Failures of bolts in helicopter main rotor drive plate assembly due to improper application of lubricant

The main rotor drive plate of the helicopter is defined as a flight safety part. Failure to follow maintenance instructions might result in an accident involving serious injury or death of crew members and/or serious damage to the helicopter. In this work, the failure of five of the shear bolts in this assembly during installation torque wrenching was analyzed. Visual inspection, fractographic and metallographic examinations, chemical analysis and mechanical testing (static tensile and hardness tests) did not indicate any deviation from the requirements of the aeronautical military standards. A specially designed bolt force sensor, which is based on strain gauge technology, was used to monitor the actual load that develops in the bolt when it is torque-wrenched into a nut. Based on comparison of the behavior of bolts that had been lubricated in different ways with antiseize material, it was concluded that the bolts failed because of improper application of the antiseize material, which led to development of overloads. (C) 2003 Elsevier Science Ltd. All rights reserved.