Patent Abstract:
An apparatus and method for retaining a damper wire used in a cathode ray tube to reduce vibration in the grill type mask assembly of a cathode ray tube. The damper wire is retained across a grill type mask by a bimetal damper spring having a first end and an opposing second end. The second end is coupled to the frame of the grill type mask assembly. A tab located proximate the first end of the damper spring is adapted to accept the damper wire that traverses the mask.

Full Description:
This invention generally relates to cathode ray tubes and, more particularly, to an apparatus and method for retaining a damper wire in a cathode ray tube to reduce vibration in a grille type mask. 
   BACKGROUND OF THE INVENTION 
   A color picture tube includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the face plate of the tube and comprises an array of elements of three different color emitting phosphors. A shadow mask, which may be either a formed aperture or a grill type mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. 
   The shadow mask is subject to vibration from external sources (e.g., speakers near the tube). Such vibration varies the positioning of the apertures through which the electron beam passes, resulting in visible display fluctuations. Ideally, these vibrations need to be eliminated or, at least, mitigated to produce a commercially viable television picture tube. 
   SUMMARY OF THE INVENTION 
   The present invention provides an apparatus and method for retaining a damper wire used in a cathode ray tube to reduce vibration in a grill type mask assembly of a cathode ray tube. The damper wire is retained across a mask by a bimetal damper spring having a first end and an opposing second end. The second end is coupled to the frame of the grill type mask assembly. A tab located proximate the first end of the damper spring is adapted to accept the damper wire that traverses the mask. In an alternative embodiment, the damper wire is “tied” to the tab such that the spring maintains a constant tension on the damper wire. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a side view, partly in axial section, of a color picture tube, including a grill type mask-frame-assembly according to the present invention; 
       FIG. 2  is a perspective view of the grill type mask-frame-assembly of  FIG. 1 ; 
       FIG. 3  depicts a prior art damper spring arrangement; 
       FIG. 4  is a cross sectional view of a prior art damper spring depicting positional movement during temperature changes; 
       FIG. 5  is a perspective view of a bimetal damper spring; 
       FIG. 6  is a cross sectional view of a bimetal spring depicting positional movement during temperature changes; 
       FIG. 7  depicts a perspective view of a bimetal damper spring having a concave first end; and 
       FIG. 8  depicts an embodiment of the invention having a damper wire tied to a respective tab. 
   

