Checking the Health of Your Rotating Machinery

Dec 1, 1999 12:00 PM, By Joseph R. Knisley, Senior Editorial Consultant

The latest in vibration analysis tools, the Fast Fourier Transform (FFT) vibration spectrum analyzer promises accuracy improvements of up to 200%.

Would you like to be able to predict the remaining life of a piece of equipment to help you plan for production shutdowns and schedule maintenance tasks? Of course! That's exactly why we're seeing manufacturers and others in the continuous-process industry adopting proactive maintenance models rather than reacting to problems as they arise.

With this trend comes a new class of sophisticated diagnostic tools that provide you with the data you need to pinpoint a problem before catastrophe strikes. Let's take a closer look at a device you can use to check the health of rotating machinery.

Rotating machines generate prominent vibration multiples of rotational frequency. Learning how to interpret these rotational frequency multiples, along with their harmonics and any sub-synchronous components, is the trick. Although various types of vibration analysis tools for motors provided users numerous benefits in the past, the Fast Fourier Transform (FFT) vibration spectrum analyzer may be your best bet today. Some users report vibration analysis accuracy improvements of 200% with such devices.

The rotational-frequency portion of the vibration spectrum, for example, provides data on bearing stability, rotor balance, alignment, and shaft clearance. Vibration frequencies generated by a rolling-element ball or roller bearings can range from the lower frequencies (20 Hz to 50 Hz), produced by interaction between bearing components, to the acoustic-emission frequencies of 1 million Hz or more.

Older analog vibration analysis equipment requires you to read gauges and tune frequencies manually. A mathematical function for converting analog information into digital information, FFT technology uses digital signals to display the information in a graphical format. By looking at a spectrum, you can see patterns or worsening conditions as they develop and repair imbalances on site with the tool.

A machine's dynamic-vibration signature may range from one component, at the frequency of a single rotating shaft, to a complex spectrum with numerous components; such as that generated by a gas turbine. To make dynamic vibration an effective indicator of machinery health, you must have a way of thoroughly analyzing even the most complex spectrum. Otherwise, the overall signal may mask obscure but important changes in the machine's signature. The key is to separate a complex vibration spectrum into manageable segments or windows.

In many plants, a condition-monitoring program is mechanically oriented with emphasis on vibration analysis and trending. However, you can also use a full-function vibration datalogger to study harmonics and current on a power system. Usually, you can perform harmonic analysis to 502 line frequency (per IEEE Standard 519).

Preventing run-to-failure is often the primary justification for implementing a predictive maintenance program. However, you can also enjoy dramatic savings when you use condition monitoring for quality control during procurement or following repairs. In new equipment, it's not uncommon to find faults such as misalignment, imbalance, and foundation flexibility resulting from improper installation.

Sidebar: High-tech Tool Proves to be Time Saver

Cleveland Electric Co.'s Service Division uses an FFT vibration spectrum analyzer to analyze its customer's equipment. The tool has a magnetically mounted accelerometer, or probe, that attaches to the motor housing. In collecting and storing the data, the analyzer's digital signal processor permits collection of 10 averages per second for a 400-line spectrum.

Predictive maintenance software allows technicians to quickly transfer data from the analyzer to their PC. According to Dave Darsey, Apparatus and Electronic Service Manager, the analyzer is useful for all motors, including induction, synchronous, DC, and wound rotor working best on motors sized 5 hp and larger. It identifies faults and conditions, such as broken-end ring, eccentricities, out-of-balance conditions, air gap off, and loose stator.

The service department maintains a database with the bearing numbers of each manufacturer, to determine if a bearing is bad, its location, and extent of damage. The database holds more than 200,000 bearing types. Although FFT analysis costs you up-front, it's warranted.