appropriate settings for shaft alignment. Larger vessels often require a Finite Element Analysis of the stern portion of the ship to estimate the extent of the hull distortion under dynamic conditions. As the shafting becomes greater in diameter it reduces flexibility, yet relative to it, the stern part of the hull is moving, sometimes sufficiently to increase misalignment to an unacceptable amount. Under these circumstances it is necessary to analyse more than just the shafting and include movement of the whole stern part of the hull. Bureau Veritas recently announced that it is developing a voluntary class notation to cover expanded criteria for shaft alignment. The Elastic Shaft Alignment (ESA) notation is targeted at vessels with large diameter shafts or other specific
propulsion configurations requiring
especially accurate shaft alignment. The ESA notation covers hull deformation, shaft speed, bearing material elasticity and lubricating film stiffness. These quasi-static calculations provide the pressure on bearing material,
reactions,
antifriction material squeezing, shaft location inside bearings and lubricating film thickness for every ship's loading condition and speed regime. The areas most critical to damage due to misalignment of the propulsion shafting are the sterntube and first, second and third engine main bearings. The sterntube bearing alignment should provide contact between the shaft and the whole length of the bearing. In cases of misalignment, there is only edge contact of the shaft with a small portion of the bearing surface. This produces a localised overload pressure on the surface of the material, resulting in failure. When the initial shafting installation
has been completed, the next phase in the alignment procedure is the practical reactions
measurement. The most common methods are: gap and sag, strain gauge, and jack-up methods. The sag and gap, and the strain gauge procedures, are indirect methods to measure the deflections and correlate shaft strain to the bearing reactions. The jack-up method uses a hydraulic jack to apply a force on the shaft and its deflection is measured with a dial gauge. Thus the loading on each bearing can be measured by adjusting its position by increasing or decreasing the height of the offset. In addition to ensuring that the shafting alignment is within acceptable limits, a further important
consideration is vibration. The
three vibration elements of torsional stress limits, lateral and axial vibration, and torsio- axial vibration (direct drives) must be within acceptable limits.
In vessels powered by slow speed two-stroke engines, the engine and propeller are directly connected without elastic couplings. The slow rotation of the crankshaft generates angular accelerations at one end of the propeller shaft system as each cylinder goes through its power stroke. At the other end, as the propeller rotates in the wake, it generates a larger thrust force on the upper side of the propeller than on the lower side. This unequal thrust produces a bending moment and angular pulsations acting on the propeller shaft. These are transmitted through each element of the propulsion system along its entire length, giving rise to resonance vibrations that need to be taken into account and may require avoiding a particular rpm band. The amplitude of the pulsation vibrations depends primarily on sea conditions, changes of hull resistance (for example, in a following sea), and variations
in propeller immersion ratio caused by wave action. Rapidly changing
A broken propeller shaft showing classic torsion fracture, in this case due to torsional vibration
direction of rotation of the engine (for example, from ahead to astern in a crash stop), creates by far the highest torque loadings in the system, sometimes up to double the loading. In addition, the thrust forces change to the opposite direction, so forces pushing flanged connections together are now trying to pull them apart! Objects striking the propeller, such as flotsam, ice, and grounding, create a shock effect producing conditions of high torque on the drive system.
A complete propeller shaft system is made up of shorter shafts bolted together using flange connections. Flange coupling bolts with an interference fit are usually used; however, as they are brought up to their prescribed torque, the bolt shank stretches, becoming narrower and the diameter of the flange bolt hole increases in accordance with the Poisson effect. The net result is that the fit is no longer an interference fit, thereby allowing relative movement or micro
dry dock in service - Waterbone
the parts, and fretting of contact surfaces is initiated. Polishing of the surfaces contributes to lowering the threshold for further micro movement. Fretting and wear of flange surfaces used to connect the individual shafts of the propeller shaft system effectively loosen the flange bolts. If this situation goes unnoticed and is not promptly rectified, one bolt will fail, then another and another, in a domino effect, causing failure of the whole propulsion system. Failure of flange bolts can be avoided by using specialised coupling fasteners
such as
A propulsion shaft system set up in the dry dock/slipway condition becomes badly mis-aligned in service as hydrostatic forces distort the hull in a hogging condition
www.mpropulsion.com
the SKF Supergrip bolts. Instead of using interference fit bolts that have to be mauled into place with a sledgehammer, the patented SKF Supergrip system offers a fast, easy, reliable method of fastening propeller shaft flanges which can also be removed, even reused, with little effort. The bolt is hydraulically expanded and tightened into the coupling bolthole, using a set of portable tools; removal is achieved in a similar manner. MP
Marine Propulsion I February/March 2012 I 51
movement between
Chris Leontopoulos, ABS Chris Leontopoulos, ABS
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