Structural dynamics / tutorial
Isolating a machine that is already installed
Transmissibility decides the mounts, and the answer is softer, not stiffer.
Teaching basis and review status
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesA 1 450 rpm fan has been bolted rigidly to a suspended floor whose vertical natural frequency is 6.2 Hz. Complaints of vibration have come from the floor below. The contractor proposes stiffening the floor. Check that before agreeing to it, then design the alternative.
- Fan speed 1 450 rpm
- Existing floor vertical frequency 6.2 Hz
- Damping 2%
- Proposed isolation mounts would give a mounted frequency of 3.0 Hz
Practice
What is the forcing frequency, in Hz?
Practice
What is the frequency ratio against the existing 6.2 Hz floor?
Practice
What fraction of the fan's force currently reaches the supports?
Practice
With mounts giving a 3.0 Hz mounted frequency, what fraction is transmitted?
One caution to carry: soft mounts have to be soft in the operating condition and still safe at start-up and shut-down, when the machine sweeps through β = 1 on its way to speed. That transient passage is brief, but it is a resonance, and mounted machines are given snubbers for exactly that reason.