Rotating machine: Infrared thermography only is not enough!

In the industrial scenario there is more than just electrical cabinets.
And even more than refractories and insulations.
There are also mechanical components and rotating machines.

And when a rotating machine heats up, because of a problem, we can find it with the infrared thermography.

This is true…absolutely true…but…
…but this is very often too late.

Because the infrared thermography shows with incredible precision and accuracy a change in the thermal map of any “measurable” surface (meaning with properties that allows to properly measure trough radiation, therefore a material without a very low emissivity or, even worse, transmissive) but this implies that the first signal of a malfunctioning will be, in fact, a “identifiable” and “detectable” variation in the thermal map.

It would be perfect, but in reality this is not the case.

In case of a bearing, for example, the factors that can cause a damage are multiple: a wrong installation, a bad lubrication, misalignment, overload, stray currents and others.

All those “mistakes” can create a damage but not all create a localise heating, during the first symptoms of the damage.

Mostly, for some of those problems, when the overheat is detectable, can be too late.

What to do, then?

Above all, in case of a Company interested in the Condition Monitoring of rotating machines, who you should trust or what to adopt?

The typical solution is to “join thermography and vibration / Shock Pulse analysis”.
Mind you…no “join vibration / Shock Pulse analysis and thermography” because in this case the infrared thermography is not the main tool but the one that will support the main techniques.

It will be a disaster to trust infrared thermography only, in case of rotating machines! 

Let’s try to better explain what is the vibration analysis and what is the Shock Pulse analysis, for non expert people.

The vibration analysis, in few words, captures the general vibration spectrum of the machine under testing, according three different directions (horizontal, vertical and axial) and evaluates the speed RMS value (for the general analysis) or decomposes the vibration spectrum in frequencies, from the lowest to the highest, according the specific measurable range and according the technician inputs (for the spectrum analysis).

The advantage of this technique, in short, is that allows a very precise analysis of a machine in case we perfectly know the technical characteristics and, above all, it is ideal in case of gearboxes.

The Shock Pulse analysis is something different.
It was born from a very old technique that skilled maintenance technicians use since many decades to understand if a bearing is in good conditions or not: they take a big screwdriver and they put the metal end on the bearing housing and their ear on the handle.

They listen to shocks.

Well…the Shock Pulse method has been invented in the sixties, in Sweden, and represents the simplest and “modular” method for bearing analysis.
The more the information, the better the outcome of the analysis.
But the same method allows a good analysis even if the information are less.

Let’s see some examples, now.

In the attached image the overheated gearmotor was installed the day before the inspection.
With the infrared thermography it isn’t normally possible to establish the reason of the overheat, but the combination of vibration and Shock Pulse analysis has indicated the problem in the supports of the conveyor that this gearmotor was driving.

Ir_36229mod

Overheated gearmotor

In the following images are reported some charts of Shock Pulse spectrum and vibration analysis of a group motor-fan that, at the infrared inspection, didn’t show any issue.

The inspection has detected a problem related to stray currents generated by the inverter and an unbalance in the fan.

Inverter spectrum shock
Shock Pulse chart for the stray current problem generated by the inverter

Inverter Spectrum
SPM Spectrum chart for the stray current problem generated by the inverter

Vib ventilatore
Vibration spectrum analysis for the fan unbalance

When the goal is the construction of a reliable Condition Monitoring system it’s impossible to count on one evaluation technique only.

The infrared thermography is a fundamental technique but isn’t the “panacea” of all problems.

And we cannot forget even other supporting monitoring techniques: the electrical motor testing, the lubricant analysis, the alignment techniques, the shaft and rotating elements balancing, and many others.

We will talk about this in the following articles.

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