Patent Application
20090097616
This application claims priority under 35 U.S.C. §119(a)-(d) to prior-filed, co-pending French patent application serial number 0758261, filed on Oct. 12, 2007, which is hereby incorporated by reference in its entirety.
1. Field of the Invention
The present disclosure relates to an X-ray tube equipped with a rotating anode cartridge comprising a reinforced sealing system. Embodiments of the claimed invention can be applied to special advantage but not exclusively in the field of X-ray tubes of an X-ray imaging system, such as an X-ray tomography or mammography system. Embodiments of the claimed invention may also be used in the field of non-destructive testing, when very powerful X-ray tubes are used.
2. Description of Related Art
In the field of radiology by X-rays, in particular, the X-rays are produced by an electronic tube equipped with an anode in rotation on a shaft. A powerful electric field created between the cathode and the anode enables the electrons emitted by the cathode to strike the anode, generating X-rays. For this emission, the positive polarity is applied to the anode via its shaft, the negative polarity to the cathode. The insulation of the assembly is assured, in particular, by dielectrics or by an enclosure, partially in glass, of the electronic tube.
When the tube is used at high power, the impact of electrons on the anode has the effect of abnormally heating up said anode. If the power is too high, an emitting track of the anode may be damaged, hollowed out with impact holes. To avoid such overheating, the anode can be rotated, so as to present, in front of the flow of electrons, a constantly renewed and always cold surface.
A motor of the tube therefore drives the shaft of the anode freely in a mechanical bearing. This bearing is situated in an anode chamber. The anode chamber is itself formed in a support of the anode. The bearing is maintained on the one hand by the anode support and maintains on the other hand the shaft of the anode.
In practice, the bearing industrially comprises conventional ball bearings, as opposed to rarely used magnetic bearings. The problem posed by rotating anodes stems from the rapid wear of the metal coating on the ball bearings, when the shaft rotates in the bearing. The lifetime is then around one hundred hours, leading to a period of use of the tube of around six months to a year. To overcome this problem, coating the ball bearings by metal, lead or silver in the form of a thin layer has been envisaged.
To reduce this premature wear of the metal layer, the invention also provides to place a lubricating film at the interface between the surfaces of the ball bearings and the shaft, between the bearing and the shaft of the anode. With this aim, a liquid based on gallium, indium and tin is poured inside the chamber. Such a liquid is chosen because it improves the coefficient of friction, it reduces the noise of impacts between the ball bearings and it increases the transfer of heat, due to the heating up of the anode, towards the fixed part, either by convection or by conduction. Other lubricating liquids are not used because they have poor degassing properties.
At present, the power demanded of electronic tubes is increasing with the aim of improving the diagnosis. This increase in power leads to an increase in the weight of the anode, up to six to eight kilograms. Consequently, the effects within the bearing become critical. Moreover, in a use in a tomodensitometer, continuously rotating at two rotations per second, the bearing undergoes an acceleration corresponding to around eight times the force of gravity g. Rotation speeds of three to four rotations per second are expected. Consequently, the lifetime of the bearing, and therefore of the tube, with ball bearings and the liquid, may be limited over time. Indeed, the liquid may lose its properties and therefore its characteristics as the heating and the friction within the bearing continue.
In addition, the use of a rotating anode must be compatible with three principal constraints. Firstly, the rotation of the anode must be as free and as perfect as possible, and simple dynamic balancing solutions must be provided to prevent the tube from vibrating when the anode rotates. Secondly, the anode must be able to be taken to a high electric voltage compared to the cathode (normally, bearings with steel ball bearings are used for this purpose). Thirdly, the heat produced by the impact of the electrons on the anode target and which propagates in the shaft must be evacuated efficiently.
Patent application FR-A-2 879 809 discloses an assembly in which ball bearings are lubricated by a gallium alloy and a sealing device of this assembly. In this assembly, an X-ray tube cartridge comprises an anode shaft fitted with ball bearings within a chamber of a fixed support. Such bearings are well suited to the considerable centrifugal accelerations undergone by the tube when it is fitted in a tomodensitometer.
The anode shaft is immersed in a liquid alloy in the chamber of the cartridge. The chamber is completely filled with this alloy. The document FR-A-2 879 809 provides that the sealing of the chamber is achieved by a sealing joint placed at the shaft outlet. An example of such a sealing joint is illustrated in
In
The sealing of such a tube will be achieved in two complementary manners. Firstly, for the vacuum tightness, when the anode shaft 10 is not rotating, a space between an interior diameter of the ring 13 and an exterior diameter of the shaft 10 at the point directly in line with this ring 13 is limited. The limit of this space is fixed by the surface tension of the alloy of liquid gallium, indium, tin metal on the material of the shaft 10 and the ring 13. The ring 13 is intended to be fixed when the shaft 10 rotates.
