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When precise fault conditions are identified this can enable replacement parts to be ordered and work planned before the machinery is shutdown for scheduled maintenance.
Even without identifying the exact cause of a fault, there is great value in simply identifying that maintenance is required, therefore avoiding catastrophic failure and the potential for secondary damage, injured personnel and excessive downtime. The latest vibration monitoring systems from suppliers such as Schaeffler and SKF extend the reach of automated vibration analysis. Non-experts are now able to set-up automated monitoring that provides a simple traffic-light system.
These systems record the vibration signature of a machine when it is known to be in a fault-free condition. Any significant change in the signature is then a potential indicator that a fault has developed. A skilled vibration analyst may then be called to identify the exact cause of the issue. For example, issues with bearings, gears, electrical faults or a lack of lubrication can all be identified from the vibration signature.
Tell-tale spikes
The latest systems can go a lot further than simply flagging up when the vibration signature has changed. Spectral analysis uses Fast Fourier Transforms to convert a vibration signal, expressed as acceleration or velocity against time, into a spectral plot giving the amplitude against frequency. This shows spikes indicating the frequencies of underlying sources of vibration.
Diagnostics is made simpler by expressing these frequencies as multiples of the shaft running speed, or in some cases the AC supply frequency. For example, unbalanced or misaligned components cause sinusoidal vibrations with frequencies at the shaft running speed or a few multiples of it. When the vibration is not excessively high and it is dominated by the shaft running speed frequency this generally indicates a healthy machine.
Vibrations at higher multiples of the shaft running speed may be caused by damaged pumps, fans or gear teeth. Harmonics are an indication of looseness, while damaged bearings produce much higher frequencies and periodic impacts.
Because of the way ball bearings and rollers precess around a shaft, these frequencies are not multiples of the shaft running speed. The number of individual balls impacting on defects in bearing surfaces increases the frequency of this type of vibration. Bearings databases contain the characteristic vibrations for faults in standard bearings. By entering the ID numbers for bearings used in a machine, the software will then look for the characteristic vibrations and flag when a bearing has developed a fault.
To do this, the software must know the shaft running speed, which can be provided by either entering a fixed value or connecting a speed sensor.
Similarly, vibration monitoring software can identify specific faults with other components if relevant information is provided. For example, a fan with a specified number of blades, a gear with a specified number of teeth or belts and pulleys.
Schaeffler’s SmartCheck works in this way, providing a simple notification of a change in the baseline vibration signature if no further information is provided. A system set up in this way can be deployed very quickly. However, it will not provide any information on possible causes of a fault, only that something has changed. Luckily, it is quite straightforward to provide additional information such as shaft running speed and the details of individual parts.
Sensitive monitoring
Standard templates are available for components such as bearings, motors, fans, pulleys and gears. The user simply fills out web-based forms to specify these components. The software first uses this information to set low pass filters and lines of resolution to ensure that monitoring will be sensitive to the changes in vibration that each component might produce.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.