There are
several different ways to measure "how much". These different ways are
known as amplitude "units". The transducer is the mechanism we use to measure
vibration and, in the case of rolling element bearings (i.e. ball bearings,
etc.), we can assume the transducer, being affixed to the bearing housing,
is going to move very close to the same amount as the shaft itself since
a rolling element bearing has very small internal clearances. The first
amplitude unit is the simplest to visualize and understand:
-
Displacement
- measures the total distance the transducer (bearing) travels back and
forth during one 'cycle' of movement (a 'cycle' is the process of moving
from one extreme to the other and back again to the starting point).
To understand
the second amplitude unit, you must first understand that if a bearing
is going to move back and forth a certain distance in a certain amount
of time (the 'period'), it is going to achieve a certain maximum, or 'peak',
speed (velocity) during that cycle. That speed is constantly changing as
it goes from 0 (when the bearing is displaced a maximum amount in one direction
and has stopped momentarily to reverse direction) to some maximum value
it achieves as it passes the center point in the movement. Once it passes
that center point, the bearing starts slowing down until the speed again
reaches 0 as it reaches the maximum displacement in the opposite direction.
-
Velocity
- measures the maximum speed the transducer achieves during a cycle.
To understand
the third amplitude unit, you must understand that to change velocity,
a body must be accelerated or decelerated. To speed a car up, you press
the "accelrator" pedal. To slow it down, you remove your foot from the
accelerator and let frictional forces (wind resistance, road surface, brakes)
take over. On machines, this "acceleration force" can be visualized as
the reaction of the bearing housing and surrounding structure
to being pushed (displaced) in one direction - the housing pushes back
because it wants to assume a neutral, or "at-rest", position.
-
Acceleration
- measures the force(s) that are causing the back and forth movement.
Now let's look
at each of these units more in-depth and see how they are inter-related. |
