Vibration Engineering

Engr. Dennis Buenviaje

Machinery Vibration
• 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.
Cont

• 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.
6 common causes of machine vibrations

Excessive vibrations on rotating equipment like pumps, gearboxes,

turbines and compressors are a clear sign that the equipment is not
functioning properly. Equipment that is showing excessive vibrations
will most likely not achieve the expected lifespan, and can be the
source of unscheduled downtime or dangerous situations. Therefore, it
is important to find the root cause of vibrations by measuring and
analysing the vibration signals.

Alignment problems
• When two or more rotating machines are connected, the correct
alignment is crucial.
Typical alignment errors are:

• Parallel misalignment:
The shaft centre lines are parallel but are not in line. This can be both
horizontal and vertical. Parallel misalignment is also known as offset
misalignment.
• Angular misalignment:
The shafts meet at a point, but are not parallel. This can be both on the
horizontal and vertical axis. Angular misalignment is also known as gap
misalignment.
• Combined parallel-angular misalignment:
A combination of both parallel and angular misalignment. Combined
parallel-angular misalignment is the most common misalignment.
Unbalance
• When the centre of gravity of a rotating object is not exactly in the centre line, it causes
machine unbalance resulting in vibration. When a machine is unbalanced, it can cause
damage to the machine itself, the foundation, pipes, etc. There are three types of
unbalance: static unbalance, coupled unbalance and dynamic unbalance.

Resonance
• Every machine has one or more resonance frequencies (natural frequency). When a
rotation frequency coincides with the resonance frequency of the machine, resonance
occurs. Resonance can have major impact.
Loose parts
• Loose bearings, loose bolts and corrosion can cause the machine to vibrate excessively.
Due to the mechanical forces in the machine, loose parts can rapidly cause damage.
Cont…
Bearing damage
In rotating machinery, we come across two main types of bearings: roller bearings and sleeve
bearings.
• A roller bearing can be damaged in several ways, each with its own vibration fingerprint:
• Damage to the inner ring
• Damage to the outer ring
• Damage to the cage
• Damage to rolling elements (e.g. cylinders, cones and needles)
• Each part of a roller bearing has its own frequency. By calculating these frequencies, it is possible to
use vibration analysis to determine whether the vibrations are a result of bearing damage.
• In contrast to roller bearings, sleeve bearings do not use a rolling element, but use a fluid (oil) film to
reduce friction. Vibrations can be caused by inaccuracies in the fluid film; if a stable oil film cannot
be formed, it can break, resulting in an oil whip or oil whirl. Additionally, this type of bearing is more
sensitive to external influences on the position of the shaft, because its position in the bearing is not
fixed.
Cont…
Damaged or worn out gears
• Gearbox vibrations are often caused by damaged or worn out gear
teeth. When gear tooth engagement involves a damaged tooth, the
force cannot be transferred as with the other gear tooth
engagements. If a gear tooth is broken, less force can be transferred
at this point of the cycle. Vibrations occur as a result.
Corrective Methods
Almost all machine vibration problems can be corrected in place. The corrective methods are
well established and are described in depth in other writings.
• Disassembly, visual inspection, cleaning, and re-assembly can fix some elusive problems
without knowing what was really done because it may go back together differently.
• Bearing replacement
• Identifying other bad parts and replacing them
• Mass balancing
• Alignment
• Lubrication. Just greasing noisy bearings can quiet them, but changing the lubrication
schedule can extend their lives.
• Structural stiffening to raise natural frequencies.
• Mass loading and stiffening can reduce any measured vibration. They will lower motion, but
this may increase local stresses at the bearings resulting in faster bearing wear. This is to be
used only as a last resort when nothing else works and measured motion must be lowered.
Judgment in Machinery vibration Analysis
• First, not all machine vibration is bad. Some small amount of vibration is useful to overcome friction and to
pump lubricants around. Resonance is usually painted as a bad character, but resonance in the wind will do
no damage to the bearings. An example is a rooftop scrubber fan mounted on springs that shakes on a
windy day at the spring natural frequency. Flexible supports allow more motion, which measures bad, but
actually does less damage to the bearings because the oscillatory energy is converted into kinetic energy
of motion rather than high contact stresses at the bearings. Vehicle wheels are a good example of this.
They are flexibly mounted on springs which allows plenty of motion, but it does less damage to the tires or
the road. We will see more compliant-mounted bearings in the future.
• A second judgment criteria is the relative change. A 2x increase (or 6 dB) above a baseline normal level
indicates that a significant physical change has occurred. A 5x increase is serious. Machine vibrations are
rarely self-healing, but it has happened where a machine has worn itself a clearance and the vibration level
dropped.
• A third judgment criteria is the rate of change. Slowly changing vibrations over a long time period (weeks)
are wear related and are not cause for alarm if lubrication is present and the temperature is stable. It will
wear more and become noisier, but it will still carry the load if lubrication is present. One strategy to nurse
a machine to the next convenient outage is to grease it every day. A high, but stable, vibration amplitude is
of concern but not an emergency if the temperature is stable. A rise in temperature at the bearings is
cause for immediate action because friction is being generated with its pernicious side effect of thermal
expansion and risk of seizure.
Cont…
• A fourth judgment criteria is operational condition. The load level has a dramatic effect on vibration
levels, especially electrical machines such as generators. It is usually assumed in analysis that the
machine has no operational problems, such as:
1. misfiring of engines
2. single phasing of motors
3. poor flow conditions of fans and pumps If no other cause of vibration is obvious, it may be time to
re-examine the operating condition of the machine.
• A fifth judgment criteria is to visualize the flow of vibratory energy. We can establish the source of
energy from the frequency signature. This tells me where it is produced and I know where it is being
measured. What lies in between? Visualize the path of vibratory energy and what components are
exposed to those oscillating forces. Finally, where is this vibratory energy dissipated?
• A sixth judgment criteria is how long the machine needs to run before a replacement comes along.
Vibration is an accumulated wear mechanism that causes fatigue failure over some time period or
number of cycles. One purpose for reducing vibration is to extend the life of machines. If this
particular machine needs to run only two more weeks before it will be replaced, then higher
vibration levels can be tolerated for a short time with low risk of failure.
REFERENCES
• https://www.reliableplant.com/Read/24117/introduction-machinery
• Engineer's Guide to Accurate Sensor Measurements