In comparing high density storage systems to conventional selective racking, an issue that often emerges is racking cost.
Block or bulk pile storage which does not rely on racking, but is supported by product packaging, is the cheapest.
Double-deep storage is the cheapest—in rack cost only—high-density racking on a cost-per-pallet position basis because it employs standard racking. Unlike other types of high-density systems it is limited to a maximum doubling of storage density over regular racking. It also requires specialized deep-reach fork truck equipment. Like the majority of high-density storage systems, double-deep racking is a last-in-ﬁrst-out (LIFO) storage system.
The next most expensive high-density storage system is drive-in or drive-through racking. On a cost per pallet position basis it is twice as expensive as double deep racking.
The key is not to become ﬁxated on the cost per pallet position. Unlike double-deep racking, drive-in racking is capable of many pallets deep storage. Thus in terms of total cost; including the building, it can be competitive with double-deep storage.
Drive-in racking is also capable of being used with standard lift trucks. Functionally, it is very similar to bulk pile storage but does not have the same stacking height limitation. Like bulk pile and double-deep it is not generally suited for order picking applications because the pallet is buried in the storage lane once the ﬁrst pallet is emptied.
In the other two types of high density storage systems; pushback and flow racking, pallets are automatically indexed to the pick face once a pallet is removed. This indexing also speeds up pallet putaway and removal particularly at upper rack levels. The lift truck can remain at the rack face and does not have to enter the lanes or tunnels of the drive-in system.
Both drive-in and bulk pile racking are limited to one SKU per storage lane or tower. Both pushback and flowracking, on the other hand, can have a different SKU for each level.
The greater number of SKUs to be stored in a drive-in system means there are a greater number of empty slots in a drive-in system compared to a pushback system. These empty slots are commonly called honeycomb. It results in at least 10 to 20 percent lower occupancy than is achievable in a pushback or flow rack system. In certain rare situations this lower occupancy is offset by the greater number of unit loads that can be stored vertically in a drive-in system.
In a flowrack or pushback system a sloped elevation is needed to provide the indexing to convey the pallet to the rack face. This can decrease usable vertical heights, particularly in deeper systems of ﬁve pallets or greater.
Another issue with drive-in systems is pallet quality, as the pallet supports the full weight of the unit load except on the edge. The travel in the tunnels also results in greater product damage and storage system damage compared to pushback or flowrack systems that index the pallet to the storage aisle.
Pushback: The concept of pushback racking has been around for over three decades. Early designs were limited to two pallets deep. These employed the basic concept that holds true today, of providing deeper storage with the advantage of indexing the rear pallet forward to the load aisle. Compared to other high-density storage options it has the following considerations and trade-offs.
Compared to drive-in or drive-through racking it is about twice as expensive per pallet position and is limited to six pallets deep. However, it is only about half as expensive as flow racking per pallet position.
And pushback is capable of multiple system design options in the vertical plane. In a four-pallet high system, the bottom two pallets can be floor-stacked and the two upper levels can employ pushback racking.
Pushback can be used effectively in an order-picking application because a large volume of product can be presented at the pick face, reducing the need for frequent replenishment. Compared to flow racking it is signiﬁcantly less expensive even compared to the newer bearingless systems. Technically it does not provide FIFO (First In, First Out) stock rotation. But by always emptying the oldest lane ﬁrst, in many applications sufﬁcient product rotation can be maintained to avoid problems, except for products with a very short life cycle.
While flow racking is the most expensive form of high density storage it is the only system that provides ﬁrst in ﬁrst out (FIFO) storage. Many users with product with a short shelf life use it to avoid loss, or in restricted space areas, like high volume manufacturing or line feed assembly. The live storage capability also permits its use in high turnover specialty applications such as staging racking. It is most susceptible to bad pallet quality and type.