Torsional Vibration in Main Propulsion Plant of a Ship
Causes of Torsional Vibration
Here torsional vibrations in a main propulsion installation based on medium speed engines, gearbox and controllable pitch propeller is considered. Torsional vibrations in this installation may be caused by:
- Power imbalance of the engine producing a greater variance of torsion produced by the crankshaft
- Operation of the engine at different speeds than normal, which may be closer to the engine critical speeds than normal
- Rotary imbalance of the main engine due to loss of a balance weight
- Damage in the gearing being transmitted to the shafting
- Propeller damage causing rotary imbalance
Effects of Torsional Vibrations
The increase in torsional vibrations would produce an increase in torsional stress. This stress would be imposed on the existing stress levels resulting from torque transmission (torsional shear stress), axial stress resulting from the propeller thrust, and the bending stresses resulting of the shaft alignment.
Hence this increase in stress levels could result in the generation and growth of cracks in the high stress areas of the transmission shaft and propeller. If left in this condition, the cracks could lead to failure of the transmission and hence loss of propulsive power.
The torsional vibrations would also increase wear on the contact faces of the gear teeth, as the contact forces would be varying affecting the lubrication of the gearing. Torsional vibrations will also distort the crankshaft and shafting, which could result in an increase in bearing loads.
How to Reduce Effects of Torsional Vibrations
The effects of torsional vibrations can be reduced by the following:
- Detuning of the engine. This is carried out by modifying the stiffness of the shaft when the output shaft starts to vibrate. When the shaft starts to torsionally vibrate the springs in the detuner will be compressed, which increases the stiffness of the coupling.
- Dampening. Once the shaft starts to vibrate the energy of vibration is reduced by the free mass moving within the fluid inside the dampener.
- Isolation. The engine can be partly isolated from the gearbox by the use of flexible couplings, whose natural dampening properties will reduce the transmitted level of torsional vibration.
- Operation of the propeller at a different pitch setting, and the propeller pitch change will change the torsional stiffness of the transmission shaft unit.
How to Modify Natural Frequency of the System
The natural frequency of the system depends upon the physical dimensions of the shaft, and its material. Thus the natural frequency can only be changed by altering the shaft dimensions. However consideration must also be given to the minimum size of the shaft required from the torque transmission limit. Thus although the shaft size could be reduced by the utilising of a higher UTS material, this would lead to an increase in possible shaft cracks.
Increasing the size of shaft components such as the flywheel would also affects the natural frequency of the shafting.