Vibration Study and Analysis Service

Technicians and operators every so often notice unusual vibrations or noises on the plant or shop floor where they work daily. In order to determine if a serious problem actually exists, they could proceed with a vibration analysis. Vibration analysis implies, state of a machine is identified on the basis of an analysis and study of vibration.

Successful application of vibration diagnosis requires in practice staff with considerable degree of knowledge and experience. Standard work of data gathering may be performed by skilled personnel without academic educations, but data assessment and processing of the machine condition is the task for an engineer who has expertise in different areas like mathematics, design of machines, dynamics, signal processing etc. and who is capable to use this expertise in context.

Why is Vibration Study and Analysis Important?

The vibration analysis is a very important technique, in terms of mechanical vibrations for machine diagnosis. It is based on the high information content provided by the machine vibration signals that are an indicator of machine condition, used for the diagnosis of faults. Vibration analysis in a predictive maintenance program, is widely used for monitoring and detection of initial and critical faults in machinery parts, like shafts, bearings, rotors, couplings, motors etc. Some problems that are usually detected by vibration analysis are: unbalance, misalignment, bent shaft, rolling bearing faults, eccentricity, resonance, looseness, rotor rub, fluid-film bearing instabilities, gear faults, belt/sheave problems.

Following are the reasons that makes vibration study and analysis important:

• Reduces equipment costs:
• Reduces labor costs
• Reduces lost production time
• Increases safety
• Increases revenue
• Increases efficiency of employee time

What is Done During Vibration Study and Analysis?

The main objectives when performing a vibration study typically fall into one or more of three categories:

• Capacity: Map the dynamic behavior of the machine from current to targeted speeds, yielding the most cost-effective approach by foreseeing problems.
• Rebuild: Evaluate how rebuilds will affect the dynamic behavior of the machine and its consequences at current or increased speeds.
• Troubleshooting: Locate and eliminate vibration sources currently having a detrimental effect on the machinery or process.

Vibration analysis is performed by Fourier transform (by its decomposition into Fourier series). All actions related to the study and analysis, given below are employed in analyzers that are utilised in vibration diagnostics.

There are different types of analyzers: operational or laboratory, with one or more channels – but the principle of their operation is always the same.

Fourier Transform

Periodical in time T, function x(t), can be stated as an infinite or endless progression:

This expression means that the original function x(t) can be composed from (infinite) number of sinusoids of different amplitudes, frequencies of which are multiplies of the fundamental frequency ω.

Coefficients an and bn are Fourier or spectral coefficients of the function x(t) and can be computed using expressions:

Discrete function x (t), which is defined on the set of N different instants of time tk (k = 1, N), can be written as a finite Fourier series:

Fourier coefficients are usually depicted in the form of amplitude cn and phase φn:

Then, the finite Fourier series can be written as:

This form of Fourier transform is called the discrete Fourier transform (DFT). The resulting Fourier series, a set of sinusoids from which the original waveform can be composed, is called a frequency spectrum.

There is a basic relationship between the length of the sample T, the number of discrete values N, sampling (or capture) frequency fs, frequency range and spectral (or frequency) resolution. Spectrum frequency range is 0−fmax, where fmax is the Nyquist frequency and ∆f is the frequency resolution (spacing between frequency lines).

An algorithm called Fast Fourier Transform (FFT) is used in up to date analyzers, where N is an integer power of number 2. In fact, the upper frequency spectral limit fmax is even more reduced in comparison with the theoretical value (e.g. for N=211=2048, only 800 frequency lines are used rather than 1024), which will be explained in the following chapter.

How is Vibration Study and Analysis Performed?

Raw time Waveform Analysis

Vibration study and analysis personnel will utilize the magnitude various signals to find out the source and nature of a problem. The most common measurement is that of vibration – in units of acceleration, velocity or displacement. Study and analysis of the raw time waveform gives helpful data for troubleshooting many issues, involving those with gearboxes. Impulsive vibration is better analyzed in the time domain.

Spectrum Analysis

This divides the total.