US2682963A - Metal cone for cathode-ray tubes - Google Patents

Description

y 1954 R. D. FAULKNER 2,682,963

- v METAL CONE FOR cA'rHoDE-im TUBES Filed Oct. 8, 1949 INVENTOR Patented July 6, 1954 METAL CONE FOR CATHODE-RAY TUBES Richard Dale Faulkner, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application October 8, 1949, Serial No. 120,400

This invention relates to improvements in cathode ray tubes having composite glass and metal envelopes. More particularly, it relates to improvements in metal shells for the bulb portions of cathode ray tubes.

As is known, there are a number of advantages to be gained by forming the envelopes of cathode ray tubes as composite metal and glass structures. One advantage is that such envelopes usually are very much lighter than all-glass envelopes of equal strength and size. Another is that since they are made of stock materials such as sheet metal and plate glass, they are much easier to fabricate, particularly in large sizes, than to cast all-glass envelopes. There is a further advantage which results from the fact that metal has much higher tensile strength than glass. It is that the envelope may be formed with a more nearly flat glass screen since the rim at the large end of the metal cone will be able .to withstand the considerable tension to which it will be subjected when the envelope is placed under vacuum. This, of course, contrasts with the known fact that all-glass bulbs should be made as nearly spherical as possible so that in the main the glass will be subjected almost exclu sively to compressive forces or, if they are not so made, then very heavy sections of glass must be used around the periphery of any flattened portion, such as a portion supporting a fluorescent screen, so that the walls supporting it can withstand the tension.

However, the manufacture of composite envelopes presents a number of its own problems and disadvantages. A first problem relates to the costliness of the metals which must be used. In composite envelopes it is usually necessary to employ special alloys for the metal portions in order to obtain satisfactory glass-to-metal seals. For example, one special alloy known as Kovar is frequently used for certain types of composite envelopes in which it is desirable that the coefficient of expansion of the metal portions be very nearly equal to that of glass portions. Similarly, other special alloys are used for other types of envelopes, they also being selected usually for their particular co-efficients of expansion. In general most of these special alloys are usually quite expensive as they include high percentages of costly metals such as chromium or nickel. While this factor may not present too serious a problem in a great variety of small receiving tubes and the like in which the amount of metal required is very small, it does in the case of large screen cathode ray tubes in which the weight of the metal shell frequently is as much as 12 or 15 pounds.

Accordingly, it has been desirable to construct the metal cone for a large screen cathode ray tube with different wall thicknesses in its different portions so as not to Waste material Where 8 Claims. (Cl. 2202.3)

greatstrength is not needed, i. e., to use different thicknesses of material according to the forces acting on its diiferentportions. For example, it is desirable to use thin material for the mantle of the cone (i. e., its sloping-wall portion) where relatively small and fairly uniform forces of atmospheric pressure are exerted and to use thicker material for its two end portions which are respectively, a large-diameter flange, in which the arch of screen disc is supported when the envelope is subjected to atmospheric pressure, and a small-diameter flange to which the neck is fastened. Moreover, in so forming the metal cone, it is necessary to avoid manufacturing processes which are as costly as the metal which is saved. For example, it is impractical to use a process including the steps of separately forming the mantle (or sloping-wall portion) of thin material and the flanges of suitably heavier material and of then welding them together since the required welds are too costly. Besides, this process entails the possibility of porosity or air holes and, for a good quality product, it requires finish-grinding.

Accordingly, it is commonly the practice to utilize a rather inexpensive spinning process by which it has been possible to attain ratios of about 2.5 or 3 to 1 between the thicknesses of flange material, i. e., that of the stock material, and that of all the mantle portion. In this process the cone is spun from a circular blank of sheet metal stock of uniform thickness in such a manner that the stock becomes reduced in thickness in its portions from which the mantle is formed.

