The first
process the data collected is put through is to convert what is an analog
signal (the transducer moving with the bearing) to a digital signal - seen
here. This is a "time domain" plot. Typically, the length of a time domain
plot will be very short - commonly in milliseconds. It is common to want
to capture 5-7 revolutions of a shaft. To capture 5 revolutions of a shaft
running (for instance) 3000 rpm (50Hz, or revolutions per second), you
would need 5/50 = 0.1 seconds = 100msecs. On that plot, you should be able
to see 5 sine waves for 5 revolutions of the shaft plus any wave shape
distortion (is it a perfect sine wave or an unusual shape) plus any higher
frequencies that might be occuring (electrical, bearings, gears).
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A 'Time Domain'
plot displays amplitude vs. time.
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However, unlike
a trend plot, the amplitude is a continuous representation of the amplitude
value.
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For instance,
if the amplitude unit for the above plot were displacement, the line would
represent the actual bearing location as it moves back and forth.
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Also unlike a
trend plot, the values can be negative or positive since, for instance,
the displacement can be on either side of a neutral, or 'at-rest' position,
and velocity or acceleration amplitudes can be in one direction or the
other (defined as the '+' and '-' directions depending on the direction
the transducer is pointing).
The time domain
is more difficult to analyze than the next plot we will discuss - the "Spectrum"
- but under certain conditions it can provide insights and information
not available on the spectrum plot. |
