For sale, is the most popular and reliable workhorse of the crusher family, the jaw crusher, has been developed in a variety of forms that probably out-rivals the variations of the gyratory crushing principle. Several different types of motion have been devised, and a great deal of mechanical ingenuity has been displayed in the design of mechanisms to generate these motions. Despite the infinite variety of details, however, most of the jaw crushers in popular usage today have actions which closely approximate or exactly duplicate one of the three types: the Blake-type crusher, the Dodge-type crusher and the single-toggle type crusher.
Rocks and stones are broken into smaller particles by at least one of the following actions: attrition, pressure or compression, impact, and shear. Most crushers apply two or more of these actions to the material they crush.
- Attrition is rubbing or grinding down by friction between material particles in the crushing chamber and between the material and the parts of the crusher (jaws, rolls, crusher heads, concaves plates, etc.). There is some of this action in all crushers. It is most effective with material that is friable, and not abrasive, that is, the material has a low silica content. Attrition is most beneficial when maximum fine material is wanted.
- Pressure or compression is a squeezing of rock between two surfaces. It is the principal means by which jaw and roll crushers accomplish their work. Crushing by compression is necessary when the material is hard, tough, and abrasive and a minimum of fines is required. It is not useful or recommended for crushing sticky materials.
- Impact is the sharp blow delivered by a hammer to the material. The hammermill and impact breaker are crushers that apply this action. Impact should be used to crush material that is not too abrasive (not much over 5% silica) and that contains a high percentage of soft stone. It also is recommended if more cubical particles are desired. Crushers designed to apply this action give well-graded products.
- Shear is an action found in gyratory and cone crushers which causes rock to break on a plane when the forces on each side of the plane act in opposite directions. It is used to reduce material, which is fairly soft and friable, when a minimum of fines is wanted. The material should not be too abrasive.
Crushers can be listed in the order of decreasing impact and increasing compression as follows: impact breaker; hammermill (high to low speed); rod mill (high speed); ball mill; single roll crusher; cone crusher; gyratory crusher; jaw crusher; and twin- or triple-roll crusher. All crushers deliver a strong impact blow or a compressive force to the rock, and attrition and shearing action, are found in every crusher.
A jaw crusher is included in the portable plant diagram shown here. Two types of jaw crushers have proven to be effective, the double-toggle crusher and the overhead-eccentric, single-toggle crusher. In both of these crusher types one jaw is stationary (the left jaw in Figure below) and the other jaw swings back and forth toward the stationary one propelled by a flywheel revolving about a shaft at the top of the jaw. The jaws converge toward the bottom of the crushing chamber. The movable jaw is closed to the crusher setting at the bottom by a toggle plate which is held against the jaw frame by spring-loaded rods attached to the main frame. Application of the closing force near the bottom of the swing jaw gives the greatest leverage and causes the greatest compressive force to be delivered to the rock near the top of the crushing chamber. But the movement of the swinging plate is smaller near the top of the jaw than near the bottom resulting in a smaller drop per stroke of the larger material near the top of the crushing chamber. Movement of the lower end of the jaw permits rapid discharge of the smaller crushed material through the bottom to permit more effective crushing of the larger rocks.
The overhead-eccentric jaw crusher has a single toggle at the bottom of the movable jaw. A rotating eccentric shaft moves the top of the swinging jaw through a circular path in the vertical plane. At the same time, the bottom of the jaw is restrained by the toggle plate to move on a crescent-shaped path. The combined effect is to force feed the rock through the crushing chamber as it is being crushed. There is greater attrition in the overhead-eccentric crusher than in the double-toggle crusher.
The jaw crusher has a rectangular top opening between the two crusher plates and the side plates. Models of jaw crushers are designated by the dimensions of the top opening of the crushing chamber in inches.