Koji Ogushi

Re-evaluation of sensitivity limit in a 10 N·m deadweight torque standard machine

We have been disseminating the national torque standard via calibration of torque measuring devices (TMDs) to the industry in the range from 0.1 Nm to 10 Nm by using a 10 Nm deadweight torque standard machine developed at NMIJ/AIST (10-Nm-DWTSM). In the previous study, the sensitivity limit of the fulcrum of the 10-Nm-DWTSM had been 2.5 μN·m, and the relative combined standard uncertainty due to the sensitivity limit of the fulcrum was 0.0025 %. In order to expand the range of a national torque standard down to 0.01 Nm in a certain precise level, we had to improve the sensitivity limit of the fulcrum. In this study, we re-evaluated the sensitivity limit of the fulcrum of the 10-Nm-DWTSM by using smaller weights and a lower nominal capacity TMD than the previous experiment. Mass of small weights were 1 mg, 0.5 mg and 0.1 mg. The high accuracy TMD with nominal capacity of 0.1 Nm was developed as a trial, and it was installed on the 10-Nm-DWTSM. The output was measured after the small weight was loaded or unloaded automatically by using the weight loading components of the 10-Nm-DWTSM. As a result, it was confirmed that the sensitivity limit of the fulcrum of the 10-Nm-DWTSM was sufficiently smaller than at least 0.1 mg. It was approximately equivalent to 0.5 μN·m as the torque unit, and the relative combined standard uncertainty limit of the fulcrum was 0.0052 % in the range from 0.01 Nm to 0.1 Nm.

DEVELOPMENT OF A NEW SMALL-RATED-CAPACITY REFERENCE TORQUE WRENCH

It is imperative that torque standard of small rated capacity is established and disseminated throughout Japanese industry. A 10 N·m dead weight torque standard machine (10-N·m-DWTSM) has been developed and evaluated at the National Metrology Institute of Japan (NMIJ), part of the National Institute of Advanced Industrial Science and Technology (AIST). By 2012, the relative expanded uncertainty of torque realized by the 10-N·m-DWTSM was estimated to be 6.6 × 10−5, with the coverage factor k begin equal to 2, in a range from 0.1 N·m to 10 N·m for calibrations of the torque measuring devices (TMDs). Calibration service for small-rated-capacity TMDs was started to disseminate the torque standard throughout Japanese industry. Here, there are two routes in the torque traceability system in Japan. One is the route for TMDs and the other one is the route for reference torque wrenches (RTWs). The torque standard in the form of RTWs has been disseminated in the range from 5 N·m to 5 kN·m by using the TSMs owned by NMIJ. There remains a strong demand to expand the calibration range of RTWs. To expand the range, we should develop the new high-accuracy small-rated-capacity RTW and evaluate its calibration method. In this study, a high-accuracy RTW (TP-5N-1109), which had a rated capacity of 5 N·m, was newly developed and calibrated with the 10-N·m-DWTSM to evaluate its characteristics. The ordinary calibration procedures adopted at NMIJ was investigated whether it was applicable to the small-rated-capacity RTWs. As a result, the TP-5N-1109 showed good performance in the creep testing, and its characteristic curves were draw for all cases of the calibration procedures. The repeatability in the calibration results was good. We clarified the problem with the calibration conditions of the small-rated-capacity RTW to calibrate it by three cases.