Dimensioning

Elastic Vibration of a Machine Foundation

Dimensioning vibration-isolating measures is the duty of the expert consultant. He or she is the only person who can judge the complex overall building dynamics. When planning elastic machine foundation isolation, special care must be taken that component resonances are avoided. Attention should also be paid to the place where equipment is installed.

The simplified view of regarding the machine as a one-mass vibrator is based on an ideal, rigid placement of the elastomer. When excitation masses are not very small compared to the mass of the foundation, it might be necessary to consider the foundation impedance.

For isolation to be effective, the following important planning parameters must be taken into account:

  1. For machines with harmonious excitation, such as building equipment, the isolation effect is determined by the ratio of the interfering frequency ω and the natural frequency of the elastic bedding (natural frequency) ω0.
  2. The degree of the isolation effect depends on the damping behaviour of the elastomer.
  3. Polyurethane foams and composite products made of rubber fibres, rubber granulates (SBR, NBR) and polyurethanes exhibit pronounced non-linear material behaviour. Consequently, it is necessary to perform precise material tests.

Typically an attempt is made to generate supercritical bedding for the machine. This means that the natural frequency ω0 is smaller than the interfering frequency ω. To achieve a physical damping effect, it is absolutely necessary that the ratio is at least √2. In point of fact, the ratio should be at least 2 to 3. The higher the selected ratio, the higher the isolation effect to be achieved.

Ideally, planning measures for the vibration isolation of machines with Regupol® or Regufoam® is conducted according to the following pattern:

  1. Calculation of the characteristic compression (without partial load factors) under the machine foundation from the dead weight of the foundation block and the dead weight of the machine.
  2. Selection of the suitable Regupol® or Regufoam® type taking the maximum static permanent load into account. 
  3. Considering the crucial interfering frequencies of the machine (in the case of active isolation) or consideration of the crucial interfering frequencies of the ambient area (in the case of passive isolation).
  4. Selection of the natural frequency taking the insertion loss / isolation effect that can be achieved and the ratio to the interfering frequency into account. An absolute requirement: a ratio between natural frequency and interfering frequency of >√2.
  5. Installation of the floating machine foundation taking points 1–4 into account.