This might well be the most frequently asked of all frequently asked questions and is rightly a subject of genuine concern to production and quality managers.
To answer the question, I am going to look to the standard BS EN ISO6789 - Assembly tools for screws and nuts - Hand torque tools. Requirements and test methods for design conformance testing, quality conformance testing and recalibration procedure. Unsurprisingly, most of us refer to it as ‘the torque wrench standard’
In 1992, ISO 6789 was very much a document covering the design and manufacture of torque tools and the requirement was that the tool should be tested at maximum capacity for 5000 cycles in each direction. No guidance was given on recalibration intervals. However, when the standard was revised to the 2003 edition, the scope was broadened to include quality conformance testing and recalibration and so became of relevance to people using torque wrenches rather than just those designing and manufacturing them. This is the first time that the standard discussed the interval for recalibration.
For those looking for a simple answer to the question posed in the title, the default period of use between recalibrations is 5000 cycles or 12 months. However, the standard recognises that many businesses will have their own procedures for the control of test devices and, as a torque wrench can be considered a test device, a company’s own procedures must take precedence over the default 5000 cycles/12 months.
The reason that there is really no simple answer to the recalibration interval question is that circumstances of use will vary widely and this will have a direct bearing on how long the torque wrench is likely to stay in calibration. Factors such as the frequency of use, setting of the wrench as a percentage of full scale, general care taken in use and storage, ambient conditions in use and storage will all have their effect. Another major consideration is the torque tolerance existing in every individual situation and degree of safety criticality of the bolted assembly. For example, a helicopter assembly company that I have visited tests their wrenches before every single use. A typical automotive garage might find this degree of control onerous and unnecessary.
The other important statement made by the standard is that if a torque wrench is subjected to an overload of 25% or more above the nominal maximum, it should then be recalibrated. For many, this might be the ultimate decider on how often your wrenches should be recalibrated. For some, it will be almost every time the wrench is used!
The draft version of the next release of ISO6789 is already in existence. The good news is that in respect of the advice on recalibration, the standard has not changed. When ISO6789 is published in early 2013 we will blog on the key changes that you should be aware of.
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