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Crushers are used to break up large pieces of rock and bulk materials. The size of the eventual pieces depends largely on the type of crusher unit used; some models are designed to merely break large stones into manageable segments, while others are engineered to reduce an entire work piece to dust. Crushers are widely used in mining, materials processing and demolition applications.

There is an extensive range of crushing units available through manufacturers, including rotor crushers, impactors, and jaw crushers. Jaw crushers use a pair of opposing jaw plates set in a V, at the base of which is an opening for crushed material to fall through. Because of the spacing of the jaw plates, the large pieces are crushed at the top of the plate, and become progressively smaller as they fall downward. This causes the finely crushed pieces to exit the system while the larger pieces remain for additional crushing.

Impactors drive various crushing arms and surfaces into the work piece to reduce its size using forces resulting from violent impact. The hammer mill, which uses a series of swinging bars to crush materials, is a common type of impact crusher. Rotor crushers involve multiple sets of rotors that spin in opposite directions to one another. As large work pieces are snagged by the rotors, they are pulled into the blades of an opposing rotor and crushed. When a piece achieves the desired size, it slides through the rotors and out of the crushing unit.

Essentially, there are four types of crushing operations: shearing, impact, compression and abrasion. Shearing is a cutting action that cleaves a work piece between the edges of two surfaces. Compression involves squeezing with a work piece between two opposing surfaces with increasing pressure until it breaks. The abrasion, or attrition, process churns materials from side to side, reducing their size—machines used for this process are typically called grinders. Impact crushing is similar to compression, except that the crushing surface is driven violently and repeatedly into the work piece.