01-05-2012, 12:07 PM
INTRODUCTION TO VIBRATION TECHNOLOGY
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VIBRATION AS AN INDICATOR OF MACHINERY CONDITION
How many times have you touched a machine to see if it was "running right"? With experience, you have
developed a "feel" for what is normal and what is abnormal in terms of machinery vibration. Even the
most inexperienced driver knows that something is wrong when the steering wheel vibrates or the engine
shakes. In other words, it's natural to associate the condition of a machine with its level of vibration.
Of course, it's natural for machines to vibrate. Even machines in the best of operating condition will have
some vibration because of small, minor defects. Therefore, each machine will have a level of vibration that
may be regarded as normal or inherent. However, when machinery vibration increases or becomes
excessive, some mechanical trouble is usually the reason. Vibration does not increase or become excessive
for no reason at all. Something causes it - unbalance, misalignment, worn gears or bearings, looseness, etc.
WHAT IS VIBRATION?
Vibration can be defined as simply the cyclic or oscillating motion of a machine or machine component
from its position of rest.
WHAT CAUSES VIBRATION?
Forces generated within the machine cause vibration. These forces may:
1. Change in direction with time, such as the force generated by a rotating unbalance.
2. Change in amplitude or intensity with time, such as the unbalanced magnetic forces generated in an
induction motor due to unequal air gap between the motor armature and stator (field).
3. Result in friction between rotating and stationary machine components in much the same way that
friction from a rosined bow causes a violin string to vibrate.
4. Cause impacts, such as gear tooth contacts or the impacts generated by the rolling elements of a
bearing passing over flaws in the bearing raceways.
5. Cause randomly generated forces such as flow turbulence in fluid-handling devices such as fans,
blowers and pumps; or combustion turbulence in gas turbines or boilers.
VIBRATION AS A PREDICTIVE MAINTENANCE TOOL
There are many machinery parameters that can be measured and trended to detect the onset of problems.
Some of these include:
1. Machinery vibration
2. Lube oil analysis including wear particle analysis
3. Ultrasonic (thickness) testing
4. Motor current analysis
5. Infrared thermography
6. Bearing temperature
In addition, machinery performance characteristics such as flow rates and pressures can also be monitored
to detect problems. In the case of machine tools, the inability to produce a quality product in terms of
surface finish or dimensional tolerances is usually an indication of problems. All of these techniques have
value and merit. However, the one characteristic that is common to practically all machines is
VIBRATION.
SUMMARY
Regardless of the application, - predictive maintenance, quality control, field service, etc. - vibration
detection and analysis techniques do work. Making them work requires the proper vibration measurement
and analysis equipment. Equally important, if not more so, is the vibration technician - an individual
properly trained to use the equipment effectively.