Assembly Planbook - April 2008 - (Page 45) of a torque wrench. On the assembly line, the debate over fastening tools has tilted in favor of torque accuracy and away from ergonomics. The growing use of transducerized DC electric tools and precision shut-off nutrunners reflect this trend. These geared tools tighten continuously and have clutches or transducers that closely measure torque. Development of these tools was stimulated in large part by Ford Motor Co. (Dearborn, MI). In the 1970s, the automaker began seeking tools that would deliver the same torque irrespective of the fastener they were used on. Ford’s requirement also prompted the development of special test equipment that later became the basis for the international standard ISO 5393, “Rotary Tools for Threaded Fasteners—Performance Test Method.” This equipment simulates tightening operations by having the tool twist a torsion bar until the shut-off torque is reached. Using a torsion bar eliminates friction and other variables inherent in tightening fasteners. During the test, the tool is mounted in a rigid fixture, even if it is normally handheld. The torsion bar test method does not handle all types of tool. ISO 5393 only covers tools that apply torque continuously. It does not apply to impact and pulse tools, which advance fasteners in discontinuous increments. That’s because the torque applied by these tools cannot be measured using conventional instrumentation. These tools are also called “discrete energy” or “discontinuous” tools, because torque is applied to the fastener from individual blows or pulses, rather than the continuous motion of a motor and gear train. Tools Get Put to the Test Tool Type Torque wrench Adjustable shut-off nutrunner Cordless impact tool with control Transducerized pulse tool Shut-off pulse tool Stall pulse tool Power Source Shut-off Method Maximum Torque Rating Manual Pneumatic Battery Pneumatic Pneumatic Pneumatic Click mechanism Clutch Electronic Valve Mechanical 200 newton-meters 55 newton-meters 160 newton-meters 100 newton-meters 45 newton-meters 90 newton-meters The author tested six types of power tool to assess their accuracy and ergonomic performance. Test Results for Medium Joint Tool Type Mean Torque Torque Range Torque Percent Standard Capability Deviation (Six Sigma Scatter) 1.2 2.1 2.4 2.7 2.3 6.1 9% 25% 19% 20% 34% 43% Torque wrench Adjustable shut-off nutrunner Cordless impact tool with control Transducerized pulse tool Shut-off pulse tool Stall pulse tool 82.7 49.8 77.9 79.1 40.2 84.7 81 to 84 45 to 55 74 to 83 75 to 84 36 to 43 79 to 94 This table shows the results of testing the tools on a medium joint. Torque values are in newtonmeters. The capability value represents total variation in applied torque. If expressed as a ± value, it should be cut in half. Test Results for Soft Joint Tool Type Mean Torque 81.7 45.8 56.3 58.1 28.5 69.2 Torque Range 81 to 83 42 to 49 55 to 58 56 to 62 26 to 32 64 to 78 Torque Percent Standard Capability Deviation (Six Sigma Scatter) 0.7 1.8 1 1.6 1.5 4.4 5% 24% 11% 17% 32% 38% Torque wrench Adjustable shut-off nutrunner Cordless impact tool with control Transducerized pulse tool Shut-off pulse tool Stall pulse tool This table shows the results of testing the tools on a soft joint. Torque values are in newton-meters. The capability value represents total variation in applied torque. If expressed as a ± value, it should be cut in half. www.assemblymag.com April 2008 / ASSE M B LY 45 http://www.assemblymag.com
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