Color Television Picture Tubes With Improved Implosion Protection System

This application relates to but is in no way dependent upon copending applications of common ownership herewith including:

Ser. No. 623,854, filed Oct. 20, 1975; Ser. No. 623,852, filed Oct. 20, 1975; Ser. No. 639,741, filed Oct. 20, 1975; Ser. No. 714,055, filed Aug. 13, 1976; and Ser. No. 632,559 filed Nov. 17, 1975.

This invention relates in general to color television picture tubes and in particular, to a system for implosion protecting such tubes. Conventionally, a color television picture tube has a glass bulb including a funnel, a flanged faceplate sealed to the flared end of the funnel, and an electron gun assembly mounted in the funnel neck for providing a source of cathode rays. The faceplate has a concave inner surface on which is deposited an electron-excitable phosphor screen. After the faceplate is sealed to the funnel, the glass bulb is evacuated and as a result, several tons of atmospheric pressure is exerted against the external surface of the faceplate. A glass bulb of this type is subject to implosion. The term "implosion" is defined by Underwriters Laboratory Incorporated as "rapid and sudden inward bursting of a high-vacuum glass envelope." It is of the utmost importance in the interest of safety to prevent the faceplate from violently shattering should it be struck for example by a heavy missile, for when a bulb implodes fragments of glass may fly forwardly from the tube into the viewing area.

Three basic approaches for implosion protecting color cathode ray tubes (CRT's) have evolved. These three approaches employ different principles of operation. One approach is implemented in systems referred to as "rimbond" systems. The rimbond system has a scalloped metal frame which surrounds the flange found on every conventional faceplate. The gap between the frame and the faceplate flange is filled with a cement -- typically an epoxy cement. In a rimbond system, the frame is not under tension. The cement holds in position the pieces of glass of a shattered faceplate long enough for air to enter the tube through the cracks formed so that pressure builds up in the tube relatively slowly. This prevents unacceptable amounts of glass from being projected forwardly from the tube although the tube may still collapse. Patents illustrating such rimbond systems are U.S. Pat. Nos. 3,485,407; 3,558,818; 3,412,203 and 3,835,250. A major drawback to such rimbond systems has been the large amounts of very costly epoxy cement needed to adhere the metal frame to the faceplate.

A second basic implosion protection approach is termed the "tension band" approach. Systems implementing this approach comprise a strap or band which is placed around the faceplate flange and put under very high tensile force. Numerous patents have been issued on various aspects of tension band systems. See U.S. Pat. Nos. 3,818,557; 3,777,057, 3,845,530; and 3,890,464. The tension band systems, however, also have several drawbacks. When the tension band is tightened about the faceplate flange, it is very likely that the glass will be scratched as the band moves across it during the tightening process. This creates flaws at the location of the scratches, increasing the possibility of cracks forming there during implosion. Also, the distribution of forces applied to the faceplate flange by the band is irregular. Specifically, the forces applied at the corners by the band are much greater than the forces applied at the sides of the faceplate flange.

The third approach is to bond a transparent protective shield over the front surface of the faceplate. Systems following this approach are commonly termed "bonded panel" systems. The bonded panel systems have no pertinence to this invention and therefore will not be discussed further.

A U.S. Pat. No. 2,222,197 to Engels discloses a CRT in which the CRT envelope comprises a curved flangeless faceplate insert in an expanded open end of a cooperating funnel. A band allegedly providing implosion protection surrounds the funnel near the open end thereof in a plane intersecting the faceplate which is ensconsed within the funnel mouth. The Engels system is quite different from the present system. No frame of any sort is provided. The implosion band environs the funnel rather than the faceplate peripheral edge. A comparison of the Engels system and the present system will reveal other important differences also.

This invention is believed to be most useful when applied to a tube having a flangeless faceplate. This tube is disclosed in U.S. Pat. No. 3,894,260, issued to the assignee of this application. The tube has a flangeless, curved glass faceplate, a concave inner surface of which receives a phosphor screen. The funnel portion of this unique tube has a convex seal land which matches and mates with the curvature of the concave inner surface of the faceplate. Since the faceplate is flangeless, and the sealing interface between the funnel and faceplate is curved rather than planar as in conventional tubes, the relevant conventional structures for implosion protection cannot be used with this tube.

It is an object of the present invention to provide for a color television picture tube an improved system for implosion protection.

It is another object of the present invention to provide an effective and low-cost implosion protection system for a novel color television picture tube having a flangeless faceplate.

FIG. 1 is a perspective view of a color cathode ray tube embodying the present invention;

FIG. 2 is an enlarged schematic fragmentary side section view of the FIG. 1 tube; and

FIG. 3 is an isolated schematic perspective view of a frame constituting part of the implosion protection system shown in FIGS. 1 and 2.

