Color picture tube

Abstract: A color picture tube has a body having a substantially rectangular front panel (11), a phosphor screen (14) formed on an inner surface of the panel (11), a shadow mask assembly including (a) a shadow mask which is detachably mounted on the panel (11) by means of a supporting assembly, and which faces the phosphor screen (14) when the shadow mask (17) is mounted on said panel, and (b) a frame (18) which sur­ rounds the shadow mask (17). The supporting assembly has stud pins (24) projecting from the four corners of the panel (11), and leaf springs (30), each of which is fixed at one end to a corresponding corner of the frame (18), and is engaged at the other end with a corre­ sponding stud pin (24) when the shadow mask assembly is mounted on the panel (11), and disengaged therefrom when the shadow mask assembly is detached from the panel (11). Each leaf spring (30) has an elastically defor­ mable portion (32) between one end portion fixed to the frame (18) and the other end portion engaged with the corresponding one of the stud pins (24). Angle β defined by the axis of elastically deformable portion (32) and an axis of the body and angle α defined by a line (38) connecting an end portion of the elastically deformable portion on the side of the frame (18) and the stud pin (24), and the axis of the body have the following relation: β < α

Description: The present invention relates to a color picture tube and, more particularly, to a supporting assembly for connecting a shadow mask to the front panel of a color picture tube. A color picture tube comprises three electron guns, a front panel, a phosphor screen formed on the inner surface of the front panel, a rigid mask frame, and a shadow mask supported by the rigid mask frame and facing the phosphor screen. The shadow mask has a plurality of apertures through which electron beams emitted from the guns pass before striking the phosphor screen. The rigid mask frame surrounds the peripheral edge of the shadow mask, thus maintaining the shape of the shadow mask. The color picture tube further comprises a supporting assembly which supports the mask frame. The mask frame can be detached from the supporting assembly, and thus too can the shadow mask. As a result of electron beam passing through the apertures of the shadow mask for some time, the shadow mask becomes heated and expands, which causes the positions of the individual apertures to change, inevitably giving rise to a undesirable phenomenon known as "doming", i.e., the mislanding of the beams. To compensate for the thermal expansion of the mask, a bimetal is incorporated in the supporting assembly. This bimetal functions to move the shadow mask as the mask thermally expands, so that the electron beams passing through the apertures of the mask can land on the desired portions of the phosphor screen. However, the bimetal cannot move the shadow mask sufficiently quickly after the mask has begun to thermally expanded, and thus mislanding of the beam occurs. Besides, incorporated of the bimetal, which is relatively heavy, renders the picture tube undesirably heavy. Japanese Patent Publication No. 46-4104 discloses a rectangular mask frame having four leaf springs which project one from each of the four corners and are connected to the four corners of a shadow mask, thereby supporting the shadow mask. As a result, when the shadow mask thermally expands, deformation of the mask frame is less than that of the mask frame surrounding the shadow mask. The use of this mask frame results in the following advantages: (1) Mislanding of the beams, do not occur concentratedly at a particular portion of the phosphor screen. (2) Mislanding of the beam, due to the vibration of the mask frame, is reduced since the springs absorb the vibration. (3) The weight of the picture tube can be reduced significantly, since the frame is thin and lightweight. Fig. 1 shows a conventional assembly for supporting the shadow mask of a color picture tube similar to the supporting assembly shown in EP-A-207 724, which is a combination made up of stud pin 24 which projects from an inner peripheral wall of panel 11, and leaf spring 2 having a hole through which stud pin 24 extends. Stud pin 24 is tapered at an angle of about 12°. Leaf spring 2 is composed of three portions, i.e., fixed portion 3 welded to frame 18 and located near phosphor screen 14; elastically deformable portion 4; and engaging portion 5 supported by stud pin 24, on its rear side, and located remote from screen 14. Leaf spring 2 has a substantially V-shaped longitudinal section. Ideally, the angle (0) defined by movable portion 4 and frame 18 should be 45° in the case of a picture tube having a beam deflection angle of 90°, and 35° in the case of a picture tube having a beam deflection angle of 110°, in order to compensate only for the mislanding of electron beams 15 which has resulted from the thermal expansion of shadow mask 17. After the picture tube of either of the above types has been in continuous use for 30 minutes or more, frame 18 begins to expand due to beam heating. Since the diameter of frame 18 inevitably increases as a result of thermal expansion, allowing shadow mask 17 to move. Consequently, as is shown in Fig. 1, apertures 19 of shadow mask 17 (indicated by a solid line) move in the radial direction of the picture tube. Nonetheless, leaf springs, which are