The bearing used for this specific case of wind turbine is ball-bearings. The ball-bearings are used for low load and high speed machines.
The ball bearings are generally manufactured from steel material. This material is preferred choice due to its high strength, and resistance of corrosion and high fatigue strength. As per standard practice, the outer race of the ball bearing is manufactured from high alloy steel bar, in most of the cases the inner races, wear rings are also manufactured from the same material. The recommended steel alloy for such manufacturing is BMS-931 and 8720H.
The manufacturing process begins with the forging of the steel bar, the material is forged into different shapes like cups, cones etc. The forging process is generally hot forging; this process shall release thermal stresses from the material during the process. The forging process can be explained as, "exposure of the steel bar in the induction heater system, the heating process is halted after the temperature reaches the melting point of the material; at this temperature maximum formability for hot forging is attained". After the completion of hot forging, the material is pierced.
The material used for the manufacturing is SAE-521000; the material is used in the form of forged rings. The centre-less grinding is applied on the material, which is extremely rough machining process. The material is then treated at high temperature, after which the hardness of the material is checked. The grinding on the material is conducted to secure smooth surface finish. During the process the material undergoes "honing and super finishing followed by washing; later anti-rush agent is applied on the object" (Bruce, 1997).
The heat treatment process is extremely critical, and during this process the ample exposure to the heat will release thermal stresses within the object. The process is conducted "to "proper wear and fatigue resistance" to the material (Groover, 1996). During this process the object is carburized, and then inserted inside the solution of carbon gas solution. The process parameters are controlled to ensure that desired heat treatment is provided, the heat treatment process shall ensure that "regulation of gas flow, furnace temperature, and process time, this shall assure accuracy, consistency and maximum efficiency of the operation". During the entire process, the object is not exposed to any harsh and cold ambient, any sudden exposure to the cold atmosphere will result in the brittleness of the object. The intent of the heat treatment is to achieve desired grain structure, which shall introduced toughness and harden-ability in the material. As per the recommended practice "the parts are cooled to room temperature, heated in a rotary furnace, oil quenched and drawn to provide a case-hardened part with controlled dimensions" (Thyer, 1991). It is then recommended that "heat treat case depth, hardness, carbon content, grain structure and retained austenite shall be verified". The entire process is conducted with rotary try furnaces, "which offers consistent environments for carburization, cool down, reheat, quench, and tempering of a continual product flow" (Groover, 1996). The entire process of ball bearing manufacturing is regarded as extremely capable, and the process is relatively cheaper and consistent against other