Determination of Tool Profile for the Milling of Three Screw Pump Rotor

partially_open2The rotors of three screw pumps are commonly machined using shaped milling cutters. The determination of the exact shape of the cutter is very important, since a high precision in the machining is required to obtain a high volumetric efficiency of the pump. This paper describes a method to determine the theoretical shape of the cutter, starting from the characteristic parameters of the pump. The rotors are modeled in space by helicoids. Then, the contact line between the tool and the workpiece is determined and this allows us to define the exact cutter profile, with a suitable reference system transformation.MIMMI G.; PENNACCHI P.Mimmi, Giovanni; Pennacchi, PAOLO EMILIO LINO MARI

Faults Identification and Corrective Actions in Rotating Machinery at Rated Speed

Malfunction identification in rotor systems by means of a model based approach in the frequency domain during long lasting speed transients (coast-down procedures in large turbo-generators), where a huge amount of vibration data at different rotating speeds is usually collected, has proved to be very effective. This paper explores the possibility to adapt this method to the situation when the vibration data are available at one rotating speed only, which in real machines is generally the normal operating speed. It results that single speed fault identification can be successful, but does not allow to discriminate between different malfunctions that generate similar symptoms. Neverthless the identification results can be used to define corrective balancing masses

Diaphragm design improvement for a metering pump

Since the membrane has to separate the process fluid from the oil circuit in metering diaphragm pumps, it is the critical component of these machines. Sometimes, depending on the operating conditions, the membrane may crack and it is necessary to disassemble the pump and replace this component part. Therefore it is important to improve the design of the membrane to avoid this. In this paper, the behaviour of a membrane mounted in a metering diaphragm pump has been simulated by using f.e.m. An accurate analysis of the results has allowed the membrane to be redesigned in a more rational way and to improve its performance

Multiple Fault Identification Method in the Frequency Domain for Rotor Systems

Fault identification in rotor systems has been studied by many authors, but the considered malfunction is one single fault only, generally an unbalance. Real machines can be affected by several different types of faults; moreover sometimes also two different faults may develop simultaneously. A model based method for identifying multiple faults acting simultaneously on a rotor system in the frequency domain is briefly described and its robustness with regards to measuring and modelling errors is evaluated, by means of numerical simulations performed on the models of two typical power plant machines: a steam turbogenerator and a gas turbogenerator

Analysis of the Instability Phenomena Caused by Steam in High-pressure Turbines

Instability phenomena in steam turbines may happen as a consequence of certain characteristics of the steam flow as well as of the mechanical and geometrical properties of the seals. This phenomenon can be modeled and the raise of the steam flow and pressure causes the increase of the cross coupled coefficients used to model the seal stiffness. As a consequence, the eigenvalues and eigenmodes of the mathematical model of the machine change. The real part of the eigenvalue associated with the first flexural normal mode of the turbine shaft may become positive causing the conditions for unstable vibrations. The original contribution of the paper is the application of a model-based analysis of the dynamic behavior of a large power unit, affected by steam-whirl instability phenomena. The model proposed by the authors allows studying successfully the experimental case. The threshold level of the steam flow that causes instability conditions is analyzed and used to define the stability margin of the power unit

Accuracy in the Identification of a Generator Thermal Bow

Model-based diagnostic techniques can be used to identify the faults that affect rotating machines. In general, the most important faults and malfunctions can be modelled by means of a suitable set of equivalent excitations that are applied to the nodes of a finite element model of the machine rotor train. Weighted least-squares error methods can be used to identify the set of forces and moments that minimize the error between experimental transient vibrations and the rotor system response obtained with the simulating model. However, the accuracy of the fault identification can be significantly influenced by the adequacy of the fault model as well as by the accuracy of the rotating machine model. This paper shows some methods, based on basic statistics, which enable the accuracy of the fault identification to be evaluated. The capabilities of the proposed methods have been proved by the results of the identification of the shaft thermalbow of a power unit generator that exhibited abnormal vibrations during machine coastdowns. The accuracy with which the location and the severity of the fault have been estimated is analyzed. In addition to this, the results obtained by the identification of the actual faults have been compared with those provided by the identification of false faults that cause similar symptoms

