Other Voices: NIOSH study highlights ergonomics of bag palletizing

Editor’s Note: The following column by John Heberger, epidemiologist at the National Institute for Occupational Safety and Health (NIOSH), Office of Mine Safety and Health Research and Sean Gallagher, associate professor of industrial and systems engineering at Auburn University, Samuel Ginn College of Engineering, is part of Modern’s Other Voices column. The series features ideas, opinions and insights from end-users, analysts, systems integrators and OEMs. Click here to learn about submitting a column for consideration.

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Many commodities are packaged and shipped in bags. Small bags usually weigh 50 pounds but in some cases can weigh up to 100 pounds and can be unwieldy depending on content. These bags are typically loaded onto pallets for transport, which can require a significant amount of manual handling by workers, involving high repetition and high loads on the back. The specific task of manual bag handling can lead to the development of musculoskeletal disorders (MSDs), especially in the lower back. This is particularly true if the design of the workplace layout is poor and does not provide appropriate materials handling aids, such as lift tables.

Researchers at NIOSH in Pittsburgh, Pa., evaluated the biomechanical demands of different work layouts when performing manual palletizing of small bags, along with the biomechanical stresses of different stacking techniques. The study examined the effect of operator position relative to the conveyor on low back stress, and the effects of control of the load (dropping vs. controlled placement) and lift destinations (high vs. low levels of pallet) on loading of the lower back. NIOSH is using the results of this study to inform the development of an audit tool for bagging operations in the mining industry.

Eight male participants tested twelve different work conditions as described above, with each condition involving eight lifts of a 25-pound bag off of a conveyor and onto a pallet. The participants stood on two force plates that captured ground reaction forces and center of pressure data for both feet. A motion capture system utilizing retro-reflective markers placed at various joints on the skin or clothing, tracked the orientation of the body and measured full body movement while completing this task.

Analysis of the collected data indicates that peak forward bending moments as well as spinal compression and shear forces are higher when the pallet is at the side of the conveyor as opposed to the end of the conveyor. In this study, placing the pallet on the side of the conveyor increased the estimated compressive loading on the spine by more than 180 pounds (a 19% increase) and spinal compression was much higher than the NIOSH suggested maximum of 764 pounds. In addition, at low levels of the pallet, controlled bag placement resulted in higher peak forward bending moments than stacking at higher levels and when dropping the bag to lower levels. In this study, which only involved lifting 25-pound bags, the difference in estimated low back compressive loading ranged from 135 to 180 pounds.

The study recommends employers, where possible, use automated palletizing of bags rather than manual. Where workers do manual palletizing, provide lift tables at the end of conveyors or other materials handling aids to reduce the likelihood of worker MSDs.

When manual palletizing is the best choice, choose lighter bags rather than heavier. Bags commonly used in industry can weigh up to 100 lbs., which leads to extremely dangerous spinal compressive forces. NIOSH suggests 51 pounds as the maximum load to be lifted under ideal conditions.

Design or modify palletizing workstations to position the pallet at the end of the conveyor rather than the side. This will help to decrease employee twisting and thus reduce low back stress. When manually loading pallets at the end of the conveyor, allow employees to drop bags into position, in particular when loading the lower levels of the pallet. This helps to decrease bending and lowers spinal compressive loading. It might also be possible to reduce the need for small bag palletizing by offering bulk bag options to customers.

For more information, see the original published article.