Which shaft extends into the music box on the link-driven swing arm operator?

The main driving sheave is the correct answer because it plays a critical role in the operation of a link-driven swing arm operator. In this context, the main driving sheave is responsible for transmitting power from the drive motor to the swing arm mechanism, thus allowing the elevator door to open and close smoothly.

The link-driven system relies on this sheave to initiate movement through a series of linkages that ultimately connect to the swing arm, which operates the door itself. By extending into the music box, the main driving sheave ensures that the rotational motion created by the drive motor translates effectively into linear motion required for door operation.

Understanding the mechanics at work in this system is essential for troubleshooting and maintenance tasks. The main driving sheave's placement and function directly influence the reliability and efficiency of the elevator door mechanism.