Wilcoxon Sensing Technologies Tel: +1 (301) 330 8811
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Frederick, MD 21701 www.wilcoxon.com
Application guide for PC420V Series LPS™ transducers
Introduction
With the availability of 4-20 mA vibration transducers, plant personnel can now input vibration
information directly to their Programmable Logic Controller (PLC) or Distributed Control
System (DCS). This capability allows plant personnel to trend overall vibration data on their
machines, correlate vibration data to plant operating conditions, and allows plant operators to
schedule maintenance tasks on machinery. However, just as other process information (such
as temperature, pressure or flow) will have limits, so also does vibration. The ISO 10816-
1:1995(E) standard can provide guidance in establishing or setting vibration limits for
machinery.
Wilcoxon’s loop powered sensors (LPS™) are self-contained 4-20 mA vibration transmitters.
They are purchased with a specific full-scale setting. The full-scale is not adjustable by the
user. The availability of a standard for vibration measurement and condition evaluation can
help to determine the best full-scale range to use in any particular monitoring environment. It
can also aid in evaluating the severity of a machine’s vibration condition.
This application guide will: review the background of machinery vibration, 4-20 mA loop
sensors, and the ISO10816-1:1995(E) standard; offer help in selecting the proper range
transducer; elaborate on installation guidelines; and provide guidance with setting vibration
limits.
It is not the purpose of this document to provide detailed instructions for analyzing machinery
vibration problems. There are many other useful publications that can aid in diagnosing
vibration problems. Links to these resources can be obtained from the Wilcoxon website at
www.wilcoxon.com or by calling Wilcoxon at 301-330-8811.
Machinery vibration
What is machinery vibration? Why does a machine have vibration? It seems as though every
treatment of vibration analysis or predictive maintenance starts with these questions. There is
a very good reason why that is so. If there is no meaningful purpose or cause for vibration then
there is no reason to bother with monitoring or measuring it. Fortunately, there are very good
reasons to measure and trend vibration.
The first question is often answered by reference to one definition or another of vibration. Here
is the definition from the Merriam-Webster Collegiate Dictionary (1996):
2
vi•bra•tion, noun (1655)
1a : a periodic motion of the particles of an elastic body or medium in alternately opposite directions from
the position of equilibrium when that equilibrium has been disturbed (as when a stretched cord produces
musical tones or particles of air transmit sounds to the ear)
b : the action of vibrating : the state of being vibrated or in vibratory motion: as
(1) : oscillation
(2) : a quivering or trembling motion : quiver
In the illustration to the right, the force applied to the “base”
translates to the “Mass” through the spring. Consider the
analogy of an automobile: if the spring is the suspension of the
auto and the if the force were acting on the tires, then the
system response would be the motion of the automobile “Mass”
over the road. Bumps in the pavement cause the auto to vibrate
in response to various pavement changes.
Machines have vibration because there is always some residual imbalance in the rotors of
motors and shafts of machines. That imbalance will act just the same as an eccentric cam on
the base of our simple spring-mass system. The imbalance will result in a sinusoidal vibration.
The rotating shaft is being held in place by the bearings the machine. Because the machine
shaft is turning, the vibration is repeating at the rate the shaft turns. This repetitious vibration is
considered to be “periodic.”
The machine bearing also acts like a spring-mass system to transmit the vibration to the
machine case. The bearing has some amount of stiffness so the shaft will be allowed to move.
The forces generated by the residual imbalance will be transmitted through the bearing
housing. The resulting motion can be measured using vibration transducers. When the signal
is observed using an oscilloscope or spectrum analyzer, it will be found to be essentially a
sinusoidal signal.
To summarize, all machines vibrate because of residual imbalance in the rotating component
(rotor or shaft). That imbalance translates into a rotating force vector that imposes itself onto
the bearing. The bearing responds by translating the force into motion because of stiffness in
the bearing housing and support structure. The resulting motion can be measured using
vibration transducers.
Other problems in the machine will also evidence themselves as vibration:
Misalignment
Bearing wear
Faulty bearing components
Rotor and air gap problems
Gear faults
Coupling faults
Mechanical looseness
Belt drive problems