As we have often observed, warehousing is nothing more than the effective management of time and space. It would stand to reason, then, that the material handling tools used in facilities would be designed to conserve both time and space. But it's never that simple.
Inevitably, trade-offs must be made. When space is at a premium, enormous creativity has been applied to finding ways to use the least amount of space possible. When time is the principal driver, it works the other way around. When both labor and real estate costs are high, the time/space economic calculation tends to fly out the window, as highly complex technology solutions are introduced into extraordinarily compact footprints.
Within the four walls of distribution centers (and in the yards), material handling equipment is broadly divided into three varieties: static equipment, mobile equipment, and tools to pick orders.
Static equipment consists primarily of items without wheels, usually storage equipment in the form of metal rack. While some varieties are movable, none are really mobile.
Let's start with racks for pallets, which come in eight basic types. "Single deep" is the most commonly used pallet rack, but it is the design that takes up the most space. "Double deep" requires a lift truck with an extending device that allows pallets to be placed two deep, and it is efficient in any warehouse that normally has at least two pallets of every item. "Self-stacking pallet rack" is a venerable technology that allows a very flexible layout and high-bay storage for products such as tires that have no packaging and no stacking strength. It is also used for some applications in the rug industry.
"Drive-in rack" and "drive-through rack" are essentially the same thing, except that the former allows entry from just one side of the installation, and the latter allows entry from either side. "Gravity flow rack" uses skate conveyor to allow product to move downhill from the top of the storage lane to the aisle. "Pushback pallet rack" allows the lift truck operator to store several pallets in a protected row by pushing the older stock uphill. "Mobile rack" is a rack installation on wheels that allows the elimination of unneeded aisles. It can be effective in cases of very limited demand for a large number of SKUs, but is both costly and inefficient for high-volume distribution.
Some storage systems are designed to hold units rather than pallets. These include shelving and cantilever rack. The shelving may be something quite similar to that found in any library. Cantilever rack resembles a metal Christmas tree, and its arms are designed to hold long products such as pipe, lumber, vinyl or aluminum siding as well as other items too awkwardly shaped to be transported on pallets.
The oldest and perhaps most critical piece of static equipment is the pallet itself. Virtually invented to accommodate the forklift truck, pallets had no standard for size or construction method until the 1960s, when General Foods demanded that pallets be built with some degree of uniformity so they could be exchanged without penalty. The passage of time has degraded the quality specifications that were built into the General Foods pallet, but its size of 48 by 40 inches has become a widely accepted standard—even in the European Union with a metric equivalent. Nevertheless, some companies and industries, notably telecommunications, continue to go their own way in defining the "standard" pallet.
On the move
The next major category is mobile equipment. Manually powered handling vehicles are the oldest devices and they are still widely used. They include two-wheeled hand trucks, four-wheeled platform trucks and hydraulic pallet movers.
The "Model T" of material handling is the counterbalanced forklift truck. It is the most widely used motorized material handling vehicle. As the technology evolved, forks were augmented with a variety of other attachments, such as clamp devices to hold cartons or paper rolls, push-pull attachments designed to handle fiberboard pallets that are called slipsheets, and multi-pallet forks designed to haul several unit loads at a time. While the earliest lift trucks were powered by internal-combustion gasoline engines, later models were developed to handle other fuel sources, including propane, diesel and CNG (compressed natural gas). Battery-powered electric vehicles became a popular replacement for internal combustion, and hybrid gas/electric power was developed for lift trucks long before it was seen in automobiles. Research is well under way toward developing fuel cells to power lift trucks.
For use in operations where space is at a premium, lift truck manufacturers have produced vehicles that are designed to run in narrow or very narrow aisles. The most extreme is the turret truck that has a device that turns the fork carriage without turning the remainder of the vehicle. These trucks can operate in an aisle just a few inches wider than the truck itself.
Tools for the picking
In most warehouses, selection of orders is the single most expensive part of the warehousing process. Conveyors and sorters are the most expensive, but also perhaps the most effective, means of reducing the labor involved in order picking. Most conveying systems are a combination of mobile and static equipment. The frame is static, but there are rollers or wheels within the frame. Gravity flow rack has two varieties, one that moves single cases and flow rack that moves full pallets.
A central question in order picking is whether to move the order picker to the part, or move the part to the picker. Carousels and automated storage and retrieval systems (AS/RS) are designed to bring the product to the picker (or to the point of use in the case of some manufacturing applications).
Carousels can be effective when there are sufficient numbers of items—of appropriate size and weight—with relatively modest demand profiles. They may be less than optimal if the application requires fewer than two or three units. AS/RS enjoyed a tax-incentive-fueled vogue in the United States some years ago, but fell out of favor as the just-in-time wave swept over industry and the tax incentives were eliminated. They have remained popular and useful in places where labor and real estate costs are high, notably Japan, Singapore, Hong Kong, and Europe, and still may be a good choice in some applications in North America.
Comprehensive systems for conveying and sorting are a must for large distribution centers supporting high-volume order fulfillment. They can involve literally miles of motorized conveyor, induction stations, recirculation lanes and readers or scanners. They also require sophisticated control systems for reliable operation. Powerful and impressive as they can be, their planning and implementation demands that they not be made too sophisticated and rigid. The pace of change in the supply chain world requires operational flexibility.
Sortation, alone, introduces a number of solution alternatives into the material handling equation. Principal varieties include pop-up sorters, push diverters, sliding shoe sorters, tilt-tray sorters, bomb-bay sorters and turnstile diverters.
Other order selection tools are more software- than hardware-driven. Options include pick-to-light, a combination of case gravity flow or motorized rack, and signal lights that allow the order selector to complete the job by simply following a trail of illuminated signals and taking the signal-indicated number of pieces.
Voice recognition is one of the most recent innovations in material handling technology. It allows warehouse workers to achieve better accuracy while at the same time increasing speed. The worker wears a headset and receives instructions on the next task. Because the technology is handsfree and eyes-free, the worker is able to move from one task to the next without paper lists or scanning guns.
Radio-frequency identification is a technology that has been around for a while. But driven by demands from retailers and the Department of Defense, it has generated more frenzied publicity than just about any other technology over the last couple of years. The key ingredient is a chip embedded in the product, in its packaging, or in its documentation. Leaders in almost every industry segment are experimenting with, or even developing, technology and process alternatives, seeking practical and costeffective applications.
So, how do you choose?
We suggest asking the following eight questions to determine the best approach to improving order picking and to implementing mechanization and control systems:
The evolution from hand-powered material handling devices to the most sophisticated conveying and sorting has almost always been based on the need to reduce the human effort required for order selection and shipping, as well as in receiving and stocking goods. The evaluation of alternatives and the acquisition of solutions demand a thorough analysis—and a rational business case. It's not a matter of what has the most bells and whistles. Depending on the application, the right solution may also be the simplest solution and the one with the lowest cost.