Failure
As mentioned, wear is a concern for moving parts in a machine. Other concerns that lead to failure include corrosion, erosion, thermal stress and heat generation, vibration, fatigue loading, and cavitation.
Fatigue is related to large inertial forces, and is affected by the type of motion that a moving part has. A moving part that has a uniform rotation motion is subject to less fatigue than a moving part that oscillates back and forth. Vibration leads to failure when the forcing frequency of the machine's operation hits a resonant frequency of one or more moving parts, such as rotating shafts. Designers avoid this by calculating the natural frequencies of the parts at design time, and altering the parts to limit or eliminate such resonance. Yet further factors that can lead to failure of moving parts include failures in the cooling and lubrication systems of a machine.
One final, particular, factor related to failure of moving parts is kinetic energy. The sudden release of the kinetic energy of the moving parts of a machine causes overstress failures if a moving part is impeded in its motion by a foreign object, such as stone caught on the blades of a fan or propellor, or even the proverbial "spanner/monkey wrench in the works". (See foreign object damage for further discussion of this.)
Read more about this topic: Moving Parts
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