Abstract:
An apparatus for maintaining tension in a mask frame of a cathode ray tube. The apparatus includes affixing a shadow mask to a pair of triboxes disposed along opposite sides of a mask frame at a predetermined tension. The triboxes are coupled to the mask frame by braces having a different coefficient of thermal expansion than the triboxes so as to compensate for the expansion of the frame during thermal cycling of the cathode ray tube. Consequently, it is possible to maintain the tension force applied to the shadow mask by the mask frame.

Description:
This invention generally relates to color picture tubes and, more particularly, to an apparatus for maintaining tension in a shadow mask of a cathode ray tube. 
     BACKGROUND OF THE INVENTION 
     A conventional color picture tube includes an electron gun for generating and directing three electron beams to the screen of the tube. The screen is located on the inner surface of the faceplate of the tube and is made up of an array of elements of three different color-emitting phosphors. A color selection electrode, also referred to as a shadow mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal, such as steel, that is usually contoured to somewhat parallel the inner surface of the tube faceplate. 
     One type of color picture tube shadow mask is a tension mask mounted within a faceplate panel thereof. In order to maintain the tension on the mask, the mask must be attached to a relatively massive support frame. Although such tubes have found wide consumer acceptance, there is still a need for further improvement, to reduce the weight and cost of the mask-frame assemblies in such tubes. 
     It has been suggested that a lighter frame could be used in a tension mask tube if the required tension on a mask is reduced. One way to reduce the required mask tension is to make the mask from a material having a low coefficient of thermal expansion. However, a mask from such material requires a support frame of a material having a similar coefficient of thermal expansion, to prevent any mismatch of expansions during thermal processing that is required for tube manufacturing, and during tube operation. Because the metal materials that have low coefficients of thermal expansion are relatively expensive, it is costly to make both the mask and frame out of identical or similar low expansion materials. Therefore, it is desirable to use the combination of a low expansion tension mask with a higher expansion support frame, and to provide a solution to the problem that exists when there is substantial mismatch in coefficients of thermal expansion between a tension mask and its support frame. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for maintaining tension in a shadow mask. The invention compensates for the thermal expansion mismatch between a low expansion material such as INVAR® which is an alloy chiefly composed of Ni and Fe, and a high expansion material such as steel used in the tension mask and the mask frame, respectively. The need to maintain tension in a tension mask is essential to the correct operation of the cathode ray tube in a color television. 
     In the present invention, the mask frame has a generally rectangular plan form with two long sides and two short sides. A corner support structure incorporating a standoff is attached to each end of each short side. Atop the standoffs is welded the long side member comprised of a triangular shaped cross-section support structures referred to herein as triboxes. The first side (leg) of the triangular triboxes are attached to the standoff and the hypotenuse faces inwardly of the mask frame. The second side (leg) of the triangular triboxes is affixed to a side of a low expansion material, such as INVAR®, blade. The blade is vertically oriented and the top of the blade supports an INVAR® mask. Attached between the short side steel mask frame and the steel triboxes is a brace formed of a low expansion material such as INVAR®. The thermal expansion mismatch and geometric separation between the low coefficient of thermal expansion brace and the steel short side of the mask frame causes the steel tribox to rotate about its stand-off when heated. The brace is sized such that the inward rotation of the tribox compensates for the expansion of the frame, thereby preserving the tension in the mask that is supported by the blade as the frame is heated. This system is specifically designed to maintain tension in the shadow mask during thermal cycling of the mask frame assembly such that uniform creep of the mask occurs and mask wrinkles caused by differential mask creep are eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view, partly in the axial section, of a color picture tube, including a tension mask-frame-assembly according to the present invention; 
     FIG. 2 is a perspective view of a tension mask of FIG. 1; 
     FIG. 3 is a partial perspective view of a section of the mask-frame assembly according to the present invention; and 
     FIG. 4 is a graphically charted diagram showing the operation principal of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a cathode ray tube  10  having a glass envelope  12  comprising a rectangular faceplate panel  14  and a tubular neck  16  connected by a rectangular funnel  18 . The funnel  18  has an internal conductive coating (not shown) that extends from an anode button  20  to a neck  16 . The panel  14  comprises a viewing surface  22  and a peripheral flange or sidewall  24  that is sealed to the funnel  18  by a glass frit  26 . A three-color phosphor screen  28  is carried by the inner surface of the faceplate panel  14 . The screen  28  is a line screen with the phosphor lines arranged in triads, each triad including a phosphor line of each of the three primary colors. A tension mask  30  is removably mounted in a predetermined spaced relation to the screen  28 . The mask may be either a tension focus mask (not shown) or a tension mask (as generally illustrated in FIG.  2 ). An electron gun  32  (schematically shown by the dashed lines in FIG. 1) is centrally mounted within the neck  16  to generate three inline electron beams, a center beam and two side beams, along convergent paths through the tension mask  30  to the screen  28 . 
