COLOUR CATHODE RAY TUBE

Abstract: In a colour cathode ray tube in which a shadow mask (18) is placed close to a phosphor screen (15) inside an envelope (14) which has an oblong-shaped panel (11), the shadow mask (18) is supported by spring members (30). The spring members (30) are each constructed from a first member (31) which is fixed so that it is easily mounted on and dismounted from the stud pins on the panel and a second member (32) which is welded to the mask frame (19) of the shadow mask (18). These members are incorporated as one by face-welding together at their ends. In this type of spring member structure, the members work together to alleviate the bending deformation which is applied to the members through the repeated mounting of the shadow mask panel which accompanies production.

Description: This invention relates to a shadow mask type colour cathode ray tube and, more particularly, to the supporting structure for the shadow mask. It is already known to support a shadow mask in the envelope of a colour cathode ray tube by employing spring members which connect the shadow mask to panel stud pins embedded in the wall of the envelope. The prior art will now be discussed with reference to the accompanying drawings, in which:- Figure 1 is a schematic cross sectional view of a part of a conventional cathode ray tube illustrating the correction for expansion; Figure 2 is a schematic cross sectional view of part of a conventional spring supporting structure; Figure 3 is a schematic cross sectional view of an alternative known improved spring supporting structure; Figure 4 is a perspective view of the spring supporting structure shown in Figure 3; Figure 5 is a front elevation of the structure of Figure 4 showing the deformation thereof; and Figure 6 is an enlarged view of a bent portion of the structure shown in Figure 4. Referring to Figure 1, Japanese Patent Publication No. 46-4104 describes the construction of a generally rectangular shadow mask which is secured at its four corners by spring members. When the spring members are employed, the shadow mask 8 moves slightly towards the phosphor screen 5 due to expansion when a significant rise in temperature occurs. A shift of the electron beam 4 also occurs caused by the relative displacement between the apertures in the shadow mask and the relevant phosphor dots on the screen and so colour reproduction is impaired. A spring member described in U.S. Patent No. 4300071 is shown in Figure 2 of this specification and the spring member 10 is constructed by bending a metal strip which is secured at one end to a frame 9 which supports the shadow mask 8 or to a rigid support secured to that frame and, at the other end it is secured to a panel stud pin 2. However, this construction has a fault in that, when the mounting and dismounting of the spring member 10 is repeated several times and it is bent as shown by broken lines 10a during the process of assembling a colour cathode ray tube, plastic deformation occurs at bend 10₁ and it does not recover its original shape. In addition, since four spring members are used to support one shadow mask and they are mass produced, it is impossible to make a completely homogeneous set. Mounting on the stud pins more than once and the heating to which they are subjected during the manufacture of the colour cathode ray tube causes any differences in the properties of the spring members to increase so that some members have an unexpected high plastic deformation. When the deformation of one of the four spring members secured to the shadow mask is greater than that of the others, the shadow mask is displaced from the desired position. As a result, the paths of the electron beams which pass through the shadow mask are shifted from the phosphor dots and there is a deterioration in the white uniformity. Even if the best stainless steel is employed for the manufacture of the spring members, this defect cannot be overcome. As a modification to this, a spring member 20, as shown in Figure 3 of the specification, can be employed. The spring member is bent in a dog leg shape from the upper end of the plate portion which is welded to the frame 9, towards the stud pin 2. In this form, when it is bent as at a broken line portion 20a for mounting or dismounting, since the whole of the upper portion bends from welding point 13, irrecoverable permanent deformation can be avoided. However, it has been proved that spring members which are formed by this kind of process of bending a continuous spring member have the fault of weakness to external mechanical shock. As shown in Figure 4, when a force F₁ from the panel stud pin (not shown) acts on the circumference of the hole 21 which accommodates the pin, and a force F₂ acts in the opposite direction to force F₁ on the surface 20₃ which is welded to the frame, permanent deformation of the spring member 20 can easily occur. This type of couple can sometimes occur when transporting colour cathode ray tubes. A study of the permanent deformation has found that the deformation of both ends 20₁ and 20₂ of the bent part of a spring member 20 due to the couple was marked. When a couple acting in the direction shown in Figure 4 was applied to spring member 20, the deformation shown by the broken line 20₄ in Figure 5 occurred. The reason for this was proved to be that the arc portion shown by arrow 20₅ in Figure 6 is liable to bend, i.e., when seen in Figure 5, the radius of the arc at end 20₁ becomes smaller and the radius at end 20₂ becomes greater. It is an object of the present invention to provide a colour cathode ray tube having improved means for supporting the shadow mask. According to the present invention, a colour cathode ray tube comprises an envelope having a generally rectangular-shaped panel carrying a phosphor screen on its inner surface; an electron gun assembly and an apertured shadow mask within the envelope; the shadow mask being positioned close to the screen and between the screen and the gun assembly; the mask being held around its periphery by a mask frame which has a plurality of spring supports by which the frame is secured to stud pins projecting inwardly from the wall of the envelope; characterised in that each spring support comprises a first member having first and third portions separated by a second portion which is inclined by an obtuse angle with the plane of the first portion and the third portion being inclined to the second portion and having an opening in it for receiving a stud pin; and a plate-like second member interposed between the mask frame and the first member and comprising first and third portions separated by an elastically deformable second portion, the first portion being welded to the mask frame and the third portion being welded to the first portion of the first member. It is convenient for the thickness of the second member of each support to be one or two times the thickness of the first member of each support. