assemblies undergo a balancing operation prior to assembly, in later stages of tube processing and subsequent heating of all components of the device, and during actual operation of the x-ray tube, expansion differences, deformation of mounting components, etc., cause imbalance, with the result that vibration and noise levels show an undesirable and sometimes marked increase.

In normal practice the anode-rotor assembly is secured to the tube bearing system by means of screws which are tightened to a predetermined torque and are often secured in a manner which should eliminate loosening during heat processing. However, in actual application, certain mechanical changes, caused by yielding, take place and result in imbalance and high displacement levels.

SUMMARY OF THE INVENTION In accordance with this invention, a greatly improved bearing-target system for rotating anode x-ray tubes has been achieved by forming interconnecting portions of the system of materials which are substantially matched to possess low thermal conductivity, low thermal expansion, and high yield strength at elevated temperature and low vapor pressures. It has been found and demonstrated that materials of the iron-nickelcobalt family or alloys thereof when used for the target nosepiece, the rotor plug, the bearing nailhead, and the mounting screws, will result in greatly reduced dis placement levels during all thermal processing of an x-ray tube.

Typically, with this invention displacement levels are decreased by an average of about db and the system remains at a well-balanced state during the operational life of the tube.

In a typical construction of an x-ray tube targetrotor-bearing assembly embodying this invention the nosepiece is made from a high melting point refractory metal, preferably molybdenum or an alloy thereof such as TZM (titanium-zirconium-molybdenum alloy containing about 99.5% molybdenum). The nosepiece is brazed to the rotor plug, which plug is made of Fe-Ni- Co alloy as are the bearing nailhead and the mounting screws by which the rotor plug is secured to the bearing nailhead.

Rotating anode x-ray tubes having target-rotorbearing assemblies made with such matched components have shown consistently lower vibration and noise levels, both after heat processing and during operational life.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objectives of this invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein FIG. 1 is a side elevational view partly in axial section of an x-ray tube embodying the invention; and

FIG. 2 is an enlarged axial sectional view of the interconnected anode-rotor-bearing assembly of the tube shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to the drawings, there is shown in FIG. 1 a rotating anode x-ray tube 10 which includes an evacuated envelope 12 having a bulb portion 14 at one end and a reduced diameter elongated neck portion 16 at its other end.

Within the envelope bulb portion 14 is an electronemitting cathode 18 which is fixed by a plate 20 to one end of the cathode support 22. Support 22 is sealed to the envelope in a hermetic fashion and from it extend leads 24 whereby the filament or filaments (not shown) in the cathode structure are connected to an external source of filament power. The cathode and its supporting structure are not shown and described in detail since they may be conventional in construction and do not in themselves constitute an important part of this invention.

The cathode 18 is located so that in operation it will direct a beam of electrons onto an inclined edge portion 26 of a disclike anode target 28. In operation of the tube, the target 28 is made to rotate about its axis so as to constantly present a new surface to the electron beam from the cathode 18. In the normal operation of a tube of this type, electrons impinging upon the surface 26 will cause the generation of x-rays which will pass out of the tube through the adjacent side wall of the envelope.

For rotation of the target 28 there is provided a shaft 30 which extends longitudinally within the envelope neck portion 16, which shaft 30 is rotated by inductive means (not shown) located in encircling relation to the envelope neck portion 16. Shaft 30 is mounted by ball bearing units 32 within a fixed hollow cylindrical axially extending support member 34 which is sealed at its lower end by a collar 36 to a reentrant end portion 38 of the envelope neck portion 16. A tubular spacer 40 extends between the outer bearing races for location purposes. The lower inner end of the bore within the support 34 is provided with a circumferential ledge 42 upon which the lower bearing outer race rests, and a set screw 44 in an upper end portion of the support 34 engages the upper edge of the upper bearing outer race for retaining the assembly in place.

Encircling the support 34 in spaced relation therewith is a rotor skirt 46 which is secured at its upper end to a rotor plug 48. Rotor plug 48 is attached by mounting screws 50 to a bearing nailhead" 52. Nailhead 52 is a ring-shaped element which is rigidly fixed as by brazing to a reduced upper end portion 54 of the bearing shaft 30.

A connecting member or nosepiece 56 has a flanged lower end which is fitted into and closely embraced by the rotor plug 48, and is brazed thereto. The other end of the nosepiece 56 extends through a hole in the center of the target 28, with an enlarged flange portion 58 thereof providing a seat upon which the target rests. The adjacent end of the nosepiece extends above the target surface and is threaded to receive a nut 60 by means of which the target 28 is firmly held against the flange portion 58. The nosepiece 56 is preferably provided with a central bore 62 to reduce material and thereby provide a heat dam, thus reducing conductivity of heat downward through the structure and into the bearings.

It will be understood that when inductive forces act upon the rotor structure, rotation thereof results. By this means the target can be rotated at high speeds such as 10000 rpm, for example.

Conventionally the nosepiece 56 is made of molybdenum, the nailhead 52 is made of stainless steel, the rotor plug 48 is made of Kovar, and the screws 50 are made of stainless steel. In some older devices the nosepiece 56 is made of molybdenum while the nailhead 52 is made of stainless steel, and the rotor plug 48 and screws 50 are made of iron. When such conventional and older devices were rotated at high speeds such as 10,000 rpm, for example, it was found that extremely undesirable vibration levels were reached, with consequent noise, especially when the parts of the devices assumed relatively high operating temperatures.

