Patent Publication Number: US-6215237-B1

Title: Color cathode ray tube with shadow mask having mask frame balanced in mechanical strength

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a color cathode ray tube. 
     In general, a color cathode ray tube comprises an envelope. The envelope has a substantially rectangular panel provided with a skirt portion at the peripheral portion of an effective surface of a curved surface, and a funnel joined with the skirt portion. On the inner surface of the panel is formed a phosphor screen which includes light absorbing layers and three-color phosphor layers formed at gaps between the light absorbing layers. An electron gun is arranged in the neck of the funnel and a deflector is mounted on the funnel. Further, a substantially rectangular shadow mask is provided in the envelope to be opposed to the phosphor screen. 
     In this color cathode ray tube, three electron beams emitted from the electron gun are deflected by the deflector and horizontally and vertically scan the phosphor screen through electron beam apertures of the shadow mask, thereby to display a color image. 
     In general, a shadow mask is comprised of a substantially rectangular mask body and a substantially rectangular mask frame. The mask body has a main surface portion consisting of a curved surface opposed to the phosphor screen and having a number of electron beam apertures formed therein, and a skirt portion provided at the peripheral edge of the main surface portion. The mask frame has side walls contacting with and welded to the skirt portion of the mask body. 
     With one of methods of supporting the shadow mask, substantially wedge-like elastic support members are fixed to the side walls at the corners of the mask frame, and the support members are engaged with stud pins provided at the skirt portion of the corners of the panel, thereby detachably supporting the shadow mask on the inside of the panel. 
     In the color cathode ray tube as described above, three electron beams which have passed the electron beam apertures of the mask body must properly land on the three color phosphor layers, respectively, in order to display an image without color deviation. Therefore, it is necessary to arrange the shadow mask at a proper alignment relationship with respect to the panel. 
     Recently, in a color cathode ray tube such as a display tube used in a terminal apparatus of a computer in response to introduction of multi-media, the arrangement pitch of three color phosphor layers is reduced in comparison with a normal color cathode ray tube, to improve the resolution, and therefore, the margin rate for beam landing is so small that color deviation easily occurs. Accordingly, more precise beam landing is required. 
     However, actually, in steps of manufacturing a color cathode ray tube, the shadow mask is repeatedly attached to and detached from the panel, and during the attaching and detaching operations, a stress applied on the shadow mask causes the mask frame to be distorted. The distortion of the mask frame further causes the mask body to be distorted, and as a result, beam landing is shifted from the predetermined three color phosphor layers, thereby causing color deviation. 
     In addition, in the cathode ray tube as described above, since the margin rate for beam landing is small, it is necessary to reduce the position shifting of the shadow mask caused by any external impact as much as possible. To reduce the position shifting of the shadow mask caused by an external impact, it is also necessary to reduce loads acting on the elastic support member itself. However, if the weight of the mask frame itself is decreased to reduce loads on the elastic support member, the mechanical strength of the shadow mask against a stress applied thereto when attaching and detaching the shadow mask in steps of manufacturing a color cathode ray tube is lowered. As a result, the position shifting of the shadow mask is caused with respect to the panel. 
     Further, with respect to the shadow mask, in order to prevent unevenness of a formed phosphor screen, the mask body is thinned so as to reduce variations of the shapes and sizes of the electron beam apertures of the mask body, thereby reducing unevenness of the phosphor screen due to variations of the electron beam apertures. However, if the mask body is thinned, the mechanical strength thereof decreases and tends to be deformed easily. 
     Also, the shadow mask is heated by collisions of electron beams and expands, causing doming. In a shadow mask formed of a low expansion-coefficient material such as invar which efficiently restricts registration of beam landing caused by the doming, a mask frame provided with a convex portion (or bead portion) has been proposed as a counter measure against deformation. 
     However, not only the mask frame of a conventional shadow mask but also the mask frame having the convex portion as described above is short of suitable mechanical strength and cannot respond sufficiently to variations of mask frames in manufacturing steps and deformation in assembling shadow masks. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been contrived in consideration of the above circumstances and its object is to provide a color cathode ray tube, which has an improved mechanical strength without increasing the weight of the mask frame and which can reduces mislanding of electron beams due to deformation of the mask frame. 
     To achieve the object described above, a color cathode ray tube according to the present invention comprises: an envelope including a substantially rectangular panel having an inner surface on which a phosphor screen is formed; a substantially rectangular shadow mask arranged in the envelope to oppose the phosphor screen; and an electron gun provided in the envelope, for emitting electron beams to the phosphor screen through the shadow mask. 
     The shadow mask includes a substantially rectangular mask body having a main surface portion provided with a number of electron beam apertures and opposing the phosphor screen, and a substantially rectangular mask frame having side walls attached to an peripheral edge portion of the mask body. The mask frame includes a long axis and a short axis perpendicular to each other, and perpendicularly crossing a tube axis, and the side walls of the mask frame includes a pair of long side walls extending in parallel with the long axis and a pair of short side walls extending in parallel with the short axis. 
     At least one of the long and short side walls has a bead. The length of the beads in a direction perpendicular of the tube axis is set in a range of 55% to 80% of a length of the at least one of the long and short side walls in the direction of the long axis, and a width of the beads in a direction of the tube axis is set in a range of 40% to 75% of a maximum width of the at least one of the long and short side walls in the direction of the tube axis. 
     Since the side walls of the mask frame are provided with beads, and the length and width of each bead are set to a proper size matched with the side wall, the mechanical strength of the shadow mask can be improved. Accordingly, deformation of the mask body and changes of the positional relationship between the panel and the shadow mask can be effectively prevented, so that it is possible to construct a color cathode ray tube which is difficult to cause misregistration of beam landing with respect to the phosphor layers. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments give below, serve to explain the principles of the invention. 