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to Figures 7 to 14 of the accompanying drawings, in which:- Figure 7 is a partial cross sectional view of an embodiment of this invention; Figure 8 is a perspective view showing an enlarged part of Figure 7; Figure 9 is a cross sectional view on the line II-II of Figure 8; Figure 10 is a partial cross sectional view showing the state when the shadow mask of an embodiment of this invention is mounted on or dismounted from the panel stud pins; Figure 11 is a perspective view illustrating an embodiment of this invention; Figure 12 is a graph of characteristic curves showing the difference for repeated tests of the initial measurement distances H of spring members by comparing an embodiment of this invention with a conventional structure; Figures 13a and 13b are side elevations illustrating the method of the spring member bending test with reference to Figure 12 in which (a) shows the state of a spring member before bending and (b) shows the bent state of a spring member; and Figure 14 is a perspective view of a spring member of an alternative embodiment of this invention. Referring to Figures 7, 8 and 9, a colour cathode ray tube has an envelope 14 constructed from a panel 11 made of glass which is essentially oblong-­ shaped or rectangular when seen by the viewer, a funnel 12 which is sealed around panel 11, and a neck 23 which is connected to the tapered section of funnel 12. On the inner surface of panel 11, a phosphor screen 15 is formed in a striped pattern of coloured phosphors which emit the three colours of red, green and blue. Facing screen 15, an electron gun 16 which emits three electron beams for exciting the phosphors is housed inside neck 23. Between screen 15 and electron gun 16, a shadow mask 18 is provided close to the screen. Shadow mask 18 is made of 0.2 mm thick iron sheet having a large number of slit-shaped apertures 17 etched in it by which the three electron beams are selectively divided on to the screen. The periphery of shadow mask 18 is secured to a relatively thin iron mask frame 19 of 0.4 mm thickness which is thicker than the plate thickness of the shadow mask. Screen 15 and shadow mask 18 are both generally oblong-shaped or rectangular and, as shown in Figure 7, spring supports 30 are welded to the outsides of the four corners of mask frame 19 which correspond to each diagonal. Each spring support 30, as shown in Figures 8 and 9, is made up of two members, each of 0.4 mm stainless steel (SU631) (precipitation-hardened stainless steel as specified by Japan Industrial Standard JIS:4305). The first spring member 31 forms a supporting section. Member 31 is formed with a fixed end 31₁ parallel to the outer wall surface of mask frame 19, an inclined portion 31₂ which forms an obtuse angle ϑ with the fixed end and which is bent from there with bending portion 31a as a basis and a supporting portion 31₃, having a supporting hole 34 to accommodate a stud pin 22 and which is extended from the inclined portion. The reasons for making ϑ an obtuse angle are so that the bend of the spring member agrees with the displacement due to the expansion of the shadow mask and also so that the strength of the spring member can be maintained at a relatively high level. The second plate-shaped spring member 32 is positioned between the first spring member 31 and the frame 19. Second spring member 32 is welded at several points along frame 19 on one of the outer walls of the four corners with a fixed portion 32₁. An elastic portion 32₂ extends from fixed portion 32₁. The welding point 32a which divides the elastic portion and fixed portion 32₁ is positioned at the point which becomes the reference position when the elastic portion deforms elastically. This elastic portion 32₂ has a specified length and, moreover, a joint portion 32₃ which extends from it. Fixed end 31₁ of first spring member 31 is fixed by welding with a surface joint to joint portion 32₃. An example of the measurements of the spring members in a 20-inch colour cathode ray tube is as follows: Spring Member 31 Plate thickness 0.35 mm; Width of fixed end 31₁ 13 mm; Length 3.5 mm; Width of inclined portion 31₂ 13 mm; Length 12.5 mm; Angle ϑ formed by the fixed end and the inclined portion from 128 to 117 degrees; Width of supporting portion 31₃ 13 mm; Length 15 mm; Centre of hole 34 is 5 mm from the boundary with the inclined portion. Second Spring Member Plate thickness 0.35 mm; Width 13 mm; Length of fixed portion 32₁ 11 mm; Length of elastic portion 32₂ 3.1 mm; Length of joint portion 32₃ 3.1 mm. In Figure 9, points X show welding positions and at each place there are two spots across the width of the plate. In particular, welding points 32a become the reference point when elastic portion 32₂ exhibits elastic deformation. When such spring members of this invention are bent for mounting and dismounting, as shown in Figure 10, since elastic portion 32₂ of second spring member 32 also bends outwards, it disperses part of the shock and so the stress does not concentrate in a specific part of the first member and permanent deformation will not occur. Moreover, when forces, for