Different
technologies can be applied for the maintenance of different category of asset.
We know that (Moubray’s PF chart) vibration is one of the earliest indicators
of start of machine failure. In this section we will cover the vibration
monitoring & analysis technology. Again human analogy becomes relevant to
the machine also. Machines are like human body. The human body has complex balance
system to maintain the internal environment of the body to a predetermined
level. This is known as homeostasis, for example it maintains the body
temperature around 98.6° Fahrenheit. Whenever there is some problem or a
breakdown in this regulatory system, this control mechanism is disturbed. This
is reflected in terms of rise in temperature, shivering etc. Similarly, if there
is some mechanical / electrical problem or a process disturbance, the machine
also expresses the problem in form of different variables like rise in
temperature, vibration, change in electrical parameters etc. Since we know that
vibration is one of the earliest indicators of the machine fault, it becomes
important for us to use the available technologies for early detection of
machine faults.
Vibration:
Everything in the nature, either static or dynamic vibrates. So, question is
what makes all these things to vibrate? The answer lies in the basic fundamentals
of Physics. Atoms are the basic building blocks of the matter and they have inherent
property of vibration. Vibration is the harmonic
motion of a machine or machine part in either side of its neutral or stationary
position & the response of a system to some internal or external excitation
/ force applied to it. So, the question is why different machines vibrate
differently? The answer lies in the machine design & fundamental conditions
like difference in mass, stiffness & damping. Even the same model &
same design machine behave differently under similar conditions. God has
created human beings & all are different from each other. Even two
identical twins have some difference & they behave differently. Similarly
no two machines can be same, even if all the design parameters are same &
this follows law of nature. This distinction makes the machine diagnostic discipline
not only a pure science but also an art which can be developed by the detailed
study of the discipline & wealth of experience on different set of
machines.
Units
of Measurement: Vibration can be measured into three units-
1. Displacement: The total distance travelled by the vibrating part, from one extreme
limit to the other. This can be explained by simple spring mass balance where displacement
is equal to the peak & bottom position travelled by the mass. The unit of
displacement is microns (Pk-Pk).
2. Velocity: Velocity is
the speed at which displacement occurs. Since the seed
is changing constantly, peak
or RMS velocity are usually selected & the
unit is mm/sec (Pk / RMS) & ips (inches per second)
3. Acceleration: Acceleration
is the rate of change of velocity. At the
extreme limit of travel of the vibrating part, acceleration is maximum or peak & most popular unit of acceleration
is - g’s (peak).
Integration of Acceleration = Velocity, Integration of Velocity = Displacement
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