Nevertheless cone manufacture according to the prior art has not proven to be entirely satisfactory. For one thing, wastage of material inthe mantle was not entirely eliminated. The reasons for continued wastage have been that on the one hand these small thinning ratios do not result in an adequately thin mantle when one starts with a heavy blank, and that on the other hand a heavy blank was used for the purpose of obtaining a rim strong enough to support the screen disc when the envelope is evacuated. The amount of reduction in thickness which can be produced by spinning is not unlimited but depends on the steepness of the cone, i. e., it be comes greater only as the top angle of the cone becomes smaller. For the wide angle cones which are most suitable for large screen kinescopes this reduction is no greater than by a factor of the order of 2 or less while if one assumes the use of cones having top angles as small as 30 for other types of cathode ray tubes, then it can be as great as by a factor of 4. For another thing, this practice has entailed considerable increase in manufacturing costs due to the occurrence of numerous pop-outs prior to evacuation. As indicated above, in composite large 3 screen kinescopes the screen disc can be made quite thin. Because of this the disc is subject to noticeable inward bending under atmospheric pressure when the envelope is evacuated. This can be objectionable, even if there is no danger of implosion, as it will introduce tensions in the glass of the finished tube and in addition will entail distortion of the glass disc which may have harmful optical effects. It has been the practice to solve this problem by using an alloy and a glass which have appropriately unequal co-efficients of expansion so that differential shrinking after the disc has been sealed to the cone will cause the flange to place the disc under compression and thereby bend it outward, i. e., in the direction to shorten both its chord and its radius of. curvature, to the end that the subsequent flattening of the disc, which will occur upon evacuation, will restore it to its original shape. It is after the disc is placed under peripheral compression by the flange and before the envelope is evacuated that the greatest number of popouts occurs.

Accordingly, it is an object of this invention to devise an improved cone for a cathode ray tube the large end of which includes a flange for carrying a convex glass screen disc and is strong enough adequately to support the perimeter thereof to prevent it from imploding when the envelope is evacuated even though the thickness of the material in the flange is no more than between 2 to 3 times that of the cone mantle while the material of the flange is thin enough not to include any substantial excess.

It is a further object of this invention to devise an improved sub-assembly for a cathode ray tube, which sub-assembly comprises a cone as set forth above and a glass screen disc sealed to the large end thereof in which the cone is so formed that there will be a reduced probability of a pop-out of the screen disc prior to evacuation.

It is a further object to devise an improved metal cone to be comprised in the bulb portion of a cathode ray tube and to carry a screen disc thereof in which the metal of the cone has a greater co-efficient of expansion than the glass of the disc and in which the end of the cone which carries the disc is so formed that it has adequate structural strength to prevent the disc from imploding, when the envelope is evacuated, by supporting the perimeter thereof under compression, even though said end of the cone is composed of materialwhich is no more than between 2 to 3 times as thick as the material composing the mantle thereof while the mantle is thin enough to not include any substantial excess of material.

It is a further object to devise an improved metal cone to be comprised in the bulb portion of a cathode ray tube and to carry a screen disc thereof in which the metal of the cone has a greater co-eflicient of expansion than the glass of the disc and in which the end of the cone which carries the disc is so formed that it has adequate structural strength to prevent the disc from imploding, when the envelope is evacuated, by supporting the perimeter thereof under compression, even though said end of the cone is composed of material which is no more than between 2 to 3 times as thick as the material composing the mantle thereof while the mantle is thin enough to not include any substantial excess of material, anclin which, moreover, said end of the cone is so formed that there will be a reduced probability of a pop-out of the screen disc during the interval which occurs after the disc is sealed to the cone and prior to evacuation of the envelope.

Other objects, features and advantages of this invention will be apparent to those skilled in the art from the following detailed description of the invention and from the drawing in which:

Figure l is a cross-sectional representation of an embodiment of the invention;

Figure 2 represents a detail section of one form of the prior art and. assists in disclosing principles underlying the present invention; and

Figure 3 shows an enlarged sectional portion of Figure l to permit comparison with the showing of Figure 2 in considering the principles underlying the present invention.

Figure 1 shows an improved metal cone [0 according to the present invention. It comprises a mantle portion ll having a large open end and a small open end and an axis of symmetry passing substantially through the centers of the open ends. At the open ends of the mantle portion II are large and small diameter flanges l2 and I3, respectively. The large diameter flange I2 includes a tapered seat I4 and a lip 15. In the assembly of a composite envelope, a screen disc [6 is joined to the large end of the cone It by being placed upon tapered seat M, in which position it is surrounded by the lip I5, and by having its edges sealed to the flange l2 by appropriate application of heat to the periphery of the disc and the flange. The details of how this is done are no part of the present invention, this being also true of such details as the angle of taper 9 of the tapered seat !4. That this is a critical angle is well known. This and ways of assembling the screen disc to the metal cone are described in detail in U. S. Patents 2,254,090 and 2,296,307.