     FIGS. 1 to 3C show a cathode ray tube according to an embodiment of the present invention, in which: 
     FIG. 1 is a longitudinal cross-sectional view showing the color cathode ray tube; 
     FIG. 2 is a perspective view schematically showing a phosphor screen, a shadow mask, and an electron gun of the color cathode ray tube; 
     FIG. 3A is a plan view of the shadow mask; 
     FIG. 3B is a cross-sectional view taken along a line IIIB—IIIB in FIG. 3A; 
     FIG. 3C is a cross-sectional view taken along a line IIIC—IIIC in FIG. 3A; 
     FIG. 4A is a plan view of a shadow mask according to a modification of the present invention; 
     FIG. 4B is a cross-sectional view taken along a line IVB—IVB in FIG. 4A; and 
     FIG. 4C is a cross-sectional view taken along a line IVC—IVC in FIG.  4 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, a color cathode ray tube according to an embodiment of the present invention will be described in detail with reference to the drawings. 
     As shown in FIGS.  1  and  2 ,a color cathode ray tube comprises a vacuum envelope  10 , and the vacuum envelope comprises a substantially rectangular panel  3  having an effective surface  1  in a form of a curved surface and a skirt portion  2  standing on the peripheral edge portion of the effective surface, and a funnel  4  joined to the skirt portion of the panel. A phosphor screen  13  is formed on the inner surface of the panel  3 , and the screen includes a number of strip-like light-absorbing layers  11 , three-color phosphor layers  12 R,  12 G, and  12 B embedded between the strip-like light-absorbing layers  11  for emitting light in blue, green, and red. Also, a substantially rectangular shadow mask  30  described later is arranged in the envelope  10  to oppose the phosphor screen  13  with a predetermined distance. 
     An electron gun  6  for emitting three electron beams  7 B,  7 G, and  7 R is arranged in a neck  5  of the funnel  4 . In addition, a deflector  8  is equipped outside the funnel  4 . In the color cathode ray tube having the construction mentioned above, three electron beams  7 B,  7 G, and  7 R emitted from the electron gun  6  are deflected by the deflector  8  and horizontally and vertically scan the phosphor screen  13  through the shadow mask  30 , thereby displaying a color image on the phosphor screen. 
     As shown in FIGS. 3A to  3 C, the shadow mask  30  comprises a substantially rectangular mask body  33  and a substantially rectangular mask frame  36  supporting the periphery of the mask body. The mask body  33  has a substantially rectangular main surface portion  31  formed of a curved surface opposing the phosphor screen  13 , and a skirt portion  32  extending from the peripheral edge of the main surface portion  31 . A number of electron beam apertures  37  are formed in the main surface portion  31 . 
     The mask frame  36  includes four side walls  34  each of which has an extending portion  35  extending from an end there and has a L-shaped cross-section. The mask body  33  is fixed to the mask frame  36  by welding the skirt portion  32  to the inner surface of the side walls  34  of the mask frame  36 . 
     According to the present embodiment, the shadow mask  30  includes beads formed in the respective side walls  34  of the mask frame  36  and projecting insides the mask frame. 
     Specifically, the mask body  33  and the mask frame  36  have a long axis (or X-axis) and a short axis (or Y-axis) which are cross each other at right angles and perpendicular to a tube axis Z. The side walls  34  of the mask frame  36  are formed of a pair of long side walls  34   a  extending in parallel with the long axis X and a pair of short side walls  34   b  extending in parallel with the short axis Y. Further, a bead  38   a  is formed at the center portion of each of the long side walls  34   a  and extends in a direction parallel to the long axis X, and a bead  38   b  is formed at the center portion of each of the short side walls  34   b  and extends in a direction parallel to the short axis Y. 
     The length L 1  of each bead  38   a  in the direction of the long axis X is set in a range of 55% to 80% of the length of each long side wall  34   a  in the direction of the long axis X. Likewise, the length L 2  of each bead  38   b  is set in a range of 55% to 80% of the length of each short side wall  34   b  in the direction of the short axis Y. The width W 1  of each bead  38   a  in the direction of the tube axis Z is set in a range of 40% to 75% of the maximum width of each long side wall  34   a  in the direction of the tube axis Z, and the width W 2  of each bead  38   b  in the direction of the tube axis Z is set in a range of 40% to 75% of the maximum width of each short side wall  34   b  in the direction of the tube axis Z. 
     The shadow mask  30  constructed in the structure as described above is detachably supported on the inside of the panel  3 , in a manner such that substantially wedge-shaped elastic support members  21  provided at corners of the mask frame  36  are respectively engaged with stud pins  22  erected on the inner surfaces of the corners of the skirt portion  2  of the panel  3 . 
     According to the color cathode ray tube constructed as described above, by providing the beads  38   a  and  38   b  at the side walls  34   a  and  34   b  of the mask frame  36  fixed to the mask body  33 , the mechanical strength of the shadow mask  30  can be increased even when the weight of the mask frame  36  is reduced. Therefore, an unnecessary unbalanced stress is not applied to the mask body  33  when assembling. the shadow mask  30 , so that deformation of the mask body  33  can be prevented. In addition, deformation of the mask frame  36  can be reduced against a stress applied when the shadow mask  30  is attached to and detached from the panel  3  in the steps of manufacturing the color cathode ray tube. As a result of this, deformation of the mask body  33  and changes of the positional relationship between the panel  3  and the shadow mask  30  can be effectively prevented, and it is therefore possible to construct a color cathode ray tube which is difficult to cause mislanding of electron beams on the three-color phosphor layers  12 R,  12 G, and  12 B. 
     With respect to the shadow mask  30  in which the bead  38   a  and  38   b  are provided at the four side wall  34   a  and  34   b  of the mask frame  36 , studies and discussions were made to the relationship between the lengths L 1  and L 2  and widths W 1  and W 2  of the beads  38   a  and  38   b,  and deformation amounts in the direction of the tube axis Z and in the direction perpendicular to the tube axis when stresses such as bending and twisting are applied to the long side walls  34   a  and short side walls  34   b  of the mask frame  36 . The results shown in the following tables 1 and 2 were obtained. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 LENGTH 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 DIRECTION 
                 DIRECTION 
                 DIRECTION 
                 DIRECTION 
                 DIRECTION 
               
