Automated warehouses tolerate far less pallet variation than a manual warehouse. A forklift driver adapts to a bent board or a skewed pallet; a stacker crane, shuttle or conveyor cannot, and a single out-of-tolerance pallet can stop an aisle. Pallets used in automated storage and retrieval systems (AS/RS) and conveyor lines therefore need tighter dimensions, higher stiffness and consistent condition than general-purpose pallets.
Where automation is sensitive
An automated pallet system has several contact points, and each imposes its own demand on the pallet:
| System element | What it does to the pallet | Typical failure caused by a poor pallet |
|---|---|---|
| Roller or chain conveyor | Supports the bottom deck on rollers or chains | Jam when a board protrudes or sags between rollers |
| Stacker crane or shuttle forks | Slide under the load and lift it | Fork collision with a protruding nail or broken board |
| Rack beams or rails | Carry the pallet on its two outer supports | Excessive deflection, pallet slips off or sticks |
| Photocells and laser scanners | Detect pallet size and position | False errors on oversize, tilted or overhanging loads |
| Transfer cars and turntables | Rotate or shift pallets | Misalignment on non-square pallets |
The result is a design discipline in which the pallet is treated as machine equipment. Germany's VDI guideline VDI 3655, Requirements on flat pallets for the use in mechanised and automated conveying and storage systems, is one reference for selecting and testing flat pallets in such systems. It sets out requirement catalogs, test methods and tolerances, and also gives guidance for developing special pallets. The edition on sale is dated November 2008.
Typical pallet requirements
Specifications are set by the system integrator or the plant operator and differ from site to site. Common categories are:
- Dimensional tolerance. Length, width and height are held within a few millimeters of nominal, and the pallet is checked for squareness (diagonal difference).
- Deck board and bottom board condition. No protruding nails, splinters or loose lead boards. Chamfers on the leading edges of bottom boards help rollers and forks to engage.
- Stiffness. A pallet that bends too far under load does not roll evenly on conveyors and may touch the rack floor or beam. Deflection is measured with the pallet loaded and supported in the way that the system supports it.
- Moisture and weight. Wet pallets weigh more and swell, changing dimensions and board gaps. Kiln-dried or heat-treated pallets are favored; see wood moisture content.
- Unit load stability. The load should be flush with the pallet edges or within a defined overhang, and wrapped securely so the unit does not lose cartons on a conveyor.
Deflection limits are application-specific. Manufacturers of racking and conveyors publish maximum bending values, often a few millimeters to a few centimeters depending on span, and the pallet must be tested in the same support configuration as in the system. Treat any quoted figure as a starting point and check the equipment data sheet.
Pallet types preferred
Three pallet types dominate automated systems:
- Plastic pallets have precise, repeatable dimensions and are easy to detect. Their stiffness may fall with temperature and load duration, and they are often fitted with steel reinforcement in racking applications.
- Block pallets of high-grade wood are used where pallet cost must stay low. They need strict grading and repair standards, as explained in EPAL quality classes, and wood-fiber or engineered alternatives are discussed in moulded and presswood pallets.
- Metal pallets provide the greatest stiffness and are used for heavy loads and harsh environments; see metal pallets.
Mixed fleets are a common source of trouble. An operation often handles incoming pallets of every condition and then places goods on a controlled pallet type inside the automated area.
Slave pallets and repalletizing
Where incoming pallets are irregular, the usual solution is to bring goods onto a slave pallet (also called a carrier pallet or captive pallet): a robust, standardized pallet that stays inside the automated system and carries the load through conveyors and racks. Goods are transferred from the supplier pallet to the slave pallet at a transfer point, and the supplier pallet is returned or sent for pooling.
Other approaches include a dedicated pallet pool for the automated zone, inspection stations at the system entrance that reject unsuitable pallets, and slip-sheet or pallet-free handling for specific products. Each solution trades capital cost, labor and floor space against reliability. Whether the receiving pallet fits within the planning of the building is usually settled at design stage, together with the choice of pallet racking systems.
Inspection stations
The standard protection for an AS/RS is an entry inspection station between receiving and the high-bay. Typical checks include:
| Check | Method |
|---|---|
| Length, width, height | Photoelectric curtains or laser scanners |
| Overhang and tilt | Light barriers or 3D vision |
| Weight and weight distribution | Load cells in the infeed conveyor |
| Pallet condition | Cameras detecting missing boards and protrusions |
| Identification | SSCC or internal barcode scan; see pallet labelling with GS1 SSCC |
Rejected pallets are diverted to a manual correction lane where the load is moved to a good pallet. The investment in inspection pays back by avoiding crane stops, which can block an entire aisle.
Maintenance and quality control
Wooden pallets in automated service should be tracked and removed from circulation when damaged, through a controlled repair program. Pallet quality inspection should cover nail protrusion, board splits and moisture at a sampling rate agreed with the system supplier. Quality systems for the producer side are covered in pallet quality control.
Frequently asked questions
Can standard Euro pallets run in an automated warehouse?
Often yes, if they are in good condition and the system was designed for them. Many automated sites accept only pallets that pass an inspection station, and reject damaged or non-standard ones.
What is a slave pallet?
A slave pallet is a sturdy, standardized pallet that remains within the automated system and carries goods that arrive on varied supplier pallets. It avoids exposing the machinery to unpredictable pallet quality.
Which guideline covers pallets for automated systems?
VDI 3655 is a German guideline on flat pallets for mechanised and automated conveying and storage systems. It provides requirement catalogs, test methods and tolerances; the system integrator's own specification should also be followed.
Sources
- VDI 3655 Requirements on flat pallets for the use in mechanised and automated conveying and storage systems — DIN Media
- VDI 3655 guideline page — VDI (Association of German Engineers)
- How Much Load Can My Pallet Carry? — Virginia Tech Center for Packaging and Unit Load Design
- ISO 445:2013 Pallets for materials handling — Vocabulary — International Organization for Standardization