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Lec6- Shafting and Propellers 14Dec22.pptx
- 1. NA-352 Marine Engineering Lecture 6:Shafting and Propellers
- 2. SHAFTING SYSTEM Main Objectives: •To transmit the power output from main engine to the propeller • Support the propeller • Transmit the thrust developed by the propeller to the ship’s hull • Safely withstand transient operating loads (e.g. high speed maneuvers, quick reversals) • Be free of harmful vibrations • Provide reliable operation throughout the operating range
- 3. TRANSMISSION SYSTEM Nogearbox
- 4. SHAFT BEARINGS Main propulsionshafting is supported by bearings which maintain the shafting in proper alignment Classification based on location: 1. Line Shaft Bearings: those bearings which are inside the watertight boundary of the hull 2. Outboard Bearings: those bearings which are outside the hull watertight boundary Classification based on Load Type: 1. Radial Bearings 2. Thrust Bearings REQUIREMENTS • Operating range: from 0 to 100 or more rpm in either direction of rotation • Reliability, as there is no redundancy for bearings and a single bearing failure may halt the propulsion system
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- 9. ENGINE-PROPELLER MATCHING • Theengine flange location and the propeller location are essential information in establishing the shafting arrangement • The location of the propeller is determined by the propeller diameter, the acceptable clearance between the propeller and the baseline of the ship, and the acceptable clearances be tween the propeller and the hull in the plane of the propeller
- 10. SHAFT ALIGNMENT • Shaftalignment is a process where the axis of power transmitting elements is made collinear • Measurement: In shaft alignment two measurements are critical: i) Offset ii) Angularity 1. Offset: Offset is defined as vertical and horizontal distance between the two shafts’ rotating axes 2. Angularity: It’s the angle between the two rotating shafts. It is generally measured in terms of slope 3. Other techniques e.g. reverse dial indicator method, laser alignment etc are also used for shaft alignment
- 11. VIBRATIONS • When amechanical system performs oscillations about an equilibrium position, due the action of a disturbing force, the system is said to be in the state of vibration • The two most noticeable effects vibration has on the ship are structural fatigue and discomfort of crew/passengers • SOURCES of SHIP VIBRATION: 1. Internal Sources: a) Main & Auxiliary Machines: Engines, Motors, Auxiliary Machinery, etc b) Unbalanced Shaft: UNBALANCE occured due to disturbed centre of mass, or/and due to shaft misalignment 2. External Sources: Due to Hydrodynamic loading on Propellers, cavitation, vortices, external moments
- 12. VIBRATION MODES • Vibrationmode is designated by number of nodes in a system • Node: A point in a vibrating system where the amplitude is zero • Antinode: A point in a vibrating system where the amplitude is maximum
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- 14. VIBRATION TYPES • Thereare three basic types of vibration which can occur in a main propulsion shafting system; these are 1. torsional, 2. Longitudinal/ Axial, and 3. Transverse vibration • It is essential that a preliminary vibration analysis of the shafting system be made in the early design stages to avoid complications later
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- 17. VIBRATION CONTROL How toavoid dangerous vibrations of the ship’s hull? By Avoiding exciting forces at frequencies close to the natural frequencies of the ship’s hull
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- 20. LATERAL VIBRATION • Thismode of vibration occurs in the direction perpendicular to the axis of rotation of the shaft • Due to the bending of the shafts, the centre of gravity of the shaft does not coincide with the ideal centreline of the shaft, therefore when the shaft rotates, the centrifugal force on the centre of gravity would cause it to shift further away from the ideal centreline, resulting in a vibratory motion called whirling of shafts • Care is to be taken by the designer that the natural frequency of lateral mode of vibration of the shaft does not coincide with the natural frequency of the engine • In such a case, resonance would result in the extreme case of whirling, which would lead the shaft to snap and cause damage or accidents Medium speed diesel engine, crankshaft fatigue fracture Figure: Eccentricity between CG and CL of shaft during whirling.
- 21. LATERAL VIBRATION • Sources: 1.Propeller Fluctuating Loads 2. Diesel Engine Excitation 3. Shaft Alignment Errors 4. Manufacturing Defects in Gearing System
- 22. PROP EXCITATION ANDADDED MASS • There are two types of excitation caused by the rotation of a propeller, and they are as follows: a. Alternating Thrust: This results in longitudinal vibration of the propulsion system b. Propeller Cavitation: Propeller cavitation results in formation of bubbles that form on the propeller blade • ADDED MASS: Added mass is the additional mass that an object appears to have when it is accelerated relative to a surrounding fluid • When propeller moves, it carries water along with it (also known as entrained water) Cavitation erosion