Vibration and Forces - Force and Moments in slow speed diesel engine
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1. Engine vibration and critical speed
2. Force and Moments in slow speed diesel engine
3. Vibration damper and detuner
4. Static and dynamic loading and balancing in engine
5. Primary and secondary forces and couple
6. Torsional vibration
7. Axial vibration details
8. Purifier vibration reasons
9. Important element in ship vibration
2. Force and Moments in slow speed diesel engine
3. Vibration damper and detuner
4. Static and dynamic loading and balancing in engine
5. Primary and secondary forces and couple
6. Torsional vibration
7. Axial vibration details
8. Purifier vibration reasons
9. Important element in ship vibration
Force and Moments in slow speed diesel engine
Diesel engine
vibratory excitation can be considered as composed of three periodic
force components and three periodic moment components acting on the
engine foundation.
The periodic force component along the axis of the engine is inherently zero, and some other components usually balance to zero depending on particular engine characteristics.
The periodic force component along the axis of the engine is inherently zero, and some other components usually balance to zero depending on particular engine characteristics.
Two distinctly different types of forces can be associated with the internal combustion reciprocating engine. These are:
- Gas pressure forces due to the combustion processes (guide force couples) and
- Inertia forces produced by the accelerations of the reciprocating and rotating engine parts (external forces).
Guide force couples
acting on the crosshead result from transverse reaction forces depending
on the number of cylinders and firing order. The guide force couples cause rocking (H-couples) and twisting (X-couples) of the engine.
The engine lateral vibration due to the guide force couples may cause resonance with the engine foundation structure. A possible solution at concept design stage may be a consideration of lateral stays (top bracings), connecting the engine's top structure to the ship hull.
The vertical force and moment, which are of primary concern with regard to hull vibratory excitation, and the transverse force and moment as well, are due to unbalanced inertial effects. For engines of more than two cylinders, which is the case of interest with ships, the vertical and transverse inertia force components generally balance to zero at the engine foundation. This leaves the vertical and transverse moments about which to be concerned.
The engine lateral vibration due to the guide force couples may cause resonance with the engine foundation structure. A possible solution at concept design stage may be a consideration of lateral stays (top bracings), connecting the engine's top structure to the ship hull.
The vertical force and moment, which are of primary concern with regard to hull vibratory excitation, and the transverse force and moment as well, are due to unbalanced inertial effects. For engines of more than two cylinders, which is the case of interest with ships, the vertical and transverse inertia force components generally balance to zero at the engine foundation. This leaves the vertical and transverse moments about which to be concerned.
The values of the moment amplitudes are usually tabulated in the manufacturer's specification for a particular engine.
The majority of low speed marine diesel engines currently in service have 6 cylinders or more. Therefore, the 2nd order vertical moment is generally considered to contribute the most to the hull vibration. However, depending on the specific number of cylinders, the first order or higher order moment can be as large as the second order moment. In that case, further consideration is to be given to the first or higher order moment.
A hull girder mode up to the third or fourth can have a natural frequency as high as the twice-per-revolution excitation of the 2nd order vertical moment. Hull girder modes higher than the first three or four have diminishing excitability and may be of less concern.
The majority of low speed marine diesel engines currently in service have 6 cylinders or more. Therefore, the 2nd order vertical moment is generally considered to contribute the most to the hull vibration. However, depending on the specific number of cylinders, the first order or higher order moment can be as large as the second order moment. In that case, further consideration is to be given to the first or higher order moment.
A hull girder mode up to the third or fourth can have a natural frequency as high as the twice-per-revolution excitation of the 2nd order vertical moment. Hull girder modes higher than the first three or four have diminishing excitability and may be of less concern.
The following steps are therefore recommended in concept design:
- The 2nd order vertical moment , is the diesel engine excitation of most concern. The potential danger is in resonating one of the lower hull girder vertical modes with a large 2nd order vertical moment. The value of second order vertical moment is to be requested from the main engine manufacturer.
- Power Related Unbalance (PRU) values may be used to determine the acceptable level of 2nd order vertical moment.
PRU=2nd order vertical moment(N-m)/Engine Power (kW)
Need for Compensator depends on the range of PRU.
PRU Below 120 Not likely needed.
PRU between 120-220 Likely needed and
PRU Over 220 Most likely needed
PRU Below 120 Not likely needed.
PRU between 120-220 Likely needed and
PRU Over 220 Most likely needed
Further attention is
recommended in cases where PRU exceeds 220 N-m/kW. The action
recommended at the initial engine selection stage may be either change
of engine or installation of moment compensators supplied by the engine
manufacturer. Otherwise, the vertical hull girder response is to be
checked by calculation within an acceptable level without installation
of compensators.
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