CATHODE RAYS TUBE WITH FRAME / MASK UNIT
FIELD OF THE INVENTION The invention relates to a cathode ray tube and more particularly to a color / frame selection mask system disposed between the glass cover. The invention finds its application in any kind of tube having a color selection mask and is more particularly adapted to tubes whose mask is held taut by the frame to which it is secured.
Conventional cathode ray tubes comprise a color selection mask located at a precise distance from inside the glass front plate of the tube, on which the red, green and blue luminophore arrangements are deposited. Under the influence of the three electron beams, each one corresponding to a given primary color, the luminophore arrangements allow the reproduction of images on the screen, the mask allows each specific ray to illuminate only the corresponding color luminophore. The color selection mask must be located and held during the operation of the tube in a precise position inside the tube. The support functions are carried out under a rectangular metal frame, generally very rigid. The frame / mask unit is mounted on the front plate of the tube by virtue of an elastic suspension means comprising at least three metal parts welded to the frame, each of the parts comprises an open and spring-forming end having the purpose of fitting into one of the metal studs integrated with the glass front plate. Usually, two arrangements of the suspension medium are used: in the middle of the sides of the frame or in its corners. The current trend is that the tubes with flat faceplate becoming flatter, are directed towards the flat plates completely flat. The manufacture of tubes comprising such faceplate comprises a technology consisting in using a flat mask, held under tension in at least one direction. These structures are described for example in the Patent of
United States No. 4827179. The color selection mask consists of a sheet of metal of a smaller thickness, generally stretched and secured to the frame with considerable mechanical strength, in such a way as to maintain the tension of the mask during the operation of the frame. tube. Since the mask intercepts more than half of the electrons emitted by the electron gun of the tube during the operation of the tube, its temperature rises rapidly, which causes the expansion of at least certain areas of the mask, this gives as a result the phenomenon of reduction in its tension and in its flattening. The mask, which is no longer flat, leads to the fact that the shock points of the three electron beams on the screen of the tube no longer correspond to the ad hoc lines of the luminescent materials, which causes the discoloration of the image reproduced in the screen. A first solution is to use a mask made of a material with a very low coefficient of thermal expansion, such as Invar, whose coefficient is within the range of 1.25 10"6 / C. Then, the mask will expand very little during the operation of the tube, which entails the guarantee of its flattening.As a result, it will be possible to use a low mask tension, for example, in the order of 5 kg / mm2, to guarantee its flattening without fear of loosening during the operation of the tube, the metal support frame will therefore have a mechanical strength in relation to the applied low voltage.A second solution is to use a regular metal mask with a higher coefficient of thermal expansion, of the order of 1.5 10" 5 / ° C, which will have to remain flat with a much higher voltage, possibly ten times the voltage that has to be applied to the Invar mask. This requires the use of supporting a frame whose mechanical strength is considerable, this mechanical strength of the frame is obtained by a considerable amount of the material used. This solution then exhibits the disadvantages of high material and high weight costs. The solution that consists of using a mask made of a material with a low coefficient of thermal expansion also has certain disadvantages, however: the tension mask can not be welded directly to the frame that holds it under tension, unless the frame is made of a material whose coefficient of thermal expansion is similar to that of the mask, and can not be made, in particular of a normal steel. This is because during the process, the unit of - ^ frame / mask is exposed to temperatures above 400 ° C; With these temperatures, the differential in the expansion of the materials, where the mask is welded to the frame can cause local welding faults and therefore, local and general changes in the tension of the mask, the modifications give way to the defects in the colors of the image reproduced on the screen of the tube. - Therefore, this solution needs to use a frame and a mask whose coefficients of thermal expansion are similar and smaller, for example, made of Invar, however, this type of material is much more expensive than regular steel, approximately three times more expensive, which considerably increases the cost of the frame and consequently of the tube manufactured.
