An ergonomic evaluation of three classes of material handling device (MHD)

Abstract

The proliferation of material handling devices (MHDs) such as articulated arms and hoists is a direct result of the staggering increases in workers compensation claims attributed to repetitive lifting. In reducing the occurrence of repetitive lifting through MHD use, however, manufacturers have opened a Pandora's Box of other issues, ranging from the asymmetric nature of torso exertions common in MHD use to various job design and motor control issues. This study evaluates the effects of some critical job design parameters such as handled load and movement distance on the movement performance of three major classes of MHD. Ten young healthy male and female subjects used three different types of common MHDs with loads of 10 and 30 kg over distances of 0.6 and 1.8 m. Peak hand forces ranged from 50–80 N for the articulated arm and hoist with overhead rail MHDs, but reached 200 N with a fixed pivot hoist MHD. As a percent of each subjects' strength, peak hand forces generally tended to remain between 30 and 50%, except for the fixed pivot hoist when moving 30 kg over a 1.8 m distance. These conditions exceeded the strengths of 80% of the subjects. Peak velocities ranged from 0.8 to 1.2 m/s with the articulated arm and hoist when moving on an overhead rail, and from 0.8 to 1.6 m/s with the fixed pivot hoist. Subjects showed the fastest movement with the fixed pivot hoist, except with heavy loads. A distinct tradeoff between movement performance and biomechanical safety was discovered. In most cases, the MHD which showed the fastest movement also required the highest stresses. In general, implementation strategies should involve selection of the fastest MHD which does not exceed accepted limits of biomechanical stress. However more sophisticated limits for dynamic back stress must be developed and must reflect the various populations in the workforce. Relevance to industry The proliferation of MHDs in industry to replace repetitive lifting tasks has created a new source of potential musculoskeletal problems. This paper evaluates the comparative movement performance and ergonomic stresses of the three major classes of MHD in an effort to provide industry with some guidance for MHD implementation.