When the shaft 10 rotates, the pressure of the liquid alloy increases. The alloy tends to escape from the chamber and to contaminate the enclosure of the tube. In this case, to confine it within the chamber, the invention provides to equip the surface of the ring 13, which is in contact or that of the shaft 10 directly in line with the ring 13, with a groove 14 of helix relief shape. The pitch of the helix is oriented so that, for a given direction of rotation of the shaft 10, the helix relief behaves like a scraper in front of the surface that rotates before it. Such a scraper tends to push the alloy back towards the chamber.
However, this type of sealing has disadvantages. Indeed, with this type of sealing joint, any small variation in the space between the interior diameter of the ring 13 and the exterior diameter of the shaft 10 leads to a loss of efficiency. Indeed, the increase in this space leads to a leak of the liquid alloy in the enclosure of the tube. A reduction in this space leads to friction.
An aim of embodiments of the invention is to remedy the disadvantages of the techniques disclosed above. To do this, embodiments of the invention propose improving the robustness of such a sealing joint.
The sealing is achieved in an embodiment of the invention in three complementary manners. Firstly, when the shaft rotates, the pressure of the liquid alloy increases. The alloy tends to escape from the chamber and to contaminate the enclosure of the tube. In this case, in order to confine it within the chamber, the invention provides to equip the surface of the ring that is in contact and that of the shaft in the region directly in line with the ring with grooves. These grooves give the liquid alloy a fluid dynamics character, thereby enabling sealing. The invention increases the surface area of the grooves by forming grooves both on the surface of the ring and on that of the shaft, thereby improving the robustness of the sealing.
Secondly, the grooves formed on the surface of the ring and on that of the shaft enable a double-sided joint to be obtained. This double sided joint makes it possible to obtain, for the vacuum tightness, when the anode shaft is not rotating, two spaces limited by the surface tension of the alloy of liquid metal between an interior diameter of the ring and an exterior diameter of the shaft. The advantage of this configuration is to cumulate the effect of the grooves on the two faces of the joint by increasing the surface area of the grooves.
Thirdly, an embodiment of the invention provides to separate the ring from the axis of rotation or the shaft, in order to have a floating ring. The degree of freedom obtained enables a translation of the ring in the axial direction. When the shaft rotates, the ring will be locked by one or several longitudinal cotters. The fact of having a floating ring enables the risk of friction to be eliminated.
Moreover, with this floating ring, the stabilization of the two spaces is achieved in a natural manner. This leads to the creation of less additional heat due to less loss of power.
More precisely, an embodiment of the invention provides an X-ray tube that comprises:
an enclosure; and
in the enclosure, a cathode, an anode situated opposite the cathode and rotating on a shaft, and a fixed anode shaft support,
Embodiments of the invention may comprise one or several of the following characteristics:
the two spaces are symmetrical.
the symmetry of these two spaces is achieved, during the design of the tube, when the ring is fixed to the anode shaft.
the symmetry of these two spaces is obtained in an automatic and natural manner, when the ring is separated from the anode shaft and becomes floating.
the shaft comprises at least one longitudinal cotter capable of locking the floating ring to the shaft, when the anode rotates.
the longitudinal cotter is a metal dowel pin.
the shaft comprises an annular part capable of reinforcing the locking of the floating ring to the anode shaft.
the grooves are helix or spiral relief shape, in which the orientation of the pitch is such that it pushes the alloy towards the chamber, when the anode rotates.
the alloy is a gallium, indium or tin alloy.
the support comprises bearings (27), particularly ball bearings.
Embodiments of the invention may best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings. These drawings are provided as an indication only and in no way limit the scope of the invention. The figures show:
When the anode 18 is supplied with high voltage, electrons are drawn from the cathode 19 and, under the effect of a powerful electric field, strike the anode track 22. Under the effect of this percussion, the anode track 22 constituted of an X-ray emitting material, emits an X-ray 23. The ray 23 exits the tube 15 through a window 24 formed in the wall 17. The window 24 is for example in glass, in a material transparent to X-rays. It is air-tight. The enclosure 16 thus formed is evacuated to form a vacuum in a conventional manner, in particular through an aspiration orifice, not shown, obstructed later by an evacuation pinch off.
