by William Goodwin

Touch a safe surface of almost any machine while it is running. Chances are you will feel some vibration. Are these normal vibrations, or are the bearings wearing out? Is a shaft out of alignment? Is an internal part out of balance? When you need to answer questions about the mechanical condition of a machine upon which production depends, vibration analysis can be a valuable tool.

Once upon a time, vibration was assessed in low-tech ways — touching and listening. Today, machinery vibration can be measured with sophisticated sensing devices and the data analyzed with powerful software. This is no simple job. It requires either a specially trained in-plant maintenance technician or an outside vibration analyst.

Vibration analysis can detect problems before a breakdown occurs by identifying machinery problems in their early stages. Diagnostic tests with sophisticated sensors and computer programs can spot gear tooth wear, imbalance, cavitation, bearing deterioration, shaft misalignment, structural looseness, destructive resonance and other problems before they cause an unscheduled shutdown for repairs or adjustments.

That’s why vibration analysis is becoming a primary predictive maintenance tool for most industrial machinery, says Ray Oliverson, vice president of HSB Reliability Technologies.

Amplitude and frequency
"When I started in this business 30 years ago, we were basically doing educated guesses about the cause of machinery vibration. Today, we have truly amazing equipment for measuring and interpreting the two primary components of vibration, amplitude and frequency," says Rich Collins, consulting engineer, Reliability Systems in San Diego, Calif.

At first, electronic vibration analysis looked only at the overall amount (amplitude) of the vibration, says Collins. It established a vibration history and trend of a piece of equipment. When vibration reached a certain level or when the trend suddenly changed, maintenance people knew that something in the machine was deteriorating. With a little luck they could identify and repair the defective parts before the machine caused a production shutdown.

Amplitude-only measurements can’t catch everything
However, amplitude-only measurements detects only a few types of problems. For example, a machine can demonstrate a constant amplitude trend and still fail suddenly from a bearing defect. That’s because a vibrating component in a machine emits sound in amplitude (strength or amount of vibrational movement) and frequency (pitch). That’s why effective vibration analysis includes looking at vibration amplitude and frequency (frequency spectral analysis).

Since each component of a machine produces a different set of vibration frequencies, determining the individual frequencies of the total vibration "signature" can help pinpoint problem areas.

"A specific combination of frequency and amplitude indicates with a high degree of accuracy the source and the extent of a machine’s vibrational problem. With the source precisely identified, repair efforts can then focus on the specific problem," Collins says.

Experts agree you shouldn’t use vibration analysis alone. It’s best used with other preventive maintenance tools such as ultrasound, infrared thermography and oil analysis.

Plants most often use vibration analysis for motors, gearboxes, tooling machines, pumps, fans, turbines, electric motors, paper machines, compressors, rolling mills and most other high-speed rotating machinery. It can point out where alignment and balance can be improved for greater efficiency. Logically, a balanced machine will run longer, produce fewer rejects and consume less energy.

Vibration analysis for high-speed machining
Vibration analysis has proved particularly helpful in high-speed machining. Based on amplitude and frequency readings, operators and engineers can determine optimum feed rates and spindle speeds to improve surface finish, reduce bench work, reduce frequency of cutter replacements and improve workpiece fixture rigidity.

Design engineers use vibration analysis to identify ways to better build equipment. It can pinpoint where more supports, elastomeric mounts or isolators, active control technologies or component redesign can reduce vibration and resonance problems.

Today, handheld units used with laptop computers can produce cost-saving, reliable results.

"Equipment for vibration analysis has improved dramatically over the past 20 years. We have seen a substantial increase in the number of companies and people working in this field," says John Piotrowski, president of Turvac Inc., a vibration consulting and training firm.

Today’s vibration analysis software can collect data automatically, detect faults, predict machinery maintenance and repair requirements, indicate trends and plot vibration frequency spectra.

The sensors and transducers that acquire this data can usually do their jobs without interfering with machinery operation. Some software programs create databases of bearing, gear mesh and structural resonance frequencies, helping analysts instantly identify the source of problems.

Because of the wide variety of machines and their complex vibration signatures, there is no "low-tech" means of accurately measuring and interpreting vibration. Vibration analysts need extensive training. Because of the extremely technical nature of this field, plants often hire this function out. The industry may even create a certification standard requiring extensive training.

Limitations
Vibration analysis sounds like a great idea, but it does have limitations. In theory, vibration analysis should increase preventive maintenance effectiveness by indicating repairs and replacements before machinery breaks down. But a simplistic approach to vibration analysis can end up costing money and time without improving maintenance efficiency.

Most vibration analysis tools rely on the easy to use and learn techniques of single-channel frequency measurement. This approach assumes a machine’s vibration profile remains constant during the measurement sequence, which means that transient, non-recurring or low-frequency vibrations will not be factored into the analysis. This can lead to bad data and in turn, bad conclusions. Multi-channel, real-time data collection, which requires more sophisticated equipment and training, is the only way to capture this important data.

Single-channel systems will catch problems such as misalignment, deteriorating bearings and imbalanced rotating components when used by a well-trained operator. But it may only identify a symptom instead of the root cause.

Another limitation: Some vibration analysis tools can’t detect or analyze problems in slow-speed machinery. However, according to the Society for Maintenance and Reliability Professionals, systems for extracting low-level vibrations from composite signals are now available.

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