Fork tine tests are meant to measure a pallet’s strength and deflection while it is being supported on fork tines (Image 1) . Testing standards require that, when determining strength, load is applied until the pallet has breakage or has excessive deformation. For this test, pallet deflection is commonly measured using string potentiometers or Linear Variable Differential Transducers (LVDT), which are connected to the pallets at various locations.

According to the ISO 8611 testing standard, the locations for these deflection measurements should be taken from the middle of the blocks or stringers, while the ASTM D1185 testing standard outlines that the location needs to be at the edge of the pallet. The difference in requirements could create up to a 0.75” difference in deflection measurement location in the case of stringer pallets and a 2.75” difference in the case of block pallets. In addition, during testing, the measurement devices are stationary while the pallet sides are bending, acting as cantilever beams, which means that the measurement location moves horizontally and vertically during testing. This results in the measurement devices deviating from vertical and creating a bias in the recorded deflection (horizontal bias).

To investigate how the measurement location affects deflection measurements, the Industrial Membership of CPULD  sponsored an undergraduate research project for the fall 2019 semester. Owen Wright, a senior, and Luke Guyre, a junior, who are both pursuing degrees in packaging systems and design, were selected for this project (Image 2 and 3).

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Image 2: Owen Wright, a senior, studying packaging systems and design.
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Image 3: Luke Guyre, a junior, studying packaging systems and design.

The purpose of this study was to understand the potential differences in deflection measurement locations and techniques during pallet bending tests using a fork tine setup. The specific objectives are the following:

  • Investigate the effect of deflection measurement locations.
  • Investigate the effect of the horizontal bias during deflection measurements.

In order to analyze these deflection measurement locations, the students completed fork tine bending tests using a wooden jig meant to simulate a pallet. The blocks for the jig were made of Douglas fir while the rest of the assemblies were built with southern yellow pine. String potentiometers were used for the deflection measurement. To evaluate the effect of the horizontal bias, a custom track was built to fit one string potentiometer and to mark the locations where the measurements were to be taken. The tests were completed on a Tinius Olsen fixed platen compression tester.

After the string potentiometers were connected to their specific locations on the jig, the platen was lowered to simulate a load being applied to a pallet. Deflection measurements were taken at the various hook-up locations, including at the edge of the pallet, 0.25” from the edge, 0.75” from the edge (simulating a stringer pallet), and 2.75” from the edge (simulating a block pallet). Initial measurements from the string potentiometers were taken at each of the locations while the pallet was under zero load. Then, when the platen was lowered 0.5”, the deflection amounts were measured. The string potentiometer was then moved along the custom track in order to make the string fully vertical and another measurement was taken. This was then done for all measurement locations. After this was completed, the platen was lowered another 0.5”, and the process was repeated.

Data analysis was then conducted in order to identify whether there were any significant differences between the deflection measurement locations and whether having the string hooked vertically to the measurement location was different then having the string angled to the measurement location (called the horizontal bias).

The results of the research revealed that the horizontal bias is not due to the stationary nature of the stringer potentiometer.

When the deflection results were analyzed, it was found that the measured deflection of the pallet decreased as the measurement location got closer to the center of the pallet’s stringer/block, both with and without horizontal bias. The data also showed that the horizontal bias is not significant due to the stationary nature of the stringer potentiometer. This study also found that certain measurement locations were consistently significantly different from each other.

The students found that there was no significant difference between taking deflection measurements at the edge of the pallet and taking them 0.25” away from the edge. Therefore, connecting deflection measurement devices a slight distance away from the edge is still an accurate measuring method. In addition, it was found that measuring the deflection in the middle of a 1.5” wide stringer (ISO) versus the edge of a pallet (ASTM) does not result in any significantly different deflection measurements. However, measuring the deflection in the middle of a 5.5” wide block (ISO) versus the edge of a pallet (ASTM) does yield significantly different deflection results (Table 1).

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Table 1: Results of the pallet simulator besting test with horizontal bias.