Authors: Eng. Potito Cordisco, Senior Project Manager, Vicoter
Eng. Mauro Terraneo, Chief Technical Officer, Vicoter
Often vibrations are just the normal effect of an operating machine and if they remain below safe and comfort levels, they should not cause undue concern. When the amplitude of the oscillations is instead higher than normal, specific investigations are needed because it could be the symptom of problems.
In simplest terms, vibration is only the back-and-forth movement of a component, induced by time-varying loads. The entity of this motion is driven by two factors: the magnitude of the input forces and the dynamics of the excited part. In heavy industrial equipment, the main sources are generally the rotating components: motors, pumps, compressors, and so on. In other machinery, e.g., ones to produce printed circuits, the main sources are instead the continuous and sharp accelerations and decelerations of the internal devices. In any case, let the forcing be due to produced or inertial loads, it is transmitted to the ground by the constraint system, producing relevant oscillations. In some cases, the floor itself amplifies the vibratory response due its resonances, potentially originating fatigue related problems to the structure and a not comfortable working environment.
Experimental vibration test campaigns, when properly done, allow to discriminate if the high amplitude perceived movements are due to too strong forces generated by the operating machine, or to the excitation of a resonance. Vicoter (www.vicoter.it) is an Italian company expert in performing in-situ recording of accelerations for problem solving in several industrial fields. Vicoter has a fifteen years long experience and it works in compliance with the higher standards, using certified and cutting-edge instrumentation and methodologies.
Due to their skills, Vicoter’s engineers were asked by a leading European EMS and ODM company to measure the operational levels generated by the in line of PCB assembling machinery, installed at the first floor of its production site. Both the vibrations transmitted to the floor and to operator stations, approximately placed at 10 m from the line, must be measured to quantify their magnitudes and to get information for designing dedicated shock dampers.
A total of thirty-eight accelerations were acquired simultaneously. An OMA (Operational Modal Analysis) was firstly carried out to determine the resonances of the floor which, if forced, could be the cause of annoying vibrations. Modal identification was performed by the state-of-art Polymax™ algorithm.
Then, the measure of operational vibrations was performed while the machines operated at different capacity. Analyses focused on low frequencies, since they are the ones typical of floors, the most perceived by men and since they are the ones that can generate possible fatigue problems on the structures.
Frequency spectra of the measurements showed that the main amount of the acceleration response is due to a modal participation, i.e., not due to tones generated by the machines. During their working cycles, the machinery generate impulses that are transmitted to the floor, which only vibrates at the frequencies of its modes with a participation dependent on the transmissibility between source and receiver.
