Isolation and control of vibrations on foundations

Vibrations are responsible for causing harmful effects to the adjoining structures, foundation and machines leading to devastating damages. Additionally, the dynamic vibrations also cause annoyance or distraction to the people working or accessing near the machine operation area. However, if the frequency ratio is managed to remain outside the critical range of 0.4 and 1.5 and if the amplitude lies within the allowance limits then damaging/harmful effects may be successfully eliminated, especially when the system us damped.

Transfer or transmission of vibration may be controlled and damaging effects can be successfully reduced to considerable amounts by isolation of the source called active isolation or by protecting absorber or receiver called passive isolation.

The measures employed are as follows;

  1. By placement or location of machine foundation far away from adjoining structures, the process is called as geometric isolation. Mechanism is that with the increase in distance between machine foundation and adjoining structure the amplitude of surface waves (R-waves) gets reduced. For this follow the ratio of foundation mass to engine mass. The specific decreasement in the amplitude can be achieved by placing the foundation to a greater depth since the R-waves tends to reduce successfully due to increase in depth.
  2. Extra masses called as dampers may be attached/added to the foundation encountering higher frequency machines in order to form a multiple degree freedom system plus for altering the natural frequency.
    • In case of reciprocating machines, considerable amount of vibrations can be reduced by counterbalancing the exciting forces through means of attachment of counterweights at the crank sides.

  3. The placement or use of absorbers helps to reduce the vibrations considerably. Absorbers may be rubber mountings, felts and corks applied amid the base and machine.
  4. Another technique can be by attaching an auxiliary mass along with a spring to the machine foundation such that the system gets transformed to two-degree-freedom system. Usable when the system is at resonance.
  5. The strength of soil may be improved via chemical or cement stabilization technique, which enhances the natural frequency of system. This technique is preferable in machines with operating frequencies.
  6. The propagation or fluctuation of waves can also be lowered by use of sheet piles, screens and trenches.