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There are four basic ways to reduce a material : Most crushers employ a combination of all these crushing methods. Impact: In crushing terminology, impact refers to the sharp, instantaneous collision of one moving object against another. Both objects may be moving, such as a baseball bat connecting with a fast ball, or one object may be motionless, such as a golf ball being driven off a tee. Attrition: Attrition is a term applied to the reduction of materials by scrubbing it between two hard surfaces. Hammermills operate with close clearances between the hammers and the screen bars, and they reduce by attrition combined with shear and impact reduction. Though attrition consumes more power and increases wear on hammers and screen bars, it is practical for crushing less abrasive materials such as pure limestone and coal. Shear: Shear consists of a trimming or cleaving action rather than the rubbing action associated with attrition. Shear is usually combined with other methods. For example, single-roll crushers employ shear together with impact and compression. Compression: As the name implies, crushing by compression is done between two surfaces, with the work being done by one or both surfaces. Jaw crushers using this method of compression are suitable for reducing extremely hard and abrasive rock. The Advantage of Closed-Circuit Crushing Closed-circuit crushing is a means of controlling product top size by screening the product and then returning oversize material to the feed end of the crusher for another pass through the machine. While it may be possible to obtain a specified top size from crushers without using a closed-circuit system, it is not always desirable. To control top size from a single crusher operating in an open circuit, material must remain in the crushing chamber until it is reduced. This can result in overcrushing a percentage of the product, with a corresponding increase in fines and a loss of efficiency. In a typical multiple-stage crushing plant with the last stage operated in closed-circuit, the primary crusher operates at a setting which produces a satisfactory feed size for the secondary crusher, so that a balance exists for the work done by each crusher.
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