To maintain the anode 18 in rotation, the tube 15 is equipped with an anode support 25. This support 25 is hollow and comprises a chamber 26. In the chamber 26, bearings such as 27 assure the anode 18 is maintained by the support 25. These bearings 27 may be ball type bearings. To resolve lubricating and heat conveyance problems from the rotation of the anode 18, it is provided to fill the chamber 26 with a liquid gallium, indium, tin alloy. The shaft 21 is maintained in the chamber 26 by the bearings 27.
Nevertheless, in a preferred manner, said sealing will be achieved in three complementary manners.
Firstly, when the shaft 21 rotates, the pressure of the liquid alloy increases. The alloy tends to escape from the chamber 26 and to contaminate the enclosure of the tube. In this case, to confine it within the chamber 26, the invention provides to equip the surface of the ring 29, which is in contact and that of the shaft 21 in the region directly in line with the ring 29, with grooves.
These grooves give the liquid alloy a fluid dynamics character, thereby enabling the sealing. The pressure of the liquid alloy in the grooves increases the mass of metallic liquid that is going to undergo the centrifugal force. This makes it possible to return the metallic liquid towards the centre of the anode.
In a preferred embodiment, these grooves are in the shape of a helix relief. They can also have a spiral shape. The pitch of the helix is oriented so that, for a given direction of rotation of the shaft 21, the helix relief behaves like a scraper in front of the surface that rotates before it. Such a scraper tends to push the alloy towards the chamber 26.
Secondly, for the vacuum tightness, when the anode shaft is not rotating, a space between an interior diameter of the ring 29 and an exterior diameter of the shaft 21 at the point directly in line with this ring 29 is limited. The limit of this space is fixed by the surface tension of the alloy of liquid gallium, indium, tin metal on the material of the shaft 21 and the ring 29. It appears that this alloy is not very wetting and that this surface tension enables a clearance of around one hundredth of a millimeter, conducive to a good rotation of the shaft 21 and moreover easy to meet industrially. The ring 29 is intended to be fixed when the shaft 21 rotates.
The grooves are formed both on the surface of the ring 29 and on that of the shaft 21, enabling a double sided joint to be obtained. This double sided joint makes it possible to obtain, when the anode shaft is not rotating, two spaces 30 and 31 limited between an interior diameter of the ring 29 and an exterior diameter of the shaft 21 at the point directly in line with this ring 29. The fact of forming grooves on the surface of the ring 29 and on that of the shaft 21 improves the robustness of the sealing. Indeed, the efficiency of the joint is inversely proportional to the square of each space 30 and 31. The advantage of this configuration, as illustrated in
However, with uniquely
When the shaft 21 rotates, the ring will be locked by one or several longitudinal cotters 32. The cotter 32 is a part introduced in the axial direction between the shaft 21 and the ring 29 to prevent any rotation between these two elements. This degree of freedom obtained and the locking by the cotter 32 of the ring 29 enables the movement of the ring and the effect of the grooves to be assured.
Curve 34 represents the loss of power in relation to variations in the limited space 30. Curve 35 represents the back pressure generated by the grooves, when the shaft is rotated.
Defects of the ring due to an unbalanced rotation or a misalignment or a circularity defect of the ring are represented in
The analysis of the curves 34 and 35 is firstly made in the case where the ring is fixed to the shaft then in the case where the ring is floating. In the case where the ring is firmly connected to the shaft, as illustrated in
Curve 35 shows that the efficiency of the joint increases with the defect. Indeed, the back pressure generated to bring the liquid alloy back towards the interior increases. As a result, the double sided joint with a fixed ring is robust by design. The back pressure depends on the dimensions of the two spaces. The more symmetrical these dimensions, the more the efficiency of the joint increases. Thus, the best practice for manufacturing the joint is to assure a symmetrical configuration of the two spaces.
However, the curve 34 shows that with a fixed ring the losses of power increase with the defect. This leads to each defect or movement of the fixed ring increasing the losses of power. This increase creates an additional energy in the joint. This brings about the creation of counter-charge to return to a more stable state.
In the case where the ring is floating, as illustrated in
With this floating ring, the stabilization of the two spaces takes place in an automatic and natural manner. This enables the creation of less additional heat due to less loss of power, compared to the example of
Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. Other embodiments will occur to those skilled in the art and are within the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0758261 | Oct 2007 | FR | national |