In accordance with this invention, it has been found that such vibration and resultant noise may be reduced by as much as five decibels by making the nosepiece 56, rotor plug 48, nailhead 52 and screws 50 of materials which are relatively closely matched to possess low thermal conductivity, low thermal expansion, and high yield strength at elevated temperatures and low vapor pressures. Such a material is the iron-nickel-cobalt family. It has been found that TZM may be used as the material for the nosepiece 56 while Kovar may be used Present Invention Part Material Exp. Coefficient Nosepiece 56 TZM Sl X lO' /oC Rotor Plug 48 Kovar 79 X l /oC Nailhead 52 Kovar 79 X l0"/0C Screws 50 Kovar 79 X l0' /oC From the above chart it will be seen that all four components of the structure have good thermal matching. Additionally, Kovar also provides lowered heat conductivity, thereby achieving a cooler bearing system.

Assemblies of prior construction have an average of about five decibels greater displacement levels and upon becoming heated become imbalanced. Such prior assemblies also tend to increase in vibration amplitudes during the operational life of the x-ray tube, believed to be caused by comprising unmatched components.

The following chart illustrates in a conventional structure the linear expansion coefficient for the referred-to components in a temperature range of about from 30-600C:

Conventional Structure Linear expansion coefficients in another older structure in the same temperature range are illustrated in the following chart:

Older Structure Part Material Exp. Coefficient Nosepiece 56 Molybdenum 51 X l0' /oC Rotor Plug 48 lron 13] X lO /oC Nailhead 52 Stainless Steel l X l0"/oC Screws 50 lron 131 X l0"/0C From the foregoing it will be understood that in accordance with the teachings of this invention an improved x-ray tube structure has been achieved by making several of the component parts thereof of materials matched as to several physical properties whereby system imbalance is greatly reduced by reduction in differential thermal expansion when the tube components are heated during tube processing and during actual operation of the tube.

It will be understood, however, that various modifications and changes may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claims. Accordingly, all matter shown and described is to be interpreted as illustrative and not in a limiting sense.

We claim:

1. An x-ray tube comprising an envelope containing a rigid anode supporting structure, a rotor structur rotatably mounted on said supporting structure, and an anode structure having a portion engaging and secured to a portion of said rotor structure for rotation therewith, said portion of said anode structure being titanium-zirconium-molybdenum alloy containing about 99.5% molybdenum, and the engaged portion of the rotor structure being of material in the Fe-Ni-Co family.

2. An x-ray tube of the rotating anode type comprising an evacuated envelope containing a fixed supporting structure, a rotatable shaft within said supporting structure, bearings between said shaft and supporting structure, a portion of said shaft extending beyond one end of the supporting structure, a rotor structure including a portion secured to said portion of the shaft for rotation therewith, and an anode structure including a portion secured to said rotor structure, said portion of the anode structure being of titanium-zirconium-molybdenum alloy containing about 99.5% molybdenum and the rotor structure and shaft being of materials in the Fe-Ni-Co family.

3. An x-ray tube of the rotating anode type comprising an evacuated envelope containing a hollow axially extending target support fixed at one end to said envelope, a shaft extending axially within said support and having one end extending beyond the adjacent end of the support, bearings rotatable mounting said shaft in said support, a nailhead on said end of the shaft, a rotor structure mounted on said shaft for rotation therewith 4. An x-ray tube as set forth in claim 3 wherein the material of said nosepiece is titanium-zirconiummolybdenum alloy containing about 99.5% molybdenum, and the material of said rotor plug and nailhead is Fe-Ni-Co alloy.

5. An x-ray tube as set forth in claim 3 wherein said rotor plug is secured to the nailhead by screws of Fe- Ni-Co alloy.

Claims

1. AN X-RAY TUBE COMPRISING AN ENVELOPE CONTAINING A RIGID ANODE SUPPORTING STRUCTURE, A ROTOR STRUCTURE ROTATABLY MOUNTED ON SAID SUPPORTED STRUCTURE, AND AN ANODE STRUCTURE HAVING A PORTION ENGAGING AND SECURED TO A PORTION OF SAID ROTOR STRUCTURE FOR ROTATION THEREWITH, SAID PORTION OF SAID ANODE STRUCTURE BEING TITANIUM-ZIRCONIUM-MOLYBDENUM ALLOY CONTAINING ABOUT 99.5% MOLYBDENUM, AND THE ENGAGE PORTION OF THE ROTOR STRUCTURE BEING OF MATERIAL IN THE FE-NI-CO FAMILY.

3. An x-ray tube of the rotating anode type comprising an evacuated envelope containing a hollow axially extending target support fixed at one end to said envelope, a shaft extending axially within said support and having one end extending beyond the adjacent end of the support, bearings rotatable mounting said shaft in said support, a nailhead on said end of the shaft, a rotor structure mounted on said shaft for rotation therewith comprising a rotor plug secured to said nailhead and a rotor skirt connected at one end to said plug and encircling said support, and a target structure attached to said rotor structure for rotation therewith comprising a target disc and a nosepiece connecting said disc to said rotor plug, said nosepiece being of titanium-zirconium-molybdenum alloy containing about 99.5% molybdenum and said rotor plug and nailhead being of Fe-Ni-Co alloy.

4. An x-ray tube as set forth in claim 3 wherein the material of said nosepiece is titanium-zirconium-molybdenum alloy containing about 99.5% molybdenum, and the material of said rotor plug and nailhead is Fe-Ni-Co alloy.

5. An x-ray tube as set forth in claim 3 wherein said rotor plug is secured to the nailhead by screws of Fe-Ni-Co alloy.