US2141924A - Electrical discharge device - Google Patents

Description

Dec. 27, 1938. H. D. MIDDEL ELECTRICAL DISCHARGE DEVICE Filed Nov. 15, 1937 F L J V I v L W 6 i m t $0M vm s nrw I H m W aww a T o 3 Patented Dec. 27, 1938 UNITED STATES PATENT OFFEE to General Electric Company,

New York a. corporation of Application November 13, 1937, Serial No. 174,388

3 Claims.

The present invention relates to vacuum discharge devices, especially X-ray tubes which employ a rotatable electrode.

In X-ray tubes for radiographic purposes it is 5 desirable to provide relative motion between the electron beam and the target surface, since such motion greatly increases the permissible energy input for a given focal area on the target and thus makes possible the generation of a very in- .o tense X-ray beam. It is difficult, however, to construct a rotatable electrode X-ray tube having a long commercially useful life because of the limitations imposed by bearing requirements.

In particular, because of the need for avoiding the presence of substances of high vapor pressure within the discharge envelope, it is not feasible to provide lubricants for the bearing surfaces.

Such surfaces are, therefore, subjected to considerable abrasion and tend to wear out after a brief period of use. This diiiiculty has been intensified in devices heretofore constructed by the fact that it has been considered necessary to make the driving element connecting with the rotatable electrode of relatively massive construction in order to obtain sufficient driving force.

This obviously increases the load imposed on the bearings and correspondingly decreases the bearing life.

It is a primary object of my present invention to provide in connection with a rotatable electrode discharge device, a driving means of which the rotating parts are of light weight and are so arranged as to minimize the bearing load.

Another disadvantage associated with the massive driving structures of the prior art consists in the relatively long periods required to bring them up to operating speed or to stop them at the termination of an operating period. In many cases the accelerating and decelerating periods may be substantially longer than the period of actual useful operation. Since these periods obviously contribute to bearing wear, it is clear that they play their part in reducing the useful life of the device.

It is a further object of my invention to pro vide a driving structure in which the rotating parts are of such low inertia as to require only short accelerating and decelerating periods.

The features of novelty which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the drawing in which Fig. 1 represents more or less schematically a discharge device suitably embodying the invention; Fig. 2 comprises a fragmentary view in section illustrating the details of one form of my improved driving structure, and Fig. 3 illustrates a modified mounting arrangement of one portion of the driving structure.

Referring particularly to Fig. l, I have shown an X-ray tube comprising an envelope 5 evacuated in a well-known manner (for example, in accordance with the procedure disclosed in Coolidge Patent No. 1,203,495) and enclosing a cooperating cathode and anode. Of these, the former comprises an incandescible filament suitably of tungsten, and an associated metal cyiindcr ii supported on a reentrant envelope stem 6. ihe cylinder 5 is provided with a recessed surface portion 1 which forms a focusing cup for emitted electrons. The anode, which is positioned in opposed relation to the cathode face, comprises a rotatable disk 9 provided with a peripherally extending target surface it, suitably of tungsten.

As is shown more clearly in Fig. 2, the anode 9 is arranged adjacent to the inner extremity of a stationary shaft l2 which extends inwardly from one end of the envelope I. Interposed between the anode and the shaft l2 there is provided an anti-friction bearing structure comprising a pair of ball bearings which are axially spaced with respect to one another and with respect to the anode. Each of these bearings comprises an inner race M, an outer race it and a plurality of interposed anti-friction elements or balls IT. The balls and at least the outer races are preferably constituted of an extremely hard, abrasion-resisting material of the type described in application Serial No. 123,222 filed January 3, 1937 in the name of George Hotaling and assigned to the same assignee as the present invention. The metals therein described comprise precipitation hardened alloys having an optimum aging temperature above the limiting temperature encountered during the manufacture of the discharge device and are further characterized by a Rockwell C hardness of at least about units. Such bearings can be operated for relatively long periods at speeds of from several hundred to several thousand revolutions per minute in the absence of a lubricating film. One example of a particular precipitation hardened alloy which has proven successful in this connection comprises a high speed steel having a typical analysis of about 0.77% carbon, about 18.5% tungsten, about 4.5% chromium, about 1.75%

vanadium, about 1% molybdenum, about 9% cobalt and the remainder iron.

In order to rotate the anode 9 at a desired speed, I provide a driving member 20, which, in the arrangement illustrated, is mechanically connected to the anode by means of a sleeve 2| ex tending between them. As shown, this sleeve is supported by the outer races l5 of the bearing structures so that the weight of the member 20 is also carried by such structures. In order to minimize the load thus imposed on the bearings, the member 29 is formed as a thin walled cylinder which consists entirely or substantially of a non-magnetic metal of sufiicient electrical conductivity to have good torque characteristics when employed as the moving element of an eddy current motor. A particular example of a material which I consider suitable for this purpose comprises copper, although numerous other materials may be employed.

From a manufacturing standpoint it is convenient to form the member 20 as a copper cup which may be spun or otherwise fabricated. With a construction having proportions such as those illustrated and in which the diameter of the cylinder is about 2% inches, I have found a wall thickness of 60 mils to be satisfactory, although other dimensions may be used.

