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Vibration is the periodic back and forth motion of the particles of an elastic body or medium. It usually arises when a physical system is displaced from its equilibrium condition and allowed to respond to the forces that tend to restore equilibrium.
The motion may be periodic, as in the motion of a struck pendulum. Or the motion can be completely random, such as the movement of a tire rolling along a rough road.
As defined in physics, vibration is the oscillating, reciprocating, or other periodic motion of a rigid or elastic body or medium forced from a position or state of equilibrium.
In some cases, vibrations can be advantageous'as with the reed in a woodwind musical instrument or in a vibratory parts hopper that dispenses discrete items from a funneled drum. But in many cases vibration is destructive. It can waste energy and create imbalances, friction, and failure in mechanical devices.
Vibration and sound studies are closely associated. Sound waves are generated by vibrating structures. These pressure waves also induce the vibration of structures or systems. Attempts to reduce unwanted noise are generally related to issues of vibration.
Free vibrations occur when a system is momentarily disturbed and free to move without restraint.
Consider a classic example such as when a weight is suspended from a spring. In equilibrium, the system has minimum energy and the weight is at rest. In contrast, if an external force pulls the weight down and releases it, the system will respond by vibrating vertically.
Consider another example of a child pulling back on a swing and letting go. The mechanical system vibrates at one or more of its natural frequencies and eventually damps down to motionlessness.
Forced vibration is when energy is added to a mechanical system'as in a system that is continuously driven by an external agency.
The periodic input can be a harmonic or a non-harmonic disturbance. A simple example is a child's swing that is pushed on each downswing. Of special interest are systems undergoing simple harmonic motion and driven by sinusoidal forcing.
This causes the phenomenon of resonance. Resonance occurs when the driving frequency approaches the natural frequency of free vibrations. The result is a rapid take-up of energy by the vibrating system, with an attendant growth of the vibration amplitude.
Damped vibration is the depletion of energy. When a vibrating system is damped there is energy loss.
Depending on the motion inputs and the application's modes of vibration, damping can be through dry-friction interfaces, mechanical devices, and arrangements that use viscous damping, and the integration of elastomeric engineered materials.
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Viscoelastic materials damp vibration through a mechanism known as hysteretic damping. As these materials are distorted, internal friction causes energy loss.
Damping vibrations help manufacturers minimize equipment downtime and boost system efficiencies.
Vibration damping can happen through proper implementation of viscoelastic components. When vibrations are damped, they are removed from the system in a couple of ways. They can be absorbed into materials that release vibration energy as small amounts of heat energy, or they can be changed to different frequencies that don't resonate with the parts of the machine.
Viscoelastic materials are often used for vibration damping. Here are a few of the most common uses.
A pad of viscoelastic material is created for moving parts of the machine. The pad sits between the parts that are causing excess vibrations.
As the parts move, it conforms to the movement to absorb some of the vibration energy. It then stores that energy and releases it as heat energy. This removes vibration from the system and keeps the machine from breaking down or making too much noise. Unconstrained damping is by far the simplest way to address vibration in machinery.
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In this type of vibration damping, the viscoelastic damping material is lined with a thin piece of flexible metal. It works similarly to unconstrained damping, but it can be more efficient.
Constrained damping in this may be used for machines that need close control of vibrations.
Directed damping, designed to keep specific wavelengths of vibrations out of a system. This is often because of the need to keep resonant frequencies from causing serious damage to machinery or motors.
Damping vibration is the dissipation of energy. Isolating vibration is the prevention of vibration transmission.
Damping is a way to limit vibrations and is essential for protecting the system in which it operates.
This is what happens with door or drawer springs, where damping prevents blows when opened/closed, preserving them and protecting the system. While on a larger scale, bridge deck damping systems have the same purpose.
Supposing there is a dynamic sinusoidal load exciting a building structure or a structure with an exact frequency, what could happen? In theory, movements would become increasingly bigger and the structure would eventually collapse. This is what happened to the Tacoma Bridge in the United States, in , a few months after its inauguration.
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