31 Shaft Rotor Cracks

transcript

Suri Ganeriwala

Spectra Quest Inc.

8227 Hermitage Road, Richmond, VA, 23228

www.spectraquest.com, suri@spectraquest.com

Shaft/Rotor Cracks

Vibration Signature Analysis

Objective of this work was to investigate the Unique

Characteristics of Shaft/Rotor Crack

using Vibration Analysis or other Techniques

Motivation:

Electric Power Research Institute estimates direct and

indirect losses (repair, replacement, and loss of

revenue) at around 1 billon US dollars in the

conventional and nuclear power industry alone.

Shaft/Rotor Crack Diagnosis

Questions:

� How Crack Signature is Different than Unbalance, Looseness, or misalignment?

� Is there an Unique Spectral/Time Signature of a Crack and if yes What is it?

� Does 2X frequency dominate the response?

� How 2X frequency amplitude varies with crack size and location?

� Does crack produce sub-harmonic excitation?

Outline

� Introduction – A quick overview of Condition Monitoring/Diagnostics

� Review of Shaft/Rotor Crack Monitoring/Diagnostics

� Details of Experimental Study done at SpectraQuest

� Results and Discussion of Results

� Cause/Sources of Confusion � Concluding Remarks

Introduction

• Condition Based Maintenance (CBM) promises

to deliver improved maintainability and

operational availability of rotating machinery

while reducing life-cycle costs.

• The three critical components of CBM are:

condition indications, diagnostics, and

prognostics

Introduction

The emphasis of the academic community has

taken a three-fold approach on machine

condition monitoring & diagnostics/prognostics :

(1) Data based methods,

(2) Model based diagnostics and prognostics, and

(3) Data mining (or probabilistic) approaches.

Academic Approach

Primarily signal based and is done in the following

• Define measurement locations on machines

• Install different types of sensors

• Create a measurement structure or database

• Collect data, perform time and frequency domain analysis and

define trend parameters

• Periodically repeat measurements and generate historic trends

• Create statistical threshold based on historical experience

• Make maintenance decision when parameters cross set

thresholds

Industrial Approach

Condition Monitoring Sensors

� Vibration Analysis

o Accelerometers and Proximity Probes

� Acoustic Emission

� Motor Current Signature Analysis

Crack Type and Mechanism

�Open Crack�Breathing Crack

�Transverse Crack�Torsional Crack

�Mixed Mode

1. Crack simulation study based on the Jeffcott rotor models

2. Modeling critical speeds/resonances and monitoring the

shift in them

3. Modeling bi-directional total frequency spectrum and

observing the change

4. Treat Crack as transient phenomenon

5. Torsinal vibration and determine shift in natural

frequencies

6. Multiple sensor approach--AE to locate the shaft crack,

and the vibration signal to determine the depth.

7. Combined fracture mechanics with rotordynamics to

calculate compliance matrix and predict spectra shift

Research Approaches

Fig. 4 Time waveform of the start-up and cost-down test for “all bolts

tight” condition.

Intensity plot of the stat-up in

the flange-simulated crack test.

Waterfall plot of the start-up and coast-down in the flange-simulated crack test

Change in the critical speed

Conditions Critical speed (Hz)

All bolts tight 65.62

One bolt loose 65.00

Two bolts loose 64.37

Three bolts loose 64.37

Change in the critical speed

Conditions Critical speed (Hz)

Intact 65.00

Filler fully tightened 62.50

Filler partially tightened 62.50

No-filler 60.00

Fig. 7 Changes of critical speeds as the crack conditions change

Spectra filled (breathing) crack

Axial Response of filled (breathing) crack

Spectra of fully open crack

Steady State at 50 Hz, Open

Outboard Bearing HorizontalInboard Bearing Vertical

Steady State at 50 Hz, Breathing

Steady State at 50 Hz

IMC-2011

Misalignment

IMC-2011

Misalignment

IMC-2011

Misalignment

Conclusions:

�The critical speed decreases with the increase in crack

�The 1X, 2X and even 3X frequency responses increases

�Increases in higher harmonics were more severe near critical speed

�No serious sub-harmonic excitation was seen

�Results are consistent with the theoretical predictions of crack.

�The results of this study do not provide a unique signature of crack

�Further research is needed to develop for modeling of rotor crack.

31 Shaft Rotor Cracks

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