Abstract:
A structure for suppressing arcing in electric current carrying anti-friction bearings operating in a vacuum environment is disclosed having opposed spaced-apart surfaces forming an effective capacitor in parallel with the bearings responsive to temperature to reduce arcing which would otherwise result from current interruption in the bearings.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to electromechanical systems which require the conduction of electrical current through an anti-friction bearing operating in a vacuum environment. More particularly, this invention has particular advantage when incorporated into rotating anode x-ray tubes having ball bearings operating within races in the vacuum environment and which conduct the anode-cathode current of the tube. 
     Such systems have experienced limited life in the past, due in part to spark erosion of the bearings occasioned by random current interruption which has been observed to occur while the equipment is rotating. Although prior art systems, including x-ray tubes, have had opposing conductive surfaces which were relatively moveable, such prior art structures have been observed to have so little capacitance as to be ineffective to suppress arcing. 
     SUMMARY OF THE INVENTION 
     This invention provides for an arc suppressor for use with an electric current-carrying anti-friction bearing operating in a vacuum environment by providing closely spaced apart first and second conductive surfaces forming an effective capacitance in parallel with the bearing, with one surface moveable with respect to the other surface while still acting like a capacitor to reduce arcing in the event of current interruption in the bearing. A further feature of the invention is that the effective capacitance increases with increases in the ambient temperature of the capacitance. 
     It has been found preferably to have the square root of the opposing area of the capacitance surfaces or plates be large in relation to the distance between the capacitance plates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially cut away view of a rotating anode x-ray tube. 
     FIG. 2 is a partial cut away section view of the rotor mounting of FIG. 1 showing a sleeve member mounted to the stationary portion of the tube. 
     FIG. 3 shows an alternative embodiment showing the sleeve member mounted for rotation with the anode mounting shaft. 
     FIG. 4 shows a further alternative embodiment for the present invention having a portion of the shaft enlarged in diameter and a sleeve mounted to the stationary part of the tube. 
     FIG. 5 shows still a further embodiment of this invention having a portion of the rotating equipment enlarged in diameter sufficient to eliminate the need for a sleeve. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a rotating anode x-ray tube 10. Tube 10 has a glass envelope 12 which provides a vacuum environment or interior 14 in which an anode 16 is mounted for rotation. 
     Referring now to both FIGS. 1 and 2, anode 16 is mechanically and electrically connected to rotor 18 and is caused to rotate by a stator (not shown) which couples a rotating field across envelope wall 12a. Electrical connection is made to anode 16 through an anode shank or stud 20. It is to be understood that anode shank 20 has an interior cylindrical aperture 22 which receives and mounts an anode shaft 24 by means of bearings 26a,b. A torque sleeve 28 preferably having high magnetic permeability is mounted on the interior surface of 30 of rotor 18. A stub shaft 32 connects anode 16 to rotor 18. A heat stop 34 preferably connects rotor 18 to anode shaft 24. 
     It is to be understood that stud 20 is electrically conductive and is mechanically bonded to wall 12a to maintain the vacuum integrity of the envelope interior 14. Bearings 26a,b, shaft 24, heat stop 34, rotor 18, and stub shaft 32 are all formed entirely or partiallly of electrically conductive material to provide for a current path to anode 16. Alternatively, only one bearing may be utilized to conduct electrical current with this invention, in applications having only one ball bearing assembly or where the second bearing is electrically insulated from the current carrying path. For example, bearing 26b may be electrically insulated and bearing 26a may be utilized to conduct electric current from shank 20 through an outer race 27a of bearing 26a, one or more balls or rolling elements 27b, and an inner race 27c to shaft 24. 
     As may be seen in FIGS. 1 and 2, anode 16, rotor 18, and associated rotatable parts make up a mechanical assembly 52 mounted for movement on inner bearing race 27c. It is to be understood that this invention has utility in any application requiring conduction of electrical current across a bearing to a mechanical assembly operating in a vacuum environment. 
