The rotational transmission errors of several assembled harmonic drive devices are precisely measured over one revolution of an output shaft by using three test pieces of each element, a ring gear, a cup gear and an oval cam. Further, angular pitch errors of the tooth spaces of the tested rigid and cup gears with a small module (0.5mm) are also precisely measured by a newly designed measuring device with a resolution of 1/1000 deg. It is found that the rotational transmission error is composed of two components. One is caused by the angular pitch error of the ring gear and varies periodically with a period of half a revolution of the input shaft. The other is caused by that of the cup gear and varies with a shorter period.
