A cathode-ray tube is evacuated to a very low internal pressure and accordingly is subject to the possibility of implosion due to the stresses produced by atmospheric pressure acting on all surfaces of the tube. This problem has been addressed in the art by providing the CRT with an implosion protection band. Such a band is used to apply a compressive force to the sidewall of a faceplate panel of the CRT to redistribute some of the forces. The redistribution of the forces decreases the probability of an implosion of the tube by minimizing tension in the corners of the panel. An implosion protection band is also beneficial because it improves the impact resistance of the tube. Glass in compression is stronger than glass which is in tension and the band causes compression in panel areas which otherwise would be in tension. Additionally, in the event of an implosion, the redistributed stresses cause the imploding glass to be directed toward the back of the cabinet in which the tube is mounted, thereby substantially reducing the probability of someone in the vicinity of the imploding tube being injured.
An implosion protection band of the shrinkfit type typically is manufactured by forming a strip of steel into a loop having the same configuration as the faceplate panel to be protected and joining the two ends of the strip on one side of the band. In some instances, the band is made by joining two identical strips on two sides to form the loop. For both types of bands, the periphery of the loop is slightly smaller than the periphery of the faceplate panel. The loop is heated to approximately 300.degree.to 500.degree. C. and the coefficient of expansion of the material causes the loop to expand to dimensions permitting the loop to be slipped around the sides of the faceplate panel. As the band cools it shrinks and tightly surrounds the panel, thereby applying the necessary implosion protection compression to the faceplate panel. The compressive force can be accurately controlled by exceeding the yield point of the metal in the band.
The ends of the strips are permanently joined by either welding or crimping. In either event, because the strip is used to apply substantial pressure to the sidewall of the tube, it is essential that the connective joint, formed where the two ends are coupled together, be sufficiently strong to withstand the tension applied to it by the band. Typically, the connective joint is designed to withstand a minimum tension of 5000 pounds ( 2268 kg). Since the tension of the band is directly proportional to the yield strength of the material and its sectional area, any increase in the yield strength of the band material that is in excess of its maximum limit, will exert a tension on the connective joint in excess of its minimum design limit and may cause the joint to fail.