Abstract:
A plurality of spaced reinforcing plates 6 are provided around the periphery of a Hip Up configured shadow mask 2 and its support frame 3 in a color picture tube assembly to strengthen and stiffen the mounting arrangement and thereby reduce Z axis deviations of the mask due to thermal deformation and/or mechanical shock.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a color television picture tube with an improved shadow mask mounting to minimize misalignments caused by thermal deformations and/or shock vibrations. 
     FIG. 1 shows a conventional color picture tube having an apertured shadow mask 2 to implement color selection or separation in the usual manner. The mask is mounted in a support frame 3, which is in turn mounted to the inner edges or skirt of a phosphor screen 1 by spring arms 4 attached to panel pins 5 extending inwardly from the screen. 
     The distance Q (FIG. 4) between the shadow mask 2 and the inner surface of the screen 1 is determined by: 
     
         Q=(L×P)/(3×S), 
    
     wherein L is the distance between the electron gun and the screen, S is the interval between the electron beams emitted from the gun, and P is the pitch or spacing between the shadow mask apertures. As may be seen from the equation, Q increases as L and P become larger and as S becomes smaller, which follows the current industry trend towards larger color picture tubes and smaller electron guns. 
     As a result of these large Q distances between the screen 1 and the shadow mask 2, the support frame 3 often projects beyond the skirt 11 of the screen as shown in FIGS. 2 and 4. Such projection creates a particular problem during the process of assembling the screen and the shadow mask; during assembly the screen faces upwardly as seen in FIG. 4 and the support frame thus receives the full weight of the screen. This leads to the deformation of the frame and thus the shadow mask, which in turn causes misalignment between the mask apertures and the phosphor dots or stripes on the screen. 
     In an effort to solve this problem the construction shown in FIG. 3 has been proposed, wherein the edge of the shadow mask, beginning at point D, is bent to form an unapertured, outwardly extending flange or step portion 22 perpendicular to the tube axis Z. Such a construction is termed a Hip Up configuration, and while it avoids the support frame projection problem, it renders the shadow mask unduly susceptible to Z axis deviations due to thermal deformation as shown in FIG. 5(A) and mechanical shock as shown in FIG. 6(A). Such deviations result from the low degree of stiffness in the vicinity of the step portion 22 and the skirt 21 of the shadow mask, which again leads to misalignment between the mask apertures and the screen phosphors. 
     SUMMARY OF THE INVENTION 
     The present invention handily avoids the drawbacks and disadvantages attendant with the prior art constructions by providing a plurality of reinforcing plates appropriately spaced around the support frame and configured to strengthen and stiffen the mounting of the flange and skirt portions of the shadow mask to the frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a screen and shadow mask of a conventional color picture tube, 
     FIG. 2 and FIG. 3 are edge sections showing two conventional configurations of a shadow mask support frame and a picture tube screen, 
     FIG. 4 is a sectional elevation showing a conventional assembly of color picture tube elements, 
     FIG. 5(A) and FIG. 5(B) are comparative edge sections showing shadow mask deviations due to thermal deformation in a conventional construction and in a construction according to the invention, respectively, 
     FIG. 6(A) and FIG. 6(B) are similar comparative edge sections showing mask deviations due to mechanical shock in the conventional and invention constructions, respectively, 
     FIG. 7 is an enlarged edge section showing a shadow mask mounting according to a first embodiment of the invention, 
     FIG. 8 is a comparative waveform diagram showing the magnitude and attenuation of shadow mask deviations due to mechanical shock for the conventional and invention constructions, 
     FIG. 9(A) and FIG. 9(B) are time plots showing electron beam deviation due to the thermal deformation of the shadow mask at the center and edge portions of the phosphor screen, respectively, for both the conventional and prior art constructions, and 
     FIG. 10, FIG. 11 and FIG. 12 are edge sections of second, third and fourth embodiments of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the first embodiment of the invention as illustrated in FIG. 7, the support frame 3, whose overall configuration would be generally rectangular, includes a flange portion 32 perpendicular to the Z axis of the picture tube and a skirt portion 31 generally parallel to the Z axis. The substantially rectangular shadow mask 2 includes a downwardly depending skirt portion 21 spot or plug welded to the shadow mask skirt at spaced points A, a planar unapertured flange or step portion 22 perpendicular to the Z axis, a similarly unapertured curved portion 23, and the usual curved or dome-shaped central portion 24 having a plurality of precisely positioned beam separation apertures. 
     In accordance with the invention a plurality of spaced reinforcing plates 6 are provided to strengthen and stiffen the mounting connections between the shadow mask and its support frame, each plate comprising a planar portion 62 overlying the step portion 22 of the shadow mask, a flange or tab portion 61 spot welded to the support frame at points B, and an upwardly angled, curved portion 63 overlying the similarly curved, unapertured portion 23 of the shadow mask and spot welded thereto at points C. The beneficial effects of the reinforcing plates in such a construction in significantly reducing Z axis or Q distance deviations of the critical central apertured portion 24 of the shadow mask due to both thermal deformation and mechanical shock are illustrated in FIG. 5(B) and FIG. 6(B), respectively. Moreover, the reinforcing plates not only reduce the magnitude of shadow mask deviations due to mechanical shock, as when the support frame is struck by a hammer, but more rapidly attenuate or dampen out any such vibrational deviations as comparatively shown by the solid curve a in FIG. 8 for a conventional mounting arrangement without any reinforcing plates and the dashed curve b for a construction embodying reinforcing plates in accordance with the invention. 
     Similar comparative curves a (solid/conventional) and b (dashed/invention) are shown in FIGS. 9(A) and 9(B) to illustrate the decreased electron beam deviations due to the thermal deformation of the shadow mask in the central and edge portions thereof, respectively, resulting from the use of reinforcing plates in accordance with the invention. Such curves evidence a beam deviation or misalignment reduction of about 20% in the central portion of the picture tube and about 60% in the edge portions thereof. 
     With a support frame thickness of from 1.2 mm to 2.0 mm and a shadow mask thickness of from 0.1 mm to 0.3 mm, which are conventional dimensional ranges in the industry, test results have indicated that the reinforcing plates should preferably be at least three times as thick as the shadow mask in order to maximize the beneficial strengthening and stiffening effects of the plates. As one example, good vibrational damping was achieved in a color picture tube having a screen width (diagonal) of 37 inches with a shadow mask thickness of 0.25 mm, a support frame thickness of 2.0 mm, and reinforcing plate thicknesses of 1.0 mm. 
     As various alternative configurations, FIG. 10 shows a second embodiment of the invention wherein the reinforcing plates 6 are disposed between the flange portion 32 of the support frame and the initial curved portion 23 of the shadow mask, FIG. 11 shows a third embodiment wherein the radially innermost portions of the reinforcing plates have been eliminated, and FIG. 12 shows a fourth embodiment wherein the reinforcing plates have a reverse bent, double L configuration. 
     Preferably just one or two reinforcing plates are provided on each of the four sides of the shadow mask/support frame mounting to avoid any wavy thermal deformations of the mask which might result from the use of too many reinforcing plates. Further, with picture tube screens having the phosphors laid out in striped rather than dot matrix patterns, the reinforcing plates need only be provided on the laterally opposite or short sides of the shadow mask since minor beam deviations in the vertical or Y axis direction are easily tolerated.