﻿ Amplitude Characteristics - What Does Amplitude Tell Us?

# Amplitude Characteristics

## What Does Amplitude Tell Us ?

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.

## What Are Amplitude 'Units' ?

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.

### Measuring Displacement Units

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.

1. The "At-Rest" position ('0') is the position the bearing would assume if the machine were de-energised
2. We'll begin with the bearing just passing the '0' point moving towards '+X'
3. The red ball indicates the amplitude level during the cycle
4. The bearing cap reaches the "X" position in both '+' and '-' directions - 1x RPM

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.

## Measuring Velocity Units

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.

## Velocity v Displacement

Let's see how displacement and velocity relate to one another at significant points in the cycle:

• The bearing is shown here at its maximum displacement of '-X'. What must the speed (velocity) be at this moment?
• Velocity must be zero since it must stop momentarily as it reverses direction
• From this moment forward until the 'at-rest' (0) position is reached, the bearing will be speeding up (the velocity amplitude will be increasing)

The bearing is passing the "at-rest" position moving in the '+' direction:

• The displacement amplitude is '0'
• Velocity is at a '+' peak since it is moving at its greatest speed in the '+' direction
• From this moment until the '+X' position is reached, the bearing will be slowing down (velocity amplitude will be decreasing)

The bearing has reached its maximum displacement in the '+' direction:

• Displacement is '+X'
• Velocity is again at 0 since it has momentarily stopped to reverse direction
• From this moment until the bearing passes the 'at-rest' position, the bearing will be speeding up (velocity amplitude increasing)

The bearing is passing the 'at-rest' position again moving in the '-' direction:

• The displacement amplitude is '0'
• Velocity is at a '-' peak since it is moving at its greatest speed in the '-' direction
• From this moment until the '-X' position is reached, the bearing will be slowing down (velocity amplitude will be decreasing)

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.

## Measuring Acceleration Units

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.

## Acceleration vs. Displacement

Let's see how displacement and acceleration relate to one another at 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 would push back (in the '+' direction) since it is trying to return the bearing to the "at-rest" position. That is a reactive force (acceleration) - it occurs in response to being displaced in one direction
• 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. The bearing is passing the 'at-rest' position moving in the '+' direction
• As the bearing passes '0', the acceleration force (amplitude) passes 0 since the bearing is located where it wants to be
• From this moment until the '+X' position is reached the bearing acceleration amplitude is increasing to a peak value in the '-' direction (remember, as it is being displaced increasingly in the '+' direction, it is increasingly pushing back towards the "at-rest" position)

The bearing has reached the '+X' position (the '+' displacement peak):

• The acceleration force (amplitude) is at a maximum pushing DOWN towards the '0' position (it's '-' peak)
• From this moment until the '0' position is reached the bearing acceleration amplitude is decreasing as the bearing approaches its '0' position (at-rest)

The bearing is passing the 'at-rest' position moving in the '-' direction.

• As the bearing passes '0', the acceleration force (amplitude) passes 0 since the bearing is located where it wants to be
• From this moment until the '-X' position is reached the bearing acceleration amplitude is increasing to a peak value in the '+' direction (remember, as it is being displaced increasingly in the '-' direction, it is increasingly pushing back towards the "at-rest" position)

As with velocity, we are only concerned with the maximum value reached - not the direction.

We use Peak signal detection.

### Further Information

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