Dynamics of Machines - Part II - IFS.pdf

changeable unbalanced mass for simulating rotating machines (for example compressors, turbines
or pumps) with an unbalance problem. Elements 6 and 7 are two acceleration sensors attached
to the second and third masses. Elements 8 and 9 are magnetic actuators built to apply forces
with different dynamic characteristics (oscillatory, random, pulse etc.) to the structure (first
floor). In that way it is possible to simulate the wave forces coming from the ocean by means
of the magnetic actuators.
The motor-disk with unbalanced masses is mounted at the top of the platform (fourth floor). The
motor has variable angular velocity from 0 to 40 Hz (2400 rpm). Due to the unbalanced masses
strong vibration amplitudes can be detected on the second and third floor of the platform.
To represent the dynamical behavior of the system a mechanical model has to be created.
Considering the range of frequencies between 0 and 40 Hz, all rotor-structure movements happen
in a vertical plane (2D motion), and an appropriate mechanical model would be the one presented
in figure 39(b). For the suggested mechanical model:
(a) (b)
Figure 39: Machine-structure dynamical interaction – (a) Laboratory prototype; (b) Mechanical
model composed of four lumped masses attached to four flexible beams, an equivalent model of a
4 D.O.F. system for analyzing the linear vibrations of the platform in the horizontal direction
due to the interaction with a machine and ocean waves.
1. MODELLING – The main information about the geometric properties of the structure
presented in figure 39(b) is given below:
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