Understanding pump vibration

Vibration is the continuous back and forth motion of an object from its neutral
position in reaction to an external force.

A simple example of vibration is when a mass is suspended on a metal spring. If
force is applied to the mass to pull it down, and then the mass is released, it will
move up and down repeatedly from its neutral position.

Now imagine the same mass but, instead of straight down, it is pulled at an angle
from the vertical position. The mass will move not only up and down, but also
sideways, to and pro, like a pendulum. In other words, the mass will vibrate both
vertically and horizontally. The same is true if it is pulled down and the spring is
twisted before it is released - it will vibrate in different directions.

If force is re-applied repeatedly the mass will continue to vibrate indefinitely until
the spring breaks due to fatigue failure.

A centrifugal pump, no matter how well-built, will always vibrate. It is the severity of
the vibration that is important. Its vibration is a complex extension of our simple
mass-and-spring example. A pump has spring-like properties and a suspended
mass - its shaft (or rotor). Its  vibration results from the displacement of its shaft
from its neutral position due to some external forces generated when the shaft
rotates. Due to the complexity of these forces, the shaft displacement occurs in
horizontal, vertical, and axial directions. If the vibration occurs indefinitely the pump
will fail prematurely depending on the severity of the vibration.

When dealing with vibration, it is important to know its severity (or magnitude) and
its frequency. Severity is a measure of how smooth or rough a vibration is, and
frequency is an indicator of what is, or are, causing the vibration.

There are many ways of measuring severity, depending on the instrument used -
it can be measured in terms of amplitude (displacement), velocity, or acceleration.

Amplitude or displacement is measured in  mils peak-to-peak.
Velocity is measured in inch per second (ips)
Acceleration is measured in gs (gravity)

Vibration readings can be either filtered (discreet), or unfiltered (overall). Filtered
vibration refers to the vibration at a specific frequency, whereas unfiltered is the
overall vibration resulting from the combination of vibrations at different
frequencies.

Vibrations in pumps with ball bearings are measured on the bearing housing,
perpendicular to the shaft, in inch per second (ips) unit, or in acceleration unit (g).

Vibrations in pumps with sleeve bearings are measured on the shaft, also in
perpendicular direction.

Common causes of pump vibration:

• unbalance of rotating parts
• misalignment of shaft, coupling, or bearings
• rubbing, or looseness of parts
• interference or eccentricity
• bent shaft
• defective bearings
• oil whip, or whirl
• acoustic, mechanical or structural resonance
• hydraulic forces
• torque variations
• aerodynamic forces
• dirt caught between mating parts

In order to identify the cause, or causes, of vibration it is important to know its
magnitude, direction, and frequency.

R: 0110-PUVI
C: operation, trouble-shooting
F: vibration basic

"Make it simple"

Related topic:
Vibration and lubricant
CENTRIFUGAL-PUMP.ORG