Productivity and Ergonomic Investigation of Bent-Handle Pliers.
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Date
2002-07-02
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Abstract
Musculoskeletal disorders (MSDs) cost industry billions of dollars in workers' compensation costs each year. Ergonomics is concerned with understanding the causes of MSDs and developing solutions to reduce these costs. Awkward postures have been implicated as a risk factor for the development of some MSDs, and a design principle to "bend the tool, not the wrist" has been advocated in many ergonomic textbooks. However, despite numerous laboratory investigations showing positive outcomes of application of this design principle to various hand tools, there is indication of lack of acceptance in industry for these bent-handle tools. In an attempt to understand the lack of industry acceptance, this investigation sought to determine if this design principle imposed constraints on users and/or negatively affected productivity, which may explain why they are not being widely used in industry.
The experiment used two different tasks (a computer-jumper installation task, and a spring assembly task) to compare the use of bent-handle pliers versus straight-handle pliers. Additionally, the effects of work surface orientation (vertical versus slanted at 45°) was evaluated, as was the effect of constraining the user's coupling of the tool. The dependent variables in the experiment were productivity and postural outcomes (arm elevation, wrist deviation in the radial/ulnar plane, and wrist deviation in flexion/extension).
An important point that must be made is that overall the results clearly suggested that the expected outcomes (both productivity and postural) are very task specific. This in itself says a lot about the general recommendation to "bend the tool", that being that the recommendation cannot be made without clearly understanding the other task characteristics involved, and that it should therefore not be proposed as a general design recommendation.
For the computer-jumper task the bent-handle pliers resulted in 5.3% faster task performance compared to the straight-handle pliers, while for the spring assembly task the performance was 4.9% faster with the straight-handle pliers. The explanation provided is that the bent-handle pliers seem to be preferable for tasks that require minimal or no tool rotation out of the sagittal plane, losing their advantage when multi-plane rotation is required. When subjects were constrained to holding the pliers with a power grip or oblique grip (modified power grip) arm elevation was reduced 50% and ulnar deviation was reduced by 12% when using the bent-handle pliers on the computer-jumper task, while on the spring assembly task ulnar deviation was reduced 22%. These results suggest that there are postural advantages to the bent-handle pliers (for the tasks used in this experiment) when the pliers-coupling is restricted to these grips. In the test of constrained versus unconstrained the results showed that for the computer-jumper task the postural benefits of the bent-handle pliers were lost when the subject could hold the straight-handle pliers any way desired, while for the spring assembly task this was not seen. In addition to showing that the postural benefits may only be seen when the pliers are held in a specific way, these results (along with others discussed in the paper) illustrate that the expectations associated with this design concept are very task-specific. Finally, removal of the coupling constraint also showed that subjects were more likely hold the straight-handle pliers in unconventional manners. In summary, it appears that the specificity of the bent-handle tool design, may make it better in very specific circumstances, but if the use requires increasing degrees of manipulation, it is less likely to be superior to a simpler design.
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hand tool design
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Degree
MS
Discipline
Industrial Engineering