inclined to shadow mask 17, push frame 18 toward phosphor screen 14, thus moving shadow mask 17 toward screen 14, whereby apertures 19 move to position 19b. Therefore, mislanding of the beams, resulting from the expansion of mask 17, is reduced to some extent, with a corresponding decrease in what is known as color purity drift. However, in a conventional supporting assembly, when frame 18 is displaced in the radial direction, as indicated by an alternate long and two short dashed line in Fig. 2, engaging portion 5 of spring 2 is inclined, and portion 4 is curved. For this reason, the shift amount of frame 18 in the axial direction becomes smaller than a design value. Thus, mask assembly cannot be moved for a sufficiently long distance in the axial direction when spring 2 is attached to frame 18 at the above-mentioned angle. In this case, apertures 19 of mask 17 cannot be located at the desired positions. Spring 2 is attached to frame 18 to have an angle ϑ larger than the above-mentioned value, e.g., to be about 70 degrees or more in a picture tube having a beam deflection angle of 90 degrees, so as to increase the shift amount of frame 18 by spring 2. However, if the angle ϑ is increased, the spring constant of spring 2 becomes too large, and it is difficult to detach shadow mask assembly from panel 11, resulting in a variety of problems in respect of manufacture and use. More specifically, in manufacturing processes of a color picture tube, an attachment/detachment of a shadow mask to/from a panel must be repetitively performed. When leaf springs are used, the attachment/detachment cannot be manually performed and must be performed by a special-purpose apparatus installed for each process. Furthermore, a large force acts on a supporting assembly, a mask frame, and a shadow mask during attachment/detachment of the shadow mask, and may deform these parts. When such strong springs are used, spring 2 is fatigued upon several attachment/detachment operations, and an initial spring constant cannot be maintained. Therefore, it is difficult to precisely maintain the shadow mask assembly by means of the supporting assembly without being drifted in the panel. The thermal expansion coefficient of a material constituting a stud pin must be substantially the same as that constituting the panel, in order to prevent the stud pin from cracking at the projecting portion. For this reason, as a material of the stud pin, an Fe-Cr alloy (Cr: 18 wt.%) is normally employed. This material is soft, i.e., has a Vickers hardness (Hv) of about 150. Leaf spring 2 is normally formed of hard stainless steel having an Hv of 380 to 500. When a rigid spring is used, the stud pin is recessed at a contacting portion, and mounting precision of the shadow mask is degraded. Therefore, in the conventional supporting assembly, the mounting angle ϑ cannot be increased, and purity drift in a continuous operation for a long period of time cannot be satisfactorily compensated. Therefore, a deterioration of color purity near a peripheral region of the screen occurs. A modified embodiment shown in US 3 492 522 has a V-shaped spring member. When the frame surrounding the shadow mask is thermally expanded the V-shaped spring becomes curved and approaches the screen. In US 3 487 251 there is disclosed a color picture tube, in which the supporting assembly detachably mounting the shadow mask on the front panel adjacent to the phosphor screen comprises at least three glass cams, that means stud pins asymmetrically projecting from the front panel along the periphery of an annular frame supporting the shadow mask. Thus, the stud pins are attached to side portions of the front panel, respectively. Moreover, deformable elements being an equivalent of spring members comprise parallel strips, are provided. When the frame is thermally expanded, the parallel strips move due to their elastic deformation. The present invention has as its object to provide a color picture tube which can satisfactorily compensate for purity drift occuring after a long use of the picture tube, and hence can improve image quality. According to the invention there is provided a color picture tube, comprising: A color picture tube, comprising: a CRT envelope having a front panel, the front panel being composed of a substantially rectangular faceplate surrounded by a skirt, and a central line axis substantially perpendicular to the faceplate; a phosphor screen formed on the inner surface of the face plate; a shadow mask assembly including (a) a shadow mask, (b) a frame surrounding said shadow mask, and (c) a supporting assembly detachably mounting the shadow mask facing to said phosphor screen, such that the shadow mask faces said phosphor screen; said supporting assembly having: stud pins projecting perpendicularly from the inner wall of the skirt, and spring members each having two ends, one end being fixed to said frame and the other end being detachably engaged with a corresponding one of said stud pins, each of said spring members including a single straight strip-shaped elastically deformable portion located between said two ends, wherein said stud pins (24) project from the four corners of said front panel (11), wherein said spring members (30,40,50,60) include an engaging portion (35) for engagement with said stud pin (24), such that when the spring members (30,40,50,60) are displaced following the thermal