18-09-2019 | Posted by Joaquín Martí
Unless you are talking about an electric toothbrush or some other equipment specifically designed to vibrate, vibrations are generally an undesirable by-product of industrial activity or environmental action. All sorts of ill effects are associated with vibrations: noise, discomfort, stresses, material fatigue, etc; eventually, vibrations tend to result in decreased efficiency, higher maintenance costs, and shortened life of equipment and structures.
For all these reasons, it is frequently necessary to estimate their size and, if excessive, reduce it to an acceptable level, which varies with the type and function of the structure or facility considered.
Common sources of vibration are traffic of pedestrians or vehicles, excitation by winds or waves, the operation of pumps and other mechanical equipment, explosions, earthquakes, impacts… In fact, just circulating fluids can cause vibrations, particularly when phase changes are involved. And the mechanisms are sometimes sophisticated: imagine the frequency of the von Karman eddies coupling with the second cantilever mode of a lamppost, or the excitation from collapsing bubbles of liquid salt, or the unconscious synchronisation of active audiences or pedestrians.
When unwanted vibrations appear, one can try to act on the source, on the transmission path, or on the resulting structural response. Principia has experience in all of them. At the source, we have dealt with vibrations in pumps for liquefied gas, in the twin ties of a tied-arch bridge, in the vibrations of a turboprop aircraft engine, and in those arising from cavitation of the liquid salts in a solar thermal plant. We acted on the transmission path on an underground military command centre, a 7th century church disturbed by traffic, an electricity generating plant, and a number of buildings near metro stations and railway lines.
The strategies for acting on the structure may be oriented to changing masses, stiffnesses and/or damping. Occasionally, we use isolation systems for seismic or other vibrations, as well as control systems.
We have acted on the structure of an auditorium suffering from active audiences, a pedestrian bridge excited by the pedestrians walking on it (where we used a passive control system), a water bottling facility, bridges and solar collectors excited by wind, the lampposts in a bridge, various buildings subjected to train-induced vibrations, seismic isolation for liquefied natural gas tanks, etc.
Not all the vibrations are bad. Exceptionally, we impose the vibrations on a system that did not have them: for example, in a large coal hopper, we used vibrations, tuned to their natural frequency, to break the coal arches that formed across the hopper, thereby interrupting the discharge.
In summary, specific knowledge and experience are required whether you want to decrease your vibration levels or to intensify them. We at Principia will be happy to provide them when needed.