Bolt tightening is an important part of the assembly process and is often performed by workers with a torque wrench. A torque wrench is not only a tool for tightening bolts but is a “torque measuring instrument”. Using a torque wrench to apply the correct amount of torque to a fastener is a critical part of assembling a high quality product.
Possible defects may be caused by man or machine: 1) the person tightening the bolt could forget to tighten a fastener or improperly use the torque wrench by not pulling through to the “click”. 2) An automated machine or the torque wrench used to tighten bolts could malfunction or loose accuracy. It may not become immediately apparent that these errors have occurred. Product quality is jeopardized and a high number of products may potentially be affected.
To prevent missed tightening, an “error proofing” system can be set up using a specialized torque wrench, either a corded limit switch version (Tohnichi’s LS Series) or a radio frequency torque wrench. For optimal freedom of movement the wireless wrench like Tohnichi’s FH256MC sends a FM radio signal to a receiver. The receiver is then connected to a “count checker”, such as Tohnichi’s model CNA-4mk2, or directly to a PLC. The system is able to count the number of times the torque wrench has been “clicked” when it reaches the torque setting. By counting the number of “clicks” it will alert the user to a missed bolt. For example, if there are 4 critical bolts and the user only tightens 3, a buzzer or light will let them know that one fastener was not tightened. If tied into the PCL, the assembly line can be stopped. The error can be remedied immediately. For any application where it is vital to confirm tightened fasteners, the FH256MC and LS wrench are invaluable tools for keeping product quality high and insuring safety for the consumer.
Verification that the tools are functioning properly and that the torque output of the wrench is accurate is also critical for proper tightening. Ideally a torque wrench checker should be set near the production line and the user can check the wrench daily prior to use. This insures that the wrench is working properly and that the torque setting is correct. Daily checking is a key point because tools that become damaged or loose accuracy can easily be detected and removed from use before any improper torque can be applied to the product. This prevents large quantity defects in the assembly process. It also functions as a training tool, linking proper tightening techniques with the exact output of the tool. The user, who checks his wrenches daily will understand more clearly that pulling the wrench steadily to the “click” at the effective length point, will create the most accurate tightening outcome.
A centralized calibration area is required to regulate the accuracy of all types of torque wrenches using a torque wrench tester. A torque wrench tester differs from a checker in that it features a loading bar that helps stabilize the wrench during the testing and adjustment procedures. A periodic calibration system should be set up according to the usage of the wrench, and the level of accuracy as demanded by each company’s quality control management systems. A minimum yearly calibration or 100,000 cycles is recommended for every torque wrench but assembly facilities usually require a weekly or monthly routine depending again on level of usage and accuracy requirements.
Utilizing error proofing methods and keeping torque tools maintained through daily checks and routine calibration inspections will insure a high level of product quality.
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