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Another factor which governs the proportions of the crushing chamber is the maximum permissible angle between the two crushing faces. This also is subject to a certain amount of variation, depending upon the character of the material, and upon the size of the machine. Crushing angles may vary between the approximate limits of 22 and 30 deg, in a line of standard gyratory crushers. It is possible to use larger angles in the larger sizes of crushers because the greater weight of the individual pieces of material tends to minimize slipping-. Furthermore, in these large machines, a difference of a few degrees in the crushing angle makes a considerable difference in the height, weight, and cost of the crusher.
It is obvious that the coefficient-of friction of the material to be crushed has a very direct bearing upon the maximum permissible crushing angle. Some slippery materials, even though quite soft, require special reduce dangle crushing chambers. In the standard gyratory crusher this is accomplished very simply by increasing the thickness of the concaves at the top, and tapering them down toward the discharge; in the jaw crusher the change is generally made by inserting a wedge-shaped filler behind the liner plates on the movable jaw.
For a given coefficient of friction, the harder the material is, the smaller will be the maximum permissible crushing angle. Extremely hard, tough materials even though their coefficient of friction be fairly high may require special reduced crushing angles to prevent excessive slipping. Anyone who has seen hard granite boulders shoot out of a gyratory or jaw crusher, or has watched a wide-angle crusher at work on hard granite or trap rock, has had a visual demonstration of this physical fact.
A certain amount of slippage occurs in all pressure type crushers, regardless of how small the crushing angle may be (that is, within practicable design limits), especially in the lower part of the crushing chamber where the pieces are smaller and lighter. Under normal operating conditions this tendency is counteracted by the weight of material in the upper part of the chamber. Another method of compensation is to "break" the angle of the lower tier of con caves to provide a more favorable crushing angle in this zone.
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