خطرات ترکیب ارگونومیک به خط متعادل کننده مونتاژ

In manufacturing, control of ergonomic risks at manual workplaces is a necessity commanded by legislation, care for health of workers and economic considerations. Methods for estimating ergonomic risks of workplaces are integrated into production routines at most firms that use the assembly-type of production. Assembly line re-balancing, i.e., re-assignment of tasks to workers, is an effective and, in case that no additional workstations are required, inexpensive method to reduce ergonomic risks. In our article, we show that even though most ergonomic risk estimation methods involve nonlinear functions, they can be integrated into assembly line balancing techniques at low additional computational cost. Our computational experiments indicate that re-balancing often leads to a substantial mitigation of ergonomic risks.

The problem of unfavorable working conditions, or poor workplace ergonomics, is an acute topic today. Ergonomic risks at the workplace cause a lot of damage on health and quality of life of workers, deteriorate economic results of employers and of the economy as a whole. In 2008, along 315,000 cases of work-related musculoskeletal disorders (MSDs, often referred to as ergonomic injuries), requiring a median of 10 days away from work, were reported in the US (Bureau of Labor Statistics, 2009). Annual compensation cost for MSDs paid by employers in the US amount to 15 to 20 billion US dollars. Moreover, occupational diseases of workers indirectly cause further cost on firms: via loss of production capacity due to absenteeism of workers, lower worker productivity and higher defect rates in work. This can be illustrated by the example of Peugeot, whose ergonomics program reduced the cycle time for the final vehicle assembly line together with a simultaneous decrease by 30% in new cases of musculoskeletal disorders (Moreau, 2003). Workplace ergonomics is becoming even more important following recent developments in legislation (EU Machinery directive, 2006/42/EC, 89/391/EEC, Occupational Safety and Health act of 1970 among others) and an on-going ageing of the workforce in most of the developed countries. Already today in assembly line production, especially in final assembly, where the share of manual labor is high, a special attention is paid to ergonomics. Most renowned companies incorporate methods for ergonomic risk estimation of working places in their production routine (Toyota Verification of Assembly Line at Toyota, GM-UAW at General Motors, AP-Ergo at Volkswagen to name a few). If ergonomic risks are detected, re-balancing of the assembly line is recommended as an effective method in the short-run (Hilla, 2006). Ergonomic aspects have been barely considered in assembly line balancing literature, though they are becoming increasingly important in practice. Few articles on this topic are those of Miralles et al. (2008) and Costa and Miralles (2009), who introduce and analyze a problem of assigning workloads to stations and to workers with different (dis-) abilities. Another article, written by Carnahan et al. (2001), examines an assignment of a certain class of tasks – gripping tasks – and their influence on fatigue and recovery dynamics of workers. However, to our best knowledge, no attempt has been made yet to incorporate ergonomic risk estimation methods used in practice into assembly line balancing models, though they are considered important by manufacturers. To close this gap, we address this important question in the present study. We provide an overview of some methods for ergonomic risks estimation, which are recommended and utilized in practice. Most of those methods are based on nonlinear functions such that incorporating them into state-of-the-art line balancing models and (exact) solution procedures is not straightforward. We propose different ways to model ergonomic aspects and a two-stage heuristic approach, based on the well-known exact balancing procedure SALOME and the heuristic meta-strategy simulated annealing. By means of this heuristic approach, we can achieve a significant reduction in ergonomic risks of workplaces at low computational cost even without increasing manufacturing capacity, i.e., number of workstations (and workers). The proposed two-stage heuristic approach, furthermore, allows for a controllable increase in manufacturing capacity considering the trade-off between increased costs from adding stations on the one hand and reduced ergonomic risks on the other hand. We precede with an overview of ergonomics tools in Section 2. A line balancing problem incorporating ergonomic risk factors, ErgoSALBP, is described and modeled in Section 3. In Section 4, we propose a two-stage heuristic, which is tested in comprehensive computational experiments in Section 5. A discussion in Section 6 concludes the paper.

In manufacturing, control of ergonomic risks at manual workplaces is a necessity commanded by legislation, care for health of workers and economic considerations. Methods for estimating ergonomic risks of workplaces are integrated into production routines at most firms using the assembly-type of production. Assembly line re-balancing, i.e., re-assignment of tasks to workers, is an effective and, in case that no additional workstations are required, inexpensive method to reduce ergonomic risks. In our article, we show that even though most ergonomic risk estimation methods involve nonlinear functions, they could be easily integrated into assembly line balancing techniques at low additional computational cost. Our computational experiments on a data set, whose ergonomic risks resemble the situation we observed in the automobile industry, indicate that re-balancing often leads to a substantial mitigation in ergonomic risks. Thus, for 50% of instances a balance with acceptable level of risks for all the stations was found without increasing the number of stations. In the cases, where an increase in number of stations was required, such a solution was found after just about 4% increase in the line capacity. Further, our computational experiments indicate that, in general, with each new working station the improvement of ergonomic risks diminishes. For a manufacturer, each additional station means a significant investment in equipment, increase in variable costs – first of all, in wages, – not to mention utilization of space of the factory, which is often a constraint resource. To perform such a step, a firm has to make a careful estimation of benefits from the reduction of ergonomic risks that one or several additional stations would bring. Assembly line balancing can give an answer, how large the expected mitigation of ergonomic risks will be for each station, as well as for the line as a whole. The above mentioned 4% increase in line capacity, necessary to keep ergonomic risks in a reasonable range (green), can be used by line managers as a rule of thumb. Currently, an intensive research is running on the overall costs of ergonomic risks of workplaces. Although these costs are firm specific and a lot of work here is still to be done, we have first insights, how large such costs could be. First of all, these costs include compensation claims and medical visits of workers. Here, Occhipinti and Colombini (2007) estimated that a 1-point increase in average OCRA index of the assembly line leads to additional 2.39% of workers suffering from upper-limb musculoskeletal disorders. Further, these are costs from deteriorated quality of work; Falck et al. (2010) in their 8-week study of a car assembly found the action costs originated in “yellow” and “red” workstations up to 9 times higher than that caused at “green” stations. The methodologies and routines for estimation the costs of ergonomic risks for a firm are being developed. A decision-making tool consulting on the reasonable measures for ergonomic improvement of workplaces with high ergonomic risks is awaited by manufacturers. Assembly line balancing models that incorporate ergonomic risks will be an essential part of such a tool. A number of questions remain for future research. Accepted estimates or methodology of estimation of the monetary equivalent of ergonomic risks is badly needed to justify measures for improvement in workplace ergonomics. Further, not only assembly line balancing, but also other production routines should be reexamined in light of possible influences on ergonomic risks. One of it is the order, in which different models of the product are processed in the assembly line (sequencing), and its consequences for ergonomics; namely, how the rules of sequencing should be modified to reduce workplace ergonomic risks. Another one is distribution of “relaxation allowances” – these are time add-ups on the task time for rest and breaks. Relaxation allowances, a part of predetermined time systems, are widely used in assembly line production. These add-ups are usually calculated proportionally to the task time. As a plausible alternative, we could interconnect the relaxation allowances with some estimation of the worker fatigue.