Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Reexamination Certificate
2000-08-02
2002-07-16
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
C313S407000
Reexamination Certificate
active
06420823
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a shadow mask and a color CRT, and more particularly to a shadow mask having a structure in which a shadow mask is provided in only one direction, and to a color CRT that uses a shadow mask of this construction.
2. Background of the Invention
We will show two references as follows.
The cited reference 1 denotes a color selecting mechanism of cathode-ray tube and frame for its color selecting mechanism which is disclosed in Japanese Unexamined Patent Application Publication No. 10106449A. This application intends to obtain a curved surface where a frame surface is single or close to this by constituting so that a surface to which a color selecting electrode thin plate is fixed is formed as a curved surface having at least one or more inflection points while proceeding to an end part from a central part.
After support members of a frame are molded by press working or the like so that the whole become respectively necessary curvature in the Y direction and the Z direction, a curved surface of a surface to which the support members are welded is formed by cutting work. The surface is a curved surface on/to which a color selecting electrode thin plate is stretched/welded, and is defined by a fifth-degree equation expressed by, for example, an equation. In this way, since the surface to which the color selecting electrode thin plate of the frame is fixed is formed as a curved surface having at least one or more inflection points while proceeding to an end part of a central part, a curved surface where a frame surface is single or close to this, can be obtained.
The cited reference 2 denotes a cathode-ray tube which is disclosed In Japanese Unexamined Patent Application Publication No.11273586A. This application intends to prevent deterioration of images due to vibration of an AG tape.
In this cathode-ray tube comprising an aperture grill having a large number of slit holes, a frame supporting the aperture grill, and a color selecting mechanism having vibration control wires tightly stretched on the aperture grill, the aperture grill are supported by the frame so as to form approximately cylindrical face with three curvature radiuses, so that the pressurizing force of the vibration control wires can largely be applied especially to the points where the AG tape is easily vibrated.
In recent years, there has been an increase in so-called flat color CRTs, in which the glass panel surface is substantially flat.
FIG. 7
shows a partial perspective cross-sectional view of a glass panel
700
for use in a flat CRT. In this drawing, the reference numeral
701
is a glass panel surface (outside of the color CRT), and
702
is the glass panel inner surface (inside of the color CRT). As shown in this drawing, even if the glass panel surface
701
is substantially flat, the thickness of the glass panel cross-section is thicker toward the periphery than at the center, and has the shape of a concave lens that forms a part of a cylinder. The first reason for this is that, if the glass thickness at the periphery were to be the same as at the center, the withstanding pressure of the glass bulb would become low, and because there is a great danger of implosion in a color CRT. The second reason is that, if the glass thickness at the periphery is the same as that at the center, the image will appear to have an unnatural dimpling effect. To prevent implosion and impart an appearance of flatness to the image, the glass thickness is made greater toward the periphery, with respect to a reference thickness at the center of the glass panel cross-section. The amount of increase in glass thickness at the periphery with respect to the center is the wedge amount W, as shown in the drawing. The wedge amount W has an appropriate value, which is dependent upon the size of the color CRT. For example, for a 19-inch color CRT, the proper wedge amount is 3 mm.
Because as noted above the glass panel inner surface
702
has the shape of a concave lens that forms a part of a cylinder, a shadow mask of the past was accordingly made as a cylinder type with a constant radius of curvature that is substantially congruent with the glass panel inner surface
702
.
FIG. 8
is a perspective view of a shadow mask structure
800
of the past. In this drawing, the reference numeral
801
is a mask frame,
802
is a shadow mask (only the periphery of which is shown, to make the inside of the mask frame
801
more visible), and
802
A is a shadow mask welding point. The shadow mask
802
is tensioned via the shadow mask welding point
802
A at two sides of the mask frame
801
.
FIG. 9
is a cross-sectional view of the shadow mask
800
of the past, in the direction of X-X′. As shown in this drawing, the shadow mask
800
of the past has a constant radius of curvature R
0
everywhere, and is cylindrical in shape. This radius of curvature R
0
is, for example, approximately 4050 mm for a 19-inch color CRT.
The above-described shadow mask
800
of the past, however, has the following problems. When manufacturing a color CRT, the shadow mask
802
is tensioned onto the mask frame
801
at room temperature (approximately 25° C.). For this reason, the shadow mask
802
maintains a normal tensioned condition, with no deformation, at room temperature. That is, the shadow mask
802
maintains a non-deformed and normal tensioned condition when the color CRT is not being used.
However, when the color CRT is in use, because an electron beam collides with the shadow mask
802
, the shadow mask structure
800
rises to a temperature of approximately 60° C. Because the shadow mask
802
is made of Invar (36% nickel iron alloy), which has a low coefficient of thermal expansion, there is almost no thermal expansion, the coefficient of thermal expansion of Invar being approximately 1.2 ppm/K at room temperature. Because the mask frame
801
, however, is made of 13 chromium stainless steel, which has a coefficient of thermal expansion approximately 10 times that of Invar, it exhibits considerable thermal expansion. Because of the difference in thermal expansion between the shadow mask
802
and the mask frame
801
, the shadow mask
802
is subjected to stress.
FIG. 10
is a plan view of the shadow mask
802
of a shadow mask structure
800
of the past, seen from the direction A. In this drawing, F indicates the distribution of stress on a side of he shadow mask
802
attributed to the difference in coefficients of thermal expansion between the shadow mask
802
and the mask frame
801
. Because the shadow mask
802
is free in the lateral direction, stress is not applied to the left and right sides thereof. In contrast to this, because the top and bottom sides of the shadow mask
802
are welded to the mask frame
801
, because of the thermal expansion of the short side of the mask frame
801
, a stress F is applied to the as shown in the drawing, this being small at the center of the side and large at the periphery of the side. Because of the non-uniform distribution of this stress F, a stress is developed in the shadow mask
802
perpendicular to the stress F, thereby pulling the shadow mask
802
downward, resulting in the deformation of the shadow mask described below.
FIG. 11
shows a plan view of the shadow mask
802
of the shadow mask structure
800
of the past from the direction A of
FIG. 8
, and the X-X′ and Y-Y′ cross-sectional views thereof. In these cross-sectional views, the condition shown is that in which the shadow mask
802
is at 25° C. (broken line) and at 60° C. (solid line). As shown in this drawing, when the temperature of the shadow mask structure rises, the shadow mask
802
is deformed by pulling in a direction that moves it away from the glass panel
700
of the color CRT. The amount of deformation is greatest near the center of the shadow mask
802
and, by experiments performed by the inventors, this was found to be approximately 100 &mgr;m near the center, as shown in the drawing. As is known, when the distance betwe
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