Filippo Cangioli

An Experimental Study of Nonlinear Oil-Film Forces in a Tilting-Pad Journal Bearing

Journal bearings have been widely used in high-speed rotating machinery. The dynamic coefficients of oil-film force affect the machine unbalance response and machine stability. The oil-film force of hydrodynamic bearing is often characterized by a set of linear stiffness and damping coefficients. However the linear oil-film coefficients with respect to an equilibrium position of the journal are inaccurate when the bearing system vibrates with large amplitudes due to a dynamic load. The study on nonlinear oil-film forces is still rare and most papers are confined to theoretical analyses. The purpose of this paper is to derive some new non-linear force models (28-co., 24-co. and 36-co. models) to identify these dynamic coefficients based on experimental data. The fundamental test model is obtained from a Taylor series expansion of bearing reaction force. Tests were performed with a nominal diameter of 100mm and a length–to–diameter ratio of 0.7 using a suitable test rig in which it is possible to apply the static load in any direction. The results show that these three models are feasible to identify the oil-film forces in which the second-order oil-film coefficients received from the 24-co. model are more stable compared to those of other two nonlinear models.Copyright

Behavior of a Tilting–Pad Journal Bearing With Different Load Directions

Tilting–pad journal bearings (TPJBs) are widely used in rotating machinery to support the rotors efficiently at elevated speeds under light/heavy loadings. Because of the importance of this machine component, many authors have published several theoretical and experimental studies, to evaluate the influence of clearance, lubricant temperature, oil flow-rate and thermal effects on behavior of TPJB. However, the investigations of the influence of loading direction on properties of TPJB are very limited. In bearing models as well as in experimental tests, the load is often assumed along the vertical direction only and the geometry of the bearing is the same for each pad, which corresponds to an axial symmetry of the bearing. This paper presents first a theoretical analysis of the influence of the load direction on both the static and the dynamic behavior of a five-pad TPJB with a non-uniform clearance: that is, the different bearing configurations in the range between load-on-pad (LOP) and load-between-pads (LBP) are investigated. Then, the analytical results are compared with experimental measurements. The tests were performed with a nominal diameter of 100 mm and a length–to–diameter ratio of 0.7, using a suitable test-rig, in which it is possible to apply the static load in any direction. The procedure for the estimation of the bearing geometry from experimental measurement of the non-uniform clearance profile is also described. The results show that the load direction has considerable effects on both the static and the dynamic characteristics of the TPJB. Besides, the influence of load directions is stronger on the dynamic characteristics of tilting pad bearing than on the static ones.Copyright

Design and Analysis of CFD Experiments for the Development of Bulk-Flow Model for Staggered Labyrinth Seal

Nowadays, bulk-flow models are the most time-efficient approaches to estimate the rotor dynamic coefficients of labyrinth seals. Dealing with the one-control volume bulk-flow model developed by Iwatsubo and improved by Childs, the “leakage correlation” allows the leakage mass-flow rate to be estimated, which directly affects the calculation of the rotor dynamic coefficients. This paper aims at filling the lack of the numerical modelling for staggered labyrinth seals: a one-control volume bulk-flow model has been developed and, furthermore, a new leakage correlation has been defined using CFD analysis. Design and analysis of computer experiments have been performed to investigate the leakage mass-flow rate, static pressure, circumferential velocity, and temperature distribution along the seal cavities. Four design factors have been chosen, which are the geometry, pressure drop, inlet preswirl, and rotor peripheral speed. Finally, dynamic forces, estimated by the bulk-flow model, are compared with experimental measurements available in the literature.

Special Signal Processing Tools for the Experimental Data of Spiral Vibrations

The occurrence of spiral vibrations in large rotating machines is not a very common phenomenon. However, this kind of shaft vibration, usually caused by rubs between rotating and stationary parts, may give rise to a rather quick and considerable change of the vibration amplitudes. Some well-known methods can be used to study and simulate spiral vibrations, however, further unconventional techniques have been developed by the authors to analyse experimental vibration data of rotating machines that are affected by this phenomenon. The results provided by these innovative techniques can allow the models used by common investigation methods to be developed and optimized. Moreover, they can also be useful to conceive effective corrective actions that can eliminate spiral vibrations. In the paper, the results obtained by applying unconventional techniques to the analysis of stable and unstable spiral vibrations that affected the dynamic behaviour of a large rotating machine, on which brush seals were mounted, are shown and discussed.

