A marine engine OEM had designed and built a starter
motor assembly for one of its engines. FEA
predicted a frequency of 36 Hz, but the assembly was complicated
and there were several hoses and cables that would add
damping. It was important to confirm the frequency, to
ensure the starter motor assembly would not be excited
by the engine frequencies, possibly leading to a fatigue
failure. Our noise and vibration
consultants were asked to do a natural frequency test.
Work done
Our noise and vibration consultants
used an accelerometer with a sound level meter to measure
the response of the assembly. Excitation came from a gentle
blow from a mallet, which is standard for this sort of
test. Our consultants recorded results on the sound level
meter and averaged to give a crisp response graph. In
this case, the frequency was 33 Hz – pretty close to the
FEA prediction – but spread over
quite a wide bandwidth by the damping from the hoses and
cables.