   To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a cathode ray tube  10  having a glass envelope  12  comprising a rectangular face plate 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 face plate  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 face plate  22 . 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 colors. A grill type mask  30  is removably mounted in a predetermined spaced relation to the screen  28 . An electron gun  32  (schematically shown by the dashed lines in  FIG. 1 ) is centrally mounted within the neck  16  to generate three in-line electron beams, a center beam and two side beams, along convergent paths through the 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 grill type 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 mask parallel a central major access, x, of the tube. The grill type mask  30  includes: strands  44  that are parallel to the central minor access y and to each other. In a preferred embodiment, the strands  44  are flat strips that extend vertically, having a width of about 0.020″ and a thickness of 0.006″. 
   It will be appreciated by those skilled in the art that although the invention is discussed in the context of grill type masks, the invention can be adapted to use formed aperture masks, tensed aperture masks, focus type masks or the like. 
     FIG. 3  depicts a prior art (U.S. Pat. No. 4,780,641) damper spring arrangement that retains a damper wire across the mask to reduce vibration in the mask. Specifically, a damper spring  50  is attached to a frame  48  of grill type mask  30 . More specifically, each damper spring  50  is comprised of a single metal and is attached to the frame  48  proximate to the two short sides  40  and  42  of grill type mask  30 . A tab  52  is disposed on each damper spring  50 . 
   A damper wire  54  extends between the damper springs  50  and contacts the surface of the grill type mask  30 . The damper wire  54  is attached to each respective damper spring  50  by sandwiching the damper wire  54  between the spring  50  and a tab  52  welded to the spring  52 . 
   Damper wire  54  is held under a high tension force of 50 N between each respective damper spring  50 . It is desireable that this tension be maintained to ensure that the damper wire  54  is always contacting the mask. Damper wire  54  is a small diameter wire made of tungsten or the like. Under a normal operating temperature of 70 degrees Celsius, each respective damper spring  50  maintains the proper tension on damper wire  54 . However, during the cathode ray tube manufacturing process, temperatures in the cathode ray tube  10  can reach temperature ranges of between 450 and 480 degrees Celsius. Because the creep threshold of the damper spring and damper wire material at the processing temperature is lower than the creep threshold at normal operating temperature and the thermal expansion of the damper wire  54  causes an increase in wire tension and spring stress at the high processing temperature, such a high temperature can cause creep strain in the damper spring or damper wire which leads to a relaxation of the damper wire tension and a resultant damper wire tension which can only be estimated from initial conditions. For instance, during high temperature processing as shown in  FIG. 4 , damper spring  50  moves from Position x to Position y exerting additional direct tension on damper wire  54  and increased bending stress on the damper spring  50 . Creep strain in the damper spring  50  will move the damper spring  50  towards Position x. When normal operating temperatures are reverted to, the permanent creep strain will position the damper spring  50  at Position z, which is inboard of Position x, and the damper wire tension is reduced. The creep threshold is about 27,000 psi at 460 degrees Celsius for a bimetal and a non bimetal spring. However, the bimetal spring has substantially lower stress at this temperature. 
     FIG. 5  depicts a perspective view of a bimetal damper spring that replaces damper spring  50  in FIG.  3 . Specifically, bimetal damper spring  56  comprises a first metallic layer  58  and a second metallic layer  60 . First metallic layer  58  comprises a metal such as carbon steel and the like disposed on an inner surface  72  of the bimetal damper spring  56 . Second metallic layer  60  comprises a metal such as stainless steel and the like, having a higher thermal expansion characteristic than the first metallic layer, disposed on an outer surface  74  of the bimetal damper spring  56 . Bimetal damper spring  56  has a thickness of between 0.008″ to 0.012″ to ensure flexibility. The first metallic layer  58  and second metallic layer  60  may be coupled with welding which can be achieved with electron beam welding or resistance welding. 
   Bimetal damper spring  56  has a first end  62  and an opposing second end  64 . Both of the ends  62  and  64  are flat. The second end  64  of each bimetal damper spring  56  is attached to the frame  48  of the grill type mask  30 . Disposed between the first end  62  and second end  64  of each bimetal damper spring  56  is a tab  52  having a first end  68  and an opposing second end  70 . The first end  68  of the tab  52  is attached to bimetal damper spring  56 . 
     FIG. 6  is a cross sectional view of a bimetal spring depicting positional movement during temperature changes. In a first embodiment of the invention, damper wire  54  is spot welded between the tab  52  and bimetal damper spring  56  at point  600 . During the cathode ray tube manufacturing process, high temperatures are achieved. Since bimetal damper spring  56  has the low expansion metal on the inner surface  74 , the bimetal damper spring  56  curls inward from Position A to Position B. Thus, unloading damper wire  54  during high temperature processing. Thereby, lowering the damper spring and damper wire stress below the creep threshold and allowing damper wire  54  tensions to be fixed before the final cathode ray tube assembly. 
     FIG. 7  depicts a perspective view of a bimetal spring  57  having a concave first end  76 . Specifically, the bimetal damper spring  57  has a curvature  78  on the first end  76 . The curvature  78  is added to first end  76  so that by aligning the apex  80  of the curvature  78  to the edge of the grill type mask  30  with the spring compressed the proper damper wire angle of elevation  82  can be achieved when the spring is released. The preferred radius of the curvature is 1.875″ degrees. The proper damper wire angle of elevation  82  is one which guarantees a tangential or slightly downward departure of the damper wire  54  from the edge of the grill type mask  30 . Such an angle of elevation guarantees proper contact is maintained with the grill type mask  30  to reduce vibration therein. Factors such as the diameter of the damper wire  54 , the degree of curvature of first end  76  and how close the bimetal damper spring  56  is to the edge of the grill type mask  30  determine the damper wire elevation  82 . Different degrees of curvature of first end  76  can be used to accommodate any type or size of cathode ray tube  10 . 
     FIG. 8  depicts a perspective view of a bimetal damper spring  86  having a damper wire  54  tied to a respective tab  52 . Tab  52  is coupled to bimetal damper spring  86  at the first end  62 . A crotch  84  exists between tab  52  and bimetal damper spring  86 . The damper wire  54  is looped around the tab  52 . Then the looped portion of damper wire  54  is secured between damper spring  86  and tab  52  by wedging the looped portion of damper wire  54  in the crotch  84 . 
   It will be appreciated by those skilled in the art that tab  52  can be an integral tab  66  formed from the body of bimetal damper spring  86 . 
   It will also be appreciated by those skilled in the art that the various embodiments of bimetal damper spring  86  can be combined. For example bimetal damper spring  86  can have a first end  76  having a curvature  78  and have damper wire  54  tied to tab  52  of bimetal damper spring  56 . 
   In another embodiment, a non-bimetal damper spring has a concave first end similar to the concave first end shown in FIG.  7 . This non-bimetal damper spring benefits from having a damper wire angle of elevation that is adjustable based on the curvature of the first end. 
   In another embodiment, a non-bimetal damper spring has a damper wire tied to a tab in the same manner as shown in FIG.  8 . As such, the damper wire is looped around the tab and the looped portion of the tab is secured by wedging the looped portion of the damper wire in the crotch. 
   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.

Technology Classification (CPC): 7