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The productivity of assembly lines is considerably affected by the health condition of assembly workers, and work-related musculoskeletal disorders (WMSDs) are common occupational diseases among assembly workers due to repetitive motions or heavy working loads. The conventional approaches to decreasing WMSD risks in the assembly lines include slowing the work-pace or applying job rotations. These adjustments usually focus on individual assembly workers at the station level but not the work allocation among the workers at the whole assembly line level, and thus may decrease the line productivity. To avoid these negative effects, some research started considering ergonomic characteristics at the line level, such as balancing ergonomic burdens by proper work assignment among workers. These previous studies incorporated physical grip demands or processing time overload into non-linear assembly line design problems, and used heuristic solution methods.
This paper presents a methodology that explicitly integrates ergonomic measures for upper extremities into linear assembly line design problems. As the ergonomic measures, this research considers a guideline of Threshold Limit Value (TLV) from American Conference of Industrial Hygienists. Linear models are developed to link work-worker assignment to the measures of hand activity and hand-arm vibration. As productivity measures, conventional assembly line design criteria are considered, such as cycle time and the number of workers. These linear models allow ergonomic and productivity measures to be integrated as a mixed-integer programming model for assembly line design. In addition, these linearization methods can be generalized in order to incorporate ergonomic measures in tabulated forms into assembly line design problems.
The analysis of the result shows the new model can effectively control the exposure levels in the upper extremity by proper work assignment compared to the conventional approaches, and does not decrease production rates considerably. This research also shows the potential to reduce the need of numerous task adjustments after assembly line design in traditional trial-and-error based assembly task adjustment.