Reliability Analysis of a Rotorcraft Transmission Bolted Connection

Reliability analysis of a rotorcraft transmission bolted connection using actual service records data is the focus of this paper. The analysis is conducted for two different bolt materials used in the gearbox to evaluate the difference in reliability. As part of a continued operational safety bolt preload assurance check, the bolted connection between the spiral bevel ring gear and first stage sun gear undergoes a periodic field inspection: the inspection is said to have failed if the application of a prescribed torque to the nut causes the entire bolt assembly to spin. Reliability analysis employs a two-parameter Weibull probability density function to model the life of the bolted connection, with parameters derived using the Maximum Likelihood Estimator method. Results for each bolt type are discussed and an observation is made as one possible explanation for disparity in reliability levels for the two different bolt materials which is supported by direct measurement of bolt tension at initial assembly. As a result of the findings in this paper, recommendations for future work, including modification of the bolted connection assembly procedure, is discussed.