Large gear is the core transmission part of mining mill, and its reliability is related to the safety and stability of the whole equipment. With the continuous increase of equipment specifications and transmission power, the large gear is also developing towards large-scale. Its diameter has exceeded 10 m and its weight is close to 100 tons. At present, there are four main types of large gears for mining mills: cast steel large gears, welded large gears, ductile iron large gears and segmented large gears. The production and manufacturing process of cast steel large gears is relatively mature, and the users of mining and grinding equipment at home and abroad have relatively high recognition of cast steel large gears. However, the solidification shrinkage of steel castings is large, large risers need to be set, the utilization rate of liquid steel is low, the production cycle is long, and the defects of slight Looseness on the tooth surface after gear opening are difficult to be completely eliminated.
Alloy steel rim, low-carbon reinforcement plate and web plate are generally welded together for welding large gears. Because the rim is rolled steel plate, the problem of rim compactness is solved. However, due to dissimilar steel welding, the welding technology is difficult. Welded large gears are less rigid than cast steel large gears, easy to deform and difficult to install and debug. Large gears with welded structure are generally used for small and medium-sized large gears with a diameter of 900 ~ 5000 mm. Nodular iron large gear has better formability than cast steel large gear, and the shrinkage of nodular iron is smaller than that of steel. It can eliminate the large riser of cast steel large gear, with high yield and low cost. However, the casting technology of nodular iron large gear with wall thickness greater than 200 mm and molten iron volume of 100 tons is difficult, which is mainly reflected in the large section nodular iron castings in the production process due to large amount of molten iron, long cooling time Slow solidification rate and rapid spheroidization decline are easy to produce graphite floating and graphite variation. At the same time, due to the slow cooling speed, a large amount of ferrite is easy to precipitate from the matrix during eutectoid transformation, which reduces the strength and hardness of the casting. In addition, it is difficult to repair the nodular iron casting after defects. This series of problems are an important obstacle to its popularization and application in large gears.
(1) The casting process creates a reasonable temperature gradient for the solidification of large gears by optimizing the riser form, reasonably setting subsidies and hanging sand chill, so as to effectively ensure the internal quality of castings.
(2) Strictly control the content of harmful elements in molten steel, through internal control of chemical composition and reducing the difference of chemical composition between steel ladles, blowing argon in the mold cavity before pouring and argon protection during pouring can improve the quality of molten steel and ensure the uniform and stable performance of castings.
(3) Through post cast heat treatment and post rough machining heat treatment, it is more conducive to improve the comprehensive performance of materials. It is an effective guarantee for the service performance of cast steel large gear of mining mill.
Segmented large gears solve the problem of partition exchange when large gears fail locally, and get rid of the dependence of manufacturing process on large casting and processing equipment. However, due to more segments, the integrity of the casting structure and the uniformity of material properties are not as good as the above three forms. Considering the impact of the cumulative processing error caused by too many mating surfaces on the large-scale gear transmission, it is required that the manufacturing and installation accuracy of each segment is very high, which also limits its large-scale application. The four forms of large gears have their own advantages, but they also have their own short plates. The choice of which form depends on the technical maturity of the design and manufacturer in this field. Through years of research, CITIC Heavy Industry has accumulated rich experience in large cast steel, large gear material selection, casting technology, smelting technology and heat treatment technology, and has established its own production, manufacturing and inspection specifications.
