Page 37 - Shock and Vibration Overview
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Analysis Overview
Figure 19: A car’s engine during idle clearly shows the 30 Hz dominate frequency which equals
twice the crank shaft rotation frequency of 15 Hz (900 RPM) where a peak is also visible.
We can use spectrum analysis of the vibration profile to indicate what the engine’s
crank shaft rotation speed was. This is a 4-cylinder 4-cycle engine. The engine
operates with two pairs of pistons moving out of phase with each other and two
piston combustions per crank shaft rotation; so the dominant frequency of the
engine’s vibration will be twice the crank shaft rotation speed (here’s a nice video on
how a 4-stroke engine works). In the FFT there is clearly a dominate frequency at 30
Hz or 1,800 RPM which tells us that at idle the crank shaft is rotating at 900 RPM (or
15 Hz) where there is also a peak in the FFT. The use of an FFT in our vibration
analysis gave clues on what was causing the measured vibration.
In many applications the vibration frequency will change with time and you can run
into trouble if you only look at the FFT. Let's zoom out of the area where the car
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