Beyond the Force: Redefining load exposure assessments of nutrunners for improved power tool ergonomics

Abstract: Reaction force exposure from handheld tightening tools (also known as nutrunners) constitutes an acknowledged contributor to musculoskeletal disorders among assembly operators, and are today not regulated by explicit limits. The research presented in this thesis aimed at contributing to the development of recommended exposure limits for, and assessments of, reaction loads from handheld right-angle and pistol-grip tightening tools.In order to address the thesis objectives, four research studies were conducted. A literature review was conducted to provide an overview of the current state of knowledge within the topic area. Knowledge gaps were identified by mapping available publications and based on those suggesting directions for further research. Thereafter, two psychophysically based experimental studies were conducted where acceptability limits related to load exposure from handheld tightening tools were derived for right-angle and pistol-grip tightening tools. Finally, an automotive manufacturer’s approach to evaluating and managing the use of handheld tightening tools was outlined by means of an interview study.As found through the literature review, four of the forty included publications had stated exposure limits (general recommendations) or acceptability limits (load acceptance as assessed by study participants) for reaction load exposure from handheld tightening tools. However, some of the reported limits did not consider relevant physical parameters, and some did not comply with modern power tool technologies. Based on this, it was suggested that researchers should emphasize physical quantities relevant to the reaction load such as impulse, express exposure limits in terms of reaction load relevant parameters (and not only the tightening torque), and further study modern power tool technologies.The experimental studies resulted in acceptability limits for right-angle and pistol-grip tightening tools expressed as screw-joint tightening torque (i.e. a task-related factor), where acceptable tightening torque limits were higher for the inertia-controlled tightening program compared to the continuous drive tightening program, in both studies. In addition, corresponding acceptable reaction load levels (i.e. the exposure) were derived, indicating load levels resulting from the tool use that the study participants assessed as acceptable for an 8-hour workday. It should be noted that the experimental times on which the acceptability limits are based were limited, and that the acceptability limits therefore should not be prescribed to full workdays.From the interview study, three main topic categories were identified based on the interviewees’ responses: ‘A holistic approach’, ‘Information and knowledge availability’ and ‘Negotiating criteria’. Within the studied automotive organization, a comprehensive approach to ergonomics assessments is incorporate, where both objective and subjective evaluations form the basis for addressing physically demanding tool use situations. Further, it was found that there are different instances where the employees lack sufficient knowledge related to the tools, and which can influence the employment of handheld tightening tools. In addition, it was found that criteria such as safety and quality could in some situations compete with ergonomics efforts. In conclusion, the findings from this thesis can contribute to the development of recommended exposure limits and evaluation methods for reaction load exposure from handheld tightening tools. Policymakers could utilize the insights presented in this thesis to form general guidelines directed at power tool manufacturers as well as tool using organizations. Through standardized guidelines, reaction loads from handheld tightening tools, which is one of the contributors to MSDs within assembly work, can be managed and reduced.