instance, F₁ and F₂, act in the width direction of a spring member due to external shock, as shown in Figure 11, a compression force will occur in one end 33a of the welded joint portion of first and second spring members 31 and 32, while a tensile force will occur at the other end 33b. However, generally the deformation due to compression and tensile force in materials from the viewpoint of material kinetics is far smaller than the bending deformation which occurs in the bending portions of the arcs shown in Figures 4 to 6. Therefore, the permanent deformation which occurs due to shock is also smaller. In fact, colour cathode ray tubes were made experimentally using spring members of conventional structures, for example, as in Figure 4, and of the structure of this invention, as shown in Figure 11. Tests were carried out by applying shock by dropping under the same conditions in each case. The fluctuation of beam landing was 102 µm for the conventional product and 56 µm for this invention. Moreover, Figure 12 shows the results of fatigue tests on spring members of 28-inch colour cathode ray tubes. This shows the results of alternate testing by repeated bending test when a stress F₃ is applied more than once to the original condition A in Figure 13a in the direction of the arrow to bend the member to condition B in Figure 13b, and by a heating process (450°C 30 minutes). In more detail, the distance H(mm) between the ends of first spring member 31 and second spring member 32 under condition A was tested by the processes mentioned above to find how much it would decrease. The solid line shows the characteristic of an embodiment of this invention and the broken line shows the characteristic of the conventional structure shown in Figure 4. From the above results, it is proved that the structure of this invention is no weaker than that of conventional products. Even after the production of colour cathode ray tubes, excellent supporting characteristics are ensured and there is little risk of undesirable deterioration of white uniformity. In another embodiment of this invention, the thickness of first spring member 31 was made 0.4 mm and that of second spring member 32 was made 0.5 mm. Thus, the second spring member was made slightly thicker. By this means, strength against shock can be improved and, furthermore, since the force required to bend the first spring member for mounting and dismounting does not increase greatly, the work of mounting and dismounting does not become more difficult. However, if second spring member 32 is made too thick, permanent deformation will occur when mounting and dismounting in the same way as for the conventional spring member shown in Figure 2. It was found experimentally that there was less permanent deformation when mounting and dismounting if the thickness of the second spring member was within the range of one to two times the thickness of the first spring member. If the thicknesses are in this range, permanent deformation will not occur, while sufficient mechanical strength is maintained. A further embodiment of this invention is the structure shown in Figure 14. The second spring member 32 is constructed having an arm 32₄ which extends from a corner section 19a of frame 19 to an adjacent section 19b. Incidentally, although the method of supporting at the corners has been described above, to carry out correction for heat expansion, the method of supporting at the mid-sections of the frame sides on the principle shown in Figure 2 is also possible, and, needless to say, this invention can also be applied for such a method. As described, a colour cathode ray tube can be produced by welding together the first and second spring members at their ends and securing the second spring member to the periphery of the frame or the mask. This always maintains the characteristic of good beam landing and permanent deformation due to mounting and dismounting or to external shock is small.

Claims: 1. A colour cathode ray tube comprising an envelope (14) having a generally rectangular-shaped panel (11) carrying a phosphor screen (15) on its inner surface; an electron gun assembly (16) and an apertured shadow mask (18) within the envelope; the shadow mask being positioned close to the screen and between the screen and the gun assembly; the mask being held around its periphery by a mask frame (19) which has a plurality of spring supports (30) by which the frame is secured to stud pins (22) projecting inwardly from the wall of the envelope; characterised in that each spring support (30) comprises a first member (31) having first (31₁) and third (31₃) portions separated by a second portion (31₂) which is inclined by an obtuse angle with the plane of the first portion and the third portion (31₃) being inclined to the second portion and having an opening (34) in it for receiving a stud pin; and a plate-like second member (32) interposed between the mask frame (19) and the first member (31) and comprising first (32₁) and third (32₃) portions separated by an elastically deformable second portion (32₂), the first portion being welded to the mask frame and the third portion being welded to the first portion of the first member. 2. A colour cathode ray tube as claimed in claim 1, characterised in that the mask frame (19) is of generally rectangular form and one of said spring supports is positioned at each corner of the mask frame. 3. A colour cathode ray tube as claimed in claim 2, characterised in that the second member of each spring support has an extension of the first portion which is welded to an adjacent portion of the mask frame. 4. A colour cathode ray tube as claimed in any preceding claim, characterised in that the thickness of the second member (32) of each spring support is one to two times the thickness of the first member (31) of the spring support.

Inventors: MURAI, RYUICHI PATENT DIVISION, NAKANE, KAZUNORI PATENT DIVISION, SONE, TOSHINAO PATENT DIVISION, URATA, HIROSHI PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: H01J29
IPC codes: H01J31, H01J29