Results of computational structural analysis at various excitation frequencies




Tip from our experts

Solving resonance problems under time pressure


Today vibration analysts have only a short time to provide a solution for production agents. Very often, vibration diagnosis and solutions need to be given on site, even if these solutions can only be used in the short term, because financial pressures are too strong if the machine is stopped. 


The real example given below is a technical illustration.



Early 2018, a series of measurements was carried out on critical geared motors using the new triaxial high-speed VIBSCANNER 2 by PRUFTECHNIK. During the series, a very high level of vibration was automatically detected by the device on one of the motors. This motor was mounted on a relatively flexible metallic framework; the PRUFTECHNIK technician suspected the presence of a resonance zone and had to find a solution as quickly as possible so that the operator, who had a large order volume at that time, could continue with production.

Motor-fan with phase marks and direction showing the 180° phase shift

Solution 1 : The machine operator was able to halt production for a few moments and our technician used the impact testing method with his VIBXPERT II on the chassis of the motor to detect resonance zones. Since he had no dedicated shock hammer on site, he positioned his sensor on the frame, programmed his VIBXPERT II to a trigger time and gave the frame a kick to activate it. The result was quite clear, and the machine was working directly in the resonance zone. New rotation speed ranges were defined with the producer in order to avoid these critical zones.

Example of a cross-channel phase measurement

Solution 2: It was not possible to stop the machine. To detect this critical zone for the rotating machine, the technician used an ingenious trick. He programmed the spectrum function of his VIBXPERT 2 to give the average « highest values » instead of the classic « linear average ». By programming a sufficient number of averages this would enable him to retain only the maximum value for each spectrum. He asked the producer to vary the rotation speed of the machine while he took the measurements. The results were available 10 min later. 


The peaks corresponded to the resonances activated in the frame, and PRUFTECHNIK was able to define with the machine control the speed ranges to be used.

These two ingenious field methods are valid and demonstrated their effectiveness using our standard equipment.


Franck Fugon

Specialist, vibration monitoring