The shape of the grinding medium can be divided into spherical shape, rod shape, short cylindrical shape, frustum shape as well as some other forms. Due to different shapes, the principles are different, which provides possibility for the selective grinding. At present, the common shapes of grinding medium are as follows.
1. Sphere. It has the best rotating performance, point-contact crushing, great crushing force, inferior accuracy, bad selective understanding, more thorough crushing effects, severe over-crushing phenomenon, especially when the size of the ball is oversize, it becomes more serious. During the fine grinding, it becomes more effective along with the enlarging of the grinding surface area. From the geometrical knowledge, we know that the spherical surface area is the smallest among the subjects with the same volume. Therefore, the ball medium is not the optimal choice.
2. Rod. Line-contact crushing. Coarse granules exist in the inbetween of the rods, there are less grinding phenomena. The product’s granularity is even. But it only performs well in the around-axis-direction rotation, unavailable towards the other directions. What’s more, its surface area is small per volume. So it is merely suitable for the coarse grinding.
3. Short cylindrical form or frustum. This type of medium shape is the variant between the rod shape and spherical shape having the merits of good rotation performance as the ball mill , large surface area as well as avoiding the over crushing phenomenon owing to line contact as the rod medium. However, its crushing force is smaller than that of the spherical medium with the same quality and its effective grinding surface area is smaller than that of the spherical medium with the same volume. They belong to short line contact lightening the over crushing which is inferior to the rod shape. They are the integrate of the ball and rod shape processing the advantages of them both. Proved by the industrial experiments, at the stage of vibration screening, short cylindrical medium with small size has a better effect than that of the spherical medium of the same quality. Under the condition of the same processing capacity, the level of unable fine grinding and over crushing decreases evidently, the qualified granular productivity increases obviously and the granularity of the products are even. Therefore, it is fit for fine grinding. There are two common short cylindrical media, one is double spherical surface the other is double plane surface. Currently, there are various kinds of the short cylindrical medium and short cone medium. The specification tends to be presented by the product of D&L of the cylinder or cone. In general, the L/D ratio is usually taken between 1.0 to 1.5, the customers can choose the minimum when the effective fine grinding is required, the maximum when the light over crushing is needed.

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