Paper ID-12 1 SIRM 2009 - 8th International Conference on Vibrations in Rotating Machines, Vienna, Austria, 23 - 25 February 2009 Some cases of rotor instability and methods of their solution Jiří Šimek Rudolf Svoboda Jiří Tůma TECHLAB Ltd. Prague, 190 00, Czech Republic j.simek@techlab.cz TECHLAB Ltd. Prague, 190 00, Czech Republic r.svoboda@techlab.cz TU Ostrava Ostrava, 70833, Czech Republic jiri.tuma@vsb.cz ABSTRACT Increasing operational speed of machines with rotors supported in fluid film bearings imposes higher demands on rotor dynamics solution. Calculation of critical speeds is important design tool, but methods of ensuring rotor stability attract increasing significance. Despite the standard design procedures of renowned producers of rotating machines, cases of rotor instabilities are still encountered in practice. Types and sources of instability are defined, some cases of rotor instability are documented and methods of their suppression are shown in the paper. KEY WORDS Rotor stability, oil whirl, oil whip, journal bearing, tilting-pad bearing, floating ring bearing, lobbed geometry, stability reserve, logarithmic decrement, labyrinth seal, antiswirl breaks 1 INTRODUCTION Still higher operational speed of machines with rotors supported in fluid film bearings brings growing problems with rotor stability. Apart from calculation of critical speeds and response to residual unbalance it is therefore inevitable to calculate rotor stability limits. This process should proceed already in the phase of machine design, because additional modifications of the rotor or bearings are difficult and not always successful. Though standard design procedures include all above-mentioned steps, cases of rotor instabilities are still encountered in practice e.g. 1, 2, 7 . Causes of instability are varying and sometimes even surprising. As excessive rotor vibration in unstable region made permanent operation of machines impossible, it is necessary to find method of their suppressing or at least reducing their intensity. Some cases of instability encountered during the author’s practice will be mentioned together with methods of their control. 2 TYPES AND SOURCES OF INSTABILITY Rotors supported in fluid film bearings exhibit basically two types of instability, both of which are characterized by subharmonic vibration with big amplitudes. Instability of „oil whirl“ type, with frequency dependent on rotational speed, occurs more likely with rigid rotors, paradoxically namely with rotors in gas bearings. For „oil whip“ instability, encountered mainly with elastic rotors, is characteristic constant vibration frequency with some – usually but not always – the lowest eigenfrequency of the system. Quite frequent in practice are cases, when instability of „oil whirl“ type converts in region of the 1 st eigenfrequency of the system to „oil whip“ type. The source of instability is in most cases the bearing support itself. Cross coupled terms of stiffness matrix, which promote journal orbit around bearing centre, prevail at some operational conditions (low load, high speed) over direct terms. Special shape of bearing surface, consisting of several areas with preload relative to the bearing centre (lemon bearing, lobbed geometry), increase stability limit, but it need not be always sufficient. Tilting-pad journal bearings are inherently stable, because cross coupling terms are at least one order (usually more than one order) lower than direct terms. However, in cases of intensive external excitation, e.g. from labyrinth seals, even tilting-pad bearings may not ensure stability of all rotors, as will by shown later. Labyrinth