Troubleshooter’s Guide to Vibration (February 01, 2007)

The application of vibration analysis should be part of any proactive total equipment management program. A regularly scheduled vibration analysis program can be carried out by in-house vibration technicians or by local contractors who are familiar with the plant’s equipment and its processes.

However, before any action takes place, the maintenance manager must first determine which machines and their components should be monitored and how often monitoring should take place. This determination is most frequently achieved by using this simple formula:

Machine Priority = Criticality X Reliability.

‘Criticality’ is a function of the importance of the machine to production goals based on a determination of numbers 1 through 10, where 10 represents the most critical machine and 1 represents the least important. Machines with a criticality of 10 might include those with the following criteria:

a) Machines in continuous operation.

b) Machines involved in a single-stream process.

c) Machines withno standby or redundant capacity.

d) Machines having little (or no) product storage capacity on either side of them.

e) Machines that handle toxic or dangerous materials.

f) Machines which operate at particularly high pressures, loads, speeds, or under abnormal conditions.

g) Machines that must run to ensure the continuation of a process, such as an air compressor that supplies instrument control air.

‘Reliability’ refers a result of a careful review of the probability of failure based on a history of the mean time between failure (MTBF) and the mean time to repair (MTTR). It is correct to assume that any machine that is operated at loads, speeds, or temperatures which exceed its design limitations will fail more frequently.

Monitoring interval

How often should the selected machines be monitored? The monitoring interval may be determined after considering the following factors:

a) Machines with a history of design flaws, premature or frequent failure of components such as bearings, or those experiencing unusual or frequent production stoppages, may require continuous monitoring, or at the very least, dailyor weekly monitoring.

b) Machines thatare operated under proper conditions and are considered reliable with a satisfactory maintenance history may only require monthly or quarterly vibration monitoring, depending upon their criticality.

c) Machines that must not break down due to production requirements, or which might create a serious safety or environmental hazard, may require monitoring on a weekly basis.

d) Machines that begin to exhibit operational problems may require being monitored more frequently, perhaps even daily or hourly, until repairs can be planned, scheduled and executed.

What is vibration?

Vibration is technically defined as the oscillation of an object about its position of rest. These oscillations are a response to various mechanical forces and are symptoms of a machine or component problem and potential failure.

The number of these cycles in a given length of time (e.g., 1 minute) is the frequency of vibration, measured in cycles per minute (cpm) or cycles per second — Hz (Hertz) related to 1 X shaft turning speed.

Displacement refers to amplitude and ‘how much’ the object is vibrating, measured in Mils (1/1,000 in.) peak to peak.

Velocity indicates ‘how fast’ the object is vibrating measured in in./second or mm/second peak.

The acceleration of the object that is vibrating is related to the forces which are causing the vibration, measured in “g’s” (1 g = 32 ft/sec2 or 9.8 m/sec2) and is reported or shown as RMS (root mean squared).

How vibration is measured

Vibration can be measured by displacement, velocity and acceleration.

Displacement (distance or movement) is generally the best parameter to use for very low frequency measurements (i.e., less than 600 cpm) where velocity and acceleration amplitudes are extremely low.

Displacement is also traditionally used for machine balancing at speeds up to 10,000 rpm or 20,000 rpm. Where stress levels or clearances are the important criteria, displacement should also be measured using a proximity transducer.

Velocity (speed) is frequently used for machinery vibration analysis, particularly where important frequencies lie in the 600-cpm to 60,000-cpm range. For most machines, mechanical condition is generally considered most closely associated with vibration velocity, which is a measure of energy dissipated and consequent fatigue of machinery components.

Overall velocity is also best for detecting a wide variety of different machinery defects occurring at the mid-frequency range, whereas displacement senses primarily low-frequency problems and acceleration primarily high-frequency defects.

Acceleration (force) is best measured when it is known that all the troublesome vibrations occur at high frequencies, that is, above 60,000 cpm. For example, in detecting high-frequency turbine blade vibration in the presence of many low-frequency vibrations, acceleration will assist in emphasizing the high frequencies. It is also important to remember that when using various transducers to monitor vibration, velocity leads displacement by 90 degrees and acceleration leads velocity by 90 degrees and displacement by 180 degrees.

