For economic reasons alone, the growing demand for basic raw materials such as ores, coal, salts and the like calls for extraction methods where an undesirably large percentage of deads is obtained as well. These contaminants require preparation of the extracted crude products, e.g. in preparation for smelting, as a substitute for household coal and power plants or as basic material for processing in various industry sectors.
Coal, for instance, must be graded according to grain sizes and rid from rocks (refuse) in separating plants. Ores must first be finely ground and then graded; salts musts be ground and graded. in some cases screen grading is sufficient for upgrading a given material.
Grading means separating solid matter according to grain size. In screen grading, the material is separated mechanically on screen plates. Extremely fine grains of less than 1 mm are primarily separated by means of air separation. In this process, an air current supersedes, in a way, the screen plates. in hydrograding (sizing in a flow of water) the different grain sizes are separaten in a flow of water.
Both in hydrograding and in air separation, variations in density and grain shape reduce the accuracy of separation. Over the past three decades, there has been enormous technical progress in the separation of materials according to grain size. The sizes of the machines used for grading, classifying and centrifuging were rapidly increased into ranges which were not considered safely feasible before.
Until far into the sixties, the maximum permissible width of these vibrating screens, which dictates their performance, was about 2.50 m. However, economic considerations led to the development of bigger, safer and easier-to-maintain vibrating machines with the objective to process both dry and moist materials, achieving the highest possible specific throughputs and the best possible accuracy of separation. Today, operationally reliable vibrating screens are up to 5.50 m wide and feature an active screen surface of approx. 40 to 45 m2 on a single tier.
The vibrating screens needed for the various grading jobs are among the most severe bearing mountings encountered in the processing technology sector. Due to the permanent alternating stress to which all machine components are exposed, and the dangerousness of their auto vibration, there was only a gradual development toward larger machines.
Rolling bearings in vibrating screens are stressed by high, mostly shock-type, loads. To compound matters, the bearings, while rotating about their own axis, perform a circular, elliptical or linear motion. This results in high radial accelerations which additionally stress the bearing, and especially the cage.
The operating speeds are usually very high so that the bearing temperatures in vibrating screen applications are 20 to 30 K higher than in normal applications.
Moreover, in screening machines, major bearing misalignments and pronounced shaft deflections must almost always be reckoned with.