Abstract: Torsional vibration measurement is important to the fault diagnosis of rotating machinery, and a big measuring error would be caused when the speed encoder is eccentric. The relationship between the actual speed and the measured speed from eccentric encoder is theoretical deduced, and the measured torsional angular velocity is derived accordingly. The components of torsional vibration are found out and whose mathematics expression are obtained. The factors influencing the amplitude of each component of the torsional vibration are discussed. It is found that the bigger the eccentricity ratio, the larger the measuring error of torsional vibration; and the measuring error increase with the rotating speed when the eccentricity ratio is constant. The measuring error of any order of torsional vibration is influenced by all the harmonic of the torsional vibration, and the bigger the interval the smaller the influence. The simulation and experimental result verified the effectiveness of the theoretical derivation, and the largest errors of the simulation and experiment are 2.83% and 5.6%, respectively.

Key words: encoder eccentricity, error analysis, torsional vibration test