DEVELOPMENT OF AN IMPROVED LIFE TENSION-TORSION STRAP

An improved life tension-torsion (T–T) strap has been developed by Bell Helicopter Textron Inc. and Lord Corporation. This new strap incorporates a corrosion-resistant material known as MP35N Multiphase alloy in place of 304 stainless steel wire. Corrosion and fatigue test results indicate the elimination of a calender life and an increase in the operational life of T–T straps for rotor blade retention. At the present time, Bell Helicopter Textron Inc. is working to qualify T–T straps which incorporate MP35N wire for all its rotorcraft that use T–T straps in their main rotor systems. This paper presents the results of the developmental test program on Bell Helicopter Model 206 Tension-Torsion straps. The tensile and fatigue test results for MP35N wire are compared with those of CRES 304 wire. The tensile test results for T–T straps are presented along with spring rate test results. A comparison is given of the humid environment fatigue test results of new versus production Model 206 T–T straps. Also, the results of accelerated corrosion tests are presented which compare T–T straps fabricated with MP35N wire to those with CRES 304 wire in salt fog and salt water alternate immersion environments.