     Referring now more particularly to FIG. 2, a conductive spacer sleeve 36 is mounted on the stationary anode shank 20. Sleeve 36 is preferably retained between bearing outer races 38a,b. Sleeve 36 has an inner cylindrical wall 40 which is in spaced apart opposed relationship to outer cylindrical surface 42 of shaft 24. An effective dielectric gap or distance 44 is formed between wall 40 and surface 42 by this construction which results in an effective capacitance or arc suppressor 43 in parallel with each of bearings 26a,b. In the event that current flowing through bearings 26a,b is interrupted, the capacitor 46 (formed by the common or shared opposing area of wall 40 and surface 42 acting as plates for capacitor 46) prevents a rapid rise in voltage across the bearings 26a,b, thus reducing or eliminating arcing which would otherwise occur in bearings 26a,b. It has been found preferable to have the ratio of the shared opposing area to the square of the distance 44 between wall 40 and surface 42 be as large as practicable. Making this ratio large will tend to maximize the high frequency bypass path formed in parallel with the open-circuit bearing, thus limiting the rate of rise of voltage across the bearing, reducing or eliminating arcing which would otherwise occur. 
     In order to provide for normal machining tolerances and to allow for thermal expansion, gap 44 is preferably in the range of 0.002 and 0.010 inches. It is a feature of this invention that the effective capacitance increases as the tube anode temperature increases. Since shaft 24 is connected to a high temperature anode 16, during operation as tube 10 heats up, shaft 24 will &#34;grow&#34; due to thermal expansion relative to sleeve 36 and shank 20. This will cause gap 44 to decrease, raising the effective capacitance. 
     Referring now more particularly to FIG. 3, an alternative embodiment of this invention provides for mounting sleeve 36&#39; on inner race 48 for rotation with shaft 24. Distance 44&#39; is the gap between spacer sleeve 36&#39; and inner cylindrical wall 22 in shank 20. 
     Referring now more particularly to FIG. 4, a further alternative embodiment provides for a spacer 36&#34; mounted in a fashion similar to spacer 36 of FIG. 2. Gap 44&#34; is formed between spacer 36&#34; and an enlarged outer cylindrical surface 42&#34; of shaft 24&#34;. In this embodiment one plate of the capacitor 46 is formed by surface 40&#34;. A bearing retainer stop ring 50 may be mounted on the end of shaft 24&#34; to prevent axial movement of bearing 26a. 
     Referring now more particularly to FIG. 5, a still further alternative embodiment of this invention permits elimination of the spacer sleeve by enlarging outer cylindrical surface 42&#34;&#39; to provide for distance 44&#34;&#39; between shaft 24&#34;&#39; and shank 20&#34;&#39;. It is to be understood surfaces 40&#34;&#39; and 42&#34;&#39; are electrically conductive. 
     The capacitance between two concentric cylinders is given by equation (1), where C is capacitance per unit length, and R 1  and R 2  are the radii of the outer and inner cylinders, respectively and ε (8.85×10 -12  farads/meter) is the permittivity of free space. ##EQU1## A generalized version of equation (1) may be found at Page 76-78 of Static and Dynamic Electricity, Smythe, McGraw-Hill (1968). 
     Equation (1) may be simplified to Equation (2), again where C is the capacitance per unit length, and 1nP is the natural log of the ratio or proportion P of the outer cylinder radius to the inner cylinder radius R 1  /R 2 . ##EQU2## 
     A simple approximation formula for the capacitance (in farads) of two closely-spaced concentric cylinders is given by equation (3), where D is the mean diameter of the cylindrical dielectric gap, d is the length of the gap (from plate-to-plate) and l is the cylinder length, all in meters. ##EQU3## 
     The invention is not to be taken as limited to all the details thereof as modifications and variations thereof may be made without departing from the spirit or scope of the invention. For example, the effective gap or distance 44 between members forming the effective capacitance of this invention may take various forms such as being stepped or tapered. There will, however, be an effective gap or distance equivalent to any such more intricate mechanical shapes. Furthermore, the geometry of the plates of the effective capacitor shunting the bearing may take various geometric shapes such as concentric cylinders or cones or parallel and opposing disks, provided that one plate of the capacitor is free to move with respect to the other plate without substantially changing the effective distance therebetween. It is to be understood that it is within the scope of this invention to have plates that may be non-concentric or non-parallel resulting in a gap which varies as a function of angular position with respect to the axis of rotation of the movable plate since such an arrangement will still have an equivalent or effective dielectric gap between such plates. It is to be further understood that it is within the scope of this invention to have a capacitance which varies as a function of the rotational position of the movable plate in a structure which is not angularly restricted, provided that the minimum capacitance contributes significantly to reducing arcing in the bearing or bearings which would otherwise occur.