BRIEF DESCRIPTION OF THE INVENTION There is a need to allow the use of a mask made of a material of low coefficient of thermal expansion associated with the frame made of conventional steel, in such a way as to obtain a frame / mask unit of low cost and under weight. To achieve this, the frame / mask unit according to the present invention comprises a color selection mask and an essentially rectangular metal frame to support the mask and hold it taut at least in one direction, the mask is made of a material whose coefficient of thermal expansion is less than the coefficient of thermal expansion of the material that constitutes the majority of the frame. Along the at least two opposite sides of the frame are disposed two metal inserts extending along and secured with opposite sides, the metal inserts are made of a material whose coefficient of thermal expansion is similar to that of the mask, the mask is secured with the frame, for example by welding two metal inserts.
BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood with the help of the following description and figures, among which: Figure 1 represents an isometric perspective view of a frame / mask unit, according to the state of the art . Figure 2 illustrates a sectional perspective view of a fourth part of the frame / mask unit in accordance with one embodiment of the invention. Figure 3 shows a metal insert that serves as the interface between the frame and the mask. Figures 4 to 6 show sections through the frame / mask unit illustrating various embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION Figure 1 illustrates a frame / mask unit in accordance with the state of the art, as described in Patent No. GB23088223. The directions along X, Y, Z, respectively, represent the directions along the horizontal and vertical axes and the main axis of the tube perpendicular to the surface of the display screen. The essentially rectangular frame 1 is composed of two sides 4 horizontal long and two vertical short sides 3. The frame 1 holds a mask 2 of color selection taut along at least one direction, parallel to the short sides of the frame. The suspensions 5 are intended to cooperate with the metal studs within the skirt of the front plate of the cathode ray tube, so as to maintain the frame / mask unit at a precise distance from the tube screen. The quality of the welding of the mask 2 along the edges of the sides 4 of the frame is what ensures that the tension of the mask remains in conformity with the predetermined values with, for example, a higher voltage in the middle 7. of the long sides and a smaller tension at the end 8 of the sides 4. The variations in the tension along the sides 4 depend on the size of the frame and its geometry. For example, a mask for a 16/9 tube format, which has a considerable length along the horizontal X axis in relation to its length along the vertical Y axis, will have to be held taut according to a higher proportion of the tension in its half 7, with respect to the tension at its ends 8, better than for a mask for a 4/3 format tube, this is carried out to ensure that the surface of the mask remains at a nominal distance from the screen of the tube without the tendency to detach by its half 7. To avoid these problems related to the thermal expansion of the mask during the steps of the pipe manufacturing process and during the normal operation of the latter, builders tend to use , to manufacture the mask, materials with a low coefficient of thermal expansion. The most used material in this sector is Invar, which consists of an iron-nickel alloy with approximately 36% nickel. In addition, the fact of using a mask made of a material such as Invar, until now, has made necessary that the material that constitutes the frame is also a material with a low coefficient of thermal expansion. This is because it is difficult to weld an Invar mask with the frame made of regular steel; also, the different thermal expansions of the two materials will compromise the resistance of the welds during the manufacturing steps of the tube, which is carried out at more than 400 ° C and possibly culminates in the failure of some of them. However, the use of a frame to avoid the aforementioned disadvantages has the disadvantage of being very expensive. Figure 2 illustrates an embodiment of the invention that does not exhibit this defect. In Figure 2 a quarter of the frame / mask unit seen perspective and in section is represented. The short sides 3 of the frame have an L-shaped cross-section, with a vertical flange 25 extending towards a plane essentially parallel to the screen. In the same way, the long sides 4 have a horizontal flange 15 extended towards the screen of the tube and a vertical flange 14 extended in a plane essentially parallel to the screen of the tube. The frame is made of a conventional steel. A mask 2, made of a material whose coefficient of thermal expansion is less than that of the frame, is stretched in at least one direction, for example the vertical Y plane. The long sides 4 of the frame exhibit a horizontal flange 15, whose