Hook spring of shadow mask frame assembly for color cathode ray tube

A hook spring of a shadow mask frame assembly for a cathode ray tube is formed by joining two metals having different thermal expansion coefficients over a predetermined range of temperatures. One ends of the hook spring is coupled with a stud pin installed at the inner side of a panel and the other end is coupled with a circumferential surface of a frame. The two metals are stainless steel and N48, and a volume ratio of the stainless steel and N48 is 5:5 to 6:4. Excessive compensation for thermal expansion of the shadow mask frame assembly can be reduced in the range of 80.degree..about.120.degree. C.

BACKGROUND OF THE INVENTION 
The present invention relates to a shadow mask frame assembly for a cathode 
ray tube, and more particularly, to a hook spring for supporting the 
shadow mask frame assembly and simultaneously compensating for gap 
variations between a fluorescent film and shadow mask due to thermal 
expansion of the shadow mask. 
Generally, as shown in FIG. 1, a color cathode ray tube (CRT) comprises a 
panel 10 on the inner side of which a fluorescent film 1i is formed, a 
funnel 20 sealedly coupled to panel 10 with an electron gun 23 and a 
deflecting yoke 24 being installed at a neck portion 21 and a cone portion 
22 thereof, respectively, and a shadow mask frame assembly 30 having a 
shadow mask 31 installed inside panel 10 and having a color selecting 
function of an electron beam radiated from electron gun 23 and a frame 32 
for supporting shadow mask 31. 
In the color CRT of such a structure, the electron beam radiated from 
electron gun 23 passes through electron beam passing holes (not shown) of 
shadow mask 31 and forms an image while landing at a fluorescent point of 
fluorescent film 11. 
To accurately land the electron beam radiated from electron gun 23 onto the 
fluorescent point of fluorescent film 11, a supporting state of shadow 
mask 31, that is, a displacement interval (.theta.) between the 
fluorescent film 11 formed on the inner side of panel 10 and shadow mask 
31 should be strictly controlled. 
Not all the electron beam radiated from electron gun 23 passes through the 
aperture formed on shadow mask 31, i.e., only about 15 to 20% thereof 
passes through the aperture. The thermions in the electron beam, which do 
not pass through the aperture collide with shadow mask 31 to heat shadow 
mask 31, thereby causing a doming phenomenon. As a result, shadow mask 31 
undergoes thermal expansion, thus displacing the aperture formed thereon. 
Accordingly, the electron beam passing through the aperture of shadow mask 
31 does not accurately land at the fluorescent point of the fluorescent 
film. 
To overcome the above troubles, as shown in FIG. 2, a hook spring 40, 
coupled with a stud pin 15 installed at the interior of panel 10 and also 
fixed to the circumferential surface of frame 32 of shadow mask frame 
assembly 30, is manufactured in the form of a bimetal made of a juncture 
of stainless steel 41 and Invar 42 having different thermal expansion 
coefficients from each other. Thus, the doming phenomenon according to the 
thermal expansion is compensated for. 
As a CRT becomes flattened and enlarged, the doming amount of shadow mask 
31 is reduced by manufacturing shadow mask 31 with Invar having a low 
thermal expansion coefficient. Thus, when a hook spring of the 
above-described bimetal type which is made of stainless steel and Invar is 
employed, the compensation for displacement according to the thermal 
expansion of the shadow mask frame assembly is excessive within CRT 
operating temperatures of 100.degree..about.120.degree. C. 
That is, as shown in Table 1, when the stainless-to-Invar volume ratio of 
the hook spring is 7:3 or 8:2, the displacement of the electron beam 
deviated from a preset track when passing the aperture which is displaced 
due to the doming of the shadow mask according to the thermal expansion 
thereof is beyond .+-.10 .mu.m, which is a test standard of for CRT's 
within a range of 100.degree..about.120.degree. C. Such a phenomenon 
indicates that the compensation by the conventional hook spring for the 
thermal expansion of the shadow mask frame assembly is excessive made. 
TABLE 1 
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DISPLACEMENT DISPLACEMENT 
AT AT 
RATIO TEMPERATURE A TEMPERATURE B 
REMARKS 
______________________________________ 
5:5 +50 .mu.m -- inferior 
6:4 +20 .mu.m -- inferior 
7:3 +5 .mu.m +20 .mu.m inferior 
8:2 0 +15 .mu.m inferior 
9:1 -10 .mu.m -- inferior 
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In Table 1, the amount of displacement of the electron beam deviated from 
the preset track after passing through the displaced aperture of the 
shadow mask is shown for the cases where the CRT operating temperature is 
80.degree. C. (temperature A) and 100.degree..about.120.degree. C. 
(temperature B). 
The above excessive compensation can be prevented by adjusting the ratio of 
cross sections of the metals constituting the hook spring. However, since 
the degree of thermal expansion of each metal is different within a 
particular temperature range, a difference in the compensation amount 
within the range of compensation temperatures occurs. Thus, it is a 
problem that the compensation for the doming phenomenon varies according 
to temperature. 
