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2010 | Buch

Cracks in Rotating Shafts Kapitel lesen Erstes Kapitel lesen

Cracked Rotors

A Survey on Static and Dynamic Behaviour Including Modelling and Diagnosis

verfasst von: Nicolò Bachschmid, Paolo Pennacchi, Ezio Tanzi

Verlag: Springer Berlin Heidelberg

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Über dieses Buch

Cracks can develop in rotating shafts and can propagate to relevant depths without affecting consistently the normal operating conditions of the shaft. In order to avoid catastrophic failures, accurate vibration analyses have to be performed for crack detection. The identification of the crack location and depth is possible by means of a model based diagnostic approach, provided that the model of the crack and the model of the cracked shaft dynamical behavior are accurate and reliable.

This monograph shows the typical dynamical behavior of cracked shafts and presents tests for detecting cracks. The book describes how to model cracks, how to simulate the dynamical behavior of cracked shaft, and compares the corresponding numerical with experimental results. All effects of cracks on the vibrations of rotating shafts are analyzed, and some results of a numerical sensitivity analysis of the vibrations to the presence and severity of the crack are shown. Finally the book describes some crack identification procedures and shows some results in model based crack identification in position and depth.

The book is useful for higher university courses in mechanical and energetic engineering, but also for skilled technical people employed in power generation industries.

Inhaltsverzeichnis

Frontmatter
Cracks in Rotating Shafts
Introduction
One of the most common incipient losses of structural integrity in mechanical structures is the development and propagation of cracks. A crack may propagate from some small imperfections on the surface of the body or inside of the material and it is most likely to appear in correspondence of high stress concentration. For example, figure 1.1 shows a crack developed in the stiffening plate of a generator casing.
Nicolò Bachschmid, Paolo Pennacchi, Ezio Tanzi
Typical Dynamic Behaviour of Cracked Shafts
Introduction
Case histories of chapter 1 have shown that the growth of a crack in a rotating shaft causes a change in its dynamic behaviour, providing that a condition monitoring system is installed and vibrations are analyzed. This chapter is focused on the typical symptoms that can be noticed in industrial rotating machinery and specifically three aspects are considered: the vibrations at rated speed, the so-called thermal sensitivity that affects cracked rotors and the vibrations during speed transients.
Nicolò Bachschmid, Paolo Pennacchi, Ezio Tanzi
Rotor Testing for Crack Detection
Dynamic and Static Tests for the Detection of Cracks in Rotors
Once the crack presence is suspected in a rotor and the rotor is removed from service, it can be tested in different ways to ascertain the presence, the position and the depth of a crack.
Nicolò Bachschmid, Paolo Pennacchi, Ezio Tanzi
Laboratory Tests on Cracked Shafts
Introduction
The results of two different laboratory test campaigns on cracked shafts are described because they illustrate two peculiar characteristics of cracked shafts: the breathing mechanism and the thermal sensitivity.
Nicolò Bachschmid, Paolo Pennacchi, Ezio Tanzi
Crack Modelling
A Review about Crack Modelling
Cracked rotating shafts have been the object of studies and investigations since the 1970s, even if some failure cases are also reported during the 1950s (see section 1.2), therefore now we have a story lasting about 50 years. Important achievements have been made during these years, the knowledge of the dynamical behaviour of cracked rotors has allowed the presence of cracks to be recognized and cracked rotors to be stopped in time before catastrophic failures. Accurate inspections revealed that cracks had already propagated up to a depth of almost 50% of the diameter in many shafts (see for instance figure 2.15), which is obviously a very critical situation. Bearing in mind the fact that it is generally believed that propagating velocity increases exponentially, in many of these cases some further days of operation would have been sufficient to provoke a catastrophic failure, with the loss of the complete machine train and high risks for people and other equipment; also the loss of production of the unit, before the replacement with a new unit, is a comparable economic loss.
Nicolò Bachschmid, Paolo Pennacchi, Ezio Tanzi
Results Obtained Using Simulations
Simulations Compared to Experimental Results
In this section some comparisons between experimental results (obtained by means of test-rigs and real machines) and simulations are shown. The calculated results are obtained with the approach described in sections 5.3.1 and 5.4.3. The good agreement between the results confirm the validity of the described approach. Other simulated results will be presented also in chapter 7 of this book, in order to highlight different peculiar behaviours.
Nicolò Bachschmid, Paolo Pennacchi, Ezio Tanzi
Some Special Effects Caused by Cracks
Effect of Transverse Cracks on Torsional and Axial Vibrations
Transverse cracks affect not only bending behaviour of rotating shafts but have effects also on torsional and axial vibrations. Torsional and axial stiffness are reduced when the crack is open. These stiffness are 1X periodical when the crack is breathing and cause parametric excitation when torsion or axial loads are applied. Moreover also coupling between shear loads and torsion and vice versa between torsion loads and bending behaviour exists. These coupling effects develop with open crack and generally disappear with closed crack. In fact when the crack is open or partially open, then the resultant of the tangential stresses due to the applied torque and/or to shear forces is eccentric with respect to the rotor axis: this causes the coupling of bending and torsional behaviour. In addition the 1X opening and closing of the crack, imposed by the bending load, generates also axial 1X displacements.
Nicolò Bachschmid, Paolo Pennacchi, Ezio Tanzi
Crack Diagnosis in Rotating Shafts
Diagnosis
Diagnosis is a procedure that allows identifying the impending fault or defect from symptom analysis. In the rotordynamics field, symptoms are generally vibrations, which are measured in correspondence of the bearings in industrial machines.
A diagnosis can be performed by skilled human operators, as it was common in the past, or automatically by means of suitable software. Different approaches can be used for automatic diagnostic procedures. In the following, two approaches are presented, along with some applications: i) a qualitative one, which does not allow defining severity and position of the identified fault; ii) a model based approach, which on the contrary identifies also severity and position of the fault.
Nicolò Bachschmid, Paolo Pennacchi, Ezio Tanzi
Backmatter
Metadaten
Titel
Cracked Rotors
verfasst von
Nicolò Bachschmid
Paolo Pennacchi
Ezio Tanzi
Copyright-Jahr
2010
Verlag
Springer Berlin Heidelberg
Electronic ISBN
978-3-642-01485-7
Print ISBN
978-3-642-01484-0
DOI
https://doi.org/10.1007/978-3-642-01485-7

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