     The tube  10  is designed to be used with an external magnetic deflection yoke, such as the yoke  34  shown in the neighborhood of the funnel to neck junction. When activated, the yoke  34  subjects the three beams to magnetic fields that cause the beams to scan horizontally and vertically in a rectangular raster over the screen  28 . 
     The tension mask  30 , shown in greater detail in FIG. 2, includes two long sides  36  and  38  and two short sides  40  and  42 . The two long sides  36  and  38  of the tension mask  30  parallel a central major axis, x, of the tube, and the two short sides  40  and  42  parallel a central minor axis, Y, of the tube. The tension mask  30  includes an apertured portion that contains a plurality of metal strips or stands  44  having a multiplicity of elongated slits therebetween that parallel the minor axis of the mask. 
     FIG. 3 depicts a partial perspective view of the mask frame assembly  300  of the present invention that supports the tension mask  30  of FIG.  2 . The portion of the assembly  300  depicted in FIG. 3 is repeated on all four corners of the mask frame assembly  300 . 
     The present invention was developed for tension masks for use in cathode ray tubes having mask frame assemblies composed of a steel mask frame  306  and a low expansion iron nickel alloy metal material, such as INVAR®, tension mask  30 . For mask frame assembly  300 , there is a large difference in thermal expansion between the tension mask  30  and the mask frame  306 . This results in excessive stresses in the tension mask  30  during tube fabrication that will cause tension loss or wrinkles. This condition is shown as line  501  in FIG.  4 . 
     The present invention provides low expansion braces  308  on the mask frame  306  to preserve, through a mechanical system, the tension levels in the tension mask  30  (line  502 , FIG. 4) at the original level of tension (line  503 , FIG. 4) when the tension mask  30  is welded to the mask frame  306 . This type of tension control is preferred over conventional detensioning of the tension mask  30  during thermal processing of the tube. The conventional detensioning (line  504 , FIG. 4) produces large gradients in tension in the tension mask  30  during thermal processing that may result in wrinkles caused by differential creep, i.e., non-uniform creep along the mask major axis induces poison effects which left unbalanced, produce wrinkles. 
     The elements of the mask frame assembly  300  comprise a set of steel hollow frame elements  324  that form the short sides of the mask frame  306  and a set of standoffs  304  attached to each end of each frame element  324 . The frame assembly  300  includes two long sides that have triangular shaped cross-section support structures, hereinafter referred to as a triboxes  310 . The steel triboxes  310  are formed from steel sheet metal in the shape of a right triangle. A first side (leg)  312  of the tribox  310  is affixed to the standoff  304 , now forming a complete rectangular frame, such that the second side (leg)  314  of the triangular triboxes  310  is perpendicular to the mask frame  306 . In this manner, the hypotenuse  316  of the triangle faces inward with respect to the frame assembly  300 . The triboxes  310  may be affixed to the steel standoffs  304  by, but not limited to, the following methods: seam, laser, spot, tack or resistance welding. 
     After the triboxes  310  have been affixed to the standoffs  304 , a blade  302  composed of a low expansion material, such as INVAR®, is vertically oriented and affixed parallel and adjacent to the second side  314  of the tribox  310 . The blade  302  is used as an interface between the mask frame  306  and the tension mask  30 . The tension mask  30  shown in FIG. 2 is welded to the top surface  322  of the blade  302 , but is not shown in this view for clarity. 
     A brace  308 , made of a low thermal coefficient of expansion material such as INVAR®, is a rectangular strip having a thickness and two respective ends  308 A and  308 B. One of the ends  308 A is affixed to a top surface  325  of the frame element  324 . The other end  308 B is affixed to the hypotenuse  316  of the tribox  310 . The brace  308  may be attached to the tribox  310  and frame element  324  by seam, spot, tack or resistance welding. A total of four braces  308  are employed around the frame to maintain tension of the tension mask  30 . 
     This mask frame assembly  300  functions as follows: during the thermal process where the picture tube is heated during manufacture or operation, the thermal expansion mismatch and geometric separation between the INVAR® brace  308  and the short side steel frame element  324  causes the triboxes  310  to inwardly rotate about the standoff  304  during heating. The braces  308  are sized such that the inward rotation of the triboxes  310  compensates for the expansion of the frame element  324 , thereby preserving the as-welded tension in the shadow mask  30 . Because the brace  308  compensates for the rotation of the triboxes  310  and the expansion of the mask frame assembly  300  is preserved, the shadow mask  30  maintains its original tension throughout and after heating. That is, the tension has not been reduced before the thermal cycle nor has it been increased after the thermal cycle. The lower linear coefficient of thermal expansion of INVAR® (1.6×10 −6 /° C.) when compared to low alloy steel (12.0×10 −6 /° C.) allows small critical parts or areas to be formed of INVAR® and provides a predominantly steel mask frame assembly  300  with the expansion characteristics of an all INVAR® mask frame. This reduces the amount of INVAR® material used during manufacture, thus providing a reduction in total cost. 
     As the embodiments that incorporate the teachings of the present invention have been shown and described in detail, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings without departing from the spirit of the invention.