Assembly Planbook - April 2008 - (Page 47) and durability, the joint consists of a high-strength aircraft bolt, hardened washers and a nut. For our test, each tool was run 20 times on a soft joint and 20 times on a medium joint, and the clamp load and torque for each rundown were recorded. The clamp load was measured by a load cell inserted in the joint. Torque was meaWith one exception, the tools tested in this study were commercially available models from established manufacturers. The exception was this prototype model of a torquecontrolled cordless impact wrench. According to the test results, the tool provided torque shut-off performance that was comparable with the best pulse tools. Photo courtesy Impact Fastening LLC sured with an electronic torque wrench using the break-forward method, or the torque at which the clamp load increases by a small increment. There is some inherent scatter in this measurement. For example, the reading can be affected by varying friction in the joint from test to test and also by variations in how much the joint is tightened further during measurement. This is why the manual torque wrench, NER The Test Approach To obtain data on the comparative accuracy of fastening tools, we tested six different models: a manual torque wrench; a pneumatic nutrunner with a shut-off clutch; a transducerized pneumatic pulse tool; a pneumatic pulse tool with a shutoff clutch; a pneumatic pulse tool with a stall clutch; and a prototype torquecontrolled cordless impact wrench. The adjustable shut-off nutrunner was included because it is representative of tools that are commonly certified through the ISO 5393 process. The three pulse tools were included to determine if there was a significant performance difference between the different types. The stall-type pulse tool was the only one that did not have an automatic shutoff, so it was run to its perceived stall point. The cordless impact tool was included to see how it compared against established tools. Because the tools had different maximum torques, each was set at about 80 percent of capacity. Before we could put the tools through their paces, we needed a test rig. To provide meaningful data, the rig had to be a bolted joint that could be tightened by all tool types. It had to provide consistent, repeatable results over time, and different joint characteristics, such as stiffness, had to be adjustable. Such a rig was designed and manufactured by P&T Precision Engineering Ltd. (Naas, Ireland). For consistency www.dorner.com/asy 8 00. 397. 8664 www.assemblymag.com April 2008 / ASSE M B LY 47 http://www.dorner.com/asy http://www.assemblymag.com
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