Patent Publication Number: US-7215071-B2

Title: Color cathode ray tube having a detensioning mask frame assembly

Description:
FIELD OF THE INVENTION 
   This invention generally relates to cathode ray tubes (CRTs) and, more particularly, to a tension mask frame assembly for CRTs capable of detensioning. 
   BACKGROUND OF THE INVENTION 
   A color cathode ray tube, or CRT, includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate panel of the tube and is made up of an array of elements of three different color-emitting phosphors. A shadow mask, which may be either a formed mask or a tension mask having strands, is located between the electron gun and the screen. The electron beams emitted from the electron gun pass through apertures in the shadow mask and strike the screen causing the phosphors to emit light so that an image is displayed on the viewing surface of the faceplate panel. 
   One type of CRT has a tension mask comprising a set of strands that are tensioned onto a mask support frame to reduce their propensity to vibrate at large amplitudes under external excitation. Such vibrations would cause gross electron beam misregister on the screen and would result in objectionable image anomalies to the viewer of the CRT. 
   The mask stress required to achieve acceptable vibration performance is below the yield point of the mask material at tube operating temperature. However, at elevated tube processing temperatures, the mask&#39;s material properties change and the elastic limit of the mask material is significantly reduced. In such a condition, the mask stress exceeds the elastic limit of the mask material and the material is inelastically stretched. When the tube is cooled after processing, the strands are longer than before processing and the mask frame is incapable of tensing the mask strands to the same level of tension as before processing. Another common problem with tension mask frame assemblies occurs when the mask strand material has a lower coefficient of thermal expansion than the mask support frame material. In such a case, tension on the mask strand increases during thermal processing causing more inelastic strain. 
   It is desirable to develop a mask frame assembly that allows tension masks to be effectively detensioned during the thermal cycle used to manufacture a CRT to mitigate stretching of the mask. 
   SUMMARY OF THE INVENTION 
   The present invention relates to color cathode ray tubes having tension masks, and particularly to a CRT having a tension mask frame assembly comprising a mask support frame constructed of a material having a first coefficient of thermal expansion. The mask support frame includes a detensioning member formed from a second coefficient of thermal expansion material and attached along the periphery of the support frame to facilitate detensioning of the mask. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example with reference to the accompanying figures of which: 
       FIG. 1  is a cross sectional view of a CRT showing a tension mask frame assembly. 
       FIG. 2  is a perspective view of the tension mask frame assembly. 
       FIG. 3  is a rear view of the tension mask frame assembly. 
       FIG. 4  is a rear view of the tension mask frame assembly shown at an elevated temperature. 
       FIG. 5  is a front diagrammatic view showing an alternate placement for the detensioning members. 
       FIG. 6  is a front diagrammatic view showing another alternate placement for the detensioning members. 
       FIG. 7  is a front diagrammatic view showing yet another alternate placement for the detensioning members. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a cathode ray tube (CRT)  1  having a glass envelope  2  comprising a rectangular faceplate panel  3  and a tubular neck  4  connected by a funnel  5 . The funnel  5  has an internal conductive coating (not shown) that extends from an anode button  6  toward the faceplate panel  3  and to the neck  4 . The faceplate panel  3  comprises a viewing faceplate  8  and a peripheral flange or sidewall  9 , which is sealed to the funnel  5  by a glass frit  7 . A three-color phosphor screen  12  is carried by the inner surface of the faceplate panel  3 . The screen  12  is a line screen with the phosphor lines arranged in triads, each of the triads including a phosphor line of each of the three colors. A tension mask frame assembly  10  is removably mounted in predetermined spaced relation to the screen  12 . An electron gun  13 , shown schematically by dashed lines in  FIG. 1 , is centrally mounted within the neck  4  to generate and direct three inline electron beams, a center beam and two side or outer beams, along convergent paths through the tension mask frame assembly  10  to the screen  12 . 
   The CRT  1  is designed to be used with an external magnetic deflection yoke  14  shown in the neighborhood of the funnel-to-neck junction. When activated, the yoke  14  subjects the three beams to magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over the screen  12 . 
   The tension mask frame assembly  10 , as shown in  FIG. 2 , includes two long sides  22  and  24 , and two short sides  26  and  28 . The two long sides  22 ,  24  of the tension mask frame assembly  10  are parallel to a central major axis, X, of the tube; and the two short sides  26 ,  28  parallel a central minor axis, Y, of the tube. The two long sides  22 ,  24  and two short sides  26 ,  28  form a continuous planar mask support frame  20  along those major and minor axes. The frame  20  comprises elongated wall portions  23 ,  25  and  27 ,  29  extending along the peripheral edge of the inside and outer surfaces of the long sides  22 ,  24  and short sides  26 ,  28  respectively. 
