Abstract:
A color cathode ray tube comprising a perforated molded mask curved inward in the middle between its two opposite sides and having round corners, and a support frame curved outward in the middle between its two opposite sides and having round corners, wherein the corners of mask are respectively brought into contact with the corners of the mask and the frame. Therefore, the alignment accuracy between a shadow mask assembly and phosphor film and the magnetic shielding of the color cathode ray tube can be improved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to color cathode ray tubes for use with color display devices in color television sets and computer monitors or the like, and more particularly to those color cathode ray tubes with improvements in shape of a shadow mask assembly used therein 
     2. Description of the Related Art 
     As shown in FIG. 1, a color cathode ray tube is configured including a glass valve comprising a panel portion  1  and a neck portion  2  as well as a funnel portion  3 , a phosphorus-coated surface or fluorescent film  4  as formed on the inner surface of the panel portion  1 , a shadow mask assembly  5  disposed at a specified location opposing and adjacent to the phosphor film, an internal magnetic shield (not shown) as formed into a shape along the inner wall of the funnel portion, and an electron gun  12  for production of three electron beams  13  toward the phosphor film  4 . There are co-provided and disposed outside of the neck portion  2  a magnet  9  for purity adjustment and a center beam static convergence adjustment magnet  10  plus a side beam static convergence adjustment magnet  11 , wherein said electron beams  13  operate exhibiting deflection in the presence of a magnetic field that is created by a deflection yoke  8  as disposed outside of the glass valve to thereby penetrate the perforated molded mask  6  letting the specified phosphor film  4  emit light. 
     In the color cathode ray tube, whether the accuracy of positional alignment of the shadow mask assembly  5  and the phosphor film  4  is good or bad can directly affect whether resultant display images are good or not in quality. More specifically, display images are deteriorated when deformation or the like takes place due to mechanical distortion and/or thermal distortion at the perforated molded mask  6  and the support frame  7  constituting the shadow mask assembly  5 . 
     FIGS. 2 to  4  show one example of the shadow mask assembly of conventional color cathode ray tube. In FIG. 2 the reference numeral  40  designates such shadow mask assembly,  41  denotes a perforated molded mask,  44  represents a support frame,  45  indicates a frame-shaped portion,  46  shows a spring, and  47  is a projection (referred to as the “dowel” hereinafter) as formed at the frame-shaped portion. The perforated molded mask  41  comprises a curved surface portion  42  which has multiple electron beam penetration holes (not shown), and a skirt portion  43  that down goes from four peripheries of the curved surface portion  4 , wherein the support frame  44  is structured from a rectangular frame-shaped portion  45  and springs  46  attached to the outer wall of the frame-shaped portion  45  for permitting attachment of the support frame  44  to inside of the panel portion (not shown) as well as a plurality of dowels  47  attached to the inner wall of the frame-shaped portion  45  for causing the inner wall of the frame-shaped portion  45  and the skirt portion  43  to come into contact engagement with each other. 
     FIG. 3 is a diagram showing a cross-sectional view at a location indicated by A-A′ in FIG.  2 . FIG. 4 is a sectional diagram at a location represented by B-B′ in FIG.  2 . The support frame  44  is of a nearly rectangle shape with nearby portions of four vertexes of such rectangle being connected by curved planes, which may resemble in shape substantially the L-shaped cross-section with the thickness t as shown in FIG.  3  and FIG.  4 . 
     The prior art shadow mask assembly  40  has the dowels  47  that are formed on the side wall of the frame-shaped portion  45  of the support frame  44  and extend toward inside of said substantial rectangle. Here, the height h of the dowels  47  as measured from inside of the sidewall of the frame-shaped portion  45  will be called the “embossing amount”. As shown in FIG.  3  and FIG. 4, the support frame  44  is of the shape which has those embossing portions of length values  1   1 ,  1   2  in the inside direction thereof. While lengthening these embossing portion lengths  1   1 ,  1   2  may enhance the mechanical strength of the support frame  44 , an excessive increase in length is not achievable due to limitations received from the cathode ray tubes effective screen size. 
