Automated Storage and Retrieval Systems: Common Mistakes Episode 1: Pallets and Loads

After more than 20 years designing and building automated warehouses, including many systems for automated storage/retrieval (ASRS) of pallet loads in deep freezers, it’s always a surprise to hear of systems that aren’t working well due to some common mistakes. 

While issues related to equipment are sometimes a factor, in many cases it is poor system design that leads to operational problems. This is Episode 1 in a series of “Common Mistakes”.

Load Size

The load type and size is an essential part of system design. In an automated pallet storage system, the loads frequently cannot be adjusted left/right and forward/backward to fit a load opening like they may in a manually operated warehouse with a forklift. The pallets are placed by automated machinery in the center of the opening and proper operating clearances must remain on all 4 sides of the load.  Most equipment on the market is not capable of dynamically shifting loads within a given footprint to fit an oversize load into a small opening. Promises of ability to do so should be met with a healthy dose of skepticism.   

Loads should be sized on the inbound transportation system to check for length, width, and height. Loads failing the size check are rejected for repair prior to induction into the system. Systems that don’t properly account for load size of products that will be stored in the system will have a high number of rejected loads that cannot enter the ASRS. While it is tempting to specify a smaller load envelope to increase pallet location density; the tradeoff is a high number of pallets that are rejected from entry into the system. This is a particular challenge with deep freeze systems where loads need to be quickly moved into storage to maintain temperature --- but with competing pressure to maximize storage positions.

Systems where load size has been specified too small are tempted to store loads that exceed the original design size, resulting in equipment and product damage and with downtime to repair.

Increasing load footprint by 1” per side increases load footprint by 8.5% - meaning there is a meaningful compromise to increasing load size in the design. However, failure to allow for adequate load size will lead to unsatisfactory system performance.

Stretch wrap is an integral part of the load and stretch wrap should be applied tight and with no loose hanging stretch-wrap “tails” that are likely to cause equipment related errors. Loose wrap and tails may be seen by sensors as obstructions, causing the automated machinery to stop.

Recommended minimum load size for food industry applications is 3” load overhang each side, meaning a 40”x48” pallet should be sized for a nominal 46” x 54” load.   There are differences between the European and North American markets when it comes to load size. In the European market, it is not unusual to see load size capped at 2” each side, while in the North American market systems with less than 3” load overhang each side struggle with excessive rejection of loads.

Many of these systems are intended to operate for 20 or 30 years, so don’t compromise on load size just to hit the numbers. 

Pallet

A good pallet is key to success of a smooth operating system. A system pallet provides the highest degree of security. System pallets can be manufactured from wood, steel, plastic, or other composite materials. The material seems to make little difference in performance; but a system pallet is a leading positive indicator of future system performance.  The system pallet will stay captive to the building or network associated with the automated material handling system.

When a system pallet is not feasible, be watchful that equipment type and system design takes into account expected pallet quality. For example, a deep-lane system that doesn’t use a system pallet may not be your best choice for reliable operation. Typical deep lane systems don’t allow for adequate pallet deflection and the sensors used for machine positioning are more dependent on condition of the bottom boards of pallets, leading to poor reliability of operation. 

A better choice when system pallets cannot be used are cranes with telescopic forks (or telescopic platens) that have more tolerance for pallet deflection and don’t rely (as much) on the condition of bottom boards for operation. A system designed without system pallets should also consider that design of inbound/outbound conveyor systems should be simplified to reduce number of transitions and transfers.

Summary advice: if automation systems are complex, a system pallet is a necessity for smooth operation. If system pallets are not possible for whatever reason, then keep transportation systems simple and use load handling devices that are most forgiving of pallet condition, such as telescopic forks.