Using transducers

The quality of the data gathered by vibration analysis is directly dependent upon proper selection and mounting of the transducer. Since the very purpose of vibration analysis is to find and eliminate (or minimize) the forcing function or frequency causing the vibration, it is absolutely imperative that the proper transducer be selected and correctly mounted in order to identify the frequencies of the forcing functions.

Transducers are used to transform mechanical vibration into an electrical signal. If the resulting data collected by vibration analysis is flawed, inaccurate, noisy or contains errors due to transducer selection, poor electrical connections or mounting, the analyst will not obtain good data. All vibration readings should be taken with the transducer mounted perpendicular to the surface of interest in the horizontal, vertical and axial directions.

Vibration signals containing high frequencies must be taken with an accelerometer tightly screwed or glued to the surface, since hand-held pressure alone cannot hold it tightly enough to the surface for it to correctly follow high-frequency motion.

Magnetic mounting on a thin film of adhesive fluid using an accelerometer is preferable to a hand-held reading, but is not as good as a hard-mounted accelerometer.

When selecting transducers, keep in mind the following points. Displacement non-contact proximeters are used to look directly at the rotating shafts of machinery, but keep in mind that the frequencies obtained will be quite low.

A displacement transducer must also be held rigidly, so that the device does not itself vibrate due to anything in its environment. A velocity transducer’s sensitivity drops off quite dramatically at speeds below about 600 rpm and may not be at all effective at measuring vibrations on low-speed machinery.

Accelerometers have the advantage of having adequate sensitivity over a wide range of frequencies. The low end is typically 1-3 Hz, while the upper end can be as high as 20 kHz. For this reason, accelerometers are often the preferred device to use.

Vibration fast facts and definitions

1. In order to ‘see’ a vibration frequency, the analyst may measure the amplitudes of displacement, velocity or acceleration, or any combination of these vibration characteristics.

The analyst’s next consideration is to select which device (transducer) should be used to ‘pick up’ the troubling frequencies, and then determine which technique will be used to analyze the sign
al(s) generated by the transducer.

The most common technique used today is Fast Fourier Transform analysis (FFT). The analyzer breaks down the complex waveforms into various amplitude levels at a variety of frequencies (forcing functions). In effect, the analyzer transforms the various waveforms into a constantly changing display of amplitude and frequencies. Most good-quality FFT analyzers are capable of viewing the frequencies in either a time domain or frequency domain.

2. The natural frequency is the frequency at which a machine prefers to vibrate when excited. It is the frequency at which the machine will vibrate when some force is applied either internally (by the frequency of another component in the machine) or externally (by something like a hammer blow).

The natural or resonant frequency of any machine or structure depends upon its mass, stiffness and damping. Change any of these conditions and the point at which a natural or resonant frequency occurs will change. This natural frequency, when excited, can cause the amplitude of the vibration to become so severe that the machine can self-destruct in a matter of minutes.

Natural frequencies may lurk in platforms, piping, pedestals and mounting pads, and are very often associated with non-rotating structures. For example, if the frequency of a motor’s rpm is equal to the resonant frequency of the motor’s mounting pad, the vibration may become severe very quickly.

3. The critical speed occurs when the rotational speed of a machine coincides with the natural frequency of the rotating shaft. This condition can be destructive and is always associated with rotating equipment.

4. The time domain is a plot of vibration amplitude versus time. It represents the ‘size’ of the vibratory motion.

5. The frequency domain is a plot of vibration amplitude versus frequency. It is called a spectrum, spectral plot or vibration signature.

6. Synchronous (or phase locked) frequencies are frequencies which occur at one to eight times the rpm of the shaft or rotor. Examples include unbalance, sheave pitch line runout, bent shaft, mechanical looseness, gear mesh frequencies and shaft centre line and bearing misalignment.

7. Subsynchronous frequencies are frequencies which occur at less than the rpm of the shaft speed. Some examples are the frequency of another component in the machine, the cage frequency of rolling element bearings, the primary belt frequency, machine rubs, and oil whirl or whip in sleeve bearings.

