By Joy M. Ebben, Ph.D, CPE
Human Factors and Ergonomic Specialist, IAC Industries, Brea, Calif.
A design goal is to allow the user to sustain a neutral position for the fingers, wrists, forearms, elbows and shoulders.
An essential part of an ergonomics program is designing a workstation for the tasks users perform. For repetitive tasks, look for characteristics of the userworkstation interface that could lead to cumulative trauma disorders (CTDs). Significant issues are posture and contact stress.
Also important in evaluating a workstation is identifying safety hazards that could cause non-cumulative types of injuries – cuts, bruises, and fractures. A user’s posture is greatly influenced by the design of the workstation’s furniture and accessories – and by the layout of the tools, piece parts, and other equipment.
A design goal is to allow the user to sustain a neutral position for the fingers, wrists, forearms, elbows, and shoulders (body parts that often suffer CTDs). Neutral postures are defined in the working draft of the ANSI Z-365, Control of Work-Related Cumulative Trauma Disorders, Part 1: Upper Extremities:
Worksurface height and reach distance affect the position of a user’s wrists, arms, and shoulders. The workstation should allow users to prioritize object placement. Generally, frequently used items should be positioned no farther than the length of the user’s forearm.
Consider the user’s arm length the limit of functional reach. How to minimize frequency and reach distance above shoulder height? Use adjustable shelves, parts cups, and equipment platforms to accommodate users’ arm lengths.
Workers should not bend the wrist to pick up piece parts. The workstation should allow parts cups to be down-angled, and subassemblies to be rotated or tilted. The goal is a neutral wrist position.
The height of both the user’s elbow and the workpiece impact the user’s neck, shoulders, and back posture. For light assembly tasks, position the user’s elbow at the same height as the assembly (the workpiece including the height of a jig platform, or subassembly).
Lower the workpiece below the elbow for tasks that require physical force; raise the workpiece above the elbow for finely detailed work, to accommodate visual or cognitive demands, and to better support the user’s arm.
The worker’s neck should not bend forward more than 20°. Excessive forward flexion of the neck and torso typically occurs when performing detailed work. Typically, the workpiece is too far away from the user’s eye, or using a magnifying lens creates an awkward viewing angle.
As the height of the workpiece changes, workers should correct elbow height. Adjusting the height of the chair or worksurface are typical methods. Both adjustments should be easy to perform, and not require special tools or training.
Another important aspect of workstation evaluation is leg clearance. Generally, the torso/ thigh angle should be 90-100º, and the thigh/ lower leg angle should be 90º. Also, allow for posture changes.
Minimize the problem by keeping worksurface thickness, and the height of jigs and other supports, to a minimum. And remove obstructions such as drawers and keyboard trays from under the work surface in the leg area.
The workstation chair is critical for proper posture. Consider the chair and workstation as an integrated system, and evaluate them together. To address contact stress and its impact on forearms, rolled front edges on the worksurface are recommended.
A workstation evaluation should also include traditional safety factors. Is the station structurally sound and stable? Can the accessories and worksurface support the expected weight loads, and unplanned loads? (Such as a worker standing on the worksurface to change a light bulb.)
What happens if a worksurface or shelf support gives way? Will it slip, and then fix in place? Or does it fall in a hazardous manner?
Is the workstation free of sharp corners, rough edges, and pinch points? Are only approved electrical components used? Are there knee or head obstructions? Are warning labels properly designed and located?
Other factors include the design of the product being assembled, and the tools, job aids, parts holders, and other equipment involved.
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