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Corrosion of propeller blade edge and repair of SAWTOOTH

Views: 3     Author: Site Editor     Publish Time: 2020-04-18      Origin: Site

A ship's propeller transmits the thrust it produces to the ship itself, which is the part that converts the Torque produced by the engine into kinetic energy. If the propeller breaks down, the normal movement of the ship will be affected to different degrees. When different faults occur, the treatment methods are different, and the proper method should be based on the reasonable analysis of the faults, that is to say, the cause analysis is also very important. The accuracy of the cause analysis directly affects the result of judging the type of fault.

 

Fault of Marine Propeller winding fishing net

 

The propeller is often entangled in fishing nets, which in turn increases the torque of the propeller, causing an increase in engine load, when overworked, and the possibility of loss of shaft or propeller lift, the propeller shaft system and propeller blades are damaged and deformed, and sometimes the vibration and noise of the propeller shaft system are increased. When there is an impact signal based on the extreme speed of the shaft system, it is proved that the mechanical operation of the shaft system is not normal. In this case, the shaft system or propeller needs to be further checked to find out the cause of the impact signal, it is possible to have the following failures:

 

1. The fin deflects

 

2. Wear or damage of Stern Shaft Flange

 

3.Damaged or missing stern bearing fixed hexagon bolt

 

4. Displacement of coupling connection Flange

 

5. Thrust bearing reverse thrust block damaged

 

6. The vibration and noise of the shaft system increase, which is caused by the fishing net debris blocking the cooling water passage of the bearing, and the shaft coupling Flange displacement which makes the shaft system parts loose.

 

Friction between propeller and accessory parts

 

Because the working environment of the stern shaft is relatively complex, the vibration of the bearing and the Stern shaft system is loud sometimes because of the poor lubrication effect and the imperfect clearance, it is also possible that the sound is caused by the incorrect installation of the sealing device, the contact between the propeller blade and the shroud, the stern shaft shroud and the propeller hub. The contact point of the propeller and its appendages may be at the propeller hub and the stern shield, and in the docking room, the shield near the hub shall be properly removed to ensure sufficient separation, so there's no more contact friction.

 

Unbalance caused by the fracture of a propeller blade

 

When the ship hull is running on the calm river surface, the sudden violent vibration of the ship hull will generally slow down the voyage, the vibration will be weakened, continue to slow down, vibration smaller.

 

Analysis of the causes of this situation: This situation is likely to happen is the propeller blade has been broken, so that the propeller out of balance, so that the hull of the violent vibration. Generally this situation may be due to poor propeller material performance, casting defects, or the blades of the propeller are seriously corroded, polluted by living things in the water, or the blades touch the cables, rocks and other objects, causing the blades to be denuded, broken, deformed, etc. , which causes the center of gravity of the blades to be unstable and out of balance, striking the stern shaft causes severe vibration of the shafting or hull.

 

In order to solve this problem, we should repair the broken blade by means of patch, repair the surface crack, cavitation erosion and corrosion systematically, and correct the deformation. To static balance, the pitch of the propeller, such as quality testing, and to meet the relevant norms.

   

Blade deformation

 

When the propeller is in operation, the blade exerts a force on the water, causing the water to reaction against the hull, which in turn causes the hull to move forward. In the long-term use of the blade, due to the material is not used, fatigue limit is not used, material deformation and fracture are over time until such a situation, and in addition to this kind of force, there are still many uncertain factors, such as water erosion makes the blade thickness thinning, this thickness thinning makes the blade deformation increased.

 

Propeller whine

 

The sound of the propeller is caused by the frequency of the random vortex generated by the blade and the frequency of the maximum amplitude structural response of the blade vibration. Here are some ways to tone it down:

 

1. Trim the trailing edge of the blades. To change the emission frequency, first of all, the thickness of the trailing edge of the blade should be changed. The structure of the trailing edge affects the frequency and intensity of the emission vortex, and determines the ratio of the transverse and longitudinal spacing of the Vortex Street. However, the sole purpose is to eliminate the singing sound, because the tail edge is weakened too thin will affect its strength, resulting in a reduction in strength to meet the needs of the work.

 

2. Increase Blade damping. Using highly damped materials to make propellers also eliminates the hum.

 

3. Near the back of the blade, about 1 / 15 of the chord length of the section, a few small metal protrusions are positioned equidistant along the radial direction and made into a semicircle.

 

4. The blade with the edge into a sawtooth-shaped structure, tooth-shaped height for the chord length of about 7% , tooth-shaped bottom edge and tooth height is equal.

 

Propeller cavitation

 

If the cavitation is near the surface of the propeller, the material of the blade will be corroded because of the long-term continuous force of the hydraulic impact. Cavitation often produces vibration and noise, and there will be spot corrosion on the metal surface. In severe cases, the dead blade will be perforated or broken, resulting in instability of the whole propeller center of gravity, vibration and noise. The solutions to these problems are as follows:

 

1. Improving the design of blade structure, increasing the flow area, reducing the thickness of blade, so that the shape of blade becomes streamline shape.

 

2. Increase the inlet angle of the blade, reduce the curvature of the blade, thereby reducing blade obstruction.

 

3. Blades are replaced with cavitation resistant materials, such as materials with high hardness, strength, toughness, and good chemical stability.

 

The Fault of marine propeller is a kind of normal fault. Because of the difference in the working conditions of different ship propellers with different materials, it is not the same as Germany to analyze the cause of the fault, the actual water surface condition and the working condition of the parts around the propeller are taken into account when dealing with the fault and analyzing it, so as to avoid judging the fault cause by experience.


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