How many turns of hoist rope must remain on the driving drum of a special purpose elevator at its maximum upward limit of travel?

For special purpose elevators, maintaining a minimum number of turns of hoist rope on the driving drum at the maximum upward travel limit is crucial for safety and operational integrity. The requirement for at least one full turn of hoist rope remaining on the drum ensures that there is still sufficient rope to prevent the possibility of the load becoming detached or the rope slipping off the drum during operation.

This single turn acts as a safety buffer. It enables the rope to stay engaged with the drum and avoids any risk associated with the potential for the rope to pay out or unravel. The guidelines for hoist ropes are designed to minimize risks related to equipment failure and to protect both the equipment and the individuals using the elevator.

In contrast, options suggesting a higher number of turns could overcomplicate the operation and may not be necessary for all types of elevators, especially special purpose elevators that are designed with specific operational parameters in mind. Therefore, one turn is the minimum required for safety and functionality.