Assembly Planbook - April 2008 - (Page 44) ASSEMBLY Fastening Torque Trade-Offs Some suppliers tout the ergonomic benefits of their tools; others emphasize the accuracy of their products. W hen choosing a power tool to install threaded fasteners, engineers have never had so many options. Clutch, pulse and impact tools are now available in pneumatic, electric and battery-powered versions. Each tool offers a different According to the author’s test results, the next best thing to a handheld electronic torque wrench is an adjustable shut-off nutrunner. Photo courtesy Atlas Copco Tools and Assembly Systems By Paul Wallace President Impact Fastening LLC Furlong, PA set of performance characteristics, but sorting out their relative benefits isn’t easy. Comparative information on each tool is just not available. We decided to remedy that situation by conducting an independent comparative study of various power tools. Before getting into the details of the study, however, let’s look at the two basic requirements for power tools. First, every power tool must be ergonomic. Many attributes make a tool ergonomic, including low torque reaction, quiet operation, and the ability to operate in confined quarters. Second, every power tool must be accurate. If an operator can safely use a tool for hours, but none of the fasteners are tightened correctly, what have you gained? To see how these two requirements play out in tool selection, consider how lug nuts are tightened. Lug nuts are typically tightened to 100 ft-lb. To apply that amount of torque with a manual wrench, an operator must exert approximately 70 pounds of force on the handle of the tool. In addition, the wheel must be locked so it doesn’t turn. Exerting that much force is fatiguing, and locking the wheel is time-consuming. As a result, lug nuts are rarely tightening manually. The same operation can be done with much less effort using an impact wrench. Little manual force is required, and the wheel doesn’t have to be locked. Given the savings in both time and effort, it’s not surprising that impact wrenches are the predominant tool for installing wheels. The ergonomic benefits are huge. But are the nuts tightened correctly? Torque accuracy with impact wrenches depends on operator skill, since these tools typically do not have torque adjustment or an automatic shut-off. Experience suggests that overtightening is common, and torque values may exceed 100 ft-lb by 50 percent to 100 percent. This is not surprising, since the torque capacity of the typical 0.5-inch impact tool exceeds 300 ft-lb. This example shows a stark choice between ergonomics and torque accuracy: between the low torque reaction of an impact tool and the high accuracy 44 ASSEMBLY / April 2008 www.assemblymag.com http://www.assemblymag.com
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