Assembly - February 2009 - (Page 45) efficiently and consistently,” adds Greenwald. “Guidelines are typically developed using published applied research studies in anthropometry, which studies measurement of the human body, muscle force generation and the other stressors. “These studies give guidance as to what percentage of the population is capable of generating an amount of force in a given posture, or how far a 5th percentile female can reach,” says Greenwald. “Design guidelines use these studies to target the extremes of the industrial population. “If a 5th percentile female can reach 25 inches, then the rest of the population can reach that far, with the exception of the smallest 5 percent of the female population,” Greenwald points out. “By designing for this 5th percentile female and the 95th percentile male, 90 percent of the industrial population is accommodated. The same is done for strength: By designing for the 5th percentile female, 95 percent of the population can perform the task.” In most cases, it is not economically feasible to design workstations for 100 percent of the population, Greenwald says. But, by designing for 90 percent to 95 percent of the population, the amount of risk workers are exposed to is minimized. Various guidelines exist for carrying, lifting, lowering, pulling, pushing, reaching and other repetitive tasks. They are typically developed by government bodies, such as the National Institute for Occupational Safety and Health (NIOSH, Washington, DC), or by private entities such as Liberty Mutual Group (Boston). “Most ergonomic guidelines are developed in a laboratory,” says Cynthia Roth, CEO of Ergonomic Technologies Corp. (Syosset, NY). “They are scientifically derived and take into consideration all the hazards found in the workplace.” Force, Posture and Movement “In ergonomics, the three biggest stressors, or risk factors, that can lead to a musculoskeletal injury are force, posture and movement (static Workplace Reaches Workplace Reaches By keeping work within reach, operators can improve their posture as they complete assembly tasks. Illustration courtesy Ergonomic Technologies Corp. or repetitive),” says Greenwald. “The more these stressors interact, such as a high-force task performed repetitively in an awkward posture, the greater the risk of injury.” Traditionally, some of the most common injuries in manufacturing are back injuries, which usually occur with manual material handling. “Recently, however, pushing and pulling tasks, and work in extreme or static postures, has become more important,” claims Thomas Waters, Ph.D., chief engineer of human factors and ergonomics research at NIOSH’s laboratory in Cincinnati. “Any task that creates high internal loads on the body joints should be considered to be important, especially when workers are exposed to combinations of these risky activities.” Even if no ergonomic guidelines existed, reviewing manual material handling tasks alone would help reduce the risk of injury, Greenwald says. The most common analysis tools used for this are the Liberty Mutual manual material handling tables and the NIOSH lifting equation. “These tools do require some training and education for successful use, however,” Greenwald points out. “A simpler concept to start with is designing for reach. By keeping work within reach, one can help the worker optimize their working posture, keeping them in a stronger, safer and more efficient position while they complete the [assembly] task.” The two main factors that can be addressed with workstations and accessories are reduced force risks and elimination of awkward postures and motions. “Workstations can be made more ergonomic by reducing reach,” claims James Anderson, western region product manager at Lista International Corp. (Holliston, MA). He says it’s important to reduce workstation depth and place tools and supplies within easy reach of operators. Sometimes, engineers specify the wrong type of workstation, which can lead to ergonomic problems. “They want too big a work surface, which results in excessive reach, and they don’t take advantage of vertical space,” explains Anderson. “If you control the height and reach of a workstation, you put work into the correct zone for maximum strength,” adds David Brodie, director of ergonomics services at Atlas Ergonomics LLC (Grand Haven, MI). “If you go outside that zone, assemblers are put in a position that requires more effort to do that work.” According to David Verrill, applications support manager at IAC Industries Inc. (Brea, CA), the use of tools at workstations is often the most significant contributor to cumulative trauma disorder. Examples of these risks www.assemblymag.com February 2009 / ASSE M B LY 45 http://www.assemblymag.com
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