Vibration monitoring 101

Vibration monitoring is one of the primary condition monitoring methods used in detecting faults in rotating machines. Experience has shown that shaft misalignment and rotor unbalance are the main sources of vibration in rotating machines. Due to advances in engineering and materials science, rotating machinery is becoming faster and lighter, as well as being required to run for longer periods of time. As a result, detection, location and analysis of faults play a vital role in the field of rotor dynamics.

Model-based fault detection is, at this time, directly employed in most areas of fault diagnosis. The model-based approach involves the establishment of a suitable process model, either mathematical or signal-based, which can estimate and predict process parameters and variables.

Rotor bearing dynamics

In many industries, the demand for high power and high speed together with uninterrupted and reliable operation is increasingly important. In this regard, the accurate prediction and control of dynamic behavior (unbalance response, critical speeds and instability) has occupied prime importance.

Bearings clearly constitute a vital component in any rotating machinery and a good understanding of their dynamic behavior is a prerequisite to the prediction of a machine’s properties. In the design of rotating parts, it is crucial that while running up to and functioning at its operating speeds, vibration does not exceed safe and acceptable levels.

An unacceptably high level of rotor vibration can cause excessive wear on bearings and may cause seals to fail. Blades on a rotor may come into contact with the stationary housing with disastrous consequences. A high level of vibration might be transmitted to the supporting structures which could generate an excessive noise level.

An understanding of a machine’s behavior when circumstances change is needed for the diagnosis of the fault and for the formulation of repair strategies and in making decisions about whether the machine is safe to run, and for how long. With reliable computer techniques it is feasible to study the dynamic behavior of machines containing high-speed rotors in greater detail and consequently obtain quantitative predictions with accuracy.

New techniques are continually being developed to cope with the demand for fault-free machinery. For instance, Halliwell (1996) showed how it is possible to measure torsional vibration with a laser torsional vibrometer, eliminating the need for cumbersome mechanical parts.

In the next article, the author explains how shaft misalignment can be another cause of vibration and discusses the most suitable fault diagnosis model.

(Siwani Adhikari is a mechanical engineer at IIT Kharagpur, India. For a detailed rendition of the modeling of unbalance, a full mathematical description and the results obtained to date, contact the author at siwani1.adhkari@gmail.com)