expansion of the frame, an end of said engaging portion (35) more apart from the screen (14) than the stud pin (24) is displaced away from said skirt of the CRT envelope, characterized in that said single straight strip-shaped elastically deformable portion (32) of the spring members (30,40,50,60) and that plane of the frame (18) to which are mounted the spring members (30,40,50,60), said plane being parallel to the central line axis, define an angle β, and a line (38) connecting the end portion of said single straight strip-shaped elastically deformable portion (32) on the side of said frame (18) with said stud pin (24) and said plane of the frame (18) to which is mounted the spring member (30,40,50,60) define an angle α, wherein the end portion of said single straight strip-shaped elastically deformable portion (32) on the side of said engaging portion (35) is more apart from the screen (14) than the stud pin (24), and wherein angle β < angle α. There is still known from JP-A-53144252 a shadow mask supporting assembly in which an attachment panel is fixed to a front panel, and a projecting part of a V-shaped elastic supporter bar is fitted to the hole of the panel. The projecting part of the bar is immovable within the hole, because the supporter bar is pushed against the attachment panel by a spring. It follows that, in this supporting assembly it is impossible to define an angle α as in the present invention. In a picture tube having a beam deflection angle of 90 degrees, angle α preferably falls within the range of 60 to 80 degrees, and most preferably, 70 degrees. Angle β preferably falls within the range of 45 to 65 degrees, and most preferably, 55 degrees. In a picture tube having a beam deflection angle of 110 degrees, angle a preferably falls within the range of 50 to 70 degrees, and most preferably, 60 degrees. Angle β preferably falls within the range of 35 to 55 degrees, and most preferably, 45 degrees. The thickness of each spring member preferably falls within the range of 0.3 to 0.8 mm. The thickness of the mask frame preferably falls within the range of 0.5 to 2.0 mm. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1 and 2 are longitudinal sectional views of a supporting assembly of a shadow mask in a conventional color picture tube; Fig. 3 is a longitudinal sectional view showing a color picture tube according to a first embodiment of the present invention; Fig. 4 is a schematic plan view of a color picture tube when viewed from the rearward; Figs. 5 and 6 are longitudinal sectional views showing a supporting assembly of a shadow mask in a color picture tube according to the first embodiment of the present invention; Fig. 7 is a view for explaining an engaging state between a spring member and a stud pin in the supporting assembly; Fig. 8 is a longitudinal sectional view showing a supporting assembly according to a second embodiment; Fig. 9 is a longitudinal sectional view showing a supporting assembly according to a third embodiment; and Fig. 10 is a longitudinal sectional view showing a supporting assembly according to a fourth embodiment. Embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. As shown in Fig. 3, a vacuum envelope of color picture tube 10 is constituted by substantially rectangular panel 11, fan-shaped funnel 12, and neck 13. Tri-color phosphor screen 14 consisting of stripe phosphor layers which emit red, green, and blue light, respectively, is deposited on the inner surface of panel 11. So-called inline type electron guns 16 which are aligned along the horizontal axis of panel 11 and emit three electron beams 15 corresponding to red, green, and blue, are disposed in neck 13. Shadow mask 17 in which a large number of slot apertures are formed is disposed to face phosphor screen 14. Substantially rectangular mask frame 18 is provided at the peripheral edge of shadow mask 17 so as to keep shadow mask 17 in position. Spring members 30 such as leaf springs, each having a through hole, are attached to the four corners of frame 18. The through holes of spring members 30 respectively receive stud pins 24 projecting from the four corners of the inner periphery of the panel 11 (Fig. 4). More specifically, when four spring members 30 are engaged with four stud pins 24, shadow mask 17 is supported inside panel 11 through frame 18. Three inline electron beams 15 are deflected by deflection device 22 outside funnel 12, scan a rectangular region corresponding to panel 11, and land on stripe phosphor screen 14 while being color-separated through slot apertures of shadow mask 17, thereby reproducing a color image. In order to prevent the electron beams from being influenced by an external magnetic field, magnetic shield 21 is engaged inside funnel 12 through frame 18. A supporting assembly of the first embodiment will be described in detail with reference to Figs. 5 to 7. Shadow mask 17 is formed of a 0.2-mm thick cold-rolled steel plate containing iron as a major component, and its peripheral edge is fixed to mask frame 18. Mask frame 18 is formed of a 0.5-mm thick cold-rolled steel plate containing iron as a major component, and its axial section is L-shaped. Each spring member 30 has a substantially N-shaped section. Each spring member 30 is formed of a 0.4-mm thick precipitation-hardened stainless steel plate having good spring characteristics, e.g., an SUS 631 thin plate. As shown in Figs. 5 and 