Equipment Vibration – The need for Tribology
Bill Lucas 2011
Conference for Industrial Tribologists
Conference for Industrial Tribologists
Vibration – Good or bad?
In simplest
terms, vibration in motorized equipment is merely the back and forth movement
or oscillation of machines and components, such as drive motors, driven devices
(pumps, compressors and so on) and the bearings, shafts, gears, belts and other
elements that make up mechanical systems.
Vibration in
industrial equipment can be both a sign and a source of trouble. Other times,
vibration just “goes with the territory” as a normal part of machine operation,
and should not cause undue concern. But how can the plant maintenance
professional tell the difference between acceptable, normal vibration and the
kind of vibration that requires immediate attention to service or replace
troubled equipment?
With a basic
understanding of vibration and its causes – and equipped with a vibration
testing device – the maintenance professional can quickly and reliably
determine the cause and severity of most machine vibration and receive
recommendations for repair. It’s all done with the intelligence built into the
tester, without the extensive monitoring and recording required for typical,
long-term vibration monitoring programs.
Vibration is
not always a problem. In some tasks, vibration is essential. Machines such as
oscillating sanders and vibratory tumblers use vibration to remove materials
and finish surfaces. Vibratory feeders use vibration to move materials. In
construction, vibrators are used to help concrete settle into forms and compact
fill materials. Vibratory rollers help compress asphalt used in highway paving.
In other
cases, vibration is inherent in machine design. For instance, some vibration is
almost unavoidable in the operation of reciprocating pumps and compressors,
internal combustion engines, and gear drives. In a well-engineered,
well-maintained machine, such vibration should be no cause for concern.
When
vibration is a problem
Most industrial devices are engineered to
operate smoothly and avoid vibration, not produce it. In these machines,
vibration can indicate problems or deterioration in the equipment. If the
underlying causes are not corrected, the unwanted vibration itself can cause
additional damage.
In this
article, we are focused not on machines that are “supposed” to vibrate as part
of normal operation, but on those that should not vibrate: electric motors,
rotary pumps and compressors, and fans and blowers. In these devices, smoother
operation is generally better, and a machine running with zero vibration is the
ideal.
Most common causes of machine vibration
Vibration can result from a number of
conditions, acting alone or in combination. Keep in mind that vibration
problems may be caused by auxiliary equipment, not just the primary equipment.
These are some of the major causes of vibration.
Imbalance –
A “heavy spot” in a rotating component will cause vibration when the unbalanced
weight rotates around the machine’s axis, creating a centrifugal force.
Imbalance could be caused by manufacturing defects (machining errors, casting
flaws) or maintenance issues (deformed or dirty fan blades, missing balance
weights). As machine speed increases, the effects of imbalance become greater.
Imbalance can severely reduce bearing life as well as cause undue machine
vibration.
Misalignment/shaft
runout – Vibration can result when machine shafts are out of line. Angular
misalignment occurs when the axes of (for example) a motor and pump are not
parallel. When the axes are parallel but not exactly aligned, the condition is
known as parallel misalignment. Misalignment may be caused during assembly or
develop over time, due to thermal expansion, components shifting or improper
reassembly after maintenance. The resulting vibration may be radial or axial
(in line with the axis of the machine) or both.
Wear – As
components such as ball or roller bearings, drive belts or gears become worn,
they may cause vibration. When a roller bearing race becomes pitted, for
instance, the bearing rollers will cause a vibration each time they travel over
the damaged area. A gear tooth that is heavily chipped or worn, or a drive belt
that is breaking down, can also produce vibration.
Looseness –
Vibration that might otherwise go unnoticed may become obvious and destructive
if the component that is vibrating has loose bearings or is loosely attached to
its mounts. Such looseness may or may not be caused by the underlying
vibration. Whatever its cause, looseness can allow any vibration present to
cause damage, such as further bearing wear, wear and fatigue in equipment
mounts and other components.
Effects
of vibration
The effects of vibration can be severe.
Unchecked machine vibration can accelerate rates of wear (i.e. reduce bearing
life) and damage equipment. Vibrating machinery can create noise, cause safety
problems and lead to degradation in plant working conditions. Vibration can
cause machinery to consume excessive power and may damage product quality. In
the worst cases, vibration can damage equipment so severely as to knock it out
of service and halt plant production.
Yet there is
a positive aspect to machine vibration. Measured and analyzed correctly,
vibration can be used in a preventive maintenance program as an indicator of
machine condition, and help guide the plant maintenance professional to take
remedial action before disaster strikes.
Characteristics
of vibration
To understand how vibration manifests itself,
consider a simple rotating machine like an electric motor. The motor and shaft
rotate around the axis of the shaft, which is supported by a bearing at each
end.
One key
consideration in analyzing vibration is the direction of the vibrating force.
In our electric motor, vibration can occur as a force applied in a radial
direction (outward from the shaft) or in an axial direction (parallel to the
shaft).
An imbalance
in the motor, for instance, would most likely cause a radial vibration as the
“heavy spot” in the motor rotates, creating a centrifugal force that tugs the
motor outward as the shaft rotates through 360 degrees. A shaft misalignment
could cause vibration in an axial direction (back and forth along the shaft
axis), due to misalignment in a shaft coupling device.
Another key
factor in vibration is amplitude, or how much force or severity the vibration
has. The farther out of balance our motor is, the greater its amplitude of
vibration. Other factors, such as speed of rotation, can also affect vibration
amplitude. As rotation rate goes up, the imbalance force increases
significantly.
Frequency
refers to the oscillation rate of vibration, or how rapidly the machine tends
to move back and forth under the force of the condition or conditions causing
the vibration.
Frequency is
commonly expressed in cycles per minute or hertz (CPM or Hz). One Hz equals one
cycle per second or 60 cycles per minute. Though we called our example motor
“simple”, even this machine can exhibit a complex vibration signature. As it
operates, it could be vibrating in multiple directions (radially and axially),
with several rates of amplitude and frequency. Imbalance vibration, axial
vibration, vibration from deteriorating roller bearings and more all could
combine to create a complex vibration spectrum.
Conclusion
Vibration is a characteristic of virtually all
industrial machines. When vibration increases beyond normal levels, it may
indicate only normal wear – or it may signal the need for further assessment of
the underlying causes, or for immediate maintenance action.
Understanding
why vibration occurs and how it manifests itself is a key first step toward
preventing vibration from causing trouble in the production environment.[1]
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