Distribution centre layout decisions : storage media and picking systems
This is the second of three articles on current trends in distribution centre design, layout and automation
We have already considered location and overall design in the first article of this series. We now need to drill down into how to become more efficient in the use of equipment, storage media and the flow of materials.
Understand the “today” situation
Before we can consider making improvements we have to understand and document how everything works now. Using reliable historical data, we can build up a database of existing warehouse material flows and equipment that is used daily. The next step is to apply any growth projections (or contractions) to this baseline. This baseline is a valuable reference point for detailed warehouse layout plans and the selection of the best storage and picking solutions.
Warehouse storage systems are a major capital investment that must fit into your warehouse floor plan as well as your budget.
How do you decide what storage media to use and what picking systems to employ?
Storage media and picking systems
In many large facilities pallet racking systems are critical to the operations but there are many variations. A common solution is single deep adjustable pallet racking with wide aisles between the racks to give maximum flexibility. Every pallet can be accessed individually allowing multiple forms of stock rotation e.g. first-in-first-out (FIFO), batch selection, management by date etc. Wide aisles allow picking from the bottom level to take place safely at the same time as pallets are being replenished. This is an efficient method of allowing pickers to pass each other, increasing productivity.
Increasing storage density
There are three main ways to improve rack storage density. Be aware that each of the options has some detrimental effect on productivity. Reaching pallets in multi-depth storage is slower, narrower aisles generally result in slower forklift operations and make it more difficult to pick at the same time, and storage at height is slower to access. The trade-off is between maximising storage capacity and managing productivity levels.
- Multi-depth racking
Double-deep racking with a forklift truck attachment or drive-in racking, push-back racking or flow-racking can be options in the right inventory conditions. High volume items where there is a limited number of SKUs work well in this solution as stock rotation flexibility is reduced.
- Reduce aisle widths
Reducing aisle widths from the standard 3m+ to a narrower <2m can greatly increase storage capacity. There are tradeoffs, however, regarding the types of racking and forklift required as well as between storage density and SKU accessibility. Narrower aisles can lead to congestion for equipment operators, leading to delays.
- Increase storage height
Where possible, eliminate wasted overhead space and use all the vertical cubic area available. Investigate suitable vertical racking solutions bearing in mind weight, safety codes and fire regulations.
Because space and labour costs vary according to the location of the DC or warehouse, each interior design and layout project is different. Therefore, the right combination of storage solutions depends on the variables present in your environment. It is recommended that you model the different options, either manual or automated, to determine the resources, capex and operating costs for each option. Automation of pallet storage and retrieval using cranes and/or pallet shuttle systems is usually cost-effective if there is sufficient throughput to operate the automation for >16 hours per day.
Picking methodologies include zone picking, single order picking, multi-order picking and batch picking. Which one is most appropriate will depend on the order profiles (units per order line, order lines per order), pick-walk capacity in terms of the number of order lines, whether orders require packing, whether we have accurate dimensional data, whether 100% checks are required.
Sorting ‘batch picked’ products into individual orders is a common method which suits certain order profiles – technology includes traditional cross-belt and ‘bomb bay’ sorters. Newer options such as pocket-sorters have developed out of hanging garment technology. Each of these solutions has its own ‘sweet spot’ in terms of order profile, types of products and scale of investment – again modelling the trade-offs is the way forward.
Automation of picking presents a huge range of possibilities. It is also a key focus for innovation as up to 50% of warehouse labour is used in picking tasks. Many technologies support order picking productivity and boost accuracy. Automated mobile racking systems are a more recent development that typically involves a fleet of robots carrying shelves to bring the product to the picker.
At the higher end are shuttle systems which retrieve product totes from a dense storage solution and present these to the picker at 1:1 or 1: many picking stations. The picker simply takes from one tote and puts to another, sometimes confirming the action with a button push or by breaking a light beam. Mobile handheld devices, often using Android™ technology and Bluetooth are used extensively for two-way communication, usually within a warehouse management system (WMS). They are used to give and receive messages or instructions via voice or text and to track and confirm orders picked.
What happens next?
Once storage and picking equipment are defined, these constitute the building blocks that need to be placed properly within the layout to provide a good overall ‘flow’. Warehouse flow concepts such as U-shaped flow and crossflow are relevant and again their appropriateness varies according to overall throughput, relative to storage.
As Warehouse and Distribution Centre Design specialists, SCCG has the knowledge and understanding of the operational requirements within an effective DC, as well as selecting the most appropriate storage and picking equipment, based on each business’ needs.
The final article in this series will look at the design and operation of inbound and outbound facilities and important but neglected areas in the warehouse.