Torsional vibration of a back-to-back gearbox rig Part 1: Frequency domain modal analysis

Abstract

The torsional vibration of a back-to-back gearbox system has been investigated experimentally and analytically. Gearboxes are typically part of a larger system, or drivetrain, which commonly includes electric motors, shafts, couplings, ball mills, turbines and generators. The dynamics of the system has been shown significantly to influence the gearbox response and requires inclusion in gearbox modelling. Assumptions are commonly made, however, to reduce the degrees of freedom of the model, and system dynamics is subsequently neglected. Receptance methods were applied to model a back-to-back gearbox system and were shown to be an effective modelling technique for geared system analysis. A detailed experimental modal analysis was performed using the swept-sine technique with an a.c. servomotor torsional exciter. Torsional excitation allowed for the dynamic response to be measured up to 1600 Hz. A multi-degree-of-freedom, frequency domain torsional model of the gearbox system was developed using receptances. The model included a combination of lumped-mass elements, and continuous shafts with distributed inertia and hysteretic damping. The modelled torsional natural frequencies were matched to the measured frequencies by the adjustment of system model parameters to achieve a high level of agreement. Matching of system natural frequencies resulted in the accurate prediction of the first nine associated torsional deflected shapes up to 1548 Hz. This paper presents the detailed results of a full torsional, modal analysis of a gearbox system, demonstrating receptance system modelling and an effective method for torsional excitation on rotating machines.