34.5 Vibration Control and Diagnostics

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34.5.1 Standards and Guidelines

Given the fact that some degree of vibration is always present in rotating machinery, some means of

judging “how much is too much” has to be established so that vibrations can be controlled within

reasonable limits. When such a judgment is not based on a scientific method, there is room for

speculation and it will depend on the one making the decision, the manufacturer, end user, the governing

authority, and so on. Since vibrations are a key indication parameter of the performance of rotating

machinery, different interest groups monitor it for a variety of purposes. It can be used as a measure of

* The displacement amplitude, velocity, acceleration, frequency, and phase angle of the

vibration signal are some of the parameters that can be used to assess the condition of a

rotating machine.

* The vibration signals generated by a typical rotating machine is complex in nature and,

therefore, requires various mathematical analysis procedures and signal averaging

techniques to reduce them to simple and interpretable forms.

Vibration in Rotating Machinery 34-39

© 2005 by Taylor & Francis Group, LLC

TABLE 34.4 Measurement Parameters and Techniques

Measurement/Technique Description When/Where Used

Acceleration, RMS When high frequency or force of

the vibration is of interest

Gear boxes, rolling element bearings,

gas/steam turbines. Mainly used

by defense and aerospace

industry

Bode/Nyquist plot Plot of displacement amplitude and phase

angle vs. speed

Observe critical speeds and instability in

machines using journal bearings

Cepstrum analysis Inverse Fourier transform of logarithmic

power spectrum

To detect families of harmonics and

sidebands in gearboxes, rolling element

bearings, and electric motors

Condition monitoring Analysis of signals generated by the

machine to determine its condition on

a continuous or periodic basis

On critical equipment where predictive

maintenance programs are used. Can

reduce equipment redundancy

Displacement peak-to-peak

(a) Absolute Absolute displacement amplitude of

rotor vibration

Where the rotor mass is very

much larger than the stator mass.

Large motors, generators, and fans

(b) Relative Relative displacement amplitude of

rotor vibration

Commonly used on machines with journal

bearings or in close clearance seals

(c) External Absolute displacement amplitude of stator

component vibration

Low speed machines (less than 1000 rpm)

Modal analysis To measure the vibration response of

a structure to an applied force.

The force can be periodic or

an impact force

To determine the modal mass, stiffness

and damping properties of a structure.

Also used to measure structural natural

frequencies

Orbit analysis The path of the shaft centerline

motion during rotation

Diagnostics of machines using journal

bearings. Provides a picture of the

motion of the journal in the bearing

Polar plots A polar graph of amplitude versus

phase at various machine speeds

Similar to Bode plots, can be used to

detect critical speeds and instability.

Modal properties can also be

extracted from polar plots

Phase angle Phase angle of vibration signal Useful in balancing, diagnosing critical

speeds, and misalignment problems

Rolling element bearing analysis

(a) Acceleration Measure the amplitude of all pass

and discreet frequency accelerations

When damage has progressed to generate

audible noise amplitudes increase.

Useful from 5 – 5 kHz

(b) Shock pulse method A high frequency resonance

technique tuned to the detector

natural frequency

Early detection of failure, measures

ultrasonic noise. Proprietary technique

(c) Envelope technique Bearing defects cause periodic

impacts, which make bearing

components resonate. Demodulation

and enveloping techniques are used to

detect the impact (fault) frequencies

Used for early failure detection (ultrasonic

noise) as well as for advanced

stages of damage (audible noise)

(d) Spike energy method Measure the broadband acceleration over

the 5 – 45 kHz range

Used for early failure detection (ultrasonic

noise) as well as for advanced

stages of damage (audible noise)

(e) Kurtosis method The normalized fourth moment of the

probability distribution of acceleration

over the 2 – 80 kHz range

Used for early failure detection (ultrasonic

noise) as well as for advanced

stages of damage (audible noise)

Run-up, run-down

analysis (waterfall/

cascade plots)

Three-dimensional plot of frequency or

time spectrum vs. time or speed

Used for diagnosing a variety of

vibration problems. Helpful in analyzing

transient signals

34-40 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

quality and workmanship or the common basis for acceptance between the user and manufacturer. From

a safety point of view, the operator can establish normal, alarm, and shutdown levels based on vibration

limits. Vibration levels are also used in making maintenance decisions in rotating machinery.

Rathbone (1939) was the first to publish guidelines for vibration limits for machinery, based on his

experience as an insurance agent. Since that time, numerous individuals, organizations, and governing

bodies have developed a variety of guidelines and standards for vibration levels in rotating machinery. A

listing of the more commonly used guidelines and standards is given in Table 34.5. It should be

recognized that these are experience-based standards, and therefore, will grow and develop with

technology.

ISO 10816 Part 1 to Part 5 is a comprehensive set of standards that has been developed for the

evaluation of vibration of rotating machinery by measuring the vibration response on nonrotating,

structural components such as bearing housings. Vibration measuring points as specified by these

standards are shown in Figure 34.14. In a similar vein, ISO 7919 Part 1 to Part 5 has been developed for

the evaluation of vibration by measuring the vibration on rotating shafts. These standards cover the most

widely used types of rotating machinery and they relate to both operational monitoring and acceptance

testing of equipment. Table 34.6 and Table 34.7 are derived from these standards and are presented as a

general guideline for vibration limits of rotating machinery. For specific details, including the limitations

of the standards, the reader is advised to refer to the relevant sections of ISO 10816 and ISO 7919

Standards.

34.5.2 Vibration Cause Identification

Vibrations are an inherent part of all rotating machinery. Vibration can be due to many causes:

improper design, practical manufacturing limits, poor installation, the effect of system environment,

component deterioration, operation outside of design limits, or a combination of the above. At times,

finding the exact cause of vibration can be quite a challenge, as several of the causes have similar

symptoms. Table 34.8 is a list of the more commonly known causes of vibration in rotating

machinery and their symptoms.

34.5.3 Vibration Analysis — Case Study

In the past, it was common practice to operate centrifugal pumps at a fixed speed and attain required

flow changes by means of throttling. This forces the pump to operate at low efficiency conditions

TABLE 34.4 (continued)

Measurement/Technique Description When/Where Used

Spectrum analysis Plot of amplitude vs. frequency of

vibration

Used for diagnostics, to determine

frequency, harmonics, side bands, beats,

transfer functions, etc., and to control

of vibrations levels at discreet frequencies

Trend analysis Vibration data collected periodically

over an extended time domain

Useful in predictive maintenance

programs in assessing machine conditions

Time averaging Averaging of time records using

triggering at the same point of the

waveform of a repetitive signal

Used in the analysis of faulty gearboxes. Can

reduce asynchronous components in the

signal and improve signal-to-noise ratio

Time-domain analysis Plot of amplitude versus time To observe amplitude modulation, beats,

impacts, transients, and phase angle.

Very useful in diagnostics

Velocity-peak or RMS Velocity amplitude of vibration signal Parameter most commonly used in many

industries to monitor vibrations.

Peak readings relate to peak stress levels

and rms to energy of vibrations

Vibration in Rotating Machinery 34-41

© 2005 by Taylor & Francis Group, LLC

TABLE 34.5 Vibration Guidelines and Standards

Year Author/

Organization

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