Cumulative tutorial 03
Isolating a machine that is already installed
Transmissibility decides the mounts, and the answer is softer, not stiffer.
A 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.
What you are given
- 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
Tutorial 03
Work the problem
4 questions
Question 1
What is the forcing frequency, in Hz?
Question 2
What is the frequency ratio against the existing 6.2 Hz floor?
Question 3
What fraction of the fan's force currently reaches the supports?
Question 4
With mounts giving a 3.0 Hz mounted frequency, what fraction is transmitted?
What this establishes
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.