horizontal edge 13 has a cross section enlarged towards the interior of the frame and has an essentially rectangular shape. A metal insert 10, for example illustrated in Figures 3 and 4, of a concave cross section, in this case essentially U-shaped, will cover the broadening portion inwardly of the frame of each of the edges 13 of two. flanges 15. The insert is secured to half 7 of each of the opposite sides 4, either by welding 17 or by riveting or bolting, the main feature is that the joining area with the side of the frame is of a small length in relation to the length of the side 4. The ends 18 of the insert extend around the joining area 7 and are then released to move, however this displacement is limited to the displacement in the XY plane, essentially parallel to the tube screen , this is due to the complementary shapes of the opposite surfaces of the edge 13 of the side 4 of the frame and of the insert 10, the edge 13 serves as a guide for the displacement of the insert. The insert is made of a material with a low coefficient of thermal expansion, preferably identical to that of the mask. The mask 2 is tensioned by welding along the vertical Y direction of the outer edges 12 of the insert 10. The tension of the mask will tend to displace each end 18 of the insert in the XY plane so far into an equilibrium position that depends of the resistance to the mechanical bending of the insert and to the initial tension of the mask. During the different steps of tube manufacturing, the frame / mask unit will be exposed to high temperatures; Then the insert and the mask will expand in the same way, since they are made of materials that have the same thermal behavior. Furthermore, the insert 10 is free to expand independently of the side 4 of the frame, whether it be the side of the joining zone 7, this joining area must extend over a small length in relation to the length of the side 4, preferably over a length that does not exceed 25% of the length of the side 4. In this way, the expansions of the frame and the mask are independent of each other. The structure of the invention illustrated by Figures 2 and 3, and in the section through the plane perpendicular to the surface of the mask by Figure 4, also present several advantages: The mask is tensed by exerting an identical force throughout of the totality of its ends, after welding its horizontal ends with the insert, the force exerted in the mask is relaxed. As indicated above, the ends 18 of the insert will bend and reach an equilibrium position for which the tension of the mask will be a maximum of half of it., near zone 7, and a minimum near the free ends 16. The distribution of the values of the tension along the weld edge can be calculated previously, and the structure of the invention makes it possible to have a manufacturing process that is simple in its installation, since the tension exerted on the mask is constant along the entire weld edge. The U-shape of the cross section of the insert cooperates with the facing surface towards the edge 13, so as to perform a guiding function in the XY plane, and to prevent the tension exerted on the ends 18 of the insert by the mask causing This end rotates by means of a lever effect, which would deform the surface of the mask. Therefore, this structure could also be used with certain advantages in frame / mask structures where the frame and the mask are made of identical materials or some that have coefficients of thermal expansion of similar values.
Figure 5 illustrates another embodiment of the invention that results from a different placement of the insert 10. The insert 10 also exhibits a preferably concave inner surface with a U-shaped cross section. The insert is disposed at the edge 51 of the flange 15, in such a way that its concave inner surface surrounds the portion of the edge facing towards the screen of the tube. The insert 10, as in the previous embodiment, is secured to the flange 15 over a zone of small length on either side of the half 7 of the flange. The mask is taut by welding with the insert along its opposite ends. In this way, the thermal expansions of the mask can be carried out independently of those of the frame. In another embodiment of the invention, illustrated in Figure 6, the insert has a solid transverse section 19 and is disposed on the outer portion of the flange 15 in such a way as to hang on the edge 51 of the flange 15. The insert 10 is secured to the flange 15 in its half 7, and the previously tensioned mask, is welded with the portion of the insert hanging from the edge 51. This embodiment has the advantage of being economical to install, and the simple shapes of the profiles of the flanges and of the inserts make it possible to reduce the cost of these parts. The described embodiments are not limited in this case, for example where the mask is tensioned in two directions, horizontal and vertical, the inserts of the type described above, advantageously can be arranged along the flanges of the horizontal sides 4 and 3 vertical sides of the frame * between the mask and the frame flanges.