     Since said prior art shadow mask assembly  40  is formed when press-machining the perforated molded mask  41  as was shown in FIG. 2 in such a manner that respective upper edges (peripheries of the curved surface section  42 ) of the skirt portion  43  are slightly curved outwardly at the center portions as compared to the corner sections thereof, the clearance with respect to a contact plane between the skit portion  43  and the frame-shaped portion  45  becomes larger at the corners of the skirt portion  43  thereby necessitating additional provision of the dowels  47  at respective corners of the frame-shaped portion  45  while at the same time suffering from a problem in that the resulting perforated area of the curved surface portion  42  with respect to the size of the support frame  44  gets narrower to the extent corresponding to an increase in clearance. 
     In said prior art shadow mask assembly  40  the contact surfaces between the skirt portion  43  and the frame-shaped portion  45  become only those layout portions of the dowels at the corners thus limiting the substantial contact regions between the skirt portion and the frame-shaped portion, which leads to a problem that the magnetic resistivity, which is important among magnetic shield characteristics of color cathode ray tubes, is significant disadvantageously at said corners of the perforated molded mask  41  thus letting any effective magnetic shield characteristics be hardly obtainable. 
     Another problem faced with said prior art shadow mask assembly  40  is that since the skirt portion  43  is designed so that respective upper edges thereof (peripheries of the curved surface portion  42 ) are formed to be slightly curved outwardly at the center portion as compared to corners, the workability remains inferior when inserting for assembly the perforated molded mask  41  into the support frame  44  while simultaneously associating with the risk of deformation of the perforated molded mask  41  during assembly processes. 
     An object of the present invention is to provide a color cathode ray tube with a shadow mask assembly offering increased robustness against deformation due to mechanical and/or thermal distortion, which structure is high in accuracy of positional alignment with an associative fluorescent or phosphor film and also excellent in magnetic shield characteristics. 
     SUMMARY OF THE INVENTION 
     The present invention has attained said object by arranging the shadow mask assembly for use with the color cathode ray tube in a way which follows. 
     The shadow mask assembly used in the color cathode ray tube of the present invention is of the inward projected shape with substantially central portions of two side lines which nearly oppose each other at a skirt portion of a perforated molded mask being curved inwardly and with four corner sections of the perforated molded mask each being outwardly curved thus resembling approximately an arc-like shape of a radius R, while substantially central portions of two opposite side lines of a support frame are each curved outwardly to have an outward projected shape with four corners of the support frame each having a substantially arc-like shape of the radius R due to outward curvature and with a dowel being substantially centrally formed on each side line of the support frame. The distal end of such dowel is placed inside of a rectangle as formed by straight lines which connects together those points at which the corners of said perforated molded mask are in contact engagement with the support frame. 
     The shadow mask assembly used in the color cathode ray tube of the present invention is such that the corners of said support frame and the corners of said perforated molded mask are contacted together whereas the dowel of said support frame and the skirt portion of said perforated molded mask are in contact engagement with each other. 
     As the shadow mask assembly used in the color cathode ray tube of the present invention is of the outward projected shape with nearly central portions of the two opposite sides of the support frame being curved outwardly, it hardly receives any limitations of the embossing amount due to the effective screen size of the cathode ray tube thus making it possible to lengthen the embossing amount of the support frame while at the same time enabling enlargement of the inward embossing amount h of the dowel as formed substantially centrally on the side of the support frame, which in turn makes it possible to increase the strength of the support frame. 
     The shadow mask assembly used in the color cathode ray tube of the present invention decreases in magnetic resistivity rendering excellent the resulting shield characteristics because of the fact that the skirt portion of the perforated molded mask and the support fame are in contact engagement with each other at four corners thereof over a wide area. 
