Patent Publication Number: US-6218772-B1

Title: Color cathode-ray tube with shadow mask mounting system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a color cathode-ray tube, and in particular to a color cathode-ray tube which is reduced in microphonic caused by vibration, deformation the at time of shadow mask assembling, and doming caused by thermal expansion of the shadow mask, and which displays images of favorable definition. 
     2. Discussion of the Background 
     In general, color cathode-ray tubes have an envelope including a substantially rectangular panel and a funnel. On the inner face of an effective portion formed by a curved face of the panel is formed a phosphor screen which is formed by three-color phosphor layers. On the inside of the phosphor screen, a substantially rectangular shadow mask is arranged to be opposed to the phosphor screen. 
     In the cathode-ray tube, three electron beams emitted from an electron gun disposed in a neck of the funnel are deflected by a deflection device mounted outside the funnel, and the phosphor screen is subject to horizontal and vertical scanning via the shadow mask. Thereby, a color image is displayed. 
     The shadow mask is provided to select the three electron beams incident on the three-color phosphor layers. In general, the shadow mask includes a nearly rectangular mask main body, and a nearly rectangular mask frame arranged along the periphery of the mask main body. The mask main body is formed by a curved face opposed to the phosphor screen. In addition, the mask main body includes a main surface portion having a large number of electron beam passage apertures, a nonporous portion located around the main surface portion, and a skirt portion located around the nonporous portion. The skirt portion is joined to side wall portions of the mask frame. 
     As for the combination of the mask main body and the mask frame, there are such a case that the side wall portions of the mask frame are joined inside the skirt portion of the mask main body, and such a case that the side wall portions are joined outside the skirt portion. Most large-sized tubes have such a structure that the side wall portions of the mask frame are attached outside the skirt portion. 
     In such a shadow mask, the distances between opposed open edges of the skirt portion in a shorter axis direction and a longer axis direction of the mask main body are set substantially equal to the distances between the side wall portions of the mask frame in the same directions. 
     In color cathode-ray tubes of recent years, it has been promoted to make an outer face of the effective portion of the panel a flat face or a curved face close to a flat face. In such color cathode-ray tubes, it is necessary to make the inner surface of the effective portion as well flat, as the outer surface of the effective portion is made flat. In the case where the inner surface of the effective portion of the panel is thus made flat, it is necessary to make the curvature of the main surface portion of the mask main body small and make the main surface portion flat or substantially flat, in order to make beam landing for the three-color phosphor layers favorable over the entire face of the screen. 
     If the curvature of the main surface portion of the mask main body becomes small, however, the tension strength of this main surface portion is lowered. If the color cathode-ray tube is incorporated into a television set in this case, then voice vibration fed from a speaker is transmitted to the mask main body. Because of resultant resonance of the mask main body, howling is apt to occur. The howling significantly degrades the image characteristics. 
     Furthermore, if the curvature of the main surface portion of the mask main body becomes small and the tension strength falls, then degradation of the color purity is apt to occur because of deformation of the mask main body caused in the manufacturing process of the color cathode-ray tube. 
     Furthermore, typically in color cathode-ray tubes, the quantity of the electron beams arriving at the phosphor screen via the electron beam passage apertures of the shadow mask is ⅓ or less of the electron beam quantity emitted from the electron gun, because of the operation principle. The rest of the electron beams mainly collide with the mask main body and heat it. Because of resultant thermal expansion of the shadow mask, such doming as to swell in the phosphor screen direction is caused in the mask main body. If the distance between the phosphor screen and the mask main body gets out of its tolerance due to the doming, the beam landing for the three-color phosphor layers deviates and color purity is degraded. 
     The shift amount of the beam landing caused by the doming largely differs depending upon the brightness of the image pattern, the duration of the pattern, and the like. Especially if a high brightness pattern is displayed locally, then local doming of the mask main body occurs, and local beam landing deviation occurs in a short time. And the local doming of the shadow mask appears especially largely in the case where the curvature of the main surface portion of the mask main body is small. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above described problems, and its object is to provide a color cathode-ray tube which is reduced in howling, deformation, and local doming of the shadow mask, and which displays images of favorable definition. 