6, fixed portion 31 of each spring member 30 is fixed by welding to the outer peripheral surface of frame 18. Movable portion 32 is formed by bending both ends of a thin plate through 55 degrees in opposite directions. One end of portion 32 is welded to an end portion (near phosphor screen 14) of fixed portion 31, and another end thereof is welded to an end portion (farther from phosphor screen 14) of engaging portion 35. More specifically, spring member 30 is attached so that angle β defined by frame 18 and portion 32 is about 55 degrees. Hole 36 is formed at the center in the widthwise direction of engaging portion 35. When the shadow mask is attached to the panel, the distal end portion of each stud pin 24 is fitted in corresponding hole 36. In this case, angle α defined by line 38 connecting the distal end central portion of stud pin 24 and bent position 33 of portion 32, and the axis of body is about 70 degrees. Stud pins 24 project from the inner periphery of glass panel 11. Each stud pin 24 has a frustoconical distal end portion, and is formed of a Cr-Fe alloy (Cr: 18 wt.%). As shown in Fig. 7, collar 37 is formed around hole 36 of each engaging portion 35 substantially along the tapered surface of corresponding stud pin 24, so that stud pin 24 and engaging portion 35 are smoothly slid each other. A case will be described with reference to Fig. 6 wherein the color picture tube of the first embodiment is operated. An alternate long and two short dashed line in Fig. 6 indicates parts positions before the operation is started, and a solid line indicates parts positions after a predetermined period of time has passed from the beginning of the operation. Electron beam radiation conditions in this case are: an anode voltage of 25 kV, an anode current of 1,400 µA, and a radiation time of 90 minutes. When electron beams are continuously radiated through the shadow mask, the shadow mask is thermally expanded in a short period of time. In a minute, mask frame 18 is heated, and the frame diameter is increased. When the diameter of frame 18 is increased by radiation for a long period of time, portion 32 of each spring member 30 is pushed toward corresponding stud pin 24, and is deformed to decrease initial angle β to angle β1. Thus, frame 18 is shifted toward the phosphor screen by a distance of (cosβ1 - cosβ)×L, and purity drift can be compensated for. Reference symbol L indicates a length between bent positions 33 to 34 of portion 32. In this case, spring member 30 is swung about hole 36 of engaging portion 35, and engaging portion 35 is inclined to compensate for the purity drift. Since angle β (β1) is smaller than angle α (α1), the elastically deformable portion 32 is located between the frame 18 and the stud pin 24. Thus, a frame pressing force applied from first bent position 33 to portion 32 and a stud pin reaction force applied from second bent position 34 to portion 32 cancel each other. Portion 32, therefore, remain straight without curving. In this manner, since purity drift can be reliably compensated, a clear image can be obtained in a continuous operation for a long period of time. When a shock is applied, offsetting of the shadow mask can be appropriately compensated for by the leaf springs. When angle α is increased, angle β can be decreased while maintaining a predetermined shift amount (correction amount). Therefore, a leaf spring having a small spring constant can be employed, and spring fatigue can be effectively prevented. For this reason, a shadow mask can be easily attached/detached to/from the panel without damaging and deforming the parts. Since the spring constant of the leaf spring can be smaller than that of the conventional one, the thickness of the mask frame can be decreased, and the picture tube can be rendered lightweight. According to the first embodiment, in a conventional 21″ color picture tube having a beam deflection angle of 90 degrees, beam mislanding of about 30 µm was found in the screen corner regions. However, according to the present invention, the mislanding can be substantially eliminated. For this reason, color purity in the screen corner regions is not deteriorated in a continuous operation for a long period of time, and a clear image can be obtained. As shown in Figs. 8 and 9, in second and third embodiments, each spring member is combinated by two plates. More specifically, in spring member 40 of the second embodiment, the fixed portion of the first embodiment is omitted, and the bent end portion of the movable portion is directly welded to frame 18. In spring member 50 of the third embodiment, both ends of the elastically deformable portion are bent in an identical direction so that they are parallel to each other. More specifically, one end portion is bent at an angle of about 125 degrees, and the bent end portion is directly welded to frame 18. As shown in Fig. 10, in the fourth embodiment, each spring member is constituted by a single plate. More specifically, in spring member 60, the both ends of the elastically deformable portion are bent at an angle of about 125 degrees in opposite directions, and one bent end is directly welded to frame 18. A hole is formed in the other bent end portion to be engaged with the stud pin 24. According to the second to fourth embodiments, the number of parts constituting the spring member can be decreased, thus improving productivity, and reducing cost.

