Impact crushers utilize impaction to reduce the size of a range of materials. Impaction enables size reduction because the striking of one body against another, referred to as a collision, causes extreme pressure in between the two opposing forces. Impactors are often used in place of a crusher, another type of size reduction equipment that uses a material that is stronger than the material being pulverized.
Impact crushers are often used for secondary pulverizing rather than primary pulverizing and are often referred to as impactors as well. Impact crushers can be used on soft, medium-hard and very hard materials, both abrasive and non-abrasive. As a result of the wide range of materials they can pulverize, a wide range of industries make use of impact crushers.
A few examples are highway construction, for the impaction of building materials and rubble; mining, to impact both mined and quarried materials; industrial manufacturing, to impact various materials for use in further processing such as burning for power generation or easier processing; and recycling, for the impaction of large materials that must be reduced in size before they can be re-used.
Although the method of pulverizing remains the same, impact crushers can be generally categorized into two main types: horizontal shaft impactors (HSI) and vertical shaft impactors (VSI). The design of a HSI crusher includes three main parts: impeller bars, curtain liners and adjustable liner plates. In a HSI crusher, materials undergo impaction when they reach the breaker-plate, at which point hammers that are fixed to the outer edge of a spinning rotor, are used to impact the material.
HSI crushers can be used for wet or dry materials, but not sticky materials, and are best suited for soft materials such as dolomite aluminum, weathered shale and clay. Although they are similar to HSI crushers, VSI crushers differ in that they feature a high speed rotor as well as a crushing chamber that is much tougher in design.
These additional features are necessary because in VSI crushers, velocity rather than hammers are used for impaction; they involve throwing the materials against hard walls at high speeds. VSI crushers can be further broken down into two more categories: shoe and anvil VSI crushers and autogenous VSI crushers. While both are well-suited for sand and gravel applications, shoe and anvil VSI crushers are better-suited for recycling applications, whereas autogenous VSI crushers are best-suited for quarried materials.