Impact Crusher

Impact Crusher

Types and Construction of Impact Crushers

The impact crusher is also known as the impact breaker. Its classification is similar to that of hammer crushers. The different types are shown in Figure 1‑37.

The two rotors of a double‑rotor impact crusher can rotate in the same direction or in opposite directions. The twin rotors can be arranged at the same horizontal level or at different levels. The most commonly used type is the single‑rotor, non‑reversible, fixed‑hammer impact crusher. However, the application of double‑rotor impact crushers has also been increasing in recent years.

Figure 1‑38 shows a domestically manufactured φ1000 mm × 700 mm single‑rotor impact crusher. This type of crusher mainly consists of a housing, a rotor, and impact plates (or breaker plates). The electric motor drives the rotor through V‑belts. Material enters the crushing chamber (the space between the rotor and the impact plates) through the feed opening, where it is subjected to repeated impact and rebound between the blow bars (hammers) and the impact plates. The fragments are struck by the blow bars on the rotor and then thrown toward the second impact plate. A second crushing chamber is formed between the second impact plate and the rotor, and the above crushing process is repeated. In addition to the repeated impact crushing by the blow bars and impact plates, the material particles also impact and crush each other within the crushing chamber, and finally the crushed material is discharged from the lower part of the crusher.

Figure 1‑37 Different types of impact crushers

a–h — Single‑rotor impact crushers;
i, j — Reversible single‑rotor impact crushers;
k — Impact crusher with discharge grate;
l–p — Double‑rotor impact crushers

In terms of structure and principle, the main differences between impact crushers and hammer crushers are as follows:

① The blow bars of an impact crusher are rigidly connected to the rotor, and the inertia of the entire rotor is used to impact the material. During crushing, the material not only gains high speed and kinetic energy and is crushed by impact against the impact plates, but also undergoes interparticle impact crushing between the material particles themselves.

Figure 1‑38 φ1000 mm × 700 mm single‑rotor impact crusher

1—Housing;2—Blow bars (hammers);3—Rotor;4—Feed opening;5—Chain curtain;6—Impact plate (breaker plate);7—Pull rod (tie rod)

② The impact plates (usually two) of an impact crusher together with the blow bars form the crushing chamber. Under the impact of the blow bars, the material travels at high speed first into the crushing chamber formed by the first impact plate, where it is crushed, and then into the second crushing chamber for further crushing.

③ Impact crushers generally do not have grate bars or screen plates. The material is crushed repeatedly by the blow bars and impact plates. The product particle size is determined by the material properties, blow bar speed, radial clearance between the blow bars and impact plates, and the machine structure.

④ One end of each impact plate is hinged on the frame, and the other end is suspended on the frame by springs or pull rods. When an uncrushable object enters the crushing chamber, the impact plate is subjected to a high pressure and swings to a certain angle around its hinge, increasing the gap between the blow bars and the impact plate so that the foreign object is discharged. After that, the impact plate returns to its original position and normal operation resumes.

Impact crushers have the advantages of high production capacity, large crushing ratio, high crushing efficiency, light equipment weight, and uniform product size. However, when crushing hard or highly abrasive materials, the blow bars and impact plates wear severely. The blow bars and impact plates are commonly made of high‑manganese steel, high‑chromium cast iron, etc. Using 15Cr2Mo1Cu high‑chromium cast iron for impact plates to crush silica stone can increase the service life by 2 to 4 times compared with high‑manganese steel.

The structural shapes of the blow bars and impact plates in an impact crusher greatly influence the crushing performance. Commonly used cross‑sectional shapes for blow bars include rectangular, I‑shaped, T‑shaped, and S‑shaped. The surface shapes of impact plates are mainly of the folded‑line type (Figure 1‑38) and the involute type (Figure 1‑39). The latter offers higher crushing efficiency because the material impacts the impact plate surface perpendicularly at every point. In practice, impact plates built from multiple circular arcs to simulate an involute curve are sometimes used (Figure 1‑39).

The technical characteristics of domestically manufactured impact crushers are listed in Table 1‑22.

Figure 1‑39 φ1250 mm × 1250 mm double‑rotor impact crusher

1—First rotor;2—Feed opening;3—Housing;4—First impact plate (first breaker plate);5—Lower impact plate (lower breaker plate);6—Second impact plate (second breaker plate);7—Spring;
8, 10—Grate bars (screen bars);9—Second rotor

Table 1‑22 Technical characteristics of domestic impact crushers