Limitations of ISO Limits

Classification based on ISO standards is done only though comparison between overall vibration value and ISO limit value. Overall vibration result is one numerical value that represents summary of vibration that is acting on vibration sensor. We do not know what the source of this vibration is. It can be coming machinery itself or it can also come from environment like large machinery running nearby. So even though overall value will be high condition of machinery will be good. That is why our company is performing detailed spectrum analysis to define exact source of vibration and condition of machinery. Below is example of vibration coming from environment.

In this example, you can see two signals taken from same machinery in same measurement point. Upper one is taken with Main Engine running and lower with Main Engine stopped.

As you can see upper one has dominant frequency at around 20Hz. This is not visible in spectrum when Main Engine is stopped. Green marker shows frequency associated with machinery running speed, so you can see, that this is the different frequency than Main Engine influence.

Overall vibration value for the upper graph is higher than for the lower, while condition of machinery is the same.

That is why sometime even if we have overall vibration results over ISO limit, we do not recommend taking any action.

Other aspect is that ISO standard limits are a starting point for machinery condition. They are set, based on research of many machineries, but they do not consider every specific machinery. Especially in marine industry, where we do not have really any stiff foundations because of steel construction, we can have sometime slightly higher vibration results than recommended in ISO limits. It is important to monitor the trend and vibration spectra in a search of faults that started to develop. If there is no wear down process it can be uneconomical to perform overhaul right away. Then we recommend monitoring the vibration and reducing measurement intervals to observe if any faults started to develop.