Motor vehicles and aircraft are held together by large numbers of threaded fasteners....Failure of these fasteners can be catastrophic and incorrectly torqued fasteners are a prime failure mechanism.

Motor vehicles and aircraft are held together by large numbers of threaded fasteners. Many of these fasteners are safety-critical. Failure can be catastrophic and incorrectly torqued fasteners are a prime failure mechanism. Examples include the wing attachment bolts on aircraft, and the fasteners for automotive steering gear, power trains and exhaust systems.

The quality assurance system in automotive assembly rests largely on the residual torque audit, according to Frank I. Skog, senior application engineer at ASI DataMyte, Inc. (Plymouth, MN). In this audit the operator measures the torque-to-turn for a sample of the fasteners in a given joint. This is repeated for hundreds of fasteners during an audit. The integrity of the audit depends on the operator using the correct technique and measuring the correct fastener.

Learning correct technique is relatively simple and straightforward. However, learning the locations of hundreds of fasteners is another matter, and failure to measure the correct fastener may allow incorrectly tightened fasteners to go undetected. A text-based description can be helpful, but learning to associate hundreds of names with hundreds of fasteners is a task that can take weeks or months for a dedicated auditor to master.

Thanks to a combination of recent technological developments, graphic-aided torque measurement can eliminate this difficult learning task, which will help reduce both error rates and training costs. Rather than relying on human memory, graphic-aided torque measurement can present the operator with an image to assist in locating the fasteners to check. Such images can be simple annotated line art imported from a CAD system. They can also be complex displays that consist of an image captured with a digital camera and enhanced using graphic editing software to show graphic instructions for torqueing fasteners to the correct specifications and in the correct sequence.

Aircraft assembly applications for graphic-aided torque measurement typically use the more complex displays, and differ from the automotive torque audit in that the use is primarily for hand tightening critical joints, says Skog. For example, 10 to 40 bolts may attach the wing of an aircraft to the fuselage. As the wing is being fitted to the fuselage, these fasteners must be tightened in a prescribed sequence to a given torque value. They will then be retightened to some higher torque, and this process is repeated until final torque is reached.

Using graphic-aided electronic torque measurement for this application offers several advantages, says Skog. First, a single electronic wrench usually can replace several click wrenches, reducing the opportunity to use the wrong tool. Second, an electronic record of the applied torque is generated for each fastener at each stage of tightening. For example, if a 19-bolt pattern were tightened in three stages, then 57 individual torque values could be captured as evidence of satisfactory assembly. Third, the operator is graphically prompted to tighten each fastener in the correct sequence, virtually eliminating the problem of the operator losing the sequence. A significant related benefit is reduced operator training time.

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