A five-aisle automated storage and retrieval system (AS/RS) with a storage/retrieval (S/R)

machine in each aisle is installed in a distribution center. Full pallet loads are stored and retrieved by the S/R machines. The input/output (I/O) point is at the end of the aisle and consists of a simple pickup and deposit (P/D) station. Pallet loads to be stored are retrieved from an inbound conveyor, delivered to the P/D station, and placed on the P/D station by lift truck; likewise, pallet loads brought to the P/D station by the S/R machines are removed by the lift truck and transported on an outbound conveyor.

It takes 0.5 minute to travel between the retrieval point of the inbound conveyor and the P/D station, 0.5 minute to travel between the P/D station and the deposit point of the outbound conveyor, and 0.5 minute to travel between the deposit point of the outbound conveyor and the retrieval point of the inbound conveyor; travel times include picking up and placing the pallet load. The time required for the S/R machine to pick up a load at the P/D station, travel into the aisle, store the load, and return to the P/D station equals four minutes. How many lift truck operators are required to service the five-aisle AS/RS and not create idle time on the part of the S/R machines?