“Trust but verify” was one of Ronald Reagan’s signature phrases. The president used it whenever he spoke of his negotiations with the Soviets during the Cold War. These days, the phrase is more applicable to the assembly line than arms negotiations. Manufacturing engineers trust power tools to install threaded fasteners quickly and accurately, but one way or another, engineers must verify that the tools truly are performing as specified.
Electronic and mechanical torque wrenches enable engineers to do just that. “Torque wrenches are used to check fasteners that have already been tightened,” says Frank Skog, product manager for ASI Datamyte Inc. (Plymouth, MN), now retired. “They ensure that the fasteners have been tightened correctly and that the joints have not loosened after assembly.”
It’s rarely necessary to check every fastener, in every joint, in every assembly. Rather, engineers check a random sample-perhaps one out of every five assemblies, or one out of 10. The frequency of such checks depends on the importance of the assembly, the softness of the joint, and the tool that installed the fastener. Safety-critical assemblies, such as automotive steering components or tie rods, are checked frequently. Similarly, gasket assemblies and other soft joints are checked often, since they are likely to relax after tightening. Joints fastened with computer-controlled DC electric tools are checked less often than joints fastened by pulse tools, because the pulsing action can interfere with torque transducers.
In the end, says Skog, how a torque audit is conducted is an economic decision. “You want to catch defects when they’re least expensive to fix,” he says. “Once the assembly goes out the door, fixing a problem is really expensive.”