<p>Transmission shafts serve as indispensable components in mechanical systems, primarily responsible for transmitting torque and motion. A comprehensive failure analysis of a transmission shaft used in a gondola was conducted. A series of experiments including visual examination, scanning electron microscopy, metallographic analysis, hardness testing, and finite element analysis were carried out to study the failure mechanism of the transmission shaft. Results revealed that the failure was a fatigue fracture originating from the diameter transition section, where the absence of a transition fillet led to significant stress concentration. Machining marks and corrosion products were observed near the crack initiation site, increasing the local stress and promoting crack initiation. The FEA simulations confirmed that the combined effect of torque and radial force enlarged the stress at the fracture location. The radial force arises from gear meshing, and particular attention must be paid to it during the design process. This study underscores the importance of design optimizations, such as adding transition fillets, to prevent similar failures.</p>