Vibration and Forces - Axial vibration details
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These stresses arise from the crankshaft being alternately compressed and stretched along its axis. Axial vibrations are longitudinal shafting vibrations and are excited by:
• Radial and tangential components of the combustion pressure and mass forces in the individual cylinders, causing unbalanced couple
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
Axial vibration details
These stresses arise from the crankshaft being alternately compressed and stretched along its axis. Axial vibrations are longitudinal shafting vibrations and are excited by:
• Radial and tangential components of the combustion pressure and mass forces in the individual cylinders, causing unbalanced couple
• Torsional vibration induced by propeller thrust
• Propeller working in the non-uniform wake field
Axial or longitudinal vibration of the crankshaft may excite the hull to vibrate through the thrust bearing. The gas pressure in the cylinder is transferred to the crankshaft as connecting rod effort. The tangential component T turns shaft and generates the shaft power. The radial component R deflects the crank webs in axial direction; In large two stroke engines, with their heavy static and dynamic load on crank pin the crank webs systematically open up and close in during one revolution of the engine. Thus, at any instant of time the number of cylinder at different phase angles are executing same type of motion but of different amplitudes.
These may be added up to excite the shaft system to resonant vibration along the axis of the shaft. An additional excitation source may come from torsional vibration and the effect of their coupling together may be more pronounced near torsional and axial critical speeds coming closer. Although shafting axial vibrations alone are rarely the cause of severe shafting damages, they are usually the cause of a vessel's hull vibration, excited by the variable force acting on the engine's trust block.
To minimize unfavorable side effects of the shafting axial vibrations, most engine builders of the low-speed diesel engines integrate an axial vibration damper into the engine casing. That way, the axial vibration damper becomes a standard building block of modern low-speed diesel engines.
An axial damper with the Torsional Detuner at the free end of the engine is shown as an example taken from engines built by New Sulzer Diesel. The transfer of oil between two chambers of the Detuner through throttle valve limit the axial amplitude of vibration. The extent of movement of the crankshaft can be measured and accordingly the throttle valve is adjusted.
Axial or longitudinal vibration of the crankshaft may excite the hull to vibrate through the thrust bearing. The gas pressure in the cylinder is transferred to the crankshaft as connecting rod effort. The tangential component T turns shaft and generates the shaft power. The radial component R deflects the crank webs in axial direction; In large two stroke engines, with their heavy static and dynamic load on crank pin the crank webs systematically open up and close in during one revolution of the engine. Thus, at any instant of time the number of cylinder at different phase angles are executing same type of motion but of different amplitudes.
These may be added up to excite the shaft system to resonant vibration along the axis of the shaft. An additional excitation source may come from torsional vibration and the effect of their coupling together may be more pronounced near torsional and axial critical speeds coming closer. Although shafting axial vibrations alone are rarely the cause of severe shafting damages, they are usually the cause of a vessel's hull vibration, excited by the variable force acting on the engine's trust block.
To minimize unfavorable side effects of the shafting axial vibrations, most engine builders of the low-speed diesel engines integrate an axial vibration damper into the engine casing. That way, the axial vibration damper becomes a standard building block of modern low-speed diesel engines.
An axial damper with the Torsional Detuner at the free end of the engine is shown as an example taken from engines built by New Sulzer Diesel. The transfer of oil between two chambers of the Detuner through throttle valve limit the axial amplitude of vibration. The extent of movement of the crankshaft can be measured and accordingly the throttle valve is adjusted.
Shafting lateral vibrations
Shafting lateral vibrations are characterized by shafting segments oscillating in a plane passing through the shaft's neutral position. For simplified description purposes, the shaft axis may be considered the shaft neutral position.
Lateral vibrations may be considered as a special case of the whirling vibrations, which represent the resultant motion of two concurrent motions, each in perpendicular planes passing through the shaft neutral position.
Lateral vibrations are mainly excited by the propeller weight, propeller induced variable forces and shafting segments' weight and imbalance. The amplitudes of lateral vibrations are generally enlarged by the increased span between the line shaft bearings. However, it must be clearly understood that a small inter-bearing distance could also provoke enlarged lateral vibrations. It is especially the case with the stern tube bearings, if the forward stern tube bearing becomes unloaded.
In general, the basic design countermeasure against the unacceptable shafting lateral vibrations is to ensure that the lateral natural frequencies are positioned sufficiently far away with respect to the propeller rotation speed.
Shafting lateral vibrations are characterized by shafting segments oscillating in a plane passing through the shaft's neutral position. For simplified description purposes, the shaft axis may be considered the shaft neutral position.
Lateral vibrations may be considered as a special case of the whirling vibrations, which represent the resultant motion of two concurrent motions, each in perpendicular planes passing through the shaft neutral position.
Lateral vibrations are mainly excited by the propeller weight, propeller induced variable forces and shafting segments' weight and imbalance. The amplitudes of lateral vibrations are generally enlarged by the increased span between the line shaft bearings. However, it must be clearly understood that a small inter-bearing distance could also provoke enlarged lateral vibrations. It is especially the case with the stern tube bearings, if the forward stern tube bearing becomes unloaded.
In general, the basic design countermeasure against the unacceptable shafting lateral vibrations is to ensure that the lateral natural frequencies are positioned sufficiently far away with respect to the propeller rotation speed.
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