     In order to achieve the above described object, a color cathode-ray tube according to the present invention comprises an envelope including a panel having a substantially rectangular effective portion, a phosphor screen formed on an inner surface of the effective portion, and a shadow mask arranged in the envelope so as to be opposed to the phosphor screen. 
     The shadow mask comprises a mask main body including a substantially rectangular main surface portion formed by a curved face having a large number of electron beam passing holes formed therethrough and having a longer axis and a shorter axis perpendicular to the longer axis; and a skirt portion raised in peripheral edges of the main surface portion. 
     The shadow mask also includes a substantially rectangular mask frame joined to the outer periphery of the skirt portion of the mask main body. 
     The skirt portion of the mask main body has a pair of longer side walls extending substantially in parallel to the longer axis, and a pair of shorter side walls extending substantially in parallel to the shorter axis. 
     Each of the longer side walls includes a pressing portion located near the shorter axis to protrude toward the mask frame and pressing the mask frame. 
     In accordance with the present invention, a length PX of the pressing portion measured in a direction of the longer axis is LX/2 or less, where LX is a length of the main surface portion of the mask main body measured in the direction of the longer axis. 
     Furthermore, a color cathode-ray tube according to the present invention comprises an envelope including a panel having a substantially rectangular effective portion, a phosphor screen formed on an inner surface of the effective portion, and a shadow mask disposed in the envelope so as to be opposed to the phosphor screen. 
     The shadow mask includes a mask main body including a substantially rectangular main surface portion formed of a curved face having a large number of electron beam passage apertures and having a longer axis and a shorter axis perpendicular to the longer axis; and a skirt portion raised in peripheral edges of the main surface portion. 
     The shadow mask also includes a substantially rectangular mask frame joined to an outer periphery of the skirt portion of the mask main body. 
     The skirt portion of the mask main body has a pair of longer side walls extending substantially in parallel to the longer axis, and a pair of shorter side walls extending substantially in parallel to the shorter axis. Each of the longer side walls includes a pressing portion formed near the shorter axis so as to protrude toward the mask frame and pressing the mask frame to generate residual internal stress in the main surface portion. 
     Furthermore, a color cathode-ray tube according to the present invention comprises an envelope including a panel having a substantially rectangular effective portion, a phosphor screen formed on an inner surface of the effective portion; and a shadow mask arranged in the envelope and opposing the phosphor screen. 
     The shadow mask comprises a mask main body including a nearly rectangular main surface portion formed of a curved face having a large number of electron beam passage apertures and having a longer axis and a shorter axis perpendicular to the longer axis; and a skirt portion raised in peripheral edges of the main surface portion. The shadow mask also includes a substantially rectangular mask frame joined to an outer periphery of the skirt portion of the mask main body. 
     The skirt portion of the mask main body has a pair of longer side walls extending substantially in parallel to the longer axis, and a pair of shorter side walls extending substantially in parallel to the shorter axis. Each of the longer side walls includes a pressing portion formed near the shorter axis to protrude toward the mask frame and pressing the mask frame. 
     In a part of the main surface portion located near the shorter axis, a curvature in a direction of the shorter axis is greater at a peripheral part of the main surface portion than at a central part of the main surface portion. 
     In the color cathode-ray tube of the present invention having the above described configuration, the mask frame is pressed by the pressing portions formed on the skirt portion of the mask main body, so that residual internal stress is generated in the main surface portion of the mask main body. As a result, the tension strength of the main surface portion is improved. It thus becomes to reduce the howling of the shadow mask, deformation of the shadow mask during manufacturing, and local doming caused by collision of the electron beams. Therefore, degradation of the color purity caused by them can be suppressed. As a result, it becomes possible to provide a color cathode-ray tube having favorable image characteristics. Especially when applied to a color cathode-ray tube flattened in the outer face of the effective portion of the panel and consequently flattened in the main surface portion of the mask main body, the color cathode-ray tube of the present invention brings about a significant effect. 
     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 hereinbefore. 
    