Driving force is imparted to the rotor 20 by the provision of means for producing a rotating magnetic field which traverses or cuts across the walls of the cylinder. Such means may comprise, for example, an electro-magnetic structure including a laminated iron core 23 and an energizing means such as a polyphase Winding 24 applied to the core. It will be understood that the moving magnetic flux produced by this structure will set up eddy currents in the walls of the cylinder 20 and by reaction with such currents will produce a torque tending to rotate the cylinder. With the structure as so far described, an impractically large core and energizing winding might be required to produce sufiicient magnetic flux to impart the desired torque to the rotating system. This diificulty may be overcome in a convenient manner by providing within the cylinder 20 an interfiting body 26 of magnetic material adapted to provide a low reluctance path for the magnetic flux. This body may be laminated as shown or may be of solid construction and is preferably constituted of iron or of a magnetic alloy of iron such as silicon steel. It may be fixedly supported on the shaft I2 as by means of a keyed connection 2'1.

By separating the magnetic and electrically conducting parts of the driving means in the manner shown, I obtain the result that only a relatively light weight rotating element need be employed, and only a slight load is imposed on the supporting bearings. Furthermore, since the member 20 does not have a heavy magnetic structure associated with it, it is inherently of low inertia so as to require only a short time to bring it up to operating speed or to decelerate it from operating speed. The particular arrangement shown, wherein the cylinder 20 is on the opposite side of the bearing structure from the anode 9, is also especially advantageous in that it permits the rotating parts to be statically balanced with respect to the bearing structure. Thus, by properly correlating the weight of the cylinder 20 with that of the anode 9, it is possible to assure that both sets of bearings shall be equally loaded and shall not be subject to unequal wear. Furthermore, due to the relatively simple form of the cylinder 20, it may be balanced dynamically with a high degree of accuracy.

Where a short tube construction is especially desired, the modified arrangement of Fig. 3 is advantageous. In this case, a stationary magnetic core body 39 is supported directly by the glass-tometal seal structure (shown at 3|) so that the core is in effect an integral part of a closure memher for the end of the discharge envelope.

This is especially desirable in that the external accessibility of the core body makes it relatively easy to obtain perfect centering of such member with respect to the stator structure. Such centering may be accomplished, for example, by the use of properly oriented brackets 33 connected at one end directly to the core 30 and at the other end to a supporting frame 34 which is mounted directly on the stator structure. A desired potential may be applied to the anode assembly through a spring contact 35 pressed into engagement with a cooperating part of the tube structure. This part may comprise, for example, the projecting end of a shaft 37, the inner extremity of which serves to support a rotating cylinder 38 and other rotating elements (not illustrated).

While I have exemplified my invention particularly by reference to particular constructions, it will be understood that various further modifications may be made by those skilled in the art without departure from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An X-ray device comprising an evacuated envelope, a cathode supported from one end of the envelope, a stationary shaft extending inwardly from the other end of the envelope, a disklike member forming a rotatable anode adjacent to the inner extremity of the shaft, a pair of ball bearings axially spaced from one another and from the anode for rotatably supporting the anode on said shaft, and means for rotating the anode without imposing a heavy load on said bearings, said means comprising a thin walled hollow cylinder of highly conducting non-magnetic material supported by the bearings, the cylinder being arranged axially at the opposite side of the bearings from the anode so as at least partially to counterbalance the weight thereof, means for producing a rotating magnetic field traversing the wall of the cylinder, and a solid stationary body of magnetic material interfitting within the cylinder and supported on the shaft independently of the bearings, said body filling substantially the entire space within the cylinder so as to provide a continuous magnetic path therethrough.

2. A vacuum discharge device comprising a. sealed envelope, a stationary and a rotatable electrode within the envelope, anti-friction bearings supporting the rotatable electrode, and means for rotating the electrode without imposing a heavy load on the bearings, said means com prising a thin-walled hollow cylinder of non-magnetic, electrically conducting material rotatably supported on said bearings and mechanically connected to the anode, means for producing a rotating magnetic flux traversing the walls of the cylinder, and a stationary solid body interfitting within the hollow cylinder and supported independently of the bearings, said body filling substantially the entire space within the cylinder and being constituted throughout of magnetic material so as to furnish a low reluctance path for the said magnetic flux.

3. A vacuum discharge device comprising a sealed envelope, a cathode within the envelope, a disk-like member forming a rotatable anode adapted to cooperate with the cathode, anti-friction bearings supporting the anode, and means for rotating the anode, the said means comprising a hollow cup-like member of non-magnetic, electrically conducting material supported on the bearings, a mechanical connection between the said hollow member and the anode, means producing a rotating magnetic flux traversing the walls of the hollow member, and. a stationary solid cylindrical body interfitting with the hollow member and peripherally spaced therefrom so as to permit the rotation of the member, the said body filling substantially the entire space within the member and being constituted throughout of magnetic material so as to provide a low reluctance path for the said magnetic flux.

HENDRIK D. MIDDEL.

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