Mobile storage-rack applications
These systems provide space-saving, high-density storage by using only one, "movable'' aisle. Here are application details.
By Staff -- Modern Materials Handling, 2/1/1998
Mobile racks provide versatile, high-density storage. They reduce the need for non-productive aisle space by placing the rack on top of wheeled carriages operating on a rail system embedded in the floor. Electric motors and controls provide organized, safe movement.Industrial-grade mo-bile racks, the subject of this article, should be distinguished from the lighter duty library or document-based systems. Industrial systems handle heavy unit or pallet loads. Some accommodate up to 400 tons per individual mobile carriage, extending to 200 ft in length and 40 ft in height.
Operated by pushbutton controls mounted on the face of each carriage, or remotely controlled from forklifts by means of radio transmitters, one or more carriages can be moved left to right automatically and simultaneously to open up a single "transposable" aisle in the position desired within the module system, see drawing.
From 2 to 10 or more mobile bases (carriages) may be placed side by side in a bank (module) with only the one moveable aisle shared among them. However, a common practice is for each bank to be composed of from 4 to 7 mobile bases. Banks are then separated by a main travel or cross aisle to facilitate movement of goods in and out of the warehouse.
A safety sweep is located at foot level along both sides of the entire length of the carriage. When contacted by an obstruction in the access aisle during movement, the carriage stops instantly. The module is reactivated with controls located outside of the access aisle. All safety sweeps are finished with visible warning striping.
In a conventional pallet rack installation, about 40% of the floor space is actually devoted to storage, while 60% is for access aisles. With mobile racking, up to 85% of the floor space is used for storage, and about 15% for the access aisle. Savings in floor space alone are obvious. The accompanying table makes similar comparisons for other common storage approaches.
How about cost?
If you just consider the cost of pallet rack, a 5-level mobile-rack system may cost $130 to $160 per pallet space, as opposed to $35 to $55 per pallet for a conventional rack system. However, that is an overly simplified and erroneous comparison, since other cost factors also come into play.
As discussed earlier, mobile racks provide significant savings in valuable floor space. Because they provide 100% access to desired picking faces, mobile installations are also highly efficient in the use of lift trucks and personnel.
The key cost to consider is the cost of storing a pallet load. Often analyzing the building and rack cost together makes sense. Keep in mind, however, that usually the mobile rack installation is classified as equipment and is depreciated over a much shorter period of time than is the building portion.
Selecting, specifying tips
In addition to using pallet racks, mobile storage units can accommodate shelving, modular drawers, containers, and cantilever and special-purpose racking. Regardless of whether unit-load storage in racks, or parts storage in shelves, is used, quicker and easier order filling is possible. The smaller operating space makes for less travel time for an operator.
When an industrial mobile storage/materials handling system is being selected, specified, and installed, factors such as floor loading (live load weights and capacity) and levelness of the installation must be evaluated.
Rails for carriage travel are permanently set into the concrete floor on leveled support pads spaced by load requirements. Rails are located directly under each rack upright member. The top surface of each rail is set flush with the existing or new floor surface. Thus, rails do not present any obstructions to lift-truck travel.
Certain technical information for this article was provided by J. H. McLain, VP operations, Storax, Inc.
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