DETC2005-85368 MODELLING OF MAGNETIC PULL IN LARGE SIZE GENERATOR

ABSTRACT This paper presents a method to analyze the dynamical behaviour of large size generators due to the magnetic pull. In rotating electrical machines, the electromagnetic radial forces acting upon rotor and stator surfaces are very large, but they are balanced when the rotor is concentric with the stator. Similarly, the tangential forces produce only an axially rotating moment. If the rotor becomes eccentric, then an imbalance of these forces occurs, so that a net radial electromagnetic force, known as Unbalanced Magnetic Pull (UMP), is developed. The models traditionally proposed in the literature to study the UMP can be considered as reliable in case of small size electrical machines supported by rolling bearings. On the contrary, in case of large size machines, such as turbo-generators supported by oil-film bearings, the approximation of circular orbits of the rotor is not acceptable. Nevertheless, the authors who have dealt with UMP in big size generators have disregarded that these rotor filtered orbits are elliptical and generally the orbit centres are not concentric with the stator. In order to provide a more realistic model and an original contribution, in this work the actual distribution of the air-gap length during the rotation will be determined in analytical terms, by taking into account the effects produced by the actual rotor orbit. The actual UMP is calculated by using the air-gap permeance approach and the simulation of the dynamical behaviour of a 320 MW generator is presented, showing the harmonic content of the UMP and the presence of non-linearities

APPLICATION OF SQUEEZE FILM DAMPERS

The level of the vibrations and the presence of instability are the two most critical aspects regarding the operations of turbomachinery. To cope with this issues that may compromise the operation of the machines, squeeze film dampers (SFD) are often used in many industrial applications. Unfortunately, many complex phenomena characterize the dynamic behavior of these compo-nents and determine the high complexity of the modeling of these components. The most relevant phenomena involved in the characterization of SFDs are indi-viduated after a comprehensive investigation of the state of the art. Among them, the oil film cavitation, the air ingestion, and the effect of the inertia are intro-duced. A modeling strategy based on the Reynolds equation is then presented. The boundary conditions to be adopted for the feeding and discharging of oil are investigated and implemented. Eventually, the finite difference model is applied to a practical example to evaluate the possibility to minimize the vibration level and to reduce the effect of the instability if a SFD is added to a rotodynamic system. Meaningful information about the modeling of SFDs is provided in this work. The critical aspects of these components and their modeling are high-lighted and discussed

Powertrain Modal Analysis for Defining the Requirements for a Vehicle Drivability Study

The powertrain of a car plays a major role in establishing the vehicle’s offered comfort due to vibrations because it is the heaviest single component installed on the chassis; therefore, when oscillating, it transmits considerable forces to the chassis, inducing unwanted vibrations. For this reason, it is important to identify some associated properties with the powertrain suspension system that describe the performance of its rigid body dynamics. In this way, we could place constraints and requirements on these quantities in order to exclude all the configurations that cause intolerable levels of vibrations, and include all the others in the analysis for further evaluation. The definition of these requirements is critical: a poor setting of requirements excludes potentially good powertrain suspension setups and includes those ones with a drivability index that is too poor. In this paper, we identify a set of quantities that show correlation with the vibration performance of the powertrain setup. A method for testing the specificity of the requirements is also shown in order to evaluate which requirements perform best when it comes to filtering engine suspension setups that provide an acceptable level of vibrations

Some remarks on breathing mechanism, on non linear effects and on slant and helicoidal cracks

The breathingmechanism of cracks in rotating shafts is accurately investigated by means of 3D non-linear models. The behaviour is then modelled by means of a much simpler approximated approach, which allows also to calculate the stiffness variation of the cracked shaft. This simple model has then been used for analysing non-linear dynamic behaviour of cracked rotating shafts, which occurs when breathing is governed by the vibration itself. Finally, also the effects of cracks which have developed according a helix, instead of developing on a cross section, due to the combined action of bending and torsion, are investigated. The breathing behaviour and still more the deflections are then influenced by the torsion. The aim of these investigations is mainly to present behaviours of full size industrial machinery, computed numerically with finite element models of the shaft