Pallets for automated warehouses: deflection, tolerances and weight

In briefThe allowed deflection is set by the system builder. As a reference, the American standard ANSI MH1 gives 12.7 mm for automated systems, against 2% of the span (20 mm over 1 m) in the ISO 8611 racking test. The pallet must also keep consistent dimensions and pass the profile check at every entry. Plastic generally keeps repeatable dimensions, but on the beams of pallet racking you need a model with runners and steel bars.

If you are designing an automated warehouse, the pallet is part of the system. Stacker cranes, shuttles and roller conveyors are built around a precise profile and a precise deflection. A pallet for an automated warehouse that falls outside those limits is stopped at the entry check or, worse, jams at height ^[3][11].

Two factors decide the choice. The first is stiffness: plastic is less stiff than wood, so on rack beams you need runners and steel bars ^[5]. The second is weight: the tare of one pallet is multiplied by thousands of locations.

Six questions before you choose

Put these questions to the system supplier before you ask for a pallet quotation.

Tolerances and profile check

Every automated warehouse has a check station at the entry. It measures height, front, rear and side overhang, load centring and weight. Pallets outside the limits go to a reject bay before they enter the system ^[11]. Some systems also inspect the empty pallet: dimensions, warped boards, damaged blocks, protruding nails ^[12].

How accurate does the pallet need to be? Part 10 of the American ANSI MH1, which covers pallets for automated systems, allows +3.2 to −6.4 mm on length and width and +3.2 to −8.3 mm on height ^[1][2]. The European CP chemical pallet specification asks for ±3 mm on length and width, −2 to +5 mm on height and no more than 10 mm difference between the diagonals ^[8].

A wooden pallet starts within tolerance and drifts out of it with use: boards break, nails lift, moisture gets in. A moulded plastic pallet has no nails or boards to replace, and every piece comes out of the same mould. That is why many integrators prefer it ^[3]. We found no public standard tolerance for plastic pallets, so ask the manufacturer for one and compare it with the system's.

Friction also matters on forks and roller conveyors. A presentation on ANSI MH1 gives a minimum static friction coefficient of 0.15; wood is around 0.45, plastic around 0.30 ^[2]. If the load is smooth or the stacker crane accelerates hard, consider a pallet with anti-slip inserts. The figures are in the guide to load securing.

Deflection and stiffness in racking

In the location, on pallet racking, the pallet is suspended between two beams, with no shelf underneath to support it in the middle. Deflection is how far the deck sags between the two supports. In a conventional warehouse it is a safety issue. In an automated warehouse it also matters for the machine: if the pallet sags too much, the shuttle cannot get under the load and the stacker crane forks hit the deck ^[5].

The limit for automation is about a third tighter than the general racking test. Most shuttles work with about half an inch of clearance, and that is where the figure comes from ^[3][4]. Asking for an even lower deflection, say 6 mm, pushes up the price, and according to one pallet manufacturer it is often unnecessary ^[4].

Why plastic needs steel

Plastic pallets rarely break: their load limit is deflection ^[5]. Plastic also gives way over time under constant load (creep), which is why ISO 8611 includes creep tests of 24 or 48 hours ^[6]. In an automated warehouse a pallet can sit in its location for months.

ISO 8611-3 defines the maximum working load for a specific load and support condition, racking included. Deflection under that load must not exceed the limit set in parts 1 and 2 of the standard ^[7]. In practice, the racking load on a data sheet applies only to the support, span and direction that were tested. In the location too, the runners sit at 90° to the beams unless the integrator specifies otherwise.

Steel bars inside the deck or the runners reduce deflection and raise the load on the beams. In a heavy pallet with runners the racking load goes from 1,000 to 1,400 kg ^[14]; in a 1200×800 with three runners, from about 1,000 to about 1,750 kg ^[15]. The pallet gets heavier in return. Racking load, orientation and drive-in are covered in detail in the guide to rackable plastic pallets.

The racking has its own tolerances too. In warehouses served by stacker cranes, the FEM 9.831 classes set, for example, beam levels within ±5 mm and beam deflection at L/300 or 10 mm in the tightest class ^[9]. The European standard on racking tolerances, deformations and clearances is EN 15620 ^[10]. The clearance between pallet and structure is calculated by the system designer, starting from the pallet dimensions you provide.

Weight multiplied by thousands

In a 10,000-location warehouse, pallet tare goes into the structural calculation.

Calculation: unit weight × 10,000. The weight of wood varies with species, moisture and construction, so it is a typical figure, not a data sheet value. A plastic pallet for beam racking weighs as much as wood or more. Its advantage is constant tare and repeatable dimensions. The lightweight pallet is in the table only for comparison, because it states no racking load ^[13].