     The shadow mask assembly used in the color cathode ray tube of the present invention is such that since the distal end of the dowel is placed inside of the straight fine which connects together those points whereat the corners of the perforated molded mask and the support frame are in contact with each other, the distal end of the dowel and the skirt portion of the perforated molded mask are close in position to each other to thereby reduce distortion occurring at the perforated molded mask when bringing the skirt portion into contact with the dowel. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     FIG. 1 is a diagram for explanation of an overview of the structure of a color cathode ray tube in the prior art. 
     FIG. 2 is a diagram for explanation of a configuration of a shadow mask assembly of the prior art color cathode ray tube. 
     FIG. 3 is a diagram showing a cross-sectional view of the shadow mask assembly of the prior art color cathode ray tube as taken along line A-A′ in FIG.  2 . 
     FIG. 4 is a diagram showing a cross-sectional view of the shadow mask assembly of the prior art color cathode ray tube as taken along line B-B′ of FIG.  2 . 
     FIG. 5 is a diagram for explanation of an overview of the structure of a shadow mask assembly of a color cathode ray tube in accordance with one preferred embodiment of the present invention. 
     FIG. 6 is a diagram showing in plan view an arrangement of the shadow mask assembly of the color cathode ray tube in accordance with the embodiment of the present invention. 
     FIG. 7 is a diagram showing a sectional view of the shadow mask assembly of the color cathode ray tube in accordance with the embodiment of the present invention as taken along line C-C′ of FIG.  6 . 
     FIG. 8 is a diagram showing a sectional view of the shadow mask assembly of the color cathode ray tube in accordance with the embodiment of the present invention as taken along line D-D′ in FIG.  6 . 
     FIG. 9 is a plan view diagram for explanation of a positional relationship of a support frame and dowels as well as a molded mask employed in the shadow mask assembly of the color cathode ray tube in accordance with the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A detailed explanation will now be given of the present invention based on some preferred embodiments thereof. 
     (Embodiment 1) 
     FIG. 5 is a diagram schematically depicting an overview of a shadow mask assembly of a color cathode ray tube in accordance with one preferred embodiment of the present invention. The shadow mask assembly  5  is equipped with a perforated molded mask  6  having a plurality of openings or holes therein, a support frame  7  for fixedly supporting this perforated molded mask  6 , and springs  7   b  for use in securing the support frame  7  to inner side surfaces of the panel. In FIG. 5 the reference character  6   a  designates a curved surface portion of the perforated molded mask;  6   b  denotes a skirt portion of the perforated molded mask;  6   b   1 ,  6   b   2 ,  6   b   3 ,  6   b   4  indicate four side lines at the skirt portion  6   b ;  7   a  is a frame-shaped portion of the support frame  7 ;  7   a   1 ,  7   a   2 ,  7   a   3 ,  7   a   4  are four sides in the frame-shaped portion  7   a ;  7   b  is a spring;  7   c , an abutting/locking protuberance member known as a dowel. 
     In FIG. 5 the perforated molded mask  6  is structured from the curved surface portion  6   a  having a great number of electron beam penetration holes (not shown) and the skirt portion  6   b  extends downwardly from the periphery of the curved surface portion  6   a . The support frame  7  is composed of the frame-shaped portion  7   a  which is formed and disposed so as to have substantially a rectangular shape by the presence of four side lines  7   a   1 ,  7   a   2 ,  7   a   3 ,  7   a   4 , and dowels  7   c  which are disposed inside of wall surfaces of the four sides  7   a   1 ,  7   a   2 ,  7   a   3 ,  7   a   4  of the frame-shaped portion  7   a , respectively. 
