Travel time model for the warehousing system with a tower crane S/R machine

This paper studies the characteristics of a warehousing system in which the storage and retrieval orders are performed by a tower crane. The crane is located at the center of the round storage area and it can rotate in both clockwise and counterclockwise directions. We develop some mathematical travel time models for this warehousing system using the characteristic of the S/R crane that the S/R device can move in radial and circumferential directions simultaneously. First, we derive travel time models for single command and dual command cycles under the randomized storage assignment rule. Then, assuming that the turnover rates for items are different from each other, we calculate expected travel time under the turnover-based assignment rule through a numerical approach. For each model some illustrative examples are presented.