The type of materials passing through warehouses varies enormously, with different sizes, weights, shapes, levels of fragility and hazard characteristics. A major benefit of unit loads such as pallets is that they enable the use of standard storage systems and handling equipment, irrespective of what is handled. Nevertheless variations in throughput and order picking patterns make it appropriate to have different types of storage system, with different operational characteristics, so that systems can be selected that most closely match the needs of the wider system within which they are to operate.
Palletized storage systems/methods
Block stacking
Block storage does not use any storage equipment. Loaded pallets are placed directly on the floor and built up in stacks, one pallet on top of another to a maximum stable height. The pallet loads must be capable of carrying the superimposed pallets, and the top of each load should be flat enough to provide a stable base for the next pallet.
Block stacking is suitable for that part of the product range where there are few product lines, each with high stock level, and where very strict FIFO movement of stock is not required. The advantages are good use of area, flexibility to change the layout of the bocks and quick to stock for rapid throughput.
Drive-in and drive-through racking
Although this is a racked storage system, it is operationally similar to block storage.
There should only be one product line in each row, and the effective utilization of the pallet positions is about 70%. The racking structure supports the weight of the pallets so this system is suitable for high stock product lines, where strict FIFO movement is not required, but where the pallet loads are not strong enough or of regular enough shape to carry superimposed loads. This system consists of vertical support frames, tied at the top, with cantilever pallet support beams at different heights.
Push back racking
This type of racking is a comparatively recent development. Like-drive-in racking it gives high-density storage and can be built to any height up to the maximum lift height of the lift trucks accessing it. Pallets can be stored up to about four deep in the racking, on either side of the access aisle. The basic operational difference between this system and block stacking or drive-in racking is the increased selectivity achieved. There should be no mix of product lines in any one lane, but there can be between the lanes in any row.
Adjustable Pallet Raking-(APR)
Adjustable pallet racking is probably the most widely used type of pallet racking, and offers free access to every pallet held. It can be built to match the lift height of any forklift truck. Unit loads other than pallets can be stored using APR, and there is a range of accessories such as drum supports and channel supports for post pallets to facilitate this.
The conventional way of laying out APR is to have one row single deep at each end of the installation, with back-to-back rows in between. This gives every truck aisle access to two rows of racking, and minimizes the number of aisles required.
APR is a flexible, versatile storage system, which gives excellent stock access. It is simple in concept, easily laid out, and damaged parts are easily replaced. It can be suitable for fast-moving and slow –moving stock, and for product lines with high or low levels of palletized stock-holding. However, APR does not make good use of volume of building volume.
Double deep Racking
If some loss of totally free access to stock can be accepted, although not nearly as severe as in block, drive-in or push back storage, space utilization can be improved using double deep racking. This supports pallets on pairs of beams as in APR, but improves space utilization by eliminating alternate access aisles, and using a double reach fork-lift truck, which can access not just one but two pallets deep into the racking.
Powered Mobile racking
Powered mobile racking is effectively single deep APR, with the racking, except the end or outer rows, mounted on electrically powered base frames. Operationally it has similar characteristics to APR, but it is slower in use, and the pallet position utilization is likely to be similar to APR at 90 to 95%. This type of storage is expensive in equipment and floor costs, and it tends to be slow in operation. However it gives very dense storage, and is suitable for the typically large number of product lines forming the ‘Pareto tail’ of a product range, where individual product lines have low stock and low throughput. It also finds use in cold-storage applications where space costs are especially high, and however temperature variations are reduced by cutting the air space in the storage area.
Pallet live storage
Live storage systems are made up of inclined gravity roll conveyors, laid out side by side and at a number of vertical levels. Pallets are fed in at the higher end and removed as required at the lower. Such a system imposes FIFO. The only accessible pallets are at the out feed end, so any one lane should only hold pallets of the same product line.
Pallet live storage systems are suitable for very fast-moving product lines. They can provide effective order picking regimes, which automatically refill empty locations, and also provide physical separation between picking and replenishment operations.
Small item storage systems
As with palletized storage systems, there is a range of different types system for holding small items. With small item storage it often happens that different systems are incorporated into one installation. For ex, drawer units and cabinets may be built into a shelving installation. Consequently the concept of standard equipment sizes and modularity is important for small item storage systems.
The following lists are some of the storage systems used for small items:
- Shelving
- Tote bins
- Drawer units
- Dynamic systems –mobile and live storage
- Mechanized systems- carousels and mini loads
