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Understanding ISO 6789 – Hand Torque Tool Manufacturers

13 Mar, 2017 | Return|

The 2017 edition of the standard is considerably modified from the 2003 edition.  The five Norbar articles are designed to explain the key differences and how the new standard is intended to be used. They are an overview, not a replacement for studying the standard. If you have questions we will be happy to try and help. Please follow the email link at the end of each article.

This second article of five has been written with hand torque tool manufacturers in mind.  Further articles for calibration laboratories, automotive end users and general industrial users follow. It will help to first read article one on the general reason for change.


Most of the requirements from the 2003 edition are carried into the new Part 1, however there are a number of additions for manufacturers of hand torque tools.

Design requirements:

There are now maximum torque values for hexagonal output drives, which is of particular interest to torque screwdriver manufacturers.

The definition of the torque range for different classes of tool is clarified and this will impact most manufacturers. One of the key changes is that the torque range now starts at the lowest marked value and calibration must start at this value.  This applies to both setting and indicating tools. Where there is no scale the manufacturer must state both the lowest and the maximum torque values at which the tool can be used.

For dial wrenches, the zero value must be marked and either the area between zero and the lowest torque value must be marked to indicate that it shall not be used, or the torque range must be marked to show that it is the usable range.

For electronic wrenches the resolution at any reading in the range must not exceed ¼ of the declared maximum permissible deviation at that reading.

One of the bigger changes comes from the uncertainty budget calculations in Part 2, because the resolution uncertainty of a hand torque tool is driven by its design. The resolution allowed by secondary scales on setting tools, different slave pointer shapes and sizes on indicating tools and digital display resolutions are all addressed to specify comparable claims of resolution.

The method of calculating the relative deviation observed at any given target torque has been returned to the formula used in the 1992 edition. The 2003 edition used the formula for relative measurement error which is an ISO defined term, but it was confusing for users. The new Part 1 continues to uses the term deviation (actually now relative deviation because it is stated in percent) and defines it as the observed value from the measuring device measured as a percentage of the target value on the torque tool. This makes sense to users because they can compare the observed relative deviation with the permissible relative deviation. In Part 2 the ISO defined term of relative standard error is used to provide consistency within calibration laboratories and because the result of the calibration is not being directly compared with the permissible relative deviation of Part 1, it does not matter that two definitions exist. The next article on Calibration Laboratories discusses this at more length.

Cycle testing is now allowed to be faster, between 5-20 cycles/minute, rather than 5-10 cycles/minute. This does speed up cycle testing but care should be taken not to overheat the mechanism and cause misleading data.

There are now statements about the use of flexible head torque wrenches and wrenches with extension handles. Flexible head wrenches can alter the torque delivered for a given setting when used at significant angles. The use of extension bars or tubes can alter the torque delivered on some torque wrenches. Manufacturers now need to inform customers about the effect on the relative deviation caused by using these features.

Quality conformance requirements:

The suitability of a measurement device now requires that the maximum measurement error of the measurement device must not exceed ¼ of the maximum relative permissible deviation of the torque tool at each target value. Uncertainty is not considered in Part 1.

The torque tool and the measurement device must reach the ambient temperature and this temperature must be recorded. This temperature is still allowed to be between 18 and 28 degrees Celsius but must be constant during the calibration to +/-1 degree.

Another important change is that the torque measurement system must not exert side loading or end loading on the torque tool. The new Norbar Torque Wrench Calibrator has a patented design to prevent this.

Also, the time to attain the last 20% of the torque application during testing has been more tightly defined according to the size of the torque tool.  This will affect the quality conformance checking performed by manufacturers. Logically small torque tools need less time to complete the last 20% of their target torque.  That does however make it harder to measure whether the load application is within specification. Torque screwdrivers have a minimum and maximum time because they are often speed dependent.

The documentation requirements are also modified to make it clear that a declaration of conformance is produced, stating whether or not the torque tool is in conformance with the requirements of the standard. The declaration has a list of items that must be shown.

Should a manufacturer also wish to produce a certificate of calibration they must issue it in accordance with the requirements of Part 2, in addition to producing a declaration of conformance in accordance with Part 1. A torque tool supplied by a manufacturer cannot just be supplied with a certificate of calibration if the manufacturer claims that its torque tools conform with ISO6789:2017 Part 1. Importantly a manufacturer cannot claim that the torque tool conforms with the requirements of ISO6789:2017 Part 2 because this part of the standard gives the requirements for calibration, not the requirements for the tool. A manufacturer could produce a tool with a declaration of conformance stating that it conforms with Part 1 and also issue a calibration certificate that conforms with Part 2.

The declaration of conformance has a date.  Retesting for conformance should take place within 12 months or 5,000 use cycles from the date of first being put into use. The standard is deliberately silent on the shelf life of a declaration of conformance because it is a statement of the performance on the day the tool was tested. It does not have a period of validity. Users must make their own decisions about the conformance of tools which are first put into use more than twelve months after the declaration of conformance.

If you have any questions please make contact with us here: ISO6789@norbar.com.

Neill Brodey

Member of ISO Working Group on ISO 6789