               
                   
                   
                 PERPENDICULAR 
                 PERPENDICULAR 
                 PERPENDICULAR 
                 PERPENDICULAR 
                 PERPENDICULAR 
               
               
                   
                 NO 
                 TO TUBE AXIS 
                 TO TUBE AXIS 
                 TO TUBE AXIS 
                 TO TUBE AXIS 
                 TO TUBE AXIS 
               
               
                   
                 BEADS 
                 APPLOX. 25% 
                 APPLOX. 50% 
                 APPLOX. 65% 
                 APPLOX. 75% 
                 APPLOX. 85% 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 BENDING OF 
                 22.7 
                 10.4 
                 6.8 
                 4.6 
                 2.5 
                 0.1 
               
               
                 LONG SIDE 
               
               
                 BENDING OF 
                 13.5 
                 6.5 
                 3.7 
                 3.4 
                 3.8 
                 4.2 
               
               
                 SHORT SIDE 
               
               
                 TWISTING 
                 0.03 
                 0.18 
                 0.1 
                 0.07 
                 0.08 
                 0.12 
               
               
                 LOAD TO 
                 0.17 
                 0.17 
                 0.07 
                 0.05 
                 0.03 
                 0.01 
               
               
                 CORNERS OF 
               
               
                 SIDE WALL 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 TUBE AXIS 
                   
               
               
                   
                   
                 TUBE AXIS 
                 DIRECTION 
                 TUBE AXIS 
               
               
                   
                   
                 DIRECTION 
                 APPLOX. 
                 DIRECTION 
               
               
                   
                   
                 APPLOX. 
                 40% TO 
                 APPLOX. 
               
