VIRGINIA TECH TEST RESULTS - PERFORMANCE OF 24x40 INCH CORRUGATED PALLETS
John W. Clarke, Marshall S. White, Ralph L. Rupert
Center for Unit Load Design
Virginia Tech
Blaksburg, Virginia
June 8, 2001
Materials
Packaging Unlimited of Louisville, Kentucky provided 10 24x40-inch quarter pallets. A description of the design is given in Table 1.
Table 1: Description of the 24x40-inch Corrugated Half Pallets from Packaging Unlimited
| Pallet Size (in.) | Stringers | Number of Hardboard Strands per Stringer | Top Deck | Bottom Deck |
| 24x40 | 3 stringers 3.00"W x 3.50"H x 24"L | 2 | 40x24 inches AAC triple wall | 40 x 24 inches CB double wall |
A photograph of the pallet design is given in Figure 1. The average pallet weight was 10.4 pounds. Stringers were glued layers of double wall CB corrugated (flutes vertical) and hardboard. Stringers were layered as follows: 2 layers DW, 1 strand HB, 4 layers DW, 1 strand HB, 2 layers DW. The pallet was a flush design. Top decks were AAC triple wall corrugated with flutes oriented perpendicular to the stringer direction. Bottom decks were CB double wall with flutes oriented perpendicular to the stringer direction. Openings in the bottom deck for pallet jack wheels measured 8" wide and 14" in length. Adhesives joined the decks to the stringers. All assembly and gluing was performed by Packaging Unlimited.
Test Methods
Stacking Strength and Stiffness
These tests were conducted according to ASTM D1185-98*, Section 8.5. This test simulates bending stresses on the pallet deck during unit-load block stack storage. Prior to testing, all pallets were conditioned at 70°F (20°C) and 90% RH, or approximately 18-20% EMC according to the procedures outlined in ASTM D1185 for paper-based pallets. Test pallets were supported on a flat 1.5-inch thick plywood sheet that simulated a warehouse floor. The test load was 1 layer of boxed sheet paper, 50 pounds per box, with 4 boxes per layer. An inflatable air bag above the layer of boxes provided the remainder of the test load.
The performance criteria was 0.25 inches maximum deck deflection and a safety factor of at least 2.0. Many loads will experience damage when decks deflect more than 0.25 inches between the stringers. In addition, pallet jacks require 3¼" tall openings between decks. The stringers on these pallets averaged 3.5 inches in height, and therefore, if decks deflected more than ¼", there would be an expected loss of functionality with pallet jacks. Initially, 2 pallets were tested to failure to estimate the rated load. Then, pallets were tested for 2 hours at the estimated rated load to determine long term deck deflection (creep). Following the ASTM 2-hour load test, the pallets were tested to failure to determine the ultimate load. The rated load was the lesser of:
- the load at which the 2-hour top deck creep deflection exceeded 0.25 inches,
- or the average load causing failure divided by a safety factor of 2.
Fork Tine Support Strength and Stiffness
These tests were conducted according to ASTM D1185-98, Section 8.5. This test simulates bending stresses on the pallet deck during forklift handling. Prior to testing, all pallets were conditioned at 70°F (20°C) and 90% RH, or approximately 18-20% EMC according to the procedures outlined in ASTM D1185 for paper-based pallets. Test pallets were supported on two simulated forktines measuring 4" wide and 40" in length. Tines were placed inside the pallet at a spacing of 14 inches inside to inside spacing. The test load was 1 layer of boxed sheet paper, 50 pounds per box, with 4 boxes per layer. An inflatable air bag above the layer of boxes provided the remainder of the test load.
Pallets were tested to failure on these simulated tines. The rated load was the lesser of:
- the load at which the top deck deflection exceeded 0.25 inches,
- or the average load causing failure divided by a safety factor of 2.
Functionality on Material Handling Equipment
The half pallets were evaluated with typical material handling equipment including the following: electric and LP forklifts, electric and manual pallet jacks, and roller conveyors (3" and 6" roller spacings). The LP forklift had tines that measured 5" in width, 1.5" in depth, and 40" in length. The electric forklift tines were 4" in width, 1.5" deep, and 40" in length. The manual pallet jack had tines that were 14.5" (inside opening) and 27" (outside span) with a lowered height of 3.25". The electric pallet jack had tines that were 12.5" (inside opening) and 27" (outside span) with a lowered height of 3.25". The roller conveyor was a Pentek belt driven system. Rollers were 2.5" diameter, and spaced on either 3" or 6" centers.
Pallets were tested on these systems under load and while empty. These are the primary systems that half pallets are likely to encounter.
Test Results
Stacking Strength and Stiffness Supporting Boxed Loads
The results of the static stacking tests are summarized in Table 2. The test setup is illustrated in Figure 2.