Intermittent Rub Caused by Carbonized Oil in a Steam Turbine

Light rubs between shaft and stationary parts are one of the most common faults in rotating machines. Sometimes, deposits of carbonized oil may form in the area close to oil-film journal bearings, especially in steam turbines. These deposits may cause the blockage of the available radial clearance between shaft and oil deflectors. In this case light rubs can be generated even starting from rather low vibration levels of the shaft. The friction forces generated by the rubs usually cause a shaft thermal bow and additional synchronous vibrations. Because of the continuous increase of vibration levels and rub-induced contact forces the blocking material can be quickly abraded. This causes a continuous decrease of the shaft thermal bow and vibration amplitudes. Therefore, intermittent onsets of high vibration levels may occur in the operating condition. These fault symptoms are ambiguous and they can make it difficult to diagnose the root cause of the malfunction. This paper shows a case history in which deposits of carbonized oil caused rubbing phenomena, in the area close to oil deflectors, and recurrent temporary peaks of the 1X vibration levels of a steam turbine. A diagnostic method that has allowed identifying the cause of the fault is described. Besides, comparisons between experimental data and numerical results obtained with a diagnostic model-based method are also shown.

Numerical Modeling of Spiral Vibrations Caused by the Presence of Brush Seals

Clearance is of paramount importance for turbomachinery manufacturers to meet today’s aggressive power output, efficiency, and operational life goals. To minimize leakages, there are various seal types used, and new sealing concepts are in development. Because of their inherent flexibility and compliance, brush seals are capable of significantly reducing the leakage, and allow sufficient geometrical margins to accommodate design and operational variations of turbomachines. Brush seals can be assembled at very tight or zero radial clearance or even with interference on the rotor to minimize the leakage. This means that the risk of contact between the rotor and the seal bristles exists, especially in case of zero clearance or interference. If the contact occurs, a hot-spot develops on the rotor and this may cause the vibration to diverge, resulting in a synchronous instability, the so-called Newkirk effect. The objective of this paper is the development of a numerical model to analyze the dynamic behavior of real turbomachines subject to thermally-induced vibration caused by light-rub of the rotor against brush seals. The model developed in the paper is based on the work of Bachschmid et al. [1]: the dynamics is analyzed in the frequency domain using the standard rotordynamic model, whereas the heat transfer analysis, to calculate the temperature distribution and the associated thermal bow, is studied in the time domain. The contact analysis has been deeply revised, aiming at estimating suitable normal and tangential force and the friction heating generated by the contact.

Development and Validation of a Bulk-Flow Model for Staggered Labyrinth Seals

As well known, the stability assessment of turbomachines is strongly related to internal sealing components. For instance, labyrinth seals are widely used in compressors, steam and gas turbines and pumps to control the clearance leakage between rotating and stationary parts, owing to their simplicity, reliability and tolerance to large thermal and pressure variations. Labyrinth seals working principle consists in reducing the leakage by imposing tortuous passages to the fluid that are effective on dissipating the kinetic energy of the fluid from high-pressure regions to low-pressure regions. Conversely, labyrinth seals could lead to dynamics issues. Therefore, an accurate estimation of their dynamic behavior is very important. In this paper, the experimental results of a long-staggered labyrinth seal will be presented. The results in terms of rotordynamic coefficients and leakage will be discussed as well as the critical assessment of the experimental measurements.

Dynamic Characterization of Milling Plant Columns

Building structures the most rigid as possible is a common practice in order to guarantee extremely precise tool spindle positioning. However, very rigid structures are usually very lightly damped and this could lead to some dynamic problems in case of resonance conditions. In the case of milling machines, due to the variety of milling programs and of cutting tool types, the machine tool is often forced by a broad frequency spectrum excitation, quite impossible to be predicted at a design stage. Thus a near-resonance working condition could easily appear.

Diagnostics of Rolling Element Bearings by Means of the Higuchy Fractal Dimension

In the field of rolling element bearing, the degradation of bearing health could be detected by means of suitable damage indexes. Band-Kurtosis index, that is the kurtosis value of the band-filtered signal, is often assumed. The critical point of this approach is the selection of a suitable filter band. In the paper, the use of a chaos metrics, namely the Higuchi fractal dimension as damage indicator is described. The trend of this index is compared with the common approach of band-kurtosis indicator for an experimental case of a rolling element bearing in which the defect developed until a permanent failure.Copyright