               
                 WIDTH 
                 NO BEADS 
                 25% 
                 75% 
                 85% 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 BENDING OF 
                 22.7 
                 4.9 
                 4.0 
                 0.1 
               
               
                 LONG EDGE 
               
               
                 BENDING OF 
                 13.5 
                 5.1 
                 4.3 
                 6.1 
               
               
                 SHORT EDGE 
               
               
                 TWISTING 
                 0.03 
                 0.08 
                 0.11 
                 0.14 
               
               
                 LOAD TO 
                 0.17 
                 0.06 
                 0.04 
                 0.01 
               
               
                 CORNER 
               
               
                 SIDE WALL 
               
               
                 PORTIONS 
               
               
                   
               
            
           
         
       
     
     Among the deformation amounts shown in Tables 1 and 2, those of the “bending of long side” show deformation amounts of the mask frame  34  where a stress in the direction of the tube axis Z is applied to the extending portion  35  of the long side wall  34   a,  those of the “bending of short side” show deformation amounts of the mask frame  34  where a stress in the direction of the tube axis Z is applied to the extending portion  35  of the short side wall  34   b,  those of the “twisting” show deformation amounts of the mask frame where stresses in the directions opposite to each other are respectively applied to the extending portion  35  of each long side wall  34   a  and the extending portion  35  of each short side wall  34   b,  and those of the “load onto corners of side walls” show deformation amounts of the mask frame where a stress in the diagonal direction is applied to the side walls. The amounts of the “load onto corners of side walls” indicates the maximum deformation amount of the portion to which the mask body  33  is to be attached. 
     As seen from the Table 1, when the length L 1  and L 2  of the beads  38   a  and  38   b  are set in a range of 50% to 85% of the length of each long side wall  34   a  and each short side wall  34   b  in the direction perpendicular to the tube axis Z, unbalance between the deformation amounts of the bending of long side and the bending of short side can be reduced, and the deformation amounts of the long and short side walls can be made substantially equal to each other. With respect to twisting and load onto corners of side walls, the deformation amount of the mask frame can be reduced. 
     It is more preferable that the length L 1  and L 2  are set in a range of 55% to 80%, wherein unbalance of the deformation amounts of the shadow mask can be further reduced and the shadow mask can be stable at a small deformation. 
     As shown in Tables 1 and 2, the mask frame having no beads has much higher deformation amounts of the bending of the long and short sides than the mask frame provided with beads. Hence, it can be understood that the mechanical strength of the mask frame is improved by providing the beads  38   a  and  38   b  and the deformation of the mask frame can be greatly reduced. 
     In case of the mask frame having no beads, the deformation amount of the bending of the long side is greater than that of the bending of the short side, due to the difference in length between the long and short side walls. However, according to the present embodiment, unbalance between the bending of the long side and the bending of the short side of the mask frame is improved and the deformation amounts of the long and short side walls can be substantially equalized to each other, where the lengths L 1  and L 2  of the beads  38   a  and  38   b  of the long and short side walls  34   a  and  34   b  are set in a range of 55% to 80% of the lengths of the long and short side walls, respectively, and where the widths W 1  and W 2  of the beads  38   a  and  38   b  in the direction of the tube axis Z are set in a range of 40% to 75% of the maximum widths of the long and short side walls, respectively. Therefore, if uneven stresses are applied to the long and short side walls, the stress applied to the long and short sides of the mask body  33  can be made substantially even so that deformation of the mask body can be reduced. 
     As described above, according to the color cathode ray tube, the mechanical strength of the shadow mask  30  can be improved, so that an unbalanced unnecessary stress is not applied to the mask body  33  and deformation of the shadow mask can be prevented even if the weight of the mask frame  36  and the like is reduced. Accordingly, deformation of the mask body  33  and changes of the positional relationship between the panel  3  and the shadow mask  30  can be effectively prevented, so that it is possible to provide a color cathode ray tube of excellent quality which is difficult to cause misregistration of beam landing on the three-color phosphor layers. 
     Note that the present invention is not limited to the above mentioned embodiment but can be variously modified within the scope of the invention. For example, in the embodiment described above, one bead  38   a  is provided for each of the long side walls  34   a  and one bead  38   b  is provided for each of the short side walls. However, as shown in FIGS. 4A to  4 C, each of the beads  38   a  and  38   b  may be divided into a plurality of bead sectors which are disposed in parallel with relatively narrow intervals. In this case, the distances between those bead sectors positioned at both ends in their lengthwise directions are set to the lengths L 1  and L 2 . The width of each of the bead sectors is set to the width W 1  or W 2 . 
     In case where each of the beads  38  is constituted by a plurality of bead sectors, the lengths of the bead sectors may differ from each other, as shown in FIGS. 4A to  4 C. 
     Also, in the above embodiment, the beads  38  are defined by projecting inwards part of each side wall of the mask frame. However, each bead may be formed by projecting outwards a part of each side wall. 
     Further, in the above embodiment, the side walls  34  is provided with beads extending in the direction toward the mask body  33  from the side of the extending portion  35 . The bead portions, however, may be provided so as to extend toward the extending portion  35  from intermediate portions of the side walls or end portions on the mask body side. 
     In the above embodiment, beads are provided at both long and short side walls of the mask frame. 
     However, beads may be provided at only one of the long and short side walls. 
     Additional advantages and modifications will readily occurs to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.