A preliminary rated load of 1100 pounds was selected based on the test to failure of replicates 1 and 2. The test load was actually 1250 pounds (1.15 times 1100) per ASTM guidelines. Two replicate pallets were tested for 2 hours under this load. The average top deck deflection exceeded the 0.25 inch performance criteria, primarily due to the top deck bending and crushing above the stringers. It is expected that, with this load, the design would pass all performance criteria at a rated load of 750 pounds, although there were not enough replicate pallets for confirmation. The average load at which the top deck flutes crushed (failure) was 2228 pounds, and ranged from 2160 to 2280 pounds. At a rated load of 750 pounds, this gives us an average safety factor of 3.0. No stringer hardboard crushing was observed at these load levels, just top deck crushing.
For rigid loads, there may be cost savings in using a lighter construction top deck and allowing the stringers to bear the weight. Pallets used in less humid environments may result in greater acceptable loads. Loads that are more flexible or concentrated than boxed paper, however, may result in lower rated loads. Users should verify the performance of this design in field trials with the actual loads before implementation.
Table 2: Results of the ASTM D1185 static stacking strength and stiffness tests of the Packaging Unlimited 24x40 inch Half Pallets (18%EMC)
Replicate ASTM 2-hour rated load (lbs)* Top Deck Deflection after 2 hours at rated load (in.) Average opening between decks at end of 2-hour full load (in.) Maximum Load at which Top Deck Flutes Crushed (lbs) 1 - - - 2160 2 - - - 2240 3 1100 0.362 3.137 2280 4 1100 0.373 3.127 2230 Average 1100 0.353 3.131 2228 Criteria - 0.250 (max) 3.250 (min) At least 2x Rated Load Results at 1100 FAIL FAIL PASS at 750 PASS PASS PASS
Dynamic Strength and Stiffness with Forklift Tine Support
The results of the dynamic forktine strength and stiffness tests are given in Table 3. The average load at which flute crushing was observed in the top decks directly above the forktines was at 1297 pounds. We divide this by 2.0 to get a rated load of 650 pounds. The stringers and hardboard did not contribute to the performance in this support mode.
In general, pallets used in less humid environments may result in greater acceptable loads. Even though the top deck flutes begin to crush at 1297 pounds, some residual strength remains. The boxed paper rested directly above the 2 forktines, and loads that are more flexible or concentrated than boxed paper may result in lower rated loads. Users should verify the performance of this design in field trials with the actual loads before implementation.
Table 3: Results of the ASTM D1185 dynamic forktine support tests of the Packaging Unlimited 24x40 inch Half Pallets (18% EMC)
Replicate Average load at which the deck flutes crush (lbs) 1 1360 2 1330 3 1200 Average 1297 Rated Load* 650
Material Handling Functionality
Pallets were tested both empty and loaded on typical material handling equipment. The load was 4 layers of boxed sheet paper, 4 boxes per layer, or a total of 800 pounds. This load was stretchwrapped. Note that this exceeds the safe rated loads given above for floor stacking and forklift handling, but these pallets were tested at ambient conditions rather than at 18% EMC. In material handling tests on forklifts, pallet jacks, and roller conveyors, the following observations were made:
- Forklifts — If forklift tines lift the pallet off-center, a column-stacked load will spill unless stabilized with stretchwrap, banding, or some other restraint. It is recommended that all loads be stabilized.
- Pallet Jacks — Some stringers were only 3.25" in height. Pallet jacks are 3.25" when lowered, and this could result in handling difficulties. Electric and manual pallet jacks entered all pallets we tested, but did scrape under the top deck. It is recommended that all stringers be at least 3.5" in height. The wheel openings on the bottom decks of the half pallets are long enough, but are only 8 inches wide. Our pallet jack wheel caught on the sides of this opening, even when centered. A 9 or 10" wide opening would improve pallet jack wheel access. Pallets were difficult to exit with pallet jacks when empty due to the relatively lighter weight.
- Roller Conveyors — The 2½" diameter rollers were spaced on 3" and 6" centers. The Half pallets conveyed well on this roller conveyor when stringers were oriented perpendicular to rollers. It did convey, but roughly, when stringers were parallel to rollers.
- The bottom decks were quickly creased by pallet jack wheels and by pallet bending when lifted. They were helpful in holding stringers in place when pallets were turned with fork tine tips (sluing).
Users of these pallets should ensure that potential warehouses and distribution systems are audited to determine if other material handling systems will be encountered.
Conclusions
A 24x40-inch Half Pallet design manufactured by Packaging Unlimited was evaluated for floor stacking strength, forktine support strength, and functionality with material handling equipment. The load tested was boxed sheet paper. At 18% EMC, the safe rated load when floor stacked was 750 pounds. The safe rated load when handled with forktines was 650 pounds. The constraint for both support modes was the top deck, and no damages were observed to the hardboard reinforced stringers. The pallet functioned well with typical industry material handling equipment including forklifts, pallet jacks, and roller conveyors.
The performance and functionality of this design will vary depending on load rigidity and equipment interfaces. Users should field test samples of this pallet design before full scale implementation.
Table 4: Summary of the ASTM D1185 Rated Loads for the Packaging Unlimited 24x40-inch Half Pallets (18% EMC)
Description Test Safe Design Strength Half Pallet* Static Floor Stack Test 750 pounds Dynamic Forklift Support Test 650 pounds