   The frame assembly  10  includes an apertured tension shadow mask  30  (shown here diagrammatically as a sheet for simplicity) that contains a plurality of metal strips (not shown) having a multiplicity of elongated slits (not shown) therebetween that parallel the minor axis, Y, of the tube. The mask  30  is fixed to a pair of support blade members  40  which are fastened to the frame  20  at mounting locations  33  (as shown best in  FIGS. 3–7 ). The support blade members  40  may vary in height from the center of each support blade member  40  longitudinally to the ends of the support blade member  40  to permit the best curvature and tension compliance over the tension shadow mask  30 . 
   As shown in  FIGS. 3 and 4 , a pair of the detensioning members  31 ,  32  are fixed to the inner peripheral surfaces of the two long sides  22 ,  24  along wall portions  25 . Each of the detensioning members  31 ,  32  are formed of a material which has a higher coefficient of thermal expansion than the frame  20 . The distance a shows a distance between the mounting locations  33  where the support blade members  40  are attached to the frame  20 . It should be understood that  FIG. 3  shows the frame  20  and support blade members  40  at room temperature.  FIG. 4  shows the frame  20  and support blade members  40  at an elevated temperature whereby the distance a between the mounting locations  33  is relatively smaller. The frame  20  is configured such that upon expansion due to elevated temperatures, the long sides  22 ,  24  are caused to bow inward toward the center of the frame  20  along the X–Y plane by the detensioning members  31 ,  32 . This effect occurs because the detensioning members  31 ,  32  have a relatively high coefficient of thermal expansion and expand faster than the long sides  22 ,  24  of frame  20  during heating. The inward bowing of the two long side  22 ,  24  has a detensioning effect on the mask  30  because the distance between the mounting locations  33  become smaller at increased temperatures thereby drawing the support blade members  40  toward each other reducing tension in the mask  30 . It should be understood that the detensioning members  31 ,  32  are preferably fastened to the long sides  22 ,  24  by welding but other suitable techniques may be utilized. 
     FIG. 5  shows a first alternate embodiment in which detensioning members  131 ,  132  are fixed to outer peripheral surfaces of the long sides  22 ,  24  along wall portions  23 . Here, the detensioning members  131 ,  132  have a relatively lower coefficient of thermal expansion than the frame  20 . The detensioning members  131 ,  132  therefore expand at a lesser rate than the long sides  22 ,  24  during heating similarly forcing the mounting locations  33  toward each other to detension the mask  30  attached to the support blade members  40  during heating. 
   Yet another alternate embodiment is shown in  FIG. 6  in which detensioning members  231 ,  232  are fixed to outer peripheral surfaces of the short sides  26 ,  28  along wall portions  29 . Here, the detensioning members  231 ,  232  each have a coefficient of thermal expansion which is relatively higher than that of the short sides  26 ,  28  of frame  20 . The detensioning members  231 ,  232  therefore expand at a greater rate than the short sides  26 ,  28  forcing them to bow outward and causing the long sides  22 ,  24  to bow inward so as to urge the mounting locations  33  toward each other thereby drawing the support blade members  40  toward each other and detensioning the mask  30  during heating. It should be understood that the detensioning members  231 ,  232  may alternatively be fixed to the inner peripheral surface of the short sides  26 ,  28  along wall portions  27  to detension the mask  30  provided the coefficient of thermal expansion of the detensioning members  231 ,  232  material is relatively lower than the frame  20  as described above with reference to  FIGS. 4–5 . 
     FIG. 7  shows yet another alternate embodiment in which each side of the frame  20  includes a detensioning member. In this embodiment, detensioning members  331 ,  332  are applied to the inner peripheral surface of the short sides  26 ,  28  along wall portions  27 . Also, detensioning members  131 , 132  are applied to the outer surfaces of the long sides  22 ,  24  as was described with reference to  FIG. 5 . Detensioning members  131 ,  132  and  331 ,  332  all have a relatively low coefficient of thermal expansion than the corresponding sides of the frame  20  to which they are attached. It should be understood that detensioning members  131 ,  132  may be fixed along the inside surface of long sides  22 ,  24  and detensioning members  331 ,  332  may be fixed along the outside surface of short sides  26 ,  28  with detensioning members having a relatively high coefficient of thermal expansion than the sides to which they are attached. 
   It should be understood that the placement of the detensioning members along the peripheral surfaces of the frame  20  discussed above are exemplary and that other arrangements may be used. In some embodiments the detensioning members may be positioned solely along the wall portions on the outer peripheral surfaces or the inner peripheral surfaces formed by the long sides  26 ,  28  and short sides  22 ,  24  of the frame  20 . Alternatively, one or more wall portions of the frame  20  may include a detensioning member having a coefficient of thermal expansion adapted to cause the distance of the mounting locations  33  to shorten as discussed above thereby drawing the mask support blades  40  toward each other to facilitate detensioning of the mask  30 . 
   The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.