Comprehensive Model and Experimental Validation of Passive Vibration Suppression for Supercritical Rotary Machines

Open Access
Author:
Haidar, Ahmad M
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
December 02, 2015
Committee Members:
  • Jose Palacios, Thesis Advisor
Keywords:
  • structural dynamics
  • rotorcraft
  • passive balancing
  • passive vibration suppression
  • supercritical shaft
Abstract:
A comprehensive physics-based model of a rotary system was formulated and experimentally validated to predict the performance and dynamic behavior of passive balancing devices. The mathematical model incorporates three-dimensional effects such as non-planar rotor bending and places little restriction on the system configuration. Special attention was placed on balancing mass interaction with other balancing masses and with the balancer track. The effects of balancing mass collisions and friction were modeled and tested. Passive balancing devices for rotary systems consist of masses that are free to move in concentric guides about the shaft axis. Under optimal conditions, notably at supercritical speeds, the balancing masses automatically assume new positions to counter any imbalance due to uneven mass distribution in the system. The balancing phenomenon occurs as a result of a 180 degree shift in the system response phase with respect to the imbalance phase when a natural frequency is traversed. Eccentric centrifugal loads on the balancing masses cause them to move in a manner that suppresses or diminishes rotor vibration. The problem is highly nonlinear and requires comprehensive modeling to achieve satisfactory prediction of the balancing behavior. In this research, a test rig was fabricated to experimentally validate the presented mathematical model. A rotating shaft with an imbalanced hub and a passive balancing device was tested at 1600 RPM, a supercritical speed. The passive balancer performance in transient and steady states was investigated under various imbalanced configurations. In the experiment, the passive balancing device on average reduced shaft vibrations by 62% at steady state. In addition, the experiment demonstrated that the passive balancer exhibits suboptimal performance when the system had low imbalance. This is opposite to what is predicted by conventional models in which viscous damping is the sole interaction between the balancing masses and the balancer track. Models available in the literature predicted the results within 68% of the experimental values. The comprehensive balancing model developed in this thesis more accurately predicts the performance of the passive balancer. The increased accuracy was accomplished through the inclusion of rolling resistance friction between the balancing masses and the balancer track, as well as the inelastic collisions between the balancing masses. By incorporating inelastic collisions, the accuracy of the predictions was improved by a factor of 6.7 (3.7% margin of error vs. 24.7%) in high imbalance scenarios. By including friction, the model prediction accuracy was increased by a factor of 3.3 (17.6% vs. 58.1%). Overall, the model presented in this thesis improved the accuracy by a factor of 3.9 when compared to published models (17.6% vs. 67.7%). Ignoring the effects of friction and mass collisions reduces accuracy of predictions significantly. The model which addresses friction and collisions was validated and predicted the performance of the passive balancer to within 17.6% of the experimental values in all tested cases.