28-11-2012, 04:18 PM
VIBRATION APPLICATION DATA
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INTRODUCTION
This application note is intended to give the reader a basic grounding in the
causes and effects of vibration, along with methods for utilising the
information obtained from measuring vibration to diagnose machine health.
It will further discuss the merits of the various methods of monitoring
vibration and explain how these can be optimised to provide the best results.
Finally, the application notes will give some examples of how vibration
monitoring can be implemented.
WHAT IS VIBRATION?
Vibration is the response of a system to an internal or external stimulus causing it to
oscillate or pulsate.
While it is commonly thought that vibration itself damages machines and
structures, it does not. Instead, the damage is done by dynamic stress, which causes
fatigue of the materials; and the dynamic stresses are induced by vibration.
Amplitude of vibration is also dependent on the “dynamic resistance” of a system.
For example, if a machine is placed on rubber mounts, the amplitude of vibration
is likely to increase due to less dynamic resistance for the same imposed dynamic
forces. The transmission of vibration to the floor and surrounding structures will be
less, but the vibration within the machine will likely increase.
However, no additional damage will be done to the machine since the same forces
(and therefore, fatigue stresses) will remain the same within this machine (as
compared to when the machine was directly mounted to the floor).
What is vibration frequency and how does it relate to a time waveform?
The figure below shows how the frequency can be calculated from the
displacement waveform, by measuring the time period (T) of one cycle and
inverting to determine the frequency Hz. This is an example of a time waveform
which plots Vibration Amplitude versus Time. This waveform is a truly sinusoidal
waveform from which direct comparisons can be made between its Peak-to-Peak,
Peak and RMS amplitudes (see section 1.5).
What is Vibration Velocity?
The velocity of the vibration is a measure of the speed at which the mass is moving
or vibrating during its oscillations. The faster a machine flexes, the sooner it will fail
in fatigue. Vibration velocity is directly related to fatigue.
Note from the example of the oscillating mass suspended from a spring in Figure 3,
that velocity reaches its maximum value (or peak) at the neutral position where the
mass is fully accelerated (acceleration is zero) and now begins to decelerate as
shown in Figure 3. Velocity is expressed as millimetres per second (mm/sec).
What is Vibration Acceleration?
When a machine housing vibrates, it experiences acceleration since it continually
changes speed as it oscillates back and forth. Acceleration is greatest at the instant
at which velocity is at its minimum. That is, this is the point where the mass has
decelerated to a stop and is about to begin accelerating (moving faster) again in
the opposite direction. Acceleration is the rate of change in velocity and is normally
measured in units of g's (where 1g = 9.81ms–2). The greater the rate of change of
velocity, the greater will be the forces (and stresses) on this machine due to the
higher rate of acceleration. At high frequencies, failure of a machine may result
from excessive forces which break down the lubrication allowing the surface of
bearings to fail (due to metal-to-metal contact). These excessive forces are directly
proportional to acceleration (Force = mass x acceleration). Acceleration is probably
the most difficult measure of vibration amplitude to grasp, but this is the parameter
most often directly measured in the field with the use of an accelerometer. Thus, it
is important to gain a good understanding of it.