Research Highlight: Evaluation of the horizontal compression forces that pallets experience during grab handling

Wooden pallets are commonly designed either by computer software, such as the Pallet Design System (PDS), or specifically to pass ISO or ASTM standards. But none of these design methods take into account the horizontal compression forces that a pallet would encounter during grab handling. Grab handling is a common method of moving palletized unit loads around construction sites or other commercial locations (Image 3), especially in the United Kingdom and across Europe. This system is preferred as it reduces the off-load time required and can be done by the delivery driver instead of needing multiple people involved or extra equipment provided by the delivery site. However, pallets are generally designed to be lifted from underneath the top deckboards by fork tines. The grab handling method uses pincher type equipment to squeeze the pallets from the sides, compressing inwards, to lift them and their product loads.

Prior to this research, it was commonly believed that these grab handling methods, designed initially for moving incompressible payloads (such as bricks or blocks on a construction site), would cause irreparable damage to any wooden pallet. There were even fact sheets distributed in the U.K. claiming that the only way to use grab handlers on palletized unit loads was “for no part of the lifting pads to touch or overlap the pallet.” The recommended usage was for the pinchers/arms to grab the product directly and not horizontally compress the pallet at all.¹

Because of this unique lifting method and due to CPULD’s long history of pallet design and customized testing options, The Pallet Loop company, based in the U.K., contacted CPULD and inquired about working with our engineers and technicians to develop a new test procedure. Their goal was to simulate the horizontal compression forces that grab handling would exert on a pallet and use the procedure to determine the payload capacity for pallets that would be grab handled. CPULD worked with Virginia Tech’s College of Engineering to design a custom apparatus to test palletized unit loads for strength and durability during grab handling conditions (Image 1).

The pallets being tested were 1070 mm x 1200 mm, non-reversable, two-way, flush, stringer class, wooden pallets built by The Pallet Loop. Pallets make up 70% of the total wood packaging materials (WPM) in the European Union. Pallets can be made from a variety of materials, but 90% to 95% of EU pallets are made from wood by 2,910 different manufacturing companies.²

The pallets were first tested in a Tinius Olson compression tester equipped with four 10,000 lb. load cells (Image 2). The pallets were positioned vertically between two rubber-block grab arms and loaded with weight by the machine until either the machine capacity was exceeded or the pallet failed . The pallets were tested both parallel and perpendicular to the direction of the stringers. The goal of the test was to determine the initial strength of the pallet. The strength of all pallets tested exceeded 24,000 lbs.

Then, in order to evaluate the ability of the investigated pallet design to survive the compressive forces during simulated grab handling, a custom testing jig was developed (Image 1). The testing jig was composed of a metal frame that contained one fixed and one movable I-beam, which were used to apply the horizontal compressive forces. A rubber-block grab arm was secured to each I-beam to simulate the actual surface that would interface with the pallet during grab handling. The horizontal load was applied by two hydraulic bottle jacks. The jacks were connected to two digital load cells to measure the applied load. The pallets were then loaded with concrete blocks to simulate a commonly shipped load in the construction industry.

In order to simulate grab handling, pressure equivalent to twice that of the payload was applied by the jacks pressing the two I-beams into the sides of the pallets. Three pallets were evaluated parallel to the stringers using a 3-ton payload, and one pallet was evaluated perpendicular to the stringers using a 1.5-ton payload. Once the required pressure was applied, the entire jig, with the palletized unit load of blocks, was lifted with a crane and held suspended for five minutes. Following this test, the pallets were inspected for damages.

The results of the evaluation of the maximum payload that can be supported by the investigated pallet design when handled by a grab handling system are presented in Table 1. The investigated pallet design was able to support 2.75 tons of a semirigid payload such as concrete blocks when lifted, parallel to the stringers, by a grab handling mechanism. But when the pallet was handled perpendicular to the stringers, the maximum payload was only 1.2 tons; the load capacity in this direction greatly depends on the fasteners used and the location or placement of the grab arms. 

Table 1. Summary of safe payload, strength, and horizontal handling pressure of the investigated pallet designs.

^{* Safe payload was calculated by dividing the safe horizontal grab handling pressure by a factor of 2 based on the manufacturer guidelines that the pressure needs to the twice the payload.
** Safe load values are calculated by dividing the average strength by a safety factor of 2 recommended by ISO 8611.}

References:

1.      PalletLink. (2019). Hydraulic and mechanical grabs – not suitable for pallets. PalletLink Datasheet Ref: 48d/JH/06/08/2019.

2.      Agra CEAS Consulting. (2012). Quantification of the economic, environmental and social impacts of introducing mandatory treatment requirements for wood packaging material circulating inside the European Union. Final Report: Framework contract for evaluation and evaluation related services – Lot 3 Food Chain. Published by Food Chain Evaluation Consortium (FCEC).