Effects of Pacing When Using Material Handling Manipulators

By Nussbaum, Maury A.; Chaffin, Don B. | Human Factors, June 1999 | Go to article overview

Effects of Pacing When Using Material Handling Manipulators


Nussbaum, Maury A., Chaffin, Don B., Human Factors


INTRODUCTION

Numerous sources have demonstrated an association between manual materials handling and work-related lost time attributable to musculoskeletal injury (e.g., Ayoub, 1982; Chaffin & Park, 1973; Marras et al., 1993). In attempts to control these types of disorders, mechanized assistive devices have been increasingly incorporated into material transfer processes. Material handling systems may be broadly classified (Sowden, Jimmerson, Joseph, & Chaffin, 1998) as positioners (lift tables, conveyors), which are used to place or orient an object, and manipulators (arms, hoists), which are used to move and/or support an object and which are the focus of the present work. With the use of manipulator assistance, the decrease in gravitational loads that the worker must counteract during manual material handling should lead to a decrease in the ensuing musculoskeletal stresses (e.g., muscle and spine forces). However, there are anecdotal reports that when mechanical devices are available they are not used, often because of concerns related to productivity. It has also been suggested (Woldstad & Chaffin, 1994; Woldstad & Reasor, 1996) that the dynamics and substantial inertia of most manipulators have been ignored in the workplace specification of these devices.

Material transfer tasks can often require additional time for completion when a manipulator is used. Because these increases in performance times lead to decreases in productivity, manipulators are often rejected even after installation in manufacturing facilities. In the absence of quantitative evaluations of the effects of manipulators and pacing, the facility planner does not have sufficient evidence to weigh the opposing effects: fatigue, injuries, or both from not incorporating manipulators versus slowdowns from using manipulators.

Previous investigations and biomechanical modeling of manipulator-assisted materials handling tasks have focused primarily on the static gravitational component. As most of the static component (i.e., weight) is reduced when using a manipulator, the remaining external loads come primarily from body segment dynamics and inertial dynamics of the manipulator + object mass. Also, manipulator-assisted operations often result in asymmetric body motions, suggesting that complex patterns of torso muscle activity, including antagonism, are probably present. Thus, improved realism in the evaluation and modeling of manipulator-assisted tasks requires the use of dynamic three-dimensional biomechanical modeling, including an accurate methodology for ascertaining dynamic muscle and spine forces.

A limited number of studies have investigated physical stresses associated with the use of manipulators, ln one study using an overhead cable-hoist system, Woldstad and Chaffin (1994) found that inexperienced participants performed rapid motions with high peak levels of push (200-500 N) and pull (150-300 N) forces. Further, these high hand forces were only minimally influenced by experimental manipulation of load, distance, target width, and friction. Resnick and Chaffin (1996) investigated symmetric and twisting tasks using an articulated arm to move objects of 0-68 kg. Results showed that higher object masses and task asymmetry increased peak hand forces. Psychophysical ratings were only moderately related to hand forces, suggesting that measured hand forces do not adequately represent limiting physical stresses during manipulator use and, thus, that the risk of musculoskeletal injury may not be controlled by voluntarily limited exertions.

Given the paucity of detailed biomechanical evaluations of material handling manipulators and, in particular, the unknown influence of task execution speed on spinal stresses, we performed an experimental study to evaluate the effects of pacing when using two types of material handling manipulators. The first experimental hypothesis was that more rapid object transfers when using a material handling manipulator would be associated with larger hand forces, altered trunk kinematics, increased spine forces, and increased muscular antagonism. …

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