Shaft Earthing Device
Overview and Function
The ship’s shafting earthing protection device is designed to prevent static electricity from causing deformation and cracking of the stern shaft, and to avoid electrochemical corrosion of crankshaft journals, bushings, etc.
For a ship to navigate, the thrust generated by the rotation of the propeller must be transmitted to the hull. The rotation of the propeller is driven by the power output by the ship’s main engine. Therefore, a reliable set of equipment for connecting the main engine and the propeller and transmitting power must be installed between the main engine and the propeller—this set of equipment is the ship’s shafting. In other words, the power output by the main engine is transmitted to the propeller through components such as the crankshaft, thrust shaft, intermediate shaft, stern shaft (propeller shaft), bearings, rigid couplings, and stern shaft stuffing boxes.
Different metal materials in seawater will generate a potential difference within a certain distance. The metal with a lower potential will be corroded by the metal with a higher potential (under the same conditions, the metal with a lower electrode potential will corrode spontaneously), and current will be generated, leading to electrochemical corrosion.
The propeller is usually made of copper, and the shafting is usually made of steel. Compared with copper, steel is an active metal, and corrosion always starts from the active metal. The potential difference between the propeller and the shafting will generate a current flowing from the propeller (copper) to the shafting (steel). If the shafting is in direct contact with the hull, this potential difference will be transferred to the hull, and no electrochemical corrosion will occur to the shafting.
The propeller and the shaft rotate with the support of bearings and are not in direct contact with the hull, but in contact through the oil film in the bearings. This cuts off the connection between the shafting and the hull, and the potential difference will flow to the main engine crankshaft journals and bushings with relatively low resistance through the breaks in the oil film, causing electrochemical corrosion of the main engine crankshaft journals and main bearings, forming small pits on the bearings. In severe cases, it will lead to excessive bearing wear, shafting vibration, bearing overheating, and even mechanical damage accidents, seriously affecting its working strength and service life. In addition, when the oil film is damaged or the lubricating oil is mixed with water, the potential difference may generate a strong current impact between the main bearings, intermediate bearings, and shafting, leading to instantaneous discharge and sparking. Sparking is a potential danger causing explosions in the main engine crankcase.
Composition and Working Principle
The shafting earthing protection device is usually composed of a copper alloy slip ring fixed on the main engine intermediate shaft, a slip ring bracket, silver-containing graphite brushes, brush holders, earthing wires, a device base, a millivolt meter, and an alarm circuit connected to the shafting earthing protection device. Generally, three silver-containing graphite brushes are provided: two are directly connected to the hull to reduce the potential difference between the shafting and the hull, and the other is connected to a millivolt meter that displays and monitors the potential difference between the shafting and the hull.
The shaft earthing protection device maintains close contact between the copper alloy slip ring and the carbon brush through a pressing device, forming a loop when the main engine is running, effectively eliminating the potential difference generated between the shafting and the hull, there by protecting the shafting and the propeller from electrochemical corrosion and avoiding the generation of electric sparks, so as to effectively protect the safety of the main engine and the shafting.
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