REASONS FOR COMPOUNDING OF MACHINERY VIBRATION
There are primarily 3 excitation sources of vibration on a ship:-
a) Wave forces
b) Machinery generated vibration:
1) Primary excitation - Which are forces and moments originating from combustion pressure and inertia forces of rotating and reciprocating masses.
2) Secondary excitation -- Stemming from a forced vibratory response in a substructure.
c) Propeller induced excitation :
1) As a result of unbalance caused by damage
2) Hydrodynamic excitation due to uneven flow of water to propeller.
If the forcing frequency of the externally applied vibration coincides with the natural frequency of the hull, resonance occurs and considerable damage can result by the compounding of overlapping excitation amplitudes.
For avoiding resonance either the frequency of the exciting external force or the natural frequency of the hull must be changed.
The vibration characteristic of low speed 2 stroke engines can be split into 4 categories:-
1) External unbalance moments:- Caused by the inertia forces origination from the rotating and reciprocating masses. The addition of counterweights and moment compensators yield good result with respect to such external unbalanced moments.
2) Guide force moments :- Caused by transverse reaction forces acting on the cross head due to connecting rod and crankshaft mechanism. Installation of the top bracing between the engine upper platform brackets and the side casing increases frequency of resultant vibration to a level where resonance will occur above normal engine speed.
3) Axial vibration :- When the crank throw is loaded by the gas pressure through the connecting rod mechanism, the arms of the crank throw deflect in axial direction of the crankshaft. The effect is minimized by the addition of an axial vibration damper.
4) Torsional vibration :- Resulting from varying gas pressure in the cylinders during working cycle and the crankshaft/ connecting rod mechanism. Torsional vibration can be dealt with introduction of tuning wheel, increase in diameter of shaft at design stage and use of material with high tensile strength.
The natural frequency of hull depends on its rigidity and distribution of masses while vibration level at resonance depends mainly on the magnitude of the external moments and the engine position in relation to the vibration of the ship.
The magnitude of forced excitation due to propeller can be reduced by better wake distribution and aft body configuration . Accordingly critical speed of engine is adjusted with natural frequency.The natural frequency may be so adjusted that resonance occurs 30-70% below specified engine speed at MCR. Conversely natural frequency may be adjusted such that resonance occurs at 35-45% above engine speed at MCR.
a) Wave forces
b) Machinery generated vibration:
1) Primary excitation - Which are forces and moments originating from combustion pressure and inertia forces of rotating and reciprocating masses.
2) Secondary excitation -- Stemming from a forced vibratory response in a substructure.
c) Propeller induced excitation :
1) As a result of unbalance caused by damage
2) Hydrodynamic excitation due to uneven flow of water to propeller.
If the forcing frequency of the externally applied vibration coincides with the natural frequency of the hull, resonance occurs and considerable damage can result by the compounding of overlapping excitation amplitudes.
For avoiding resonance either the frequency of the exciting external force or the natural frequency of the hull must be changed.
The vibration characteristic of low speed 2 stroke engines can be split into 4 categories:-
1) External unbalance moments:- Caused by the inertia forces origination from the rotating and reciprocating masses. The addition of counterweights and moment compensators yield good result with respect to such external unbalanced moments.
2) Guide force moments :- Caused by transverse reaction forces acting on the cross head due to connecting rod and crankshaft mechanism. Installation of the top bracing between the engine upper platform brackets and the side casing increases frequency of resultant vibration to a level where resonance will occur above normal engine speed.
3) Axial vibration :- When the crank throw is loaded by the gas pressure through the connecting rod mechanism, the arms of the crank throw deflect in axial direction of the crankshaft. The effect is minimized by the addition of an axial vibration damper.
4) Torsional vibration :- Resulting from varying gas pressure in the cylinders during working cycle and the crankshaft/ connecting rod mechanism. Torsional vibration can be dealt with introduction of tuning wheel, increase in diameter of shaft at design stage and use of material with high tensile strength.
The natural frequency of hull depends on its rigidity and distribution of masses while vibration level at resonance depends mainly on the magnitude of the external moments and the engine position in relation to the vibration of the ship.
The magnitude of forced excitation due to propeller can be reduced by better wake distribution and aft body configuration . Accordingly critical speed of engine is adjusted with natural frequency.The natural frequency may be so adjusted that resonance occurs 30-70% below specified engine speed at MCR. Conversely natural frequency may be adjusted such that resonance occurs at 35-45% above engine speed at MCR.
Comments