To increase the efficiency of storage space utilization, various types of racks are used in warehouses. The most common in practice are free-standing frame racks without shelves, ranging from several to over a dozen meters in height, with diverse structures. Palletized Load Units (PLUs), formed on various types of loading pallets, are stored in them.
Placing these units into racks and retrieving them is done using powered mechanical transport equipment. We are therefore dealing with the cooperation of mechanical devices, most often various types of powered industrial trucks or stacker cranes, with stationary devices, which are fixed frame racks. During such cooperation, minor or major collisions often occur.
Their frequency depends mainly on the “human factor” and the technical culture they demonstrate. It must be realized that the choice of equipment also has some influence on this. However, it is mainly the human who operates the mechanical equipment and decides on the method and quality of forming the palletized load units that subsequently enter the racks. Therefore, it is the human’s psychophysical disposition that determines the number of collisions that occur. It is unlikely that where we are dealing with the cooperation of mobile and stationary devices, it is possible to completely avoid collisions. As a result of the collisions that occur, permanent deformations of generally thin-walled rack elements are possible and do occur. The number of deformed elements increases with the duration of rack operation, which causes a radical increase in the risk to work safety in their vicinity. The elements most often subject to deformation are:
– load-bearing frame elements, in particular: front uprights, lower bracing elements, and base plates,
– load-bearing beams, both front and rear,
– protectors shielding the uprights or load-bearing frames at the ends of the racks.
The anchor bolts of the load-bearing frame base plates and protectors become loose and/or damaged. During operation, other bolted connections joining other rack elements also become loose (examples of permanently deformed rack elements are shown in photos 1 – 4).
The appearance of permanent deformations in any rack element, manifesting as its deformation, indicates that the yield strength of the material from which it was made has been clearly exceeded at that point. This means that the load state assumed during design for the damaged element has changed. It is precisely this change in the load state that makes the deformed area a hazard zone, where – faster than elsewhere – the yield strength may be exceeded again, or operation is already taking place at that limit. A catastrophe can occur at any moment, even with tragic consequences for people. Practice has shown that permanent deformations of rack elements often occur even during incorrectly performed assembly.
The article comes from the magazine “Logistyka” 1/2008.