    
     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 given below, serve to explain the principles of the invention. 
     FIGS. 1 to  8  show a color cathode-ray tube according to a first embodiment of the present invention, in which: 
     FIG. 1 is a sectional view of the color cathode-ray tube, 
     FIG. 2 is a top view of a mask main body of the color cathode-ray tube, 
     FIG. 3 is a sectional view taken along a line III—III in FIG. 2, 
     FIG. 4 is a sectional view showing the mask main body and a mask frame of a shadow mask before assembling, 
     FIG. 5 is a sectional view showing the mask main body and the mask frame of the shadow mask after assembling, 
     FIG. 6 is a diagram for explaining a change in curvature of a curved face of the mask main body occurring between before and after the mask main body is attached to the mask frame, 
     FIG. 7A is a plane view of the above described shadow mask, 
     FIG. 7A is a sectional view taken along a line VIIB—VIIB in FIG. 7A, 
     FIG. 7C is a sectional view taken along a line VIIC—VIIC in FIG. 7A, and 
     FIG. 8 is a plane view schematically showing the mask main body; 
     FIG. 9A is a plane view of a mask main body in a color cathode-ray tube according to a second embodiment of the present invention, 
     FIG. 9B is a sectional view taken along IXB—IXB in FIG. 9A; 
     FIG. 10A is a plane view of a mask main body in a color cathode-ray tube a third embodiment of according to the present invention, 
     FIG. 10B is a sectional view of the mask main body taken along the X-axis in FIG. 10A, 
     FIG. 10C is a sectional view of the mask main body taken along the Y-axis in FIG. 10A; 
     FIG. 11A is a sectional view showing a mask main body and a mask frame before assembling a shadow mask according to a modification of the present invention, and 
     FIG. 11B is a sectional view showing the mask main body and the mask frame of the shadow mask after assembling. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereafter, embodiments of a color cathode-ray tube according to the present invention will be described in detail. 
     As shown in FIG. 1, the color cathode-ray tube has a vacuum envelope  10 . The vacuum envelope  10  includes a substantially rectangular panel  22  and a funnel  23 . The panel  22  includes an effective portion  20  formed of a curved face, and a skirt portion  21  provided at the periphery of the effective portion. The funnel  23  is joined to the skirt portion  21 . On an inner surface of the effective portion  20  is formed a phosphor screen  24  which includes three-color phosphor layers emitting blue, green, and red light, and light absorbing layers. A substantially rectangular shadow mask  25  described later is arranged inside the phosphor screen  24  with a predetermined interval. 
     An electron gun  29  for emitting three electron beams  28 B,  28 G, and  28 R is arranged in a neck  27  of the funnel  23 . In the color cathode-ray tube, the three electron beams  28 B,  28 G and  28 R emitted from the electron gun  29  are deflected by a magnetic field generated by a deflection device  30 , which is mounted outside the funnel  23 , to scan the phosphor screen  24  horizontally and vertically via the shadow mask  25 . As a result, a color image is displayed. 
     As shown in FIGS. 1 to  3 , the shadow mask  25  includes a substantially rectangular mask main body  32  opposed to the phosphor screen  24 , and a substantially rectangular mask frame  33  joined to the periphery of the mask main body  32 . The shadow mask  25  has a center C through which the tube axis Z of the color cathode-ray tube passes, and a longer axis X and a shorter axis Y passing through the center and perpendicular to each other. 
