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
oil whip, or whirl
acoustic, mechanical or structural resonance
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.