Claims: A color picture tube, comprising: a CRT envelope having a front panel. (11), the front panel (11) being composed of a substantially rectangular faceplate surrounded by a skirt, and a central line axis substantially perpendicular to the faceplate; a phosphor screen (14) formed on the inner surface of the faceplate; a shadow mask assembly including (a) a shadow mask (17), (b) a frame (18) surrounding said shadow mask (17), and (c) a supporting assembly detachably mounting the shadow mask facing to said phosphor screen (14), such that the shadow mask faces said phosphor screen; said supporting assembly having: stud pins (24) projecting perpendicularly from the inner wall of the skirt, and spring members (30,40,50,60) each having two ends, one end being fixed to said frame (18) and the other end being detachably engaged with a corresponding one of said stud pins (24), each of said spring members (30,40,50,60) including a single straight strip-shaped elastically deformable portion (32) located between said two ends, wherein said stud pins (24) project from the four corners of said front panel (11), wherein said spring members (30,40,50,60) include an engaging portion (35) for engagement with said stud pin (24), such that when the spring members (30,40,50,60) are displaced following the thermal expansion of the frame, an end of said engaging portion (35) more apart from the screen (14) than the stud pin (24) is displaced away from said skirt of the CRT envelope, characterized in that said single straight strip-shaped elastically deformable portion (32) of the spring members (30,40,50,60) and that plane of the frame (18) to which are mounted the spring members (30,40,50,60), said plane being parallel to the central line axis, define an angle β, and a line (38) connecting the end portion of said single straight strip-shaped elastically deformable portion (32) on the side of said frame (18) with said stud pin (24) and said plane of the frame (18) to which is mounted the spring member (30,40,50,60) define an angle α, wherein the end portion of said single straight strip-shaped elastically deformable portion (32) on the side of said engaging portion (35) is more apart from the screen (14) than the stud pin (24), and wherein angle β < angle α. A tube according to claim 1, characterized in that each of said spring members (30,50,60) has a substantially N-shaped longitudinal section. A tube according to claim 1, wherein each of said spring members (40) further includes an engaged portion detachably engaged with said corresponding one of said stud pins (24), the strip-shaped elastically deformable portion (32) and the engaged portion being connected together to form a substantially V-shaped spring. A tube according to claim 1, characterized in that each of said spring members (30, 40, 50) comprises a leaf spring composed of a plurality of thin plates. A tube according to claim 1, characterized in that each of said spring members (30, 40, 60) comprises a leaf spring formed by bending both ends of a thin plate in opposite directions so that both end portions are parallel to each other. A tube according to claim 1, characterized in that each of said spring members (50) comprises a leaf spring formed by bending both ends of a thin plate in an identical direction, so that both end portions are parallel to each other. A tube according to claim 1, characterized in that each of said spring members (60) comprises a leaf spring formed by bending a single thin plate. A tube according to claim 1, characterized in that a through hole (36) is formed in a free end portion of each of said spring members (30, 40, 50, 60), and is engaged with a corresponding one of said stud pins (24). A tube according to claim 8, characterized in that each of said stud pins (24) has a frustoconical shape. A tube according to claim 8, characterized in that a peripheral edge portion of the through hole (36) is bent to form a collar (37). A tube according to claim 1, characterized in that one end of each of said spring members (30, 40, 50, 60) is fixed by welding to said frame (18).

Inventors: INOUE MASATSUGU C O PATENT DIV, NAKANE KAZUNORI C O PATENT DIV, TOKITA KIYOSHI C O PATENT DIVI, INOUE, MASATSUGU C/O PATENT DIVISION, NAKANE, KAZUNORI C/O PATENT DIVISION, TOKITA, KIYOSHI C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H01J29
IPC codes: H01J31, H01J29