Basic Concepts of Vibration
What Causes Machine Vibration?
Almost all machine vibration is due to one or more of these causes:
(a) Repeating forces
(b) Looseness
(c) Resonance
Why is Vibration Monitoring important?
Any motion that repeats itself after an interval of time is called vibration or oscillation.
The swinging of a pendulum and the motion of a plucked string are typical examples of
vibration. The theory of vibration deals with the study of oscillatory motions of bodies and
the forces associated with them.
A vibratory system, in general, includes a means for storing potential energy (spring or
elasticity), a means for storing kinetic energy (mass or inertia), and a means by which
energy is gradually lost (damper).
The vibration of a system involves the transfer of its potential energy to kinetic energy
and of kinetic energy to potential energy, alternately. If the system is damped, some energy
is dissipated in each cycle of vibration and must be replaced by an external source if a state
of steady vibration is to be maintained.
What is Vibration ?
Vibration is a physical movement or oscillation of a Mechanical part about a reference position
The cyclic or
oscillation motion of a machine or machine component from its position of
rest.
Vibration is the
response of a system to an internal or external stimulus causing it to
oscillate or pulsate.
A measure of response
of bearing house to internal or external forces causing it to vibrate.
What Causes Machine Vibration?
Almost all machine vibration is due to one or more of these causes:
(a) Repeating forces
(b) Looseness
(c) Resonance
Why is Vibration Monitoring important?
We can hear
vibration! Ø Vibration can generate sound!
We can sense
vibration! Ø Vibration can generate heat!
We can see vibration! Ø Vibration can move objects!
“Vibrations and high
Dynamic is a symptom”
The aim is to find the root cause for having a longer life cycle of the machines
The aim is to find the root cause for having a longer life cycle of the machines
Components in a vibrating system have three properties of interest. They are : MASS (weight), ELASTICITY(springiness) and DAMPING (dissipation). Most physical objects have all three properties, but in many cases one or two of those properties are relatively insignificant and can be ignored (for example, the damping of a block of steel, or in some cases, the mass of a spring).
The property of mass (weight) causes an object to resist acceleration. It also enables an object to store energy, in the form of velocity (kinetic) or height (potential). You must expend energy to accelerate a mass or to lift a mass. If you decelerate a moving mass, or drop a lifted mass, the energy it took to accelerate it or to lift it (as applicable) can be recovered.
The property of elasticity enables an object to store energy in the form of deflection. A common example is a spring, but any piece of metal has the property of elasticity. That is, if you apply two equal and opposite forces to opposite sides of it, it will deflect. Sometimes that deflection can be seen; sometimes it is so small that it can't be measured with a micrometer. The size of the deflection depends on the size of the applied force and the dimensions and properties of the piece of metal. The amount of deflection caused by a specific force determines the "spring rate" of the metal piece. Note that all metals (in the solid state) have some amount of elasticity.
You must expend energy to deflect a spring. The spring stores most of the energy required to deflect it. When you release a deflected spring, the stored energy can be recovered.
The term damping is frequently misunderstood. The property of damping enables an object to DISSIPATE energy, usually by conversion of kinetic (motion) energy into heat energy. The misnamed automotive device known as a "shock absorber" is a common example of a damper. If you push on the ends of a fully extended "shock absorber" (so as to collapse it) the rod moves into the body at a velocity related to how hard you are pushing. Double the force and the velocity doubles. When the "shock" is fully collapsed, and you release your hand pressure, nothing happens (except maybe you drop it). The rod does not spring back out. The energy (defined as a force applied over a distance) which you expended to collapse the damper has been converted into heat which is dissipated through the walls of the shock absorber.
The RESONANT FREQUENCY of an object (or system) is the frequency at which the system will vibrate if it is excited by a single pulse. As an example, consider a diving board. When a diver bounces on the end of the board and commences a dive, the board will continue to vibrate up and down after the diver has left it. The frequency at which the board vibrates is it’s resonant frequency, also known as it’s natural frequency. Another example is a tuning fork. When struck, a tuning fork "rings" at it’s resonant frequency. The legs of the fork have been carefully manufactured so as to locate their resonant frequency at exactly the acoustic frequency at which the fork should ring.
A WAVEFORM is a pictorial representation of a vibration. For illustration purposes, Figure 1 shows waveform representing the instantaneous torque of a typical 8-cylinder engine.
The term FREQUENCY occurs often in the discussion of vibration. The frequency of a waveform (torque variation, in this case) is the number of times per second that the waveform repeats itself, while the order of a waveform is the number of times that the waveform repeats itself during a particular event (such as one revolution of a crankshaft).
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