     The mask main body  32  integrally includes a substantially rectangular main surface portion  31  formed of a curved face opposing the phosphor screen, and a skirt portion  36  erected along the periphery of the main surface portion. The main surface portion  31  includes a porous portion  34   a  having a large number of electron beam passage apertures  34 , and a nonporous portion  35  disposed around the porous portion  34 . The skirt portion  36  has one pair of longer side walls  37   a  extending in parallel to the longer axis X, and one pair of shorter side walls  37   b  extending in parallel to the shorter axis Y. 
     In the present embodiment, a pair of notches  38  are formed in each of the longer side walls  37   a  of the skirt portion  36  with interposing the shorter axis Y between the notches. Each of the notches  38  extends from the periphery of the nonporous portion  35  to the edge of the opening side of the skirt portion. In each longer side wall  37   a , a part sandwiched between the pair of notches  38  and located near the shorter axis Y is raised so as to protrude outside and form a pressing portion  36   a . A distance Pd between tips of the pressing portions  36   a  is longer than a length LY of the main surface portion  31  along the shorter axis Y by 2 da (Pd−LY=2 da). A length PX of each pressing portion  36   a  in the direction of the longer axis X satisfies the relation PX≦LX/2, where LX is the length of the main surface portion  31  measured along the longer axis X. The length PX of each pressing portion  36   a  is suitably set in the range of LX/2 in accordance with the curvature and the tension strength of the curved face of the mask main body  32 . Each pressing portion  36   a  is formed symmetrically about the shorter axis Y. 
     As shown in FIG. 4, the mask frame  33  has a pair of longer side walls  39   a  extending in parallel to the longer axis X, and a pair of shorter side walls  39   b  (only one of the shorter side walls is illustrated) extending in parallel to the shorter axis Y. Each side wall has an inner overhang portion, and has an L-shaped cross section. A distance FY between inner surfaces of the one pair of longer side walls  39   a  along the shorter axis Y is substantially equal to the length LY of the main surface portion  31  of the mask main body along the shorter axis Y. The distance FY is smaller than the distance Pd between the pair of pressing portions  36   a , that is, FY&lt;Pd. 
     As shown in FIG. 5, in the case where the mask main body  32  is to be joined to the mask frame  33 , the pair of pressing portions  36   a  of the mask main body  32  are deformed elastically in such a direction as to make them approach each other, and in this state these pressing portions and other parts of the skirt portion  36  are put into the inside of the longer side walls  39   a  and the shorter side walls  39   b  of the mask frame  33 . Then a plurality of regions of the skirt portion  36  are welded to the inner surfaces of the longer side walls  39   a  and the shorter side walls  39   b  of the mask frame  33  to join the mask main body  32  to the mask frame  33 . The shadow mask  25  is thus formed. In this state, the pair of pressing portions  36   a  elastically abut against the inner surfaces of the longer side walls  39   a  of the mask frame  33  and press the effective portion  34  of the mask main body from both sides thereof in the direction of the shorter axis Y. 
     The shadow mask  25  having the above described configuration was used as a shadow mask for color cathode-ray tube, for example, having a screen aspect ratio of 16:9 and a diagonal dimension of 66 cm. The following Table shows the dimensions Pd, LY, FY and PX in this case as compared with the conventional shadow mask. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 
               
