Phase opposition to stop unwelcome vibrations

Wood / Misc /
Wood / Misc /

Pickin’ up bad vibrations ? We’re givin’ you the solution !

We make our equipment vibrate to move products around. Downside: these vibrations are also partially or fully transferred to underlying structures and foundations. To help our customers manage these “bad” vibrations we communicate in an early design phase the dynamic charges of our equipment. Civil engineers can then calculate the proper structures and/or foundations to absorb these charges. But despite these precautions, sometimes vibrations would still propagate and cause hindrance or damage to surrounding structures.
Our machines can be designed to reduce the “bad” vibrations to a minimum: the use of isolation springs and counterbalance weights on natural frequency equipment are some of the available options. But what if the equipment has none of these options, gets installed, and at start-up a 3-storey office building at 100m starts shaking severely? It happened to one of our customers in the wood industry! The equipment consisted of two vibrating tables leading wood from the yard towards 2 vibrating conveyors feeding a chipper.
A detailed study by an external expert showed that foundations were calculated correctly and that the vibrations in the office were not caused by resonance. The cause was a high groundwater level that was serving as a vibration propagator. Computer screens were shaking so bad on the 3rd floor that people couldn’t work!
So we were asked to come up with a solution…
After long discussions we decided to put a phase opposition in place between the two vibrating tables. We had done this successfully before, but with 2 identical conveyors positioned in line. This was very different since one table had roughly double the weight of the other and the tables were installed at a 60° angle to each other. We knew for sure that the situation would improve, but not to what extend.
The motors were equipped with VFDs, drive shafts with proximity switches, and a Delta-Phase® type control panel was installed. The design speeds of the tables were only 20 rpm different from each other. Both tables were tuned to the same speed, but with exact opposite phase. The result after fine-tuning and start-up was stunning: the vibration level in the office building had almost completely disappeared. Exact words in a mail from the customer: “The vibrations were aligned so well against each other that only experts can feel them! The project team and the management are very satisfied with this implementation”.
We have other solutions to deal with “bad” vibrations, depending on the type of equipment, the industry and the site conditions.

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