In a completed kinescope, the lower portion Ila of the mantle II will be subjected primarily only to the moderate and fairly uniform compressive forces which are exerted upon it by the atmosphere. For this reason, this portion of the mantle may be made of very thin material and still have adequate structural strength. Composite envelopes, in which the mantle thickness is of the order of .04", when placed in a compression chamber, have withstood as much as 60 to 75 lbs. per square inch of external air pressure with the inside of the envelope under hard vacuum. However, the flanges l2 and I3 and the portions of the mantle which are adjacent thereto are subjected to much less uniform and, in some cases, much greater forces. For example, the large diameter flange is subjected to tension when the atmosphere presses in on the screen disc l5 and tends to flatten it out, and the small flange is subjected to very complex strains and stresses whenever a kinescope tube is lifted by its neck. For these reasons it has been apparent for some time, as was indicated above, that the cone should be formed employing rather heavy sections of material for the flanges.

Some attempts have been made to obtain mantle walls thin enough to not include any excess metal by using blanks of quite thin stock to begin with. While, as was expected, little difficulty was experienced with implosions of the mantle walls (even under test at four atmospheres), considerable difliculty was experienced with implosions of screen discs. Apparently when the large-diameter flange was made of such thin stock it was not strong enough to support the periphery of the glass disc under compression. Therefore it became customary to use heavier stock.

This choice was based on more than mere cost considerations. Any substantial susceptibility to implosion represents a highly dangerous condition which, moreover, due to age fatigue of the glass, increases as the tube becomes older. However, in the matter of cost the use of the heavier stock has left much to be desired both because of the wastage of material in the mantle and becauseof the costliness of the numerous pop-outs. The pop-outs wereaccepted'as not involving any danger for the eventual consumer since they almost entirely cease to occur after evacuation. In the matter of cost the only alternatives which have seemed available were to try to see if one could reduce the number of pop-outs by using even heavier stock, and, if this proved success ful, to Weigh any saving attained thereby against the increased wastage of expensive alloy, or simply to accept the manufacturing shrinkage occasioned by pop-outs as cheaper than the use of more metal. Though there was some evidence of a reduced number of pop-outs with thinner cones, it was not considered possible to use them since this would result in a dangerous finished product.

I have discovered that this was an incorrect conclusion. I made tests to determine if there was any possibility of devising a light cone of such improved structural design as simultaneously to be free of excess material in the mantle; to not incur a substantal number of discpopouts prior to evacuation; and to afford a tube not unduly susceptible to implosions, i. e., of such improved structural design that it would no longer be necessary to accept wastage of material and pop-outs, as the price for avoiding disc implosions. I found that of several influences which take part in causing implosions there are some which can be eliminated or reduced without necessarily increasing the effective total of those which take part in causing pop-outs, in other words, that the causes for the two difiiculties are not as closely related as has been supposed.

Fig. 2 shows a fragmentary view of a cone in which all of the mantle is thin-walled, i. e., in which the mantle is thin all the way up the sloping side of the cone to the inner perimeter of the large-diameter flange I 2. his figure also shows a fragmentary portion of the screen disc 16.

As pointed out above, in a composite largescreen kinescope it is possible to employ a relatively thin and flat screen disc since the metal flange into which it is sealed can stand considerable tension and therefore can serve as a circular abutment to retain the flat arch of the disc under compression. It is correct to say that in general pop-outs occur if the glass at or near to the glass-to-metal seal is not strong'enough to withstand this compression.