               
                   
                   
               
               
                   
                 Pd (mm) 
                 LY (mm) 
                 FY (mm) 
                 PX (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Present 
                 337 
                 331 
                 331 
                 250 
               
               
                 embodiment 
               
               
                 Prior Art 
                 331 
                 331 
                 331 
                 — 
               
               
                   
               
            
           
         
       
     
     As shown in the Table, Pd is substantially equal to FY in the conventional shadow mask. In the shadow mask of the present embodiment, however, Pd is significantly larger than FY. 
     As for the curved face of the main surface portion  31  of the mask main body  32  before attaching it to the mask frame  33 , the curvature is large in the center region as illustrated by a broken line  41  in FIG.  6 . After joining the mask main body  32  to the mask frame  33 , in the curved face of the main surface portion  31 , the curvature becomes small in the center region as illustrated by a solid line  42  in FIG. 6, according to a simulation result. 
     The tension strength of the curved face of the mask main body  32  typically becomes large as the curvature becomes large. Furthermore, it is known that local doming of the shadow mask  25  becomes small as the curvature of the curved surface becomes large. 
     In the color cathode-ray tube according to the present embodiment configured as described above, the mask main body  32  of the shadow mask  25  has a pair of pressing portions  36   a . In the direction of the shorter axis Y of the mask main body  32 , therefore, the curvature of the main surface portion  31  of the mask main body  32  in the center region after assembling of the shadow mask is smaller at the center region and larger at the region near the periphery of the mask main body than before the mask main body  32  is attached to the mask frame  33 , as illustrated by a solid line  42  in FIG.  6 . 
     However, the distance Pd between opening edges of one pair of pressing portions  36   a  is set larger than the distance FY of the longer side walls  37   a  of the mask frame  33 . These pressing portions  36   a  are fitted between the longer side walls  37   a  of the mask frame  33  while the distance Pd is compressed. As a result, the longer side walls  37   a  are pressed. Therefore, the pressing portions  36   a  generate large residual internal stress in the curved face of the mask main body  32 , increase the tension strength of the curved face of the mask main body, and maintain a strength at a sufficiently high level close to the strength before shadow mask assembling. Furthermore, in the peripheral part of the main surface portion  31  of the mask main body  32 , it is possible to increase the curvature and generate the residual internal stress. 
     Therefore, it is possible to reduce the howling of the shadow mask  25 , the deformation of the shadow mask in the manufacturing process of the color cathode-ray tube, and local doming of the mask main body  32  caused by collision of the electron beams, and it is possible to effectively suppress the degradation of the color purity. As a result, a color cathode-ray tube having favorable image characteristics can be provided. 
     Furthermore, the shadow mask  25  in the present embodiment is formed so that the curvature of the mask main body along the shorter axis Y will become larger than that along the longer axis X in the central region of the mask main body  32 , as shown in FIGS. 7A to  7 C. In this case, the doming suppression effect of the mask main body  32  can be improved. 
     As shown in FIG. 8, this is owing to the fact that the dimension LY of the main surface portion  31  of the mask main body  32  in the direction of the shorter axis Y is shorter than the dimension LX thereof in the direction of the longer axis X, and to anisotropy of the shadow mask  25 . In other words, in the case where, in the mask main body  32 , a large number of electron beam passage apertures  34   a  are formed in rows in a direction parallel to the short axis Y, a plurality of continuous straight bridge portions  44  extending in the direction of the shorter axis Y and having no electron beam passage apertures  34   a  are present. In the direction of the longer axis X, however, a continuous straight bridge portion is not present as represented by a line  45 . Therefore, the shadow mask  25  has anisotropy. If the curvature values are substantially equal, therefore, increasing the curvature of the short axis direction brings about a larger doming suppression effect. 
     Furthermore, when the shadow mask  25  is applied to a color cathode-ray tube in which the external face of the effective portion  20  of the panel  22  is made to be substantially flat or a curved face close to flat in order to improve the visual recognition, and consequently the main surface portion  31  of the mask main body is flattened, a significant effect is obtained. Furthermore, when the shadow mask  25  is applied to a color cathode-ray tube having an aspect ratio of 16:9 which is long sideways, a significant effect is obtained. 
