Vibration in diesel engine
Vibrations in diesel engine are most complex as magnitude and direction of forces creating the vibrations vary throughout one revolution.
Mathematical approach is necessary and prudent.
The firing forces in a slow running create large low frequency vibration.In contrast the blade of a turbocharger have high frequency but low magnitude vibration.Both can cause a components failure.
Tha failure that results on vibration is almost universally fatigue failure which accounts for the greatest (proportion ) machinery component failure in engineering components.
Vibrations can normally be separated into ( one ) of the two forms.One is NATURAL VIBRATION ,Which is a function of the material itself and resistance to movement.The other firm of vibration is the FORCED Vibration,Which is a result of the frequency with which applied force occurs.
E.g. :- A 6 cylinder engine Rotating at 100 R.O.M will have a frequency of 6 ×100 = 600 Hz.
The main problem arises when the natural and forced frequency coincide, Resonance is said to Occur.The Forcing Frequency acting at same time and in the same direction tends to amplify the natural frequency substantially to such an extent that the strength of the material may no longer be able to withstand the stressing.Ultimately fatigue failure occurs,with cracks passing through the material until insufficient area is left to carry the load and complete failure takes place.
The so called critical speed is that at which torsional forces created by the firing impulses and the reactions from the propellers synchronise with the natural frequency of the shaft system.