>You can test
>the natural frequency of the shaft by hanging from a chain or such and
>"pinging" it with a light hammer, and measure the noise frequencies.
in my opinion this test would provide no useful information about what's
going on inside the engine.
for a start the first and last main bearings 'terminate' the ends of the
crank and stop it from moving freely and so create a different standing wave
pattern when the crank is 'pinged'. the other main bearings in effect
divide the crank into a number of shorter cranks each with the ends
terminated, their own standing wave pattern and presumably a higher resonant
frequency.
the dampening effect of the big ends (imagine putting a finger on a
vibrating guitar string) would also vary the free air resonance of the
crank.
and don't forget all this is dynamic; subject to twisting, and bending.
a better way to see what frequencies are produced would be to run the engine
with some sort of microphonic device attached (piezo sensor perhaps) and
look at the output with an audio spectrum analyser.
might sound far fetched; but not beyond a basic home computer and some
software to do this.
derek [1960 herald, fan on water pump :) ]
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