     A length LY′ of the main surface portion  31  of the mask main body  32  in the direction of the shorter axis Y after assembling of the shadow mask  25  as shown in FIG. 5 is shorter than the distance FY between the inner surfaces of the opposed side wall portions  39  of the mask frame  33  by about 2 db. The magnitude of db is set to a value required to hold the curved face of the mask main body  32  which is needed to accurately land the electron beams on the phosphor screen through the electron beam passage apertures  34   a  of the mask main body  32 . 
     A color cathode-ray tube according to a second embodiment of the present invention will now be described. 
     In the above described first embodiment, each pressing portion  36   a  of the mask main body  32  is formed by forming a pair of notches  38  in each of the longer side walls  37   a  of the skirt portion  36  with interposing the shorter axis Y of the mask main body  32  between the notches and raising the portion sandwiched between the pair of notches to protrude outside. In the second embodiment, as shown in FIGS. 9A and 9B, each pressing portion  36   a  is formed by making a portion of the longer side wall  37   a  of the skirt portion  36  located near the short axis Y protrude outside smoothly by press molding or the like. 
     In the first and second embodiments, the pressing portions  36   a  are provided only on parts of the mask main body  32  located near the shorter axis Y, that is, only on the longer side walls  37   a  of the skirt portion  36 . As in a third embodiment shown in FIGS. 10A to  10 C, however, pressing portions  37   b  may also be formed on those portions of the mask main body  32  which are located near the longer axis X, that is, on the shorter side walls  37   b  of the skirt portion  36 . In this case, each pressing portion  36   b  is formed, in the same way as the pressing portion  36   a , by forming a pair of notches, which are not illustrated, in each of the shorter side walls  37   b  on both sides of the longer axis X and raising the portion sandwiched between the pair of notches to protrude outside. As for the skirt portion  36  located near the longer axis X, a distance PI between opening edges of the pair of pressing portions  36   b  is set larger than the length LX of the main surface portion  31  of the mask main body measured along the longer axis X, that is, P 1 &gt;LX. Furthermore, a length PY of each pressing portion  36   b  in the direction of the shorter axis Y is set so as to satisfy the relation PY≦LY/2, where LY is the length of the main surface portion  31  in the direction of the shorter axis Y. The length PY of the pressing portion  36   b  is suitably set in the range of LY/2 in accordance with the curvature and the tension strength of the main surface portion  31 . The pressing portions  36   b  are formed symmetrically about the longer axis X. 
     The configuration of remaining portions are the same as that of the above described embodiments. The same portions are denoted by like reference numerals, and detailed description of them will be omitted. 
     In the third embodiment of the above described configuration as well, operation effects similar to those of the first embodiment can be obtained. Specifically, by providing the pressing portions  36   a  and  36   b  on the mask main body  32 , it is possible to generate large residual internal stress in the main surface portion  31  of the mask main body by the action of the pressing portions and enhance the tension strength of the main surface portion, when the mask main body is attached to the mask frame  33 . Further-more, when the shadow mask  25  is applied to a color cathode-ray tube in which the external face of the effective portion of the panel is made to be substantially flat or a curved face close to flat in order to improve the visual recognition, and consequently the main surface portion of the mask main body is flattened, a significant effect is obtained. Furthermore, when the shadow mask  25  is applied to a color cathode-ray tube incorporating a large-sized shadow mask, a significant effect is obtained. 
     The present invention is not limited to the above described embodiments, but within the scope of the present invention, various modifications can be applied. For example, in the above described embodiment, the length LY′ of the main surface portion of the mask main body in the direction of the shorter axis Y after assembling of the shadow mask is set shorter than the distance FY between a pair of longer side walls  39   a  of the mask frame  33 . Alternatively, the mask main body  32  may be constructed so that the length LY of the main surface portion  31  of the mask main body  32  in the direction of the shorter axis Y, before assembling of the shadow mask  25 , is slightly greater than the distance FY between the inner surfaces of the longer side walls  39   a  of the mask frame  33  as shown in FIG. 11A, and the length LY′ of the main surface portion  31  in the direction of the shorter axis Y, after assembling the shadow mask, is shorter than the length LY in the direction of the shorter axis Y before assembling. In this case, it is a matter of course that the distance Pd between the opening edges of the pair of pressing portions  36   a  provided on the skirt portion  36  of the mask main body  32  is set greater than the distance FY between the inner surfaces of the longer side walls  39   a  of the mask frame  33 . 
     Furthermore, the main surface portion  31  of the mask main body  32  may be formed so as to be greater than the distance between the inner surfaces of the shorter side walls  39   b  of the mask frame  33  not only in the direction of the shorter axis Y but also in the direction of the longer axis X. 
     Additional advantages and modifications will readily occur 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.