1. Field of the Invention
The present invention relates in general to a cathode ray tube, more particularly, to a cathode ray tube mounted with a reinforcing band for improving landing stability of electron beams and howling of shadow mask and reinforcing an explosion-proof function.
2. Discussion of the Background Art
FIG. 1 illustrates the structure of a related art cathode ray tube, and FIG. 2 is a cross-sectional view taken along line A–A′ in FIG. 1.
As shown in FIGS. 1 and 2, the related cathode ray tube includes a panel 3 having a fluorescent screen 6 formed on an inner surface, a funnel 5 connected to a seal edge portion 4 of a skirt portion 2 of the panel 3, an electron gun 14 mounted on a neck portion 12 of the funnel 5, emitting electron beams 13, deflection yokes 15 for deflecting the electron beams 13 emitted from the electron gun 14, a shadow mask 8 for selecting colors of the electron beams 13, and a mask frame 9 for supporting the shadow mask 8.
The mask frame 9 is connected to a stud pin 10 formed on the skirt portion 2 of the panel 3 by means of mask springs 11.
Also, a reinforcing band 17 straps around an outer peripheral portion of the skirt portion 2 of the panel 3 in order to prevent CRT implosion by external impacts, and a lug 18 to be incorporated in a TV set is also formed on the outer peripheral portion of the skirt portion 2 of the panel.
The Z—Z line in FIG. 1 indicates a tube axis.
As depicted in FIG. 3, the reinforcing band 17 is usually mounted on a portion where the stud pins 10 are formed.
However, when the reinforcing band 17 is mounted on where the stud pin 10 is formed, a howling phenomenon occurs to the shadow mask 8 due to voice vibrations generated from a speaker mounted on the outside of the cathode ray tube.
To be more specific, the voice vibrations generated from the speaker are transmitted to the reinforcing band 17, and to the stud pin 10 formed inside of the skirt portion 2 of the panel 3, and eventually to the mask frame 9 and the shadow mask 8 connected to the stud pin 10.
As a result, the howling phenomenon occurs to the shadow mask 8, and electron beams do not land stably, eventually deteriorating a picture quality.
As FIG. 4 shows, clamping power of the reinforcing band 17 is influenced by the outer peripheral length of the skirt portion 2 of the panel 3 and the inner peripheral length of the reinforcing band 17. The clamping power of the reinforcing band 17 influences in turn picture quality characteristics of the cathode ray tube.
FIG. 4 illustrates clamping power errors of the reinforcing band. More details on the drawing are now followed.
Referring to FIG. 4, Lm denotes an inner peripheral length of the reinforcing band 17, ±β denotes tolerance errors in a manufacturing process for the reinforcing band, Lg denotes an outer peripheral length of the skirt portion 2, and ±α denotes tolerance errors in a manufacturing process for the skirt portion. Here, the reinforcing band 17 has a maximum clamping power at a point Pa, and a maximum clamping power at a point Pb.
Generally, the reinforcing band 17 is expanded by heating. The reinforcing band 17 is welded to the skirt portion by stress applied thereon when the reinforcing band is strapped around the outer peripheral portion of the skirt portion 2. And, the reinforcing band 17 becomes clamped by cooling. However, depending on temperature and material characteristics, there can be errors, and these errors eventually change the clamping power of the reinforcing band.
Errors of the clamping power of the reinforcing band 17 contribute to the difference in compressive stress and the deviations in landing of electron beams.
After all, a picture quality of the cathode ray tube is severely deteriorated. This problem becomes worse, however, as shown in FIG. 3, if the reinforcing band 17 is mounted on an outer surface of the skirt portion where the stud pin 10 is formed.
In addition, compared to a traditional cathode ray tube having a convex outer surface, an inner surface of radius of curvature (Ri in FIG. 3) of the panel whose outer surface is substantially flat is increased, and this in turn increases the curvature radius of the shadow mask 8, thereby lessening the strength of the shadow mask 8.
Particularly, in case of using tint glass having less than 51% of light transmittance, the difference between a thickness at the central portion (Tp) of the panel and a thickness at the peripheral portion (Tw) of the panel causes about 15 fL of brightness reduction to the peripheral portion. Thus, the thickness at the peripheral portion (Tw) should be reduced.
