Storage device

The most commonly used warehouse storage device is the so-called single-deep pallet rack (see Figure 1). As the name implies there is only one pallet stacked at each level and the pallets are retrieved with forklifts. For higher levels it is suitable to use a reach truck or a high-level order picker. The pallets on the bottom level are easily accessible with for instance a pallet jack.

Figure 1

Double-deep racks or pushback racks are twice as deep as single-deep pallet racks and have two pallet slots at each location. Double deep racks increase the storage density but each SKU requires two dedicated slots in order to avoid moving the frontmost pallet. The design follows the last-in-first-out retrieval LIFO; and the solution requires a specific forklift with double-reach forks. Storage and retrieval require less labor than for single deep racks. 

Narrow aisle racking is a variation of single-deep racking but the aisle width is considerably smaller. The use of narrow aisle racking will increase the storage density but consequently decrease the number of forklifts that can operate in the area. Narrow aisles are designed to fit one specific forklift that must operate alone, resulting in a limited picking capacity. Hence, narrow aisles are not recommended for items that are retrieved frequently.  A design that follows the first-in-first-out (FIFO) retrieval is the flow racks (see Figure 2). The pallets are stacked on one side and retrieved on the other. In between there are flow racks that, with the help of gravity, move the pallets to the front-most position. The system enables the stacking of pallets or cartons in multiple depths or lanes and requires the least labor of all the systems mentioned. 

Figure 2:  Flow-through racks

A simple and low investment stacking is the block-stacking, where stackable pallets are placed directly on the floor. The problem however, is the man-hours required to rearrange pallets when retrieving pallets that eventually end up in the middle.

Allocation

Items can either have a dedicated slot or dynamically allocated slot. With dedicated slots all items have a specific location whereas items with dynamically allocated slots are positioned where there currently is an available slot – regardless of the previous location. Allocation and storage devices can be a strategic important decision and rethinking the allocation strategy can save both time and reduce costs. The areas that should be reconsidered before allocating items, such as frequency of use, volume, and destination