8. Non-synchronous frequencies are those that are higher than 1 X shaft rpm, but not synchronous to the shaft’s frequency. In addition, harmonics may be present that are not at the rotational frequency of the shaft. Examples include electrical fault frequencies such as cracked or loose rotors or rotor bars, drive belts and rolling element bearing frequencies excluding the cage frequency. (A harmonic is a frequency that is an integer multiple of a given frequency).

Vibration problem solving: A seven-step process

1. Identify the problem (is it a vibration?). Noise is often mistaken as vibration. An example is the electrical field in a conductor, which causes it to vibrate, causing an annoying noise. Frequently a noisy process, such as gas flowing through a compressor system, is mistaken for vibration. Be certain the problem is caused by a vibration before any correction is attempted.

2. Gather information about the system. Sketch the machine system, including gathering all data concerning belts, bearings, number of blades and gears. Gather the machine history, such as “what was the last thing done to the machine?” Ask the operator to describe any changes to machine operation or operating conditions. Any recent change, including maintenance activity, could be the direct cause of the problem.

3. Determine the forcing function (frequency) that is causing the vibration. Common forcing functions, depending upon the design of the machine, are:

* Rotor or shaft frequencies (unbalance, misalignment, bent shaft, etc.)

* Bearing fault frequencies

* Rubs (such as on a mechanical seal or sleeve bearing)

* Drive belt frequencies

* Gear mesh frequencies

* Blade pass frequencies

* Pump impeller frequencies

* Harmonics (or multiples) of these frequencies

* Resonance within the system or a resonant frequency of the machine itself

* Frequencies caused by oil whirl or whip

* Electric motor problems.

4. Determine where to collect data and what equipment to use. Collect data in the correct plane to properly evaluate the problem. The axial plane is along the centre line of the shaft. Some forcing functions, such as misalignment, have their greatest effect in the axial plane. The radial plane, on the other hand, is perpendicular to the axial plane and two radial readings must be taken, vertical and horizontal. Forcing functions such as unbalance, rubs, loose bearings or oil whirl dominate the radial plane.

With regard to the necessary equipment, if sleeve bearings are present in the machine, the problem will probably be in the low-frequency range and proximity probes measuring displacement would be appropriate. If, on the other hand, you are analyzing a gearbox, the use of accelerometers may be required.

5. Take vibration data. The vibration data taken with a transducer must be converted into some meaningful form for proper diagnosis. The two primary forms of display are time domain and frequency domain. The time domain display shows how the waveform amplitude varies over a specific period of time.

A time domain waveform is useful in determining whether the vibration frequency is random or periodic (a non-periodic signal, such as electric arcing, will not transform into the frequency domain). If the time domain display has spikes, it indicates impacts such as those associated with anti-friction bearings or gear impacting.

For more effective or complete interpretation, it is better to view the wave form in acceleration.

The time domain display (see Figure 1) is also a powerful aid in analyzing the vibration condition as it changes over time, temperature and load.

The frequency domain display is a plot showing the frequencies and amplitudes of periodic components in the signal. The frequency domain display is commonly called a spectrum and is used to measure the relative amplitude of particular machine components, harmonic relationships, and the separation of close frequencies and the precise location of frequencies.

For more effective or complete interpretation, it is helpful to view the spectrum in velocity (see Figure 2).

6. Analyze the vibration data. Using the data collected, determine the forcing functions (cause or causes) of the vibration and treat the most severe problem first if there is a more than one forcing function frequency.

7. Make recommendations for correcting the vibration problem. Use a vibration severity chart (Table 1) and vibration identification chart (Table 2) to assist with a determination of problems and corrections.

A thorough knowledge of the machinery and their individual components, experience and a full understanding of the diagnostic equipment used in the plant are all necessary in order to make sound recommendations for correcting vibration problems.

Click here to view the tables from this story

Lloyd (Tex) Leugner is the principal of Maintenance Technology International Inc. of Cochrane, Alta., a company that specializes in the resolution
of maintenance problems and provides training for industry. He can be reached at 403-932-7620 or texleug@shaw.ca.

Part 2 of this report will be published in the April 2007 issue.