However, if the peripheral portion of the panel 3 gets thinner, the radius of curvature of the inner surface of the panel 3 is increased, and this causes an increase of the radius of curvature of the shadow mask 8 disposed at a predetermined distance (Tmp) from the inner surface of the panel 3. When these happen, the strength of the shadow mask 8 is lessened, being more subject to the influence of external impacts.
FIG. 5 shows different thicknesses of a panel with and without a tint glass.
Since the thickness at the peripheral portion of a tint glass (Pt) is relatively thinner than a clear glass (Pc), as shown in FIG. 6, the radius of curvature of the inner surface (Ri) of the tilt glass (Pt) is greater than that of the clear class (Pc), and the strength of the shadow mask 8 is lessened even more.
Therefore, the howling phenomenon due to voice vibrations of the speaker, and clamping power difference (error) of the reinforcing band 17 play a more negative role in landing of electron beams.
FIG. 7 describes stress being applied to the inside of a cathode ray tube in vacuum state.
When the inside of the cathode ray tube is in vacuum state, compressive stress (σc) is applied to the central portion of the panel 3, and tensile stress (σt) is applied to the peripheral and skirt portions of the panel 3.
If the cathode ray tube in such state is broke down by external impacts, fragments of the panel 3 are first sucked into the cathode ray tube because of atmospheric pressure, and split to the outside later.
The above phenomenon is called ‘accidental implosion phenomenon’. Thus, the reinforcing band 17 constricts the skirt portion 2 to minimize splitting of glass fragments at the time of CRT implosion.
FIG. 8 shows how stress changes according to a reinforcing band mounted on the skirt portion.
Referring to FIG. 8, if the reinforcing band 17 is mounted on a skirt portion 2 of a panel having a predetermined outer surface radius of curvature (Ro), clamping power (P) of the reinforcing band 17 acts upon the skirt portion 2, thereby lessening compressive stress at the central portion of the panel 3.
More specifically, as the compressive stress is lessened by mounting the reinforcing band 17, a variation (hv) of the panel before the reinforcing band 17 is mounted thereon is changed to a variation (hb) of the panel after the reinforcing band 17 is mounted thereon.
What happens here is that the compressive stress is cancelled by the clamping power (P) of the reinforcing band 17. Therefore, even when CRT implosion accidentally occurs due to the lessened compressive stress at the central portion of the panel 3, much less fragments are split outward.
FIG. 9 illustrates how clamping power of a reinforcing band acts upon a skirt portion of a panel.
As depicted in FIG. 9, when the reinforcing band 17 is mounted on the skirt portion 2 of the panel 3 having a predetermined radius of curvature (Ro) on its outer surface, the clamping power (P) of the reinforcing band 17 is divided into a vertical component of power (σn) and a horizontal component of power (σp), and their resultant power (σr) is applied along the curvature radius of the panel 3.
Thus, the compressive stress acting upon the central portion of the panel 3 because of atmospheric pressure is cancelled by the resultant power (σr), and as a result, the compressive stress is lessened, as discussed in FIG. 8.
However, the above operation is not always equally applied to a substantially flat outer surface.
FIG. 10 describes changes in stress after mounting a reinforcing band on a panel having a substantially flat outer surface, and FIG. 11 illustrates how clamping power of a reinforcing band acts upon a panel having a substantially flat outer surface.
As shown in FIG. 10, a variation (hb) of compressive stress acting upon a central portion of the panel 3 where the reinforcing band is mounted on the panel having a substantially flat outer surface is rather increased, in contrast with what is shown in FIG. 8.
That is, thanks to the clamping power (P) of the reinforcing band, the compressive stress after mounting the reinforcing band 17 is much improved, compared to before mounting the reinforcing band 17.
This phenomenon occurs because the clamping power (P) of the reinforcing band 17 is divided into a vertical component of power (σn) and a horizontal component of power (σp), as shown in FIG. 11, and their resultant power (σr) is applied to the inward of a cathode ray tube because the outer surface of the panel 3 is substantially flat.
That is to say, it is hard to expect the same effect, namely improving the explosion-proof characteristic of a cathode ray tube, from mounting the reinforcing band 17 on the panel having a substantially flat outer surface with the one on the panel whose outer surface is curved,