     As shown in FIG. 6, the perforated molded mask  6  of the illustrative embodiment is machined, when forming the skirt portion  6   b  by press machining techniques, in a manner such that with regard to the skirt portion  6   b  of the four sides  6   a   1 ,  6   a   2 ,  6   a   3 ,  6   a   4 , a respective one of the central or midway portions of these four sides is curved inwardly (in the center direction of the perforated molded mask  6 ) relative to the corner sections of the perforated molded mask  6 . In addition, each corner of the perforated molded mask  6  is of substantially an arc-like shape of a radius R which is curved outwardly. The corners of the frame-shaped portion  7   a  are also formed into a nearly arc-like shape with radius R being curved outwardly in a manner similar to the corners of the perforated molded mask  6 , while causing the corners of the perforated molded mask to be in internal contact therewith The four sides  7   a   1 ,  7   a   2 ,  7   a   3 ,  7   a   4  of the frame-shaped portion  7   a  are molded so that each central or “midway” portion is curved outwardly than the corners. 
     A cross-section of the midway portion (C-C′ part in FIG. 6) of the frame-shaped portion  7   a  of the support frame  7  is illustrated in FIG. 7, while a cross-section of the corner (D-D′ part in FIG. 6) of the frame-shaped portion  7   a  of the support frame  7  is depicted in FIG.  8 . As shown in FIG. 7, the midway portion of the frame-shaped portion  7   a  has its thickness t and exhibits in general the L-shaped cross-section having an extension or “overhang” of the length  1   3  which extends inwardly while providing the dowel  7   c  of the convex shape that is projected inwardly. Additionally, although as shown in FIG. 8 the corner of the frame-shaped portion  7   a  also has the thickness t and exhibits the nearly L-shaped cross-section having an overhang of length  14  that extends inwardly, no dowels are provided at the corners in the embodiment shown herein. 
     See FIG. 9, which shows details of a layout of the dowel  7   c  and the frame-shaped portion  7   a  of the support frame  7  as well as the perforated molded mask  6   b  of the illustrative embodiment. The frame-shaped portion  7   a  of the support frame  7  is of the shape with its almost center or midway portion curved outwardly. Any one of the corners of the perforated molded mask  6   b  and the corners of the frame-shaped portion  7   a  of the support frame  7  is of substantially the arc-like shape with the radius R. The corners of the perforated molded mask  6  and corresponding corners of the frame-shaped portion  7   a  are internally contacted together at portions P-P′ and Q-Q′ respectively. The dowel  7   c  is disposed at substantially the central or midway portion of the frame-shaped portion  7   a , wherein its distal end is placed inside (in the center direction of the perforated molded mask  6 ) of a straight line that connects together specified points P, Q at which the corners of the frame-shaped portion  7   a  of the support frame  7  are in internal contact with the corners of the perforated molded mask  6 . As the shadow mask assembly  5  of the subject embodiment is designed such that the distal end of the dowel is closer in position to the skirt portion of the perforated molded mask in the way stated above, any distortion remains less which can take place at the perforated molded mask during assembly through securing by soldering techniques or the like this skirt portion to the dowel. In addition, since the skirt portion of the perforated molded mask and the support frame are brought into contact with each other at four corners over a large area, the resultant magnetic resistivity decreases letting the magnetic shield characteristics be excellent. 
     As the shadow mask assembly of the embodiment of the present invention is arranged including the support frame with nearly midway portions of two opposite sides being curved outwardly into an outward projected shape, it becomes possible to increase the inward embossing amount of the dowel as formed substantially centrally on the side of the support frame. As a result of such arrangement, it was possible to increase the mechanical strength or robustness against twist of long and short sides of the support frame  7  as well as any possible thermal distortion. 
     (Embodiment 2) 
     This embodiment is similar to the embodiment 1 except that dowels are additionally provided at four corners of the support frame  7  of FIG. 6, which are secured by soldering techniques to the perforated molded mask  6 . In this embodiment also, distortion stays less which can occur at the perforated molded mask when securing for assembly the skirt portion to the dowels by soldering or the like. Furthermore, as the skit portion of the perforated molded mask and the support frame are contacted together at four corners over a wide area, the resulting magnetic resistivity decreases letting the magnetic shield characteristics become excellent Simultaneously, it was possible to increase the strength of the shadow mask assembly  5  against mechanical and/or thermal distortion.