Diagnostic Tools For Successful Field Balancing

Today's portable instrumentation has come a long way from that of yesteryear. This article highlights features to look for when you're trying to determine the equipment best suited for your needs.

As anyone having experience with machinery diagnostics knows, vibration can be caused by a broad range of problems. These can include worn bearings, misalignment of components, mechanical looseness, improper or damaged foundations, hydraulic and aerodynamic forces, resonances, etc. The most common problem, though, seems to be unbalance.

Unbalance (i.e. an uneven distribution of mass around an axis of rotation) can result when individual components have not been properly balanced prior to assembly, from errors due to the assembly of these components, or both.

The high centrifugal forces generated by unbalanced rotors during operation can lead to premature bearing failure, fatigue fractures, foundation deterioration and shaft deformation, to name just a few. Unbalanced rotors also can present a safety hazard to personnel. For these reasons, well-balanced machinery is a necessity for any maintenance program.

Unlike balancing a rotor in the controlled, predictable environment of a balancing machine, field balancing presents a number of unique challenges. Not the least of these is the need to first determine if the vibration is actually the result of an unbalance. Making the decision to balance without first verifying that an unbalance condition exists may result in wasted time and money.

To ensure successful field balancing, today's vibration analyst needs the type of tools that will quickly and efficiently allow him/her to verify that an unbalance actually exists, and at what operating conditions balancing is best attempted. When evaluating these tools for your specific needs, you'll want to look for the following capabilities.

Measurement of overall vibration
The simplest vibration measurement is the "overall" vibration, which represents the sum of the energy content of all vibrations at all frequencies. Anyone who has worked with machinery of any kind has consciously or unconsciously measured its overall vibration.

If you've ever put your hand on a machine and thought about whether its vibration is high or low, you've made a judgment of overall vibration. Using an instrument to assign a value to that which you feel with your hand allows you to compare your machine with similar machines.

Often, the decision to conduct a vibration analysis begins with someone questioning the severity of a machine's overall vibration.

Measuring the overall vibration at various points on the machine allows for comparison with local and international standards (ISO, API, DIN, etc.). If this comparison concludes that the levels are excessive and further analysis shows that field balancing is required, the first step is to document overall vibration. In fact, regardless of the methods employed to resolve a vibration issue, documentation of the overall vibration is always the initial step.

Frequency analysis (the FFT function)
Arguably the most valuable tool in the vibration analyst's arsenal is the ability to separate a measured overall vibration into its individual components. This is most commonly done using an instrument's "FFT" (Fast Fourier Transform) function. Employing the FFT function results in a spectrum showing the individual vibration amplitudes and their associated frequencies. The beauty of an "FFT spectrum" (as shown in Fig. 1) is that it allows the vibration analyst to see the frequencies that represent the most severe vibration. Correlating these frequencies with a machine's components, or the interaction between components, makes it possible to pinpoint the problem.

While high overall vibration can result from a multitude of problems, each having its own signature on a spectrum, high vibration due to unbalance occurs at the rotational frequency of the component that actually is out of balance. It goes without saying that balancing without there being an unbalance problem is a waste of time and effort.Therefore, an FFT spectrum is essential in determining whether balancing is the proper course of action as opposed to drive alignment, bearing replacement, foundation repair, etc.

Tracking function
Using a reference sensor, such as a photocell, the tracking function's bandpass filter locks onto that vibration frequency corresponding to the running speed of a rotating machine, following it as it changes. Tracking this vibration component (i.e. amplitude and phase) during run-up or coast-down helps one see how the rotor responds at various speeds (Fig. 2). In addition to being a necessity for Bode and polar plots, the tracking function makes it possible to determine where, for example, a system resonance might be, thereby helping the analyst avoid this speed when balancing.

Time waveform function
(Oscilloscope Function)
The FFT capabilities of today's analyzers often cause the value of the oscilloscope function (Fig. 3) to be overlooked. The oscilloscope has an advantage in that, unlike the FFT function, it provides an almost un-damped, instantaneous response to the vibration signal. This makes it useful in the identification of transient, short-duration events such as shocks and impacts.

Influence from unstable, irregular vibrations caused by such things as mechanical looseness, transient impacts, etc., can negatively affect the outcome of a field balancing job. Identification and resolution of these problems, therefore, is very important prior to balancing. The oscilloscope function also is useful for identifying sensor problems.

Balancing
The "art" of balancing has come a long way from the days when vectors were plotted by hand on polar graph paper.Whether static (single plane) or dynamic (dual plane) balancing is needed, today's instrumentation provides the analyst with an array of powerful user-friendly tools, all designed to get the job done as quickly and efficiently as possible. Some of the best software tools:

• Let the analyst obtain up to four measurement points. A variety of combinations are possible, such as simultaneous horizontal and vertical measurements, or two horizontal and two vertical measurements. This unique "optimization" feature allows unbalance vibrations to be recorded at up to four locations, and reduced to a minimum by balancing in one or two planes. This feature is ideal when an operator needs to simultaneously measure the effect of field balancing efforts at other locations on the system.
• Display data in both polar and component form.
• Provide the freedom to define a rotor's available correction locations, whether equally or unequally spaced.
• Afford the ability to store a rotor's influence coefficients in the instrumentation's memory, thereby negating the need to re-calibrate when future balancing is required.
• Allow the storing of the machine description, sensor positions, date and time and the uploading of all data to a PC.

PC upload capability
Whether it's for your own records or those of your customer, the ability to upload all recorded data to a PC is vital. The PC environment affords the user further data analysis and management capacity. Using PCbased software, the analyst can, for example, create expert balancing reports. These reports might incorporate such tools as Bode and Nyquist plots, cascade/waterfall spectra, etc. Compatibility with Windows Office Suite programs such as Word and Excel are important features as well. MT

George Allen is manager of Balancing and Vibration Analysis Services for Schenck Trebel Corporation. Roland Kewitsch is product manager for the company's portable vibration analysis and condition monitoring equipment.