Calibration of reference torque wrenches using a 10 N m deadweight torque standard machine

• The 10-N m-DWTSM with new holders is capable of calibrating small capacity RTWs.
• The new holder with the aerostatic shaft improved the reproducibility.
• The calibration method of large capacity RTW is applicable to small capacity RTWs.

Within the torque traceability system in Japan, there are two processes, one for “pure torque” loading without any parasitic components, and the other for “torque wrench” loading with unavoidable transverse force and bending moment. The torque standard for reference torque wrenches (RTWs), which is a reference standard for first-grade accredited laboratories, has been disseminated in the range from 5 N m to 5 kN m using other torque standard machines at the National Metrology Institute of Japan (NMIJ). In this study, in order to expand the range of the torque standard for RTWs, we designed new holders allowing small-rated-capacity RTWs to be calibrated using the 10 N m deadweight torque standard machine (10-N m-DWTSM). We also studied the calibration method for small-rated-capacity RTWs according to the guidelines that have been adopted for medium and large RTWs. Experiments conducted using two kinds of small-rated-capacity RTWs showed that the 10-N m-DWTSM equipped with new holders can calibrate small-rated-capacity RTWs, and the calibration procedure ordinarily adopted for medium to large capacity RTWs is also applicable to the calibration of small-rated-capacity RTWs. The newly designed holder with an aerostatic shaft resulted in a smaller relative standard uncertainty due to reproducibility with a change in mounting position compared with that obtained using the conventional holder with a solid shaft.