When touching an operating machine, for instance a car or a lawnmower, you will feel vibration; this is a repetitive or “cyclical” back and forth movement.
Amplitude is a measure of the amount of movement, with the amount of movement is linked to the severity of the vibration.
Quantifying amplitude can be displayed as "displacement" – the measure of total distance the transducer travels back and forth during one full 'cycle' of movement, moving from one extreme to the other and back again to the starting point. "Velocity" is also defined, which is the maximum speed the transducer achieves during a cycle. The third unit, "Acceleration" measures the force that is causing the back and forth movement; defining the pushing action which is taking place.
The three units are all inter-related.
Displacement measures the length of the "trip" back and forth from (in this case) +X to -X (2X total distance).
This example shows the bearing at various points during a single cycle with the transducer oriented vertically.
We want to know the total length of the "trip" being made by the bearing (+X to -X = 2X distance).
This amount is a measure of the STRESS that the bearing structure is being subjected to - how much is it being bent back and forth ? It is sensitive to the likelihood of a stress failure occurring.
Since we want to know the total distance being moved (stress being endured) We use Peak-to-Peak signal detection.
Technically, velocity measures how much the displacement is changing over a period of time - inches or millimetres per second, which in the case of linear motion (e.g. a car on a highway), velocity can be fairly constant. With sinusoidal motion, however, the velocity is constantly changing as the displacement changes. As a result, measuring velocity amplitude over time generates a sine wave just as measuring displacement amplitude does.
Let's see how displacement and velocity relate to one another at significant points in the cycle:
The bearing is passing the "at-rest" position moving in the '+' direction:
The bearing has reached its maximum displacement in the '+' direction:
The bearing is passing the 'at-rest' position again moving in the '-' direction:
With displacement (stress), we were concerned with the total distance of the "trip". With velocity, we are only concerned with the maximum speed (velocity amplitude) reached during that trip - the direction is meaningless. Therefore...
We use Peak signal detection (not Peak to Peak).
Whereas displacement is a measure of the likelihood of a stess related failure...
Velocity measures the how often the stress is being applied (distance over time).
Velocity is measure of the likelihood of fatigue being the mode of failure.
Acceleration measures the rate of change of velocity. There are two things that will cause an object (bearing) to change velocity - a 'Pushing Action' similar to you pushing open a heavy door. If you were to push a pillow block bearing "X" distance from its 'at-rest' position, it would push back – it is this acceleration force that we are measuring.
A 'Striking Action' is more similar to hitting a nail with a hammer. This action can be extremely destructive since it can cause structural flaws (cracks, for instance) to develop. Consider a jack hammer. It is the striking action that breaks up the cement.
Let's see how displacement and acceleration relate to one another at significant points in the cycle:
The bearing has reached the '+X' position (the '+' displacement peak):
The bearing is passing the 'at-rest' position moving in the '-' direction.
As with velocity, we are only concerned with the maximum value reached - not the direction.
We use Peak signal detection.