Elsevier

Applied Ergonomics

Volume 31, Issue 3, 1 June 2000, Pages 227-237
Applied Ergonomics

Motion times, hand forces, and trunk kinematics when using material handling manipulators in short-distance transfers of moderate mass objects

https://doi.org/10.1016/S0003-6870(99)00062-9Get rights and content

Abstract

The risk of musculoskeletal injury associated with manual materials handling tasks has led in part to the use of material handling manipulators, yet there is limited empirical data to facilitate selection, design, and evaluation of these devices. A laboratory study of two types of mechanical manipulators (articulated arm and overhead hoist) was conducted of short-distance transfers of moderate loads, and the influence of various task parameters and transfer method on motion times, peak hand forces, and torso kinematics was obtained. Use of manipulators increased elemental motion times for symmetric sagittal plane transfers by 36–63%, and asymmetric transfers (in the frontal plane) by 62–115%, compared to similar transfers performed manually. Peak hand forces were significantly lower with both manipulators (40–50%), and approximately 10% higher for asymmetric versus symmetric transfers. Overall torso kinematics were grossly similar with and without a manipulator. These results suggest that for self-paced job tasks, moderate mass objects will be transferred slower over short distances and with lower levels of external (hand) forces when using mechanical aids. These simple effects, however, were influenced by object mass and transfer height.

Introduction

Numerous investigations have demonstrated the association between unassisted manual material handling and increased risk of musculoskeletal injury (e.g. Ayoub, 1982; Chaffin and Park, 1973; Marras et al., 1993), occurring particularly in the low back and upper extremities. Notwithstanding the increased mechanization of contemporary manufacturing processes, manual methods of product transfer, palletizing/depalletizing operations, and assembly, remain the common way of working in many industries. In some industries, such as baggage handling, health care, and food and part distribution, frequent manual handling of products or goods can be expected to continue. In contrast, there are increasing reports of situations where mechanized assist devices, while available, have not been used to advantage because of lack of information regarding their effectiveness in reducing musculoskeletal injury risk, as well as a lack of guidelines which are based on ergonomically sound design principles.

Material handling devices are available in a variety of technologies, differ broadly in price, and may range in level of engineering complexity from simple `pendulum’ hoists to pneumatic powered articulated arms with built-in braking systems and constant balance capabilities. We discriminate here (following Sowden et al., 1998) between positioners (e.g. lift tables and conveyors), and manipulators (e.g. arms and hoists); the emphasis of this study is on the latter.

The potential benefits of assisting in the material handling process through use of mechanical manipulators are readily apparent. The basic function of manipulator devices is simple: eliminate the magnitude of the static (gravitational) load that the worker must handle, with an expected reduction in musculoskeletal stresses. Traditional investigations and biomechanical modeling of material handling tasks have focused on this static component (Resnick and Chaffin, 1996, Resnick and Chaffin, 1997), and static loads are often employed as the basis for manual handling limits (Waters et al., 1993).

It has been suggested previously (Woldstad and Chaffin, 1994), however, that the potential benefits of various manipulators have not been fully realized. A manipulator may be unsuccessful because its use requires significant additional accelerative and decelerative forces, which can be compounded if increased time to perform a task is not provided. Woldstad and Chaffin (1994) reported that for moderate loads, manipulators are often discarded after installation, and that workers did not always report decreases in perceived workload using them. They suggested that the fatigue experienced by workers using a manipulator is related to dynamic forces resulting from large system inertias and forced pace production.

If a significant time penalty is incurred when using manipulators, this would be especially deleterious in jobs with relatively short cycle times. Here the added incremental times required for acquiring, moving, placing, and disengaging objects may be sufficient to overturn, in the mind of the designer, any potential musculoskeletal benefits. Time, along with the initial purchase and maintenance of a manipulator system, can lead to substantial operating cost. In the absence of appropriate research studies, a cost-benefit analysis for manipulator use cannot be given in any detail. This lack of information provides the primary motivation for the present study, the focus of which is two-fold: (1) provide data to aid in the design and evaluation of alternate manipulator types; and, (2) demonstrate whether, and to what extent, manipulators offer relief to the musculoskeletal system when handling moderate loads.

A few previous laboratory studies have investigated physical loads associated with the use of common manipulators. Resnick and Chaffin (1996) found high peak hand forces during horizontal pushing and pulling of objects supported on hoists or articulated arms at about elbow height when moved in self-paced conditions. They noted that higher loads and asymmetry generally led to increased peak hand forces. A biomechanical analysis using a static model (Resnick, 1993), however, suggested only minimal low-back injury risk for self-paced horizontal transfers at elbow height. A second study by these authors (Resnick and Chaffin, 1997) examined movement times, speed, and peak hand forces and found that certain combinations of device and load yielded tradeoffs between movement performance and musculoskeletal stress. Woldstad and Chaffin (1994) also showed that with increasing mass of an object and/or manipulator, peak accelerations and decelerations in a movement will normally decrease (i.e. the operator will slow down as mass increases). In contrast, during time constrained operations (e.g. paced work) the operator will necessarily have to exert higher peak forces on the manipulator as the mass increases. These studies have demonstrated a complex relationship within and among the type of manipulators and task parameters. They further indicate that answers to such questions as whether a manipulator will be beneficial, or what type will be optimal, are not obvious.

A variety of objective measures were compiled in this experimental study as a function of critical task parameters. This study addressed the following hypotheses: (1) the use of a manipulator will significantly alter motion times, required hand forces, and trunk kinematics, when compared with comparable tasks performed manually; and, (2) these measures will differ under different task conditions and will depend on the type of manipulator employed.

Section snippets

Methods

The investigation focused on what are considered `elemental’ activities involved in the acquisition, transfer, and placement of material. Since earlier studies indicated that peak stresses were incurred at the beginning and ending periods of a load transfer motion, this study emphasized these `transitional’ aspects of manipulator-assisted object transfers, as opposed to any prolonged carry or long-distance transfer operations. Given the peak musculoskeletal loads arising from accelerations of

Example Data

An example of the processed trial data is illustrated in Fig. 2. For this ARM-assisted transfer, the subject exerted hand forces primarily in the y (fore-aft) and z (up-down) directions. Moderate forces were also seen in the x (right-left) direction, resulting from radial motion of the ARM links. This behavior can also be seen in the small levels of horizontal plane (twisting) motions of the torso, which are attempts at compensating for the ARM dynamics. Given the complex nature of the hand

Discussion

Given the relative paucity of available data concerning the effects of manipulator assistance during load transfers, and the difficulty this imposes in designing, evaluating, and selecting these devices, an experimental study was undertaken to provide data facilitating such goals. The investigation focused on the transitional events involved in transferring objects over short distances in typical material handling scenarios. These transitions, involving object acquisition and placement, were

Acknowledgments

This work was supported by grants from Ford Motor Company. All experiments were performed in the Center for Ergonomics, The University of Michigan. Thanks to Catherine Sowden of Ford Motor Company for assistance in the experimental design.

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