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  • Analysis of cause of failure of Marine propeller

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    Analysis of cause of failure of Marine propeller

    Marine propeller is transmitted the thrust to the ship itself, it is a component that converts the torque of an engine into kinetic energy. If the propeller break down, the normal process of the ship will be surffered different influences. When happened different troubles, the dispose approaches are different and choose appropriate methods should base on the appropriate analysis of the troubles. That is to say that reason analysis is very important. The accuracy of the reason analysis can directly affect the result of the fault type.

    1. The failure of a ship propeller entangling a fishing net

    The propeller is always be interwined by the fishing net, it will add the torque in this kind of situation, consequently causeed an increase of the engine load. When overload working, it is possible to shop the laminar or propeller

    (1).The cover fins happen to deforme

    (2). Shafting stern shaft flange wear or damage.

    (3). stern bearing fix hex bolts damaged or missing

    (4). Joint coupling flange takes place displacement

    (5). Thrust bearing reverse thrust block damaged

    (6). The vibration noise of shafting increases. On the one hand, it is the cooling water channel of the bearing which is blocked by the debris of the fishing net. The second is the flange displacement of the shaft coupling, which makes the shaft parts loose.

    2. Friction of contact points between propeller and accessory parts

    The working environment of the stern axis is relatively complicated. Bearings and stern shafts sometimes have poor lubrication effect, reasons causing shafting vibration clearance is not perfect and produce large noise, it may also be because its sealing device installed right way, blade and blade fairing, stern shaft shielding created in contact with the propeller hub. Propeller and auxiliary parts of the contact point is likely to appear in the propeller hub with a stern shaft cover, the docking examination room, for make sure have enough space, will be appropriate to remove protective near the hub, to ensure that contact friction will not happen again.

    Vibrating violently, the crew will slow down, the vibration will weaken, continue to decelerate and the vibration will become smaller.

    The reason for this situation is that it is likely that the propeller blade has been broken and the propeller has lost its balance, causing the hull to vibrate violently. Generally this may be because the propeller material performance is poor, the casting defects, or propeller blade is severe corrosion, the water pollution or propeller blade touching the rope, such as reef – the propeller blade erosion, rupture, deformation and so on reasons, led to the blade of propeller center of gravity be unsteady lost her balance and knock to the stern shaft, shaft or the vibration of the hull is very serious. For that to happen is to fill up the method to fix the broken blade to crack, cavitation and corrosion on the surface of the repair of the defect of system, the distortion correction back again. The quality inspection of the static balance and propeller is required to meet the requirements of the relevant fan.

    2. Blade distortion

    When the propeller is working, the blades and the water produce a force that forces the water to work against the hull so that it can move forward. Blade in the process of long-term use, as a result of material no, fatigue limit, also need not material deformation and fracture are there with the extension of time until to produce this kind of situation, and in addition to the force, there are still many uncertain factors, such as water erosion makes the blade thickness thinning, the thickness of the thin blade deformation increase.

    3. Propeller singing

    The sound of the propeller is caused by the frequency of the random vortex of the blades and the frequency of the maximum vibration amplitude of the blades. These energy causes the impeller to vibrate. There are several ways to reduce the tone of the sound:

    (1). Trim the trailing edge of the blade. In order to change the frequency of distribution, the thickness of the trailing edge of the blades should be changed first, the structure of the tail side will affect the frequency and intensity of the distribution vortex, and the transverse and longitudinal spacing of the vortex street formed. However, the elimination of sound as the sole purpose, because the trailing edge is weakened too thin will affect its strength, resulting in the decrease of intensity, which can not meet the work needs.

    (2). Increase the damping of the blades. The use of high damping materials to make propellers also eliminates the sound.

    (3). In the vicinity of the back of the blade, the length of the section string is 1/15 or so, and the small metal material is fixed along the radial and other Spaces, and it is made into a semicircle.

    (4). The blade is replaced by the serrated structure with the shape of the shape of the top of the string, and the base of the tooth is the same as the height of the tooth.

    4. Propeller cavitation

    In the work of the propeller, if the cavitation is near its surface, the blades of the blades will be corroded by the long-term constant force of the hydraulic impact. Cavitation tend to produce vibration and noise, and can appear in the metal surface pitting corrosion, serious when can die blade perforation or burst, causing the whole propeller center of gravity be unsteady, vibration and noise. The solutions to these problems are as follows:

    (1). Improve the design of the blade structure, increase the flow area, reduce the thickness of the blade, and make the shape of the blade into a streamlined shape.

    (2). Increase the inlet Angle of blade, reduce the bending of blade, thus reducing blade obstruction.

    (3). The blade material is changed into an anti-cavitation material, such as material with high hardness, strength and toughness, and its chemical stability is better.

    Ship propeller occurrence of failure is a kind of normalized failure. Different ship propeller due to different material in your work situation, which is different and cannot equate to analyze the causes of the fault. Fault and analysis in specific processing when the water in the actual situation and propeller parts around the work has taken into account, to avoid relying on experience to determine the cause of the problem.

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