Whereas the invention may be implemented in color cathode ray tubes of various types, it is preferably embodied in a tube of the nature shown in FIG. 1. The tube 2, as illustrated in FIG. 1, has an envelope comprising a funnel 4 sealed to a flangeless faceplate 6. The novel construction of the faceplate 6 without a flange permits economies in manufacture of the envelope and simplified and economical screening and assembly processes. The faceplate 6 has a curved configuration which may be spherical, multi-radial, cylindrical, or of other suitable curvature. The faceplate 6 has a convex front surface 8 connected to a concave rear surface 10 by a peripheral edge surface 12. The edge surface 12 is contoured, that is, the edge surface portions along sides of the faceplate depart from and return to a plane connecting the four corners of the faceplate.

The funnel 4 has a convex seal land, herein intended to mean a seal land which lies on an imaginary curved surface, which surface curvature may be spherical, multi-radial, cylindrical, or of other suitable curved configuration. The seal land of the funnel 4 is curved to match and mate with the concave rear surface 10 of the faceplate 6 along a sealing interface 16. The seal land of the funnel 4 is hermetically bonded to the rear surface 10 of the faceplate 6 by a devitrifying glass solder, herein termed a "frit material" 22.

The concave rear surface 10 of the faceplate 6 is here shown as being slightly larger than the wide end of the funnel 4 to which the faceplate 6 is attached. Thus, when the tube 2 is assembled, the faceplate overhangs the funnel slightly. Alternatively, the faceplate edge surface 12 may be flush with the outside surface of the funnel 4.

The present invention will now be described. As described, relevant prior art system used on conventional flanged faceplate tubes followed either of two approaches: 1) the "rimbond" approach wherein a portion of the outside surface of the faceplate flange is cemented to a surrounding variable depth frame; and 2) the "tension band" approach wherein the flange is compressed tightly by a high tension band. This invention involves a unique approach to implosion protection. By this invention, there is provided an edgebond implosion protection system for a color CRT in which the thickness dimension of a tube face is bound up and held by cemented frame. In a preferred embodiment, in the interest of minimizing costs, the frame is preferably shallow and of uniform depth around the tube. A novel edgebond implosion protection system constructed according to this invention is illustrated in FIGS. 1-3.

A high tensile strength frame 14 surrounds and overlies the peripheral edge surface 12 of the faceplate 6. The frame 14 is shown in isolation in FIG. 3. The frame 14 has a contour corresponding generally to that of the edge surface 12 of the faceplate 6, that is, a contour in which the sides of the frame 14 depart from and return to a plane (shown at 23 in FIG. 3) connecting the four corners of the frame 14, the direction of departure of the frame 14 from the said plane 23 being the same as the direction of departure of the sides of the faceplate edge surface 12 from a plane connecting the four corners of the faceplate 6.

Surprisingly, it has been found that effective implosion protection is provided by a frame which is shallow and which has a substantially uniform front-to-back depth. As will become evident, a shallow frame according to this invention offers economies not only in frame cost, but more importantly in the amount of costly cement required to bond the frame to the tube (to be described in more detail hereinafter). With a shallow frame edgebond system according to this invention for a 23 inches diagonal tube, only about 1/4 lb. to 1/2 lb. of epoxy cement is believed to be necessary to provide adequate implosion protection. The small amount of epoxy cement necessary results in significant savings in the manufacture of color television picture tubes since suitable epoxy cements cost in the order of $.90 per pound.

The frame 14 has a depth which is preferably of sufficient magnitude to overlie the sealing interface 16 of the faceplate 6 and the funnel 4 as well as the peripheral edge surface 12 of the faceplate 6. Although not necessary, it is preferable also that the frame 14 have a lip 17 which overlies a small marginal portion 15 of the convex front surface 8 of the faceplate 6.

To assemble the system, the frame is placed in its correct position with respect to the faceplate 6, and a cement 18, preferably an epoxy cement, is introduced in the gap between the frame 14 and the edge surface 12 of the faceplate 6. A gasket 20 disposed between the lip 17 and the marginal portion 15 of the surface 8 prevents the cement 18 from flowing onto the convex front surface 8 of the faceplate 6; other suitable methods also may be used to prevent the cement 18 from flowing onto the front surface 8.

As illustrated in FIGS. 1 and 3, mounting tabs 25 for attaching the tube 2 to a cabinet may be included as part of the frame 14. In the preferred embodiment the tabs 25 extend from the corners of the frame 14 and have provisions such as holes for permitting attachment of the tube to the cabinet. Alternatively, these tabs could be placed anywhere about the frame or formed in a way which would best suit the type of cabinet being used.