It has been supposed that more particularly the reason for the seal breakage was greater stretching of the top portion of the large-die ameter flange 12 than of its bottom portion, i. e., of its lip l5 than of its tapered seat l4, so that the outermost portion of the flange tilts axially backward in the direction T about a center of rotation such as F, pulling the tapered seat i i away from the seal glass at the periphery of disc I E. Then,.according to this supposition, upon the fracture of the seal, the compressive forces were suddenly released, allowing the flange to tilt axially forward to its original shape at the same time projecting the disc out of the cone. Consistently with this, the reduction in pop-outs, which was occasioned by the use of lighter metal, could be explained as the result of a reduction in the differential stretching of the lip l5 and the tapered seat M by an increase in that of the latter and the possibility existed that pop-outs might also be reduced through the use of thicker metal for reducing differential stretching of the lip and the tapered seat by a decrease in the former.

Another supposition has been that seal breakage would probably increase if a quite-thin flange were used since excessive stretching thereof might lead to movement of the seal-glass on the underside of the periphery of the disc with respect to the adjacent top surface of the tapered seat 14, causing the former to creep on the latter until it sheared away.

I have found instead that the principal reason for pop-outs is to be found in excessive rigidity of the flange, more specifically in its inability to tilt rather than in any tendency for it to'do so. When an assembly comprising a cone it and a disc 16 is allowed to cool after sealing, the differential shrinking by which the disc is placed under compression reduces both its chord and its radius of curvature. This causes the periphery of the disc 16 to tilt in the direction T about a fulcrum corresponding to F If the flange l 2 is flexible enough to tilt with the periphery of the disc a fracture of the seal is unlikely. However, a heavy flange is not able to do so, and the seal-glass at the periphery of the disc [6 therefore breaks at the fulcrum point.

Although a flange which is thin enough to be flexible will stretch more than a rigid one, it appears that a great deal of stretching can .be withstood without fracturing the seal, probably because of the fact that while it is occurring the metal actually is not receding from the glass and therefore the seal is not under tension. How- 'ever, it should be remembered that there must be a limit to this as excessive stretching is a principal reason for disc implosions.

of the large-diameter end of the mantle l i when,

at the same time, it is subjected to atmospheric pressure). If that end of the mantle is thinwalled it will spread out radially in the directions R and will also exert outward forces on the flange I2. For this reason it has proven advantageous to thicken the upper portion of the mantle II in the manner shown in Figs. 1 and 3.

If a cone of this type is made of light material, the flange will be flexibleenough to lessen popouts and, since the flange is freed of the burden of acting as a re-inforcement for the largediameter end of the mantle, the cone will be strong enough to avert disc implosions. In this way, the material may be so light that the cone will not include any excess of material in all of the mantle portion Ha even if this portion is not thinned to less than one-half the flangematerial thickness by the operation of spinning.

I have tested under external pressures of between 45 and 60 pounds per square inch, i. e., of upwards of three atmospheres, evacuated envelopes whose cones were formed according to the present invention of such thin stock that there was no excess of material in the mantle portion I la when the walls thereof were one-half as thick as the large-diameter flange, and I found them to offer satisfactory resistance to disc implosion.

In practice, this improved design has resulted in saving a considerable amount of costly alloy, e. g., about two pounds of metal for the cone of a 16-inch tube, in addition to a substantial reduction of manufacturing losses due to pop-outs. In addition, it is expected to permit the use of a cheaper alloy which contains less chromium and has a higher co-efficient of expansion. The use of this cheaper metal will cause an increase in the differential shrinkage between the large-diameter flange and the screen disc and therefore there will be greater compression and outward bending of the disc. While it is expected that the improved cone shown herein will permit a cone-and-disc assembly to withstand this compression without entailing a pop-out, it is obvious that this would certainly not be true if cones of the prior art type were used.

It is still within the scope of the invention to provide the mantle portion l lb with a thickness intermediate the thickness of the mantle portion Ha and the rim portion I2.

The flange [2, by a proper choice of sheet stock, may be flexible enough to obtain an acceptable reduction of pop-outs even if the stock is not the thinnest permissible in-so-far as the strength of the mantle portion Ila is concerned. Because of this it is possible, where an increased safety factor against disc implosion is preferred to maximum economy, to use stock of an intermediate thickness which will afiord a sufiiciently flexible flange, very strong peripheral support for the disc, and only a slight excess of wallthickness for the mantle.

