Pallets for automation: conveyors, AS/RS and robotic palletizers

Automated conveyors, stacker cranes, shuttles and robotic palletizers handle pallets without a person to notice a cracked board or a proud nail. A pallet that a forklift driver would move without thinking can jam a conveyor, trip a photo eye or fail in a high-bay rack. This guide lists what automation asks of a pallet and how to put those requirements into a pallet spec.

#Why automation is harder on pallets

A forklift adapts. An automated system assumes every pallet has the same footprint, height, flatness and bottom structure, and it carries the pallet on narrow supports (rollers, chains, rails, forks) that sit in fixed positions. The pallet is also often racked in an automated storage system with tighter deflection limits than a conventional rack.

#Conveyors: bottom deck orientation

The bottom of the pallet must bridge the conveyor's supports.

#Roller conveyors

On a roller conveyor, the pallet's bottom contact members (bottom deckboards, runners, stringers or a perimeter base) should run in the direction of travel, perpendicular to the rollers, so each member rests on several rollers at once. A common design rule is that the pallet should be supported by at least three rollers at all times, which sets the maximum roller pitch for a given pallet length.

A bottom board that runs parallel to the rollers can drop between them or ride unevenly, producing bumps and noise.

#Chain conveyors

On a chain conveyor, two or more chain strands run in the direction of travel. The bottom members must run across the chains so every board bridges the strands. A stringer pallet traveling with its stringers across the chains can have its bottom boards lined up with the chains and ride poorly.

#What this means by pallet type

Perimeter-base block pallets are common in automated distribution because they convey in either direction and allow four-way entry for forks and pallet jacks. The template for this page is a perimeter-base 48 × 40 block pallet.

#Gaps and openings

• Bottom deck gaps should be small enough that boards cannot drop between rollers and that transfer mechanisms do not catch. Wide gaps between bottom lead boards and interior boards, as on notched stringer pallets, are a common problem.
• Lead boards carry the pallet onto and off each conveyor section. Full-width, solid bottom lead boards help transfers.
• Chamfers on bottom boards help pallet jacks but should not interfere with conveyor transfers; check with the system supplier.
• Fork openings must suit the stacker crane or shuttle forks, which are often specified by the system supplier.

#Flatness, squareness and dimensions

The Uniform Standard for Wood Pallets sets general assembly tolerances: overall size +1/4 in / −1/2 in, flatness 1/4 in corner to corner, and diagonal difference no more than the greater of 1.5% or 1 in. Many automated systems need tighter values, particularly for height and flatness. Ask the system supplier for:

• maximum and minimum length, width and height;
• flatness of the bottom and top decks;
• squareness;
• maximum overhang of the load past the pallet.

Put those numbers in the pallet spec and in the receiving inspection.

#Stiffness and deflection

Automated storage and retrieval systems usually rack pallets on rails or narrow supports and move them on shuttle or crane forks. Deflection limits are tight because a sagging pallet can hit the level below or be misread. A limit of 0.5 in (13 mm) or less is commonly cited for automated systems, compared with about 2% of span for ordinary racking. See pallet racking and deflection.

To meet a tight deflection limit, a wood pallet needs deeper or more numerous stringers or stringerboards, thicker bottom boards, or stiffer, drier lumber. Plastic pallets for AS/RS often include steel reinforcement.

#Wood, block or plastic?

Moisture matters for wood pallets in automation: green lumber shrinks as it dries, changing dimensions and loosening nails. Kiln-dried lumber gives more stable dimensions. See pallet lumber moisture.

#Pallet quality at the gate

Many automated sites put a pallet check or inspection station before the system: sensors measure dimensions and profile, and rejected pallets are diverted or restacked onto a good pallet (sometimes with a pallet inverter). Common reject causes:

• broken, missing or protruding bottom boards;
• protruding nails or loose blocks;
• oversize or overhanging loads;
• loose stretch wrap tails or labels hanging below the deck;
• pallets outside height or flatness limits.

See pallet inspection.

#Pallet dispensers and robotic palletizers

Pallet dispensers and stackers separate empty pallets from a stack, usually by lifting all but the bottom pallet by its fork openings or deck edges. They need consistent entry heights, flat decks and square stacks. Robotic palletizers place cases to programmed coordinates, so they need the pallet in a known position with a flat top deck and consistent board positions.

#Specifying a pallet for an automated system

Include in the RFQ:

1. The conveyor types and the pallet's travel direction on each.
2. Roller pitch or chain positions, and transfer points.
3. Dimensional limits and flatness from the system supplier.
4. The storage support condition and deflection limit.
5. Maximum load weight and load dimensions.
6. Lumber moisture condition and quality grade.
7. Acceptance inspection and reject criteria.

The custom pallet spec guide covers the rest. In the designer, you can start from the perimeter-base block template and set the bottom style and board layout to match your system.