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Vibration Analysis: Where to Start

Vibration analysis is a useful way to monitor machine health. Many plants have implemented regular vibration analysis plans to ensure the condition of their machines is being checked at regular intervals and so, if an issue comes to light after spectral analysis, a solution can be pursued.

When analyzing vibration, it is important to be familiar with several terms.
These are:
  • Frequency
  • Displacement
  • Velocity
  • Amplitude
These are all terms that are used in vibration analysis on a spectrum. For example, the dominant frequency shown on a spectrum after data has been collected from the rotating machine will determine what kind of fault the machine is experiencing. This fault could be unbalance, misalignment, looseness, bearing failure, or a mix of two or more of these.

The vibrations emitted from a machine are generally comprised of the primary frequency of a machine, which can usually be found at the rotating speed of the machine, as well as its harmonics and other sources within the machine. All of these vibrations add up to form a spectrum, which is a wide range of signals that need to be analyzed either by a vibration analysis software like Accurex and/or by a vibration analysis expert.

Sometimes, when a forcing frequency coincides with a system natural frequency, resonance will occur. Engineers normally try to design around resonances because they can cause a dramatic amplitude increase which could result in premature or catastrophic failure of the system.

Let’s look at a few examples of common machine faults and how they usually appear on the spectrum and where they will manifest.
  • Unbalance–occurs at rotational frequency equal to 1 X RPM of the out of balance part
  • Misalignment– typically causes angular and/or offset problems in couplings
  • Mechanical looseness–caused by loose rotating parts or excessive play in machine mountings
  • Belt issues–vibration analysis will typically identify rubbing, wear, and ultimately misalignment
  • Bearing failure–bearing problems indicate issues at high frequency and at low amplitude
When taking vibration analysis readings, it is important to be consistent in order to get accurate data.
Here are several things you should implement when taking vibration analysis readings:
  • Always take readings from the same point(s) on the machine
  • Ensure other conditions (speed, loading, etc.) are consistent when readings are taken
  • Always note the machine speed (RPM) at the time the reading was taken
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