The improved cone of the present invention makes possible a simplification in a required operation of annealing the cone-and-disc assembly after the disc has been scaled into the largediameter flange. According to conventional annealing practices the sub-assembly would be placed in an annealing oven at a temperature 1 high enough to cause the release of strains in the glass without softening it; and thereafter the sub-assembly would be cooled nearly all the Way down to room temperature by gradual equilibrium-cooling in which the temperature of the oven is so gradually reduced (or the sub-assembly is so gradually moved along an oven in which a falling temperature gradient exists) that all portions of the sub-assembly would drop in temperature uniformly despite their unequal conductivities, thicknesses, etc. Assuming that the metal has a somewhat greater co-efficient of expansion than the glass, the following will take place during equilibrium-cooling: the temperature of the sub-assembly will drop down to the setting-point of the glass without the disc being placed under compression by differential shrinkage since in that temperature range glass will simply be displaced by the contracting flange; as the temperature of the sub-assembly drops below the setting-point differential shrinkage will gradually place the glass disc under increased compression bending it outward and shortening its radius of curvature by reducing its chord.

In the past it has been considered unfeasible to move a structure including glass directly from an annealing oven to surroundings at room temperature as soon as the glass has been cooled to below the setting-point. For example, it has been considered objectionable for sub-assemblies of the kind in question because the metal portions, which are thin and highly conductive, would tend to give up their residual heat very much more rapidly than the glass disc, which is thick and rather non-conductive, so that at least transiently the magnitude of differential shrinking would be very much greater than that intended for the sub-assembly when all of its parts are at the same (room) temperature and thata resulting temporary excess of compression on the disc would very much increase the total number of pop-outs. A more general objection to quick removal from an'annealing oven has been that if, upon its removal from the oven for cooling, the structure is placed on a rack or other support any glass portions which come into contact therewith will cool more rapidly than the other glass portions and thus potentially harmful strains may be set up.

However, by actual test and contrary to expectation, I have found that it is quite feasible to move such a sub-assembly directly from the annealing oven to surroundings at room temperature immediately after it has cooled below the setting-point of the glass. Moreover I have found that this is particularly true if the subassembly includes the improved cone of the present invention.

It appears that the reasons for the more general objection do not apply if one uses an external cooling rack in which only the metal cone is in contact with the rack. If this is done there is no difiiculty as to strains being set up at the point of support since such strains in the metal cone, as distinguished from similar strains in glass, are of minor importance. When the subassembly is so set out to cool the cone affords an ideal form of carrier for the disc since its fiange carries the disc symmetrically around its entire perimeter.

The following is a suitable way of using this shortened annealing process for sub-assemblies for 16-inch cathode ray tubes whose metal cones have the particular dimensions set forth above and are made of an alloy including 28% of chrome and 72% of iron and whose glass discs are made of soda lime silica glass (one form of ordinary window glass) which is of an inch thick and can be obtained under the trade name Clearlight as manufactured by the Fourco Glass Co.:- immeditaely after the sub-assembly comes from the sealing fires, i. e., when the glass in the perimeter of the disc is strain-free, place it in an annealing oven which is maintained at a temperature of between 535 and 5'75 degrees centigrade, preferably near to 550 degrees; keep it there for at least five minutes; and then move it directly to surroundings at room temperature. It is neither necessary to vary the oven temperature over the five-minute period nor to establish a temperature gradient in the oven along a path of travel for the sub-assemblies. If desired, the sub-assembly may be allowed to remain in the oven for longer than the five minutes and/or a higher initial temperature may be used so long as the take-out temperature is in the region above mentioned. Ijowever, if the initial temperature is above that for take-out it is obvious that either the temperature of the oven must vary over time or a temperature gradient must 9 be established within the oven and the sub-assembly moved in accordance with the direction of the gradient.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated certain specific applications for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or uses indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. A metal shell for a cathode ray tube envelope comprising as a unitary structure a frustoconical mantle having a substantially uniform wall thickness, a large-diameter flange at the large end of said shell having a wall thickness which is greater than that of said mantle, said mantle including a peripheral portion directly interconnecting said large-diameter flange and said mantle and having a thickness intermediate the thicknesses of said flange and said mantle.

