Development of a Rotordynamic Test Rig for Slender Shafts with Large Disks

Abstract

The main focus of this thesis is the review and update of a rotordynamic test rig which originally was unsafe to run at high speeds. The rig was recently developed at the department of marine technology at NTNU. The original production tolerances was not met during the development of the rig, which resulted in high driving forces that could excite the rotor into dangerously large amplitude oscillation. The aim with the thesis is to model the physics of the rig and use the simulation results to improve its design, making it possible to run advanced experiments at higher speeds.

Three different mathematical models are developed. A four dimensional lumped parameter model will help to study the stability of the rotor. A model based on the assumed modes method is used to calculate the frequency response. The last model is based on the finite element method and includes shaft bow in the simulation. The simulation results are used to implement O-ring damping in the bearings, making the rig run stable at high speeds.

The mathematical analysis predicted the vibrational amplitude to be 30 times as large as the value measured on the rig at the first critical speed. This is most likely because of the unpredictable character of the rubber material used in the bearing damping. Other factors such as aerodynamic drag may also contribute to a stronger damping force on the physical test rig.

The rig has currently been run to 4200 rpm without any significant problems. With two disks, one of them in an overhung configuration, the rig is capable of showing two eigenmodes, both visible to the naked eye.