Bringing Direct Bolt Preload Measurement to the Aerospace Industry

Many helicopter components are held together with fastened joints that include threaded bolts. Bolt preload is important for keeping fastened joints from loosening or sliding. In the aerospace industry, bolt preload is typically set by applying a specified torque. Common procedures for clamping a joint include the use of a calibrated torque wrench to apply the specified torque. While a torque wrench will display the torque applied to a clamped bolt assembly, the preload (or bolt tension) must be inferred. However, a significant factor relating applied torque to the acquired bolt tension is the friction between the bolt threads and joint interface. Tension measurement techniques are available in the aerospace industry, but they are not common. A small amount of contamination or lubricant can significantly alter the torque-tension relationship. Under the Future Advanced Rotorcraft Drive Systems (FARDS) agreement, Bell Helicopter, IntellifastTM, and the Army Aviation Development Directorate (ADD) - Aviation Applied Technology Directorate (AATD) developed and tested a technology that directly measures the tension in the bolted flexure joints of a KAFLEX drive shaft coupling. The main purpose of testing was to validate bolt tension measurement to demonstrate the Technology Readiness Level (TRL) suitable for flight test. Testing performed under the FARDS agreement for this new technology included measuring the variation in bolt preload when set to a specific torque and then dynamically testing the joint to simulate flight time. Correlation was obtained between joint movement and the reduction in bolt preload. This paper summarizes the test results and shows the importance of directly measuring bolt preload in a fastened assembly to ensure joint integrity.