06.10.2009
To calibrate torque transducers by means of the comparison procedure, so-called reference transducers (Figure 1) are needed which furnish traceable torque reference values. These reference transducers, in turn, are calibrated in high-grade facilities in which the reference transducer are subjected to other – normally smaller - cross forces and bending moments than in the comparison procedure. The influences of these unequal conditions were investigated for typical examplary situations with the aid of special devices in order to give recommendations for the uncertainty calculation in the calibration laboratories concerned.
The investigations showed that the orientation of a reference transducer can, by all means, have a decisive influence on the result. Deviations of detected 0.03%, which seem small, are, however, too large in the case of an expanded (k = 2) measurement uncertainty of 0.02% which some calibration laboratories claim to have. The measure is exceeded even more when larger cross forces or bending moments lead to measured deviations of 0.07% for torque transducers or to 0.35% for torque wrenches. Just the loading of a calibrated torque wrench used as a reference in the exactly opposite direction can lead to a deviation of 0.1%. A coupling which is eccentric by z leads to changes which scale with the ratio of this eccentricity to the lever arm length l for the introduction of force. According to the lever principle, the influence of side forces is proportional to 1/l.
The results of the investigation of these effects were presented within the scope of a lecture held on the occasion of the XIX IMEKO Word Congress which took place in Lisboa from September 6 to 11, 2009. They were also included in a publication which is contained in the proceedings of the meeting and can be downloaded from the IMEKO website www.imeko.org.
Figure 1: Loading of the torque reference transducer with additional force and moment components
Contact person:
Andreas Brüge, Department 1.2, WG 1.22, E-mail: andreas.bruege@ptb.de