To Guided Aisle or not to Guided Aisle? That is the question

Cantilever Racking Storage of Timber: Guided system or free path?

Cantilever Racking is the most common method of racked storage for both Hardwood and Softwood Timber products. The system of legs or columns to give them their correct term have a number of arms attached to them by a variety of means (by pins, bolts or rarely welded) that protrude clear of the columns. The columns are connected together for lateral stability by a series of cross braces and tie beams. This gives an uninterrupted run of storage ideal for accommodating the long product lengths that timber is stocked in. Other methods include Vertical Storage (often called A-Frames) where timber is stored by being leaned upright against a series of beams or Pigeon Hole Storage where timber is stored horizontally in small compartments.

The rise in use of Cantilever Racking in the Timber industry can be correlated directly to the trend for end users to purchase only that amount of timber for which they have an immediate use. The tightening of credit facilities and the requirement for companies to tightly control their cash flow and stock levels means that they often no longer take full packs of material that may take several weeks or even months to consume. Therefore there have been significant increases in multi drop deliveries with a much greater number of smaller orders being processed. This in turn puts pressure on the order picking systems with lower cycle times necessary.

To achieve the levels of selectivity needed Cantilever Racks are used extensively to store and aid selection of individual lengths to meet those increasing numbers of smaller and varied orders. This is either directly from the rack or by bringing a pack stored at high level down to the ground with the pack then returned to its original location after selection has taken place.
The two alternative options conventionally used are Guided Aisle Cantilever Rack systems or Free Path Cantilever Rack systems. In a Guided Aisle system the mechanical handling equipment, usually a multi directional side loading reach truck , is guided down the aisle, the space in between two opposing runs of racks, via guide rollers fixed to the truck running between rails. The rails may be floor fixed or fixed to the bases of the racks or a combination of both. In a Free Path system the truck is solely under the control of the operator whilst operating in the aisle.

Guided Aisle systems offer the maximum density of storage particularly if the full height of the warehouse is utilised. Modern handling equipment now allows storage of timber at heights of up to 12 metres and whilst this presents no technical issues it can present a psychological barrier generally in the timber industry where sideloaders with 3.6 to 4.2 metre lift heights have been used for many years. In such systems the product is often withdrawn from the rack location and driven to a picking station where the required items are removed prior to the pack being returned to its rack location. Ground based picking at lower levels onto timber trolleys or the forks of the lift truck is available although care must be taken to ensure that health and safety issues that inevitably arise with such situations are addressed. The aisle is effectively closed to other equipment during such operations and cannot be accessed until all picked stock is moved out of the system.

Free Path systems whilst offering lower storage densities generally offer wider aisles that can allow simultaneous picking and replenishment and retrieval of full packs subject to suitable and fully risk assessed operational procedures. They tend to be lower in height than guided systems and aisle widths can vary from 2400mm for Side Loading trucks to 6000mm plus for Counter Balance Fork Lift trucks. Where Fork Lift trucks are utilised the aisle width is often determined by the product length to be stored rather than by the physical dimensions of the truck as the products have to be turned through 90 degrees in the aisle to be loaded into the racks. The benefit of these wider aisles is that products can be lowered to floor level on the forks of the truck and picking carried out in situ with much lower travel distances required than with guided aisle systems. The pack can be replaced quickly and easily into its allotted location as it is already correctly aligned laterally. There is still a need for the picked product to be taken to the order assembly area or despatch area and this may be carried out by an alternative piece of handling equipment or by the same truck used to decant the pack. Of course if the same truck is used the time advantage over the guided aisle system is lost and the floor area utilised is very much greater.

The main advantage offered by a free path system is generally when wider aisles are used and secondary stock movement systems are employed for picked products to meet peak demand cycles. In contrast, such utilisation of additional labour in a guided system is only possible if additional and expensive handling equipment is available and its specialised nature makes it almost impossible to hire in on a short term basis. Thus if this is a regular feature of trading for a company then capital equipment that is only used on an infrequent basis is needed. Therefore the trade off is on-going higher costs of space against costly and underutilised capital equipment. An interesting argument perhaps between the Operations Director and Financial Director?