Where the tare counts:

• Racking and foundations: the pallet location is calculated on gross weight, pallet included. In a clad-rack warehouse the racking also carries the roof and walls, and the design follows FEM 10.2.02 ^[9].
• Stacker crane: the machine's capacity includes the pallet. With a net load of 1,000 kg, 9 kg less tare is 0.9% of the gross weight, which can decide whether you stay within a capacity class.
• Energy: here the gain is small. Our estimate: lifting an extra 9 kg over 20 m takes 9 × 9.81 × 20 ≈ 1.8 kJ. At 3,000 movements a day, the throughput of a 13,000-location system described by a forklift manufacturer ^[17], that comes to about 1.5 kWh a day, before friction and regenerative braking.

A plastic pallet also has a constant weight. Wood does not: the CP specification notes that fresh timber exceeds 30% moisture ^[8]. If the profile check weighs the load ^[11], a pallet that is always the same makes the check more reliable.

Roller and chain conveyors: runner orientation

On roller conveyors the pallet must rest on several rollers at once. The bottom boards or runners should be at right angles to the rollers, in the direction of travel ^[4]. A board that barely spans two rollers makes the load vibrate; one that spans seven runs smoothly ^[3]. If the runners are parallel to the rollers, the pallet can rock and the cartons slide outwards ^[4].

That is why a pallet for an automated warehouse almost always has continuous runners underneath, three or five, or a closed perimeter base. A bottom that covers both length and width lets the pallet travel in either direction ^[4]. The separate feet of nestable pallets do not work on roller conveyors.

Chain conveyors and 90° transfer tables have their own rules: the chains run under the runners in a set direction. Get the station drawings from the integrator and check the pallet orientation at each one.

Cold, fire, RFID

Automated cold stores

In the cold, the material matters. HDPE works down to about −45 °C, PP down to about −23 °C ^[18]. Below those limits the manufacturer generally states no performance, and in an automated system a pallet that fails at height stops the aisle. See the guide to pallets for cold stores.

Fire protection

Data sheet DS 8-24 from an industrial insurer treats plastic pallets that are not FM Approved as unexpanded plastic, with more demanding sprinkler protection; approved pallets are protected like wood ^[19]. In a high-bay warehouse this affects the fire protection design, so talk to your insurer before you choose the pallet. Classes and tests are in the guide to fire-safe plastic pallets.

RFID

In plastic pallets the transponder can be embedded in the material, where it is protected from dirt and impacts. It reads without line of sight and can be rewritten ^[20]. In an automated warehouse it is used to track empty pallets and to withdraw those that have done too many cycles. Technologies and costs are in the guide to RFID pallets.

Pallet specification for the system supplier

Send this list to the automated warehouse integrator and ask for it back filled in. Then compare it with the pallet data sheet.

1. Accepted pallet type: nominal dimensions, 2-way or 4-way entry, runners or perimeter base, entry direction.
2. Tolerances on length, width, height and diagonals, in mm.
3. Maximum load overhang per side and maximum unit load height.
4. Maximum gross weight per pallet location and per stacker crane cycle.
5. Support in the location: beams, cantilever arms or rails; clear span in mm; pallet orientation.
6. Maximum allowed deflection under load, on the support in point 5, and for how long.
7. Clearance under the pallet for forks or shuttle.
8. Roller and chain conveyors: roller pitch, chain position, runner orientation at each station.
9. Minimum friction coefficient required between pallet and forks, or between pallet and load.
10. Operating temperature, minimum and maximum.
11. Fire class required by the sprinkler design.
12. Identification: barcode, RFID, label position.
13. Rejection criteria for used pallets: when a pallet must be taken out of circulation.

The full procedure for choosing a plastic pallet, including outside automation, is in the guide how to choose a plastic pallet.

The flat pallet ROLL offers for beam racking

Among the flat pallets ROLL offers, the one designed for pallet racking is the PP69: 800×1200, 16 kg, closed flat runners, three steel bars, 4-way entry, 1,200 kg in racking and 5,000 kg static distributed load according to the manufacturer's data sheet ^[16]. It sits between the medium pallet without steel and the 25 kg heavy one. Before you adopt it in an automated system, check its data sheet against the specification above: deflection, tolerances and runner orientation must be approved by the warehouse builder.

The three flat pallets offered by ROLL

ROLL does not sell generic plastic pallets. Among flat pallets it offers only these three, each for a specific use.

For pallet racking ROLL PP69 1200×800 with 3 steel bars, 16 kg; 1,200 kg racking load according to the manufacturer’s datasheet. Designed for rack storage, with the pallet spanning two beams.

For hygienic environments ROLL PPH1 H1 type with closed runners, no steel bars, washable. Made from recycled material: not for direct food contact.

For floor handling ROLL PP08 Closed flat deck, 4-way entry, 9 feet: for moving pallets on the floor with a pallet truck. Not designed for racking.

Found a mistake or an outdated figure? Write to us at info@roll-packaging.com