An important and radically different aspect of this edgebond implosion-protection structure is that the frame 14 and frame-contained cement 18 embrace and bind up the entire edge surface 12 of the faceplate 6. In a preferred execution, the cement 18 embraces and binds up not only the entire edge surface 12 but a small portion 15 of the convex front surface 8 and the sealing interface 16 of the faceplate 6. It is desirable that the cement cover the sealing interface for two reasons. First, the implosion protection afforded by the system is improved. Second, the epoxy cement being a good electrical insulator, insulates the sealing interface and obviates the customary wrapping of the sealing interface with insulative tape. In effect, the edgebond structure according to this invention embraces and binds up the actual thickness dimension of the faceplate 6 thereby effectively holding together the pieces of the faceplate when it is shattered long enough to allow air to enter the tube 2 slowly. The internal pressure in the tube is thus caused to increase gradually, preventing unacceptable amounts of glass fragments from being thrown forwardly from the shattered tube.

Underwriters Laboratory Incorporated ("UL") sets the standards for implosion protected cathode ray tubes for television receiving equipment. The test employed by UL is generally as follows: The color television picture tube is mounted in a test cabinet enclosure of a specified size (depends on the size of the tube). The cabinet is supported on a 30 inches high, rigid, table-like test stand. Two barriers each 1/2 inch thick, 9-178 inch high and 72 inches long are placed on edge on the floor in front of the test stand. The barriers are located at distances of 3 ft. and 5 ft., respectively, from the plane of the front enclosure of the cabinet. The three areas bounded by the barriers are indicated as follows: Zone 1: 0-3 ft., Zone 2: 3-5 ft., Zone 3: 5 ft. and beyond. The ball impact test is defined as follows. An impact is to be applied to any point on the face of the tube 1-1/2 inch away from the edge of the screen area and is to be obtained from a solid, smooth, steel sphere 2 inches in diameter and weighing approximately 1.18 lbs. The sphere is to be suspended by a suitable cord and allowed to fall freely as a pendulum from rest through a distance necessary to cause it to strike with an impact of 5 foot-pounds. The cabinet supporting the cathode ray tube is to be placed so that the surface tested is vertical and in the same vertical plane as the point of support of the pendulum. When a tube is tested as described above, the amount of glass thrown forward shall not exceed the following. First, there shall be no single piece of glass weighing more than 1/2 oz. in Zone 2; second, the total weight of all the pieces of glass in Zone 2 shall not exceed 1-1/2 oz.; and third, there shall be no single piece of glass in Zone 3 weighing more than 0.05 oz.

One embodiment (for a 23 inches diagonal tube) of the invention has been constructed. From the preliminary test results, it is expected that the tube will successfully pass the implosion protection tests of the Underwriters Laboratory Incorporated (described above). This embodiment comprised a steel frame with a thickness in the range 0.025 to 0.030 inch and a uniform depth of approximately 5/8 inch. The frame had a lip 17, of approximately 1/4 inch overlapping a small marginal portion of the front surface of the faceplate. The gap between the frame 14 and the edge surface 12 of the faceplate 6 was about 1/8 inch, the edge surface of the faceplate having a width of 0.450 inch. A thixotropic epoxy, type A, was introduced into the gap so as to cover the sealing interface 16 as well as the marginal portion 15 of the front surface 8 of the faceplate. A gasket 20 of foam rubber tape approximately 1/8 inch thick was used with an adhesive for attaching it to the frame 14.

From preliminary tests, this tube is expected to pass the afore-discussed UL test.

In applications where it may be desired to further increase the implosion protection capability of the disclosed system at somewhat greater cost, the uniform depth of the frame can be increased e.g., to 1-1/4 inch so that the frame overlies a small marginal portion of the mouth of the funnel 4. The cement 18 would cover also the marginal portion of the funnel and would then bind up this portion of the funnel as well as the entire edge surface 12, the sealing interface 16 and the small marginal portion 15 of the faceplate 6.

The invention is not limited to the particular details of construction of the device depicted and other modifications and applications are contemplated. For example, whereas the above-depicted embodiment included an approximately spherical or multi-radical faceplate, the invention may be applied to other types of tubes having a contoured faceplate edge surface and sealing interface -- e.g., a tube having a flangeless cylindrical faceplate. Whereas in the preferred embodiment, the frame 14 has been disclosed as being of uniform depth, it need not be perfectly uniform so long as it is relatively shallow and is contoured as above-defined. Certain other changes may be made in the above-described device without departing from the true spirit and scope of the invention herein involved. It is intended therefore that the subject matter in the above depiction shall be interpretative as illustrative and not in a limiting sense.