Low frequency faults in rotating machines include:

• Out of Balance
• Misalignment
• Looseness

These faults usually show up as particular patterns of vibration frequency and some example are given here.

Out of Balance

When a rotor is out of balance its centre of gravity doesn’t coincide with its centre of rotation. This uneven distribution of mass can be due to manufacturing defects, or can come about during operation due to changes in the rotor (e.g. broken or missing fan blades or pump rotors). This effectively results in a mass being spun round at the speed of the rotor and hence a vibration at a frequency corresponding to the running speed of the rotor. The further away the centre of gravity is from the centre of rotation the bigger the vibration.

All machines will have some degree of out of balance due to uneven distribution of mass but if the out of balance is excessive it can result in large potentially damaging levels of vibration. This can then lead to premature bearing failure and looseness of parts which may then present a hazard to operators and other equipment.

If a machine is showing high levels of vibration at its running speed, and only its running speed, it is quite likely to be running out of balance.

Figure 1 – Typical vibration time waveform of an out of balance rotor

A typical time waveform for an out of balance rotor vibration, measured here in mm/s, is shown in Figure 1. This shows a clear sinusoidal waveform with very high peak amplitude of approximately +/- 30 mm/s and a period of 20 milliseconds, which corresponds to the rotor running speed of 50Hz (3000 RPM).

If we look at the frequency spectrum of this waveform, as shown in Figure 2 below, it can be clearly seen that there is a large frequency component present at 50 Hz with an average (RMS) amplitude of 16 mm/s. This figure is significantly higher than the acceptable levels for even a large machine and clearly indicates that an out of balance rotor is the problem.

Figure 2 – Typical vibration frequency spectrum from an out of balance rotor

Misalignment

By comparison with out of balance, misalignment often shows up as a “double beat” in the vibration signal giving rise to “M’s” and “W’s” in the time waveform, as shown in Figure 3 below.

Figure 3 – “M’s” and “W’s” in the wave shape typical of a misalignment issue

This in turn can give rise to double peaks in the frequency spectrum where a large vibration signal at the second harmonic of the run speed may be present.

Figure 4 – double peak in frequency spectrum as a result of misalignment

This situation can be due to the effect of two offset centres of gravity passing the sensor (as opposed to one in the case of unbalance).

Looseness

Machine looseness tends to show up as a series of harmonics of the run speed as the loose parts “rattle” and excite various resonant modes in the machines structure.

Figure 5 – looseness showing up as harmonics of the machine run speed

Looseness is also usually relatively easy to detect by simple inspection of the machine!