What is the purpose of the fillet on a turned down shaft?

The purpose of the fillet on a turned down shaft is primarily to relieve stress. In mechanical design, sharp corners or abrupt changes in cross-section, such as where a shaft is reduced in diameter, can create stress concentrations. These stress concentrations can lead to cracks or failure under load. The fillet, which is a curved transition from one surface to another, helps distribute these stresses more evenly across the component. By providing a smoother transition, the fillet significantly reduces the likelihood of stress concentration, prolonging the life of the shaft and contributing to its overall reliability.

While other options may seem relevant, they do not directly address the primary functional aspect of a fillet on a shaft. Enhancing aesthetic appearance is a secondary consideration, weight reduction is not a primary function of a fillet, and while it may influence the grip of couplings, that is not the main purpose of the fillet itself. The key aspect is its effectiveness in stress management, making it a critical feature in ensuring mechanical integrity.