SUMMARY OF THE INVENTION 
To solve the above problems, it is an object of the present invention to 
provide a hook spring of a shadow mask frame assembly, by which 
differences in compensation amounts within the range of compensation 
temperatures can be reduced. 
Accordingly, to achieve the above object, there is provided a hook spring 
of a shadow mask frame assembly for a cathode ray tube which is formed by 
uniting two metals having different thermal expansion coefficients over a 
predetermined range of temperatures. One end of the hook spring is coupled 
with a stud pin installed at the inner side of a panel and the other end 
is coupled with the circumferential surface of a frame. The two metals are 
stainless steel and N48, and a volume ratio of the stainless steel and N48 
is 5:5 to 6:4.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIG. 1, a color CRT comprises a panel 10 on the interior of 
which a fluorescent film 11 is formed and a stud pin 15 is installed, a 
funnel 20 sealedly coupled with panel 10 and provided with an electron gun 
23 and deflection yoke 24, respectively, at a neck portion 21 and cone 
portion 22 thereof, a shadow mask frame assembly 30 fixed to the interior 
of panel 10, and a hook spring 50, opposite ends of which are coupled with 
stud pin 15 and the circumferential surface of frame 32 of shadow mask 
frame assembly 30, respectively. 
The hook spring 50 is made of bitmetal each component of which has a 
different thermal expansion coefficient. As shown in FIG. 3, the bimetal 
hook spring 50 is comprised of a first member 51 formed of stainless steel 
and a second member 52 formed of N48, being mutually united. A volume 
ration of the two metals is preferably 5:5 through 6:4. Here, material N48 
(manufactured by Metalimphy Produits of France) is an alloy consisting 
essentially of, in weight %, at most 0.02% C, at most 0.25% Si, 0.3-0.6% 
Mn, at most 0.007% P, at most 0.002% S, 47-48.5% Ni, and a balance of Fe. 
The operation of the hook spring having such a structure according to the 
present invention will now be described. 
In the color CRT adopting the hook spring according to the present 
invention, an electron beam radiated from electron gun 23 passes through 
the aperture formed in shadow mask 31 of shadow mask frame assembly 30. 
Then, the beam lands on the fluorescent film of panel 10 and forms an 
image by exciting the fluorescent film. In doing so, shadow mask frame 
assembly 30, being heated by thermions which do not pass through the 
aperture of the shadow mask, undergoes thermal expansion. The thermal 
expansion of shadow mask frame assembly 30 causes the doming phenomenon of 
the shadow mask 31 as shown in FIG. 4 and, the displacement interval 
decreases to the amount shown by reference numeral 100. On the other hand, 
as frame 32 for supporting shadow mask 31 is thermally expanded by means 
of the heat transferred from shadow mask 31, the displacement interval 
increases to the amount shown by reference numeral 200. While the 
thermally expanded hook spring transfers the shadow mask frame assembly 30 
toward panel 10, the displacement interval alters to the amount shown by 
reference numeral 300. 
Here, when the volume ratio of a first member 51 formed of stainless steel 
which is a high thermal-expansion metal and a second member 52 formed of 
N48 which is a low thermal-expansion metal is adjusted to 5:5.about.6:4, 
the excessive compensation of the shadow mask frame assembly can be 
reduced during CRT operation. That is, since first and second members 51 
and 52 of the hook spring 50 each have a slightly different slope of 
temperature as shown in FIG. 5, variation in the compensation amount 
throughout the operating temperature range of the CRT can be reduced. 
Thus, a rapid change of the compensation amount within the compensation 
temperatures of 100.degree..about.120.degree. C. can be prevented. 
The results of experiments carried out by the present inventor are shown in 
Table 2. Here, stainless steel and N48 are united using four different 
volume ratios (stainless steel-to-N48). 
TABLE 2 
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DISPLACEMENT DISPLACEMENT 
AT AT 
RATIO TEMPERATURE A TEMPERATURE B 
REMARKS 
______________________________________ 
5:5 0 +5.mu.m superior 
6:4 -5.mu.m +1.mu.m superior 
7:3 -20.mu.m -- inferior 
8:2 -25.mu.m -- inferior 
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In Table 2, as in Table 1, the amount of displacement of the electron beam 
deviated from the preset track after passing through the displaced 
aperture of the shadow mask is shown for the cases when the CRT operation 
temperature is 80.degree. C. (temperature A) and 100.degree. 
C..about.120.degree. C. (temperature B). 
It is noted from the above result that when the volume ratio of stainless 
steel (i.e., a high thermal expansion metal) and N48 (i.e., a low thermal 
expansion metal) in the hook spring is 5:5 and 6:4, the experimental 
result is superior. Particularly, when the ratio is 5:5, the displacement 
amount of the electron beam due to thermal expansion of the shadow mask is 
minimized to 0 .mu.m and +5 .mu.m at 80.degree. C. and 
100.degree..about.120.degree. C., respectively. 
As described above, the hook spring of the shadow mask frame assembly of 
the present invention using stainless steel and N48 can improve 
compensation of the doming of the shadow mask. Also, the resolution of a 
CRT adopting the present invention is enhanced.