2. A metal cone for a cathode ray tube envelope comprising as a unitary structure a frustoconical mantle including a mantle-portion adjacent the smaller end having a substantially uniform wall thickness, a large-diameter flange at l the larger end of said cone having a wall thickness which is greater than said mantle, said mantle also comprising a peripheral portion adjacent said flange having a thickness substantially equal to the thickness of the large-diameter flange and directly attached to the large-diameter flange, said peripheral mantle portion extending an appreciable distance from said flange.

3. A cathode ray tube having an envelope comprising a metal shell including a frusto-conical mantle, a large-diameter flange attached to the large end of the mantle in which the large-diameter flange has a wall thickness greater than the main portion of the mantle, the portion of the mantle adjacent to and extending an appreciable distance from said large-diameter flange having substantially the same thickness as said flange, a glass screen disc having its periphery sealed into the large-diameter flange, each surface of the disc having an approximately spherical curvature, and the disc being retained under compression within the large-diameter flange.

4. A cathode ray tube having a composite glass and metal envelope, the metal portion of said envelope comprising as a unitary structure a shell having a small open end and a large open end and an intermediate mantle with an axis of symmetry passing through said open ends, said mantle including a first mantle-portion at the smaller end thereof and having a substantially uniform wall thickness, a second mantle-portion at the larger end of the mantle and having a wall thickness greater than that of said first mantle-portion, a large flange formed at the large end of said mantle and extending outwardly from the outer surface of said shell at a small angle with the perpendicular to said axis of symmetry, said flange having a wall thickness of the same order as that of said second mantle-portion, and a glass screen andclosure member sealed at its edges to said flange.

5. A cathode ray tube having a composite glass and metal envelope, the metal portion of said envelope including a metal cone, the small-diameter portion of said cone having a substantially uniform wall thickness, a large-diameter flange formed at the large end of said cone and having a wall thickness greater than that of the small diameter portion, the portion of said cone adjacent said flange being of the same order of thickness as said flange, said flange comprising a portion inclined at almost a right angle to a plane transverse of the longitudinal axis of said cone and a second portion inclined at an obtuse angle to said plane, and a glass screen closure member sealed to the inside portions of each of said inclined portions of said flange.

6. A metal cone for a cathode ray tube envelope comprising a unitary structure having a frustoconical mantle including a mantle portion ad-- jacent the smaller end of said cone having a substantially uniform wall thickness, a largediameter flange at the larger end of said cone having a wall thickness which is greater by a factor between 2 and 3 than the thickness of said mantle portion adjacent the smaller end, said mantle comprising a peripheral portion adjacent said flange having a thickness substantially equal to the thickness of the large-diameter fiange'and directly attached to said large-diameter flange.

7. A metal cone for a cathode ray tube envelope comprising a unitary structure having a frustoconical mantle including a mantle portion adjacent the smaller end of said cone having a substantially uniform wall thickness, a large-diameter flange at the larger end of said cone having a wall thickness which is greater by a factor between 2 and 3 than the thickness of said mantle portion adjacent the smaller end, said mantle comprising a peripheral portion adjacent said flange having a thickness substantially equal to, the thickness of the large-diameter flange and directly attached tosaid large-diameter flange, said flange including one portion forming on its inside an obtuse angle to a plane transverse of the longitudinal axis of the cone and directly joining said mantle and a second portion forming on its inside almost a right angle with said plane.

8. A metal shell fora cathode ray tube envelope, said shell comprising as a unitary structure, a tubular mantle member including a large opening at one end and a smaller opening at the other end thereof and an axis of symmetry passing through said openings, said mantle portion having a substantially uniform wall thickness, a large flange at the large end of said shell extending outwardly from the outer surface of said shell at a small angle with the perpendicular to said axis of symmetry, said flange having a wall thickness greater than that of said mantle, said mantle portion also including a peripheral portion adjacent said flange having a thickness substantially equal to the thickness of said flange and directly attached to said flange, said peripheral mantle portion extending an appreciable distance from said flange.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 151,435 Ripley May 23, 1874 1,184,813 Birdsall May 30, 1916 1,922,087 Hiester Aug. 15, 1933 1,939,356 Lingren Dec. 12, 1933 1,963,008 Weeks June 12, 1934 2,189,261 Bowie Feb. 6, 1940 2,254,090 Power Aug. 26, 1941 2,296,579 Seelen Sept. 22, 1942

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