In analyzing the vibration of a machine, which is a more or less complex mechanical system, it is useful to consider the sources of vibration energy and the paths in the machine that this energy takes. Energy always moves, or flows, from the source of the vibration to the energy absorber where it is converted into heat. In some cases, this may be a very short path, but in other situations, the energy may travel relatively long distances before being absorbed.
The most important absorber of energy in a machine is friction, which can be sliding friction or viscous friction. Sliding friction is represented by relative motion between parts of the machine, and an example of viscous friction is the oil film in a journal bearing. If a machine has very little friction, its vibration level tends to be fairly high, for the vibration energy builds up due to the lack of absorption. On the other hand, a machine with greater inherent friction will have lower vibration levels because the energy is absorbed quickly. For example, a machine with rolling element bearings (often called anti-friction bearings), generally vibrates more than a machine with sleeve bearings, where the oil film acts as a significant absorber of energy. The reason that airplane structures are riveted together rather than being welded into a solid unit is that the riveted joints move slightly, absorbing energy by sliding friction. This keeps vibrations from building up to destructive levels. Such a structure is said to be highly damped, and the damping is actually a measure of its energy absorption capability.
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