18.06.2013
Torque wrenches are, among other things, used to tighten wheel nuts and screws on a car. Thereby, the torque is specified by the manufacturer of a vehicle. These wrenches are available in a triggering version ("setting torque wrenche") and, to an ever increasing extent, they are also put on the market as indicating wrenches with an electronic display. For the calibration of these measuring instruments, DIN/EN/ISO 6789 is applied. This standard, however, has some weak points as far as the investigated influencing factors are concerned, and a measurement uncertainty calculation is completely lacking. For the revision, which is currently being carried out, PTB has published some proposals in cooperation with the DKD.
The proposals were presented within the scope of a congress of IMEKO, the International Measurement Confederation. The contribution [1] can be downloaded from IMEKO's website.
Some of the points of the currently valid standard from 2003 which have to be improved are the following:
- The measuring instruments should be calibrated in the entire specified measurement range (so far, this has only been done from 20 % of the full value onwards).
- It should be possible to control calibration facilities with the aid of one's own signals and displays (up to now, the display of the tool has had to be used).
- Only ONE calibration result should be calculated (according to the current standard, there are five to ten single deviations).
- The measurement results should take into account the influence of the lever length (for torque wrenches), the influence of the connection profile (frequently square drive), and the influence of the resolution of the display or of the scale.
- Measurement uncertainties should be assigned to the measurement results. For the calculation of these uncertainties, proposals should be contained in the new version of the standard.
Some of these influencing quantities are not easy to determine, and the costs for the calibration would drastically increase if they had to be measured for each measuring instrument. On the other hand, such an effort is not necessary. From the experts' point of view, it would be sufficient to assume, for example, model-typical values for these quantities which would be determined once – preferably by the manufacturer – and made public via the Internet. The technical committee "Torque" of the German Calibration Service (Deutscher Kalibrierdienst, DKD) has taken first steps in this direction: at present, an interlaboratory comparison with 36 participants is taking place. Within the scope of this comparison, each participant must determine the parameters "lever length" and "connection profile" – and that on a total of three different measuring instruments. Also, there are agreements within DKD that accredited laboratories which work in accordance with DIN/EN/ISO 6789 must carry out measurement uncertainty calculations already today within the scope of such calibrations.
We hope that the proposals for the amendments mentioned above, as well as other proposals which have not been mentioned here, will be taken into account by the ISO committee in the current revision of the standard. The main goal is to consider important influencing quantities that are known and to prescribe a measurement uncertainty determination which is, in principle, stipulated for all measurements and calibrations.
References:
[1] D. Röske, ISO 6789:2003 Calibration Results of Hand Torque Tools With Measurement Uncertainty – Some Proposals, Proc. Of the XX IMEKO World Congress „Metrology for Green Growth“, September 914, 2012, Busan, Republic of Korea, 
http://www.imeko.org/publications/wc-2012/IMEKO-WC-2012-TC3-O34.pdf
Contact person:
Dirk Röske, FB 1.2, AG 1.22, E-Mail: dirk.roeske@ptb.de