Dynamics of Cracked Shafts

Abstract The ever-growing interest of the modern rotordynamicist in the early detection of rotor cracks in turbomachinery has been the direct result of multiple catastrophic experiences that industry has faced in recent times due to cracked rotors. The complete failure of the rotor due to crack propagation is easily recognized as one of the most serious modes of plant failure. Even though the past decade has witnessed some laudable attempts that have been moderately successful in detecting cracked rotors, this aspect has not received the attention it warrants. A complete test rig has been designed and constructed for experimental research on the response characteristics of cracked rotors, the results of which will permit increased confidence in detecting the presence of rotor cracks in turbomachinery. The rig is capable of testing cracked shafts under the effect of lateral and coupled lateral/torsional vibrations. Conventional vibration signature analysis has been employed for the purpose of crack detection. This paper presents the details of the rig capabilities and results from the unbalance excitation technique applied for crack detection. The response of a cracked shaft differs markedly from that of an uncracked shaft when subjected to a known unbalance. This paper shows that unbalance excitation is a promising tool for cracked shaft detection.

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Typical Dynamic Behaviour of Cracked Shafts

Cracked Rotors ◽

10.1007/978-3-642-01485-7_2 ◽

2010 ◽

pp. 17-35 ◽

Cited By ~ 1

Author(s):

Nicolò Bachschmid ◽

Paolo Pennacchi ◽

Ezio Tanzi

Keyword(s):

Dynamic Behaviour ◽

Cracked Shafts

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An Experimental Study of Breathing Mechanism for the Transversely Cracked Shafts

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.338.547 ◽

2011 ◽

Vol 338 ◽

pp. 547-552

Author(s):

He Len Wu ◽

Zhong Yi Cai ◽

Ke Qin Xiao

Keyword(s):

Fatigue Crack ◽

Diagnostic Techniques ◽

Transverse Cracks ◽

On Line ◽

Stiffness Variation ◽

Heavy Loads ◽

Opening And Closing ◽

Cracked Shafts ◽

Breathing Mechanism ◽

Line Crack

Shaft fatigue crack is one of the most common defects in rotating equipment, due to its extensive operation with continuous heavy loads. Finding an efficient way to evaluate the true stiffness variation due to the crack rotation is the key step to develop both on-line and off-line crack diagnostic techniques. This study analyzed time-variant bending stiffness of elastic shafts with experimentally-induced fatigue, welding and wire cut transverse cracks. It was found that crack gap has a significant effect on the opening and closing behaviour of the transverse crack. As in the case of a cut crack, large crack gap could completely prevent the crack from closing during rotation. A fatigue crack without a clear gap shows a typical opening and closing behavior. Further, it remains fully closed within a small angular range and most of time it is partially closed. It was also observed that both switch and harmonic models cannot describe periodic stiffness variation well enough to represent the actual breathing function of the fatigue crack.

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A Generalized Approach to the Dynamics of Cracked Shafts

Journal of Vibration and Acoustics ◽

10.1115/1.3269850 ◽

1989 ◽

Vol 111 (3) ◽

pp. 257-263 ◽

Cited By ~ 8

Author(s):

Changhe Li ◽

O. Bernasconi ◽

N. Xenophontidis

Keyword(s):

Finite Element Method ◽

Finite Element ◽

Experimental Method ◽

The Finite Element Method ◽

Nonlinear Formulation ◽

Local Stiffness ◽

Advanced Model ◽

Cracked Shafts ◽

Element Method

A new analytic-experimental method for describing crack breathing and obtaining shaft local stiffness change is presented. A generalized nonlinear formulation using the finite element method, which can be widely used for both horizontal and vertical cracked shafts, is derived. Some results previously explicited in other studies are confirmed and clarified. This advanced model can serve as a solid basis for further studies.

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Deflections and Strains in Cracked Shafts due to Rotating Loads: A Numerical and Experimental Analysis

International Journal of Rotating Machinery ◽

10.1080/10236210490447728 ◽

2004 ◽

Vol 10 (4) ◽

pp. 283-291

Author(s):

N. Bachschmid ◽

E. Tanzi

Keyword(s):

Experimental Analysis ◽

Cracked Shafts

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A Brief Review on Dynamics of a Cracked Rotor

International Journal of Rotating Machinery ◽

10.1155/2009/758108 ◽

2009 ◽

Vol 2009 ◽

pp. 1-6 ◽

Cited By ~ 31

Author(s):

Chandan Kumar ◽

Vikas Rastogi

Keyword(s):

Fatigue Crack ◽

Lagrangian Formulation ◽

Catastrophic Failure ◽

Time Study ◽

Cracked Rotor ◽

Main Emphasis ◽

Cracked Shafts

Fatigue crack is an important rotor fault, which can lead to catastrophic failure if undetected properly and in time. Study and Investigation of dynamics of cracked shafts are continuing since last four decades. Some review papers were also published during this period. The aim of this paper is to present a review on recent studies and investigations done on cracked rotor. It is not the intention of the authors to provide all literatures related with the cracked rotor. However, the main emphasis is to provide all the methodologies adopted by various researchers to investigate a cracked rotor. The paper incorporates a candid commentary on various methodologies. The paper further deals an extended Lagrangian formulation to investigate dynamics of cracked rotor.

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A Two-Disk Extended Jeffcott Rotor Model Distinguishing a Shaft Crack from Other Rotating Asymmetries

International Journal of Rotating Machinery ◽

10.1155/2008/846365 ◽

2008 ◽

Vol 2008 ◽

pp. 1-11 ◽

Cited By ~ 2

Author(s):

Xi Wu ◽

Jim Meagher

Keyword(s):

Torsional Vibration ◽

Nonlinear Models ◽

Vibration Response ◽

Full Spectrum ◽

Side Load ◽

Torsional Coupling ◽

Asymmetric Rotor ◽

Lateral Modes ◽

Vibration Signatures ◽

Cracked Shafts

A mathematical model of a cracked rotor and an asymmetric rotor with two disks representing a turbine and a generator is utilized to study the vibrations due to imbalance and side load. Nonlinearities typically related with a “breathing” crack are included using a Mayes steering function. Numerical simulations demonstrate how the variations of rotor parameters affect the vibration response and the effect of coupling between torsional and lateral modes. Bode, spectrum, and orbit plots are used to show the differences between the vibration signatures associated with cracked shafts versus asymmetric shafts. Results show how nonlinear lateral-torsional coupling shifts the resonance peaks in the torsional vibration response for cracked shafts and asymmetric rotors. The resonance peaks shift depending on the ratio of the lateral-to-torsional natural frequencies with the peak responses occurring at noninteger values of the lateral natural frequency. When the general nonlinear models used in this study are constrained to reduce to linear torsional vibration, the peak responses occur at commonly reported integer ratios. Full spectrum analyses of theXandYvibrations reveal distinct vibration characteristics of both cracked and asymmetric rotors including reverse vibration components. Critical speeds and vibration orders predicted using the models presented herein include and extend diagnostic indicators commonly reported.

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