Let's see how displacement and acceleration
relate to one another at the significant points in the cycle:
-
The bearing is shown here at its maximum
displacement of "-X". If you were pushing the bearing housing down to this
position, in which direction would the bearing housing be pushing back
?
-
The housing structure in this position
will be pushing back in the "+" direction since it is trying to return
the bearing to the "at-rest" position.
-
As mentioned before, the "amplitude"
we measure is a combination of many variables but what do we actually measure
?
-
The bearing is achieving a certain peak
velocity once per cycle. That velocity is a combination of the amount of
movement (displacement) and the time it takes for one complete cycle (from
which we calculate frequency). The less time a cycle takes, the higher
the frequency of the vibration and the more force it requires to generate
a particular peak velocity. In other words, going from 0 velocity to 1
in/sec (25 mm/sec) 1000 times a minute requires a lot less force than doing
it 100,000 times per minute. The forces being applied to make that happen
may destroy a component before metal fatigue (what velocity is sensitive
to) even becomes a factor.
-
That makes acceleration an amplitude
unit that is particularly sensitive to the likelihood of a component failing
due to the forces being applied to it due to the machine's operation.
-
When either displacement peak is reached,
an acceleration peak is reached in the opposite direction.
-
From the moment shown until the '0'
position is reached, the acceleration amplitude decreasing.
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