Experiments to model the rotordynamic behavior of rub rings and bumpers

Abstract

Rub-impact between rotors and stators is a widely studied phenomenon due to its deleterious effects on rotating machinery. When large amplitudes of vibration are developed in rotors due to either imbalance or triggered instabilities, the possibility of contact between the rotor elements and static components of these machines will always arise. Therefore, rub rings or rotordynamic bumpers are incorporated to protect hardware, such as seals, compressor blades and bearings by preventing abnormal contact conditions of parts in motion with stators. In this project, flexibly supported rub rings have been tested to evaluate their capability to reduce the amplitude of vibration developed by a rotor running throughout its first critical speed. New materials with improved characteristics against wearing and suitable coefficients of friction were selected to build the rub rings. Materials with low coefficients of friction for the rub rings were used in an attempt to avoid the generation of dry friction whirl, which is a deleterious operating condition of rotating machinery undergoing rubbing effects. For the rub rings support, a material made out of woven metal strands was used. This material has shown promising characteristics providing predictable stiffness and damping for other applications such as bearing dampers. The testing of the proposed bumper design, carried out for different rotor configurations, showed a particular rotor response characteristic. For most of the cases the bumper reduced the rotor response amplitude at its critical speed during the coast-downs. However, for the run-ups the bumper performed in a less regular and effective manner. After investigating some of the bumper design aspects that would cause this characteristic it was concluded that the system was showing a behavior such as has been described in the literature for vibrating systems with a hardening spring characteristic. Computer simulations support the experimental results and this explanation. The mathematical model is a single-wheel rotor mounted on flexible supports with the wheel surrounded by a flexibly supported stator simulating the presence of the bumper.