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When is Too Little Too Much? VNA Storage Applications

As the heightened demand for industrial real estate continues to drive construction and rent costs to new record highs, expanding warehouse operations are pressured to do more with the space they already have.  Companies welcome anything to reduce the size of a distribution center, cut cost, and lower fixed overhead.  One readily available option that can assist in those reductions is a very narrow aisle (VNA) storage system.

VNA storage can increase the storage density of a facility dramatically, without sacrificing selectivity.  However, the application requires upfront analysis to ensure the VNA storage system does not become a facility bottleneck, constraining a distribution operation from achieving the throughput its customers require.  There are several key factors to keep in mind when evaluating a VNA storage system to ensure you take maximum advantage of its storage density advantages and avoid the pitfalls of throughput limitations.

Most VNA systems have aisles in 6-feet to 5-feet, 6-inch range.   Yes, there are systems with narrower aisles than that, but let’s focus on the more common VNA applications.  Turret or swing reach lift trucks generally service aisles in the six-feet to five-feet, six-inch range. We call them turret trucks because the forks of the vehicle turn 90 degrees to the left or right and allow a pallet to be put away in rack storage positions without the truck turning.  Either a fixed rail system or a floor-embedded wire guidance system allows the trucks to move very rapidly and safely in the aisle without hitting any of the racks to which they operate in such close proximity.   This allows the truck to move only straight forward and backward in the very narrow aisle—an aisle at least three feet narrower than the narrowest reach truck aisle.  Those savings are significant, even after allowing for the higher price point of the turret lift trucks.

The density increase, however, requires a solid understanding of the operations storage throughput requirements, coordination of your workload planning for pallets moving in and out of your VNA storage system, and the ability to ensure volume is spread across the aisles of the storage area.

While very dense, the very nature of a VNA system does not allow you move more product in and out than the VNA turret trucks can support.   While every operation must ensure it has the proper amount of lift vehicles in the building, a VNA building must ensure that the storage configuration has enough aisles to accommodate the combined inbound and outbound peak volume requirements.  The very narrow aisles only allow for one turret truck per aisle in most applications, while conventional layouts allow multiple trucks to work the same aisles.  If the combined inbound and outbound storage volumes at peak demand have more trucks than there are aisles in the VNA layout, then the operation encounters an unacceptable bottleneck to fulfilling operational and, most importantly, customer demand.

A pickup and delivery (P&D) system maximizes the effectiveness of lift trucks in a VNA storage system.  The P&D configuration generally consists of extending every other row of single deep rack an additional rack bay or two at the end of each aisle.  This allows conventional lift equipment to drop inbound pallets in one of the P&D locations as well as take outbound pallets from the P&D positions after the turret truck has deposited them there.  This allows the turret or swing reach to do what it does best – move up and down a narrow aisle at high speed, and put away and retrieve pallets in the VNA system.  Minimizing aisle changes will increase the throughput capability of the turret lift truck and the P&D system allows for that.   The P&D system, however, requires solid work load planning to keep the production of the turret trucks and the conventional lift trucks servicing the P&D stations in balance.  Not enough conventional trucks feeding the P&D stations and the turret trucks will be starved or need to change aisles too frequently.  Too many conventional trucks feeding the P&D stations (or not enough active turret trucks) and the P&D stations will fill up, and the turret trucks will not have enough positions in the P&D stations to effectively output pallets, and the conventional trucks will not have enough positions to drop off inbound pallets.  If the fastest moving product is all stored in one very narrow aisle, the demand on the turret truck in that aisle may far outstrip the number of pallets that can be pulled from, or put away in, that aisle in the timeframe required.  In all these instances, a constraint that may unacceptably limit the volume that can be moved in and out of storage will be reached.

The significant density advantages available in a VNA storage system requires attention to the details of operational storage demands compared with the limitations of the VNA system you are considering.   Attention to the throughput requirements, balancing the volume across aisles, and planning the staffing assignments to balance the number of vehicles servicing the P&D positions with the number of active turret trucks ensure realization of all the density advantages of a VNA storage system, without suffering from any of the possible throughput constraints.     VNA storage density does not come without costs, but properly applied, those costs are far less than the benefits reaped from a VNA storage application.

—Bryan Jensen, St. Onge Company

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