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. 

Kraftexponering från handhållna åtdragningsverktyg (även kallade mutterdragare) bidrar till belastningsskador bland montörer, och är idag inte reglerade med explicita exponeringsgränser. Syftet med avhandlingen var att bidra till ett underlag för utveckling av rekommenderade exponeringsgränser för, och utvärdering av, reaktionskrafter från handhållna åtdragningsverktyg (vinkeldragare och pistolverktyg).

Fyra forskningsstudier har genomförts inom ramen för denna avhandling. Först genomfördes en literaturstudie där kunskapsläget inom ämnesområdet sammanställdes och kunskapsluckor identifierades. Baserat på utfallen föreslogs riktning för fortsatt forskning. Sedan genomfördes två experiementella studier baserade på psykofysiska metoder där acceptansgränser relaterade till reaktionskraftsbelastning härleddes för både vinkelverktyg och pistolverktyg. Slutligen undersöktes en fordonstillverkares tillvägagångssätt vid utvärdering av handhållna åtdragningsverktyg via en intervjustudie.

Genom literaturstudien framkom att fyra av det 40 inkluderade publikationerna hade uttryckt exponeringsgränser eller acceptansgränser för reaktionskraftsexponering från handhållna åtdragningsverktyg. Genom granskning av publikationerna konstaterades att gränsvärderna av olika anledningar vara otillräckliga. Baserat på de identifierade behoven föreslogs det att man inom vidare forskning bör fokusera mer på de mindre undersökta fysikaliska parametrar som är relevanta för reaktionskraftsexponeringen, så som impuls, samt att exponeringsgränser bör uttryckas i termer av exponeringen i sig och inte enbart i termer av åtdragningsmoment (dvs. moment i skruvförbandet). Dessutom föreslogs att moderna verktyg bör studeras vidare då de teknologiskt skiljer sig från traditionella luftdrivna åtdragningsverktyg.

De experimentella studierna resulterade i acceptansgränser för vinkelverktyg samt pistolverktyg uttryckta i åtdragninsmoment, där de accepterade momentnivåerna var högre för det ”tröghets-kontrollerade” åtdragningsprogrammet än det ”segdragande” åtdragningsprogrammet i båda studierna. Dessutom härleddes motsvarande reaktionskraftsnivåer (dvs. exponeringen från verktyget). Acceptansgränserna speglade de belastningsnivåer som studiedeltagarna bedömde vara acceptabla med avseende på hållbar arbetsbelastning över en arbetsdag. Det bör påpekas att de experimenttider som ligger till grund för de framtagna acceptansgränserna var korta och att acceptansgränserna därför inte bör tillämpas på hela arbetsdagar. 

Genom intervjustudien konstruerades tre övergripande teman baserade på intervjuerna med studiedeltagarna: ”Ett holistisk tillvägagångssätt”, ”Informations- och kunskaptillgång”, och ”Förhandlingskriterier”. Inom den studerade organisationen tillämpas en ergonomiutvärdering där både objektiva och subjektiva utvärderingar ligger till grund för beslut kring fysiskt belastande användning av handhållna verktyg. Vidare uppmärksammades att det förekommer ett flertal tillfällen där de anställda saknar adekvat kunskap kring handhållna verktyg, och att detta i sin tur kan påverka vilka och hur verktyg  används. Dessutom noterades att kriterier som säkerhet och kvalitet i vissa situationer kan hamna i konkurens med ergonomiinsatser. 

Resultaten från den här avhandlingen kan bidra till att utveckla rekommenderade exponeringsgränser och utvärderingsmetoder för reaktionskrafter från handhållna åtdragningsverktyg. Beslutsfattare kan nyttja de insikter som presenteras i denna avhandling till att utforma riktlinjer riktade till både verktygstillverkare, och till de organisationer inom vilka handhållna verktyg frekvent används. Genom standardiserade riktlinjer kan reaktionskrafter, samt exponeringen för reaktionskrafter från handhållna åtdragningsverktyg reduceras. Därmed bör avhandlingen på sikt kunna bidra till en minskad förekomst av belastningsskador orsakade av monteringsarbete. 

Reaction forces from nutrunner tools constitute a risk of developing MSDs. However, recommendations for sustainable reaction force levels are lacking. The aim of this study was to inform recommendations regarding reaction load exposures from right-angle nutrunners. Through a psychophysics approach, experienced assembly workers subjectively assessed reaction loads when using a nutrunner in six combinations of tool tightening strategy, work-pace and screw-joint stiffness. Electromyography, tool and joint parameters were measured. Regardless of tightening strategy, joint stiffness and work-pace combinations, no large differences in acceptable tightening torque, peak reaction force, and handle displacement were observed. However, acceptable jerk and impulse differed substantially between the TurboTight (R) (high-acceleration) and QuickStep (R) (conventional) tightening strategies. Although the TurboTight (R) strategy overall showed reduced peak muscular activities compared to the QuickStep (R), the participant-rated acceptable torque levels were similar, plausibly due to TurboTights' high jerk levels. Jerk and impulse are hypothesized to influence the perception of reaction loads.

Assembly workers using handheld powered tightening tools are repetitively subjected to reaction loads resulting from the tool handle. Despite the vast amount of research, tool manufacturers and OSH practitioners still lack well-grounded recommendations and benchmarks for ergonomics evaluations of reaction loads. This review provides an overview of research investigating reaction loads and operator physical demand, and identifies gaps in current means of evaluating reaction load exposure from tightening tools. A scoping review was conducted, scanning for quantifications of reaction loads as well as reported indications of MSD risks following tightening tool use. Suggested maximum exposures for reaction loads were further identified. The most commonly investigated quantities among the 40 publications included, were peak reaction force/torque (70%) and handle displacement (70%), followed by impulse (23%). Pneumatic tools were studied at greater extent (65%) than electric tools (30%). Three studies presented a relationship between reaction load exposure and physiological changes, i.e. edema, tissue oxygenation and blood volume changes, indicating an elevated risk of developing MSDs. Four publications proposed values for maximum reaction load exposure, expressed as either reaction torque, handle displacement, or tightening torque. To conclude, strategies for developing recommendations for reaction load exposure are suggested. Less investigated reaction parameters, e.g. impulse, can carry additional relevant information regarding exposure. Exposure values should be expressed as physical quantities of the reaction load rather than tightening torque. Recommendations are further needed for various tool handle-configurations, emphasizing the increasingly used electric tools. Such recommendations can eventually contribute to reducing MSDs resulting from tightening tool use.