Torque auditing detects fastener relaxation, which occurs to some extent in all fastened joints. For correctly designed joints, relaxation is small and can be virtually ignored. Relaxation in safety-critical joints, however, must always be corrected.
Audit frequency always depends on the product being assembled. Fasteners in safety-critical joints are always audited more often than less critical joints ,say, screws in a photocopying machine.
A joint audit is done not only to verify that the fastener was torqued within specifications , but also to make sure the plant’s assemblers and fastener-installation process are reliable.
Simply because a power tool tightens to programmed settings is not proof that resultant joints have proper clamp force.
Manufacturers pay close attention to three factors that can cause joint relaxation and increase the need for torque auditing. These include the temperature in and around a joint, the presence of lubricants, and multiple layers and materials used in the joint.
“Painting can cause joint relaxation. A boat engine manufacturer customer was getting complaints about loose bolts in their engine head gaskets. It turned out that the heat generated while painting the engines caused some slight bolt relaxation. So the company now performs torque audits before and after the paint has dried.
Auditing isn’t cheap, so never disregard any auditing results. The results might show that three auditors get three different torque readings on the same type of fastener, or that one or more tools cannot install fasteners to required specifications. So be prepared to invest in properly training auditors to minimize differences in audit results or buying better tools.
Manufacturers also need to establish efficient routes for each auditor. Automakers tend to have each auditor cover a few assembly stations and work within a few feet of assemblers. Many Tier 1 manufacturers do likewise. Torque auditing is required on all critical and warrantied fasteners during each shift.
In general, noncritical fasteners do not need to be torque audited. Rather, an auditor will perform visual inspection to make sure these fasteners are installed. Noncritical fasteners tend to be screws of all sizes,nonmetal screws and screws installed in nonmetal surfaces.
However, some screws are critical fasteners, and they can be difficult to torque audit without the correct tool. An electronic or manual wrench or screwdriver can be used, but must be fitted with a socket or bit that fits very snugly over or in the head. This is especially critical for slotted screws. Also, the tool’s internal or
external transducer or sensor must be able to measure torque in either kilogram-meters, kilogram-centimeters, in-ozs or in-lbs.
Usually, the best way to audit tiny screws in computers and electronic circuit boards is to use a transducerized manual Screwdriver. These screws are similar in size to those used in spectacles. Auditing must be done very carefully. Just 1 in-lb of torque will strip the threads.
Auditing critical plastic fasteners can be challenging. The standard breakaway test with plastic fasteners is not the ideal method. This is because it is extremely difficult for any auditor, including those that are skilled and well-experienced, to understand peak down torque.
Peak down torque is the force required so the threads start to break free. Torque then dips briefly before climbing to its peak. To obtain a more accurate torque measurement, especially with small plastic fasteners, it is usually recommended that auditors use a specialized breakaway torque tester. A motorized breakaway torque tester captures peak down, peak and yield torque of screws during auditing. It turns the screw at a consistent speed, applies continuously increasing torque and helps the auditor visually determine the breakaway point. By unifying these three actions, the tool enables an auditor to perform the breakaway test without overtorquing. In certain models torque values are automatically displayed . In conclusion all auditing requires a trained auditor using the right tool. However, the use of accurate fastening tools doesn’t eliminate the need for a torque audit.
Acknowledgements – Various technical literature from “world wide web”