Impact

The bottom of the PCC Reversible Impactor is open, allowing sized materials to pass through almost instantaneously. Liberal clearance between hammers and the breaker blocks eliminates attrition. Crushing is by impact only.

In crushing, impact is the sharp, instantaneous impingement of one moving object against another. Both objects may be moving (a bat connecting with a ball), or one object may be motionless (a golf ball hit off a tee).

Two Variations of Impact

Gravity impact:  An example of gravity impact is coal being dropped onto a hard surface such as a steel plate. This method is most often used to separate two materials which have different friability. The more friable material is broken, while the less friable material remains unbroken. Separation is then done by screening. The PCC Bradford Breaker is an example of a crusher which uses gravity impact only.

Dynamic impact: Material dropping in front of a moving hammer (both objects in motion) illustrates dynamic impact. When crushed by gravity impact, free-falling material is momentarily stopped by the stationary object. But when crushed by dynamic impact, the material is unsupported and the force of impact accelerates movement of the reduced particles toward breaker blocks and/or other hammers. Dynamic impact is the crushing method used by PCC Impactors. Dynamic impact has definite advantages for the reduction of many materials and is specified under the following conditions:

• When a cubical particle is needed.
• When finished product must be well graded, and meet intermediate sizing as well as top and bottom specifications.
• When ores must be broken along natural cleavage lines in order to free and separate undesirable inclusions (such as mica in feldspars).
• When materials are too hard and abrasive for hammermills, but where jaw crushers cannot be used because of particle shape requirements, high moisture content or capacity.

Attrition

When a PCC Non-Reversible Hammermill is used for reduction, material is broken first by impact between hammers and material and then by shear and a scrubbing action (attrition) of material against screen bars.

Attrition means the reduction of material by scrubbing it between two hard surfaces. Attrition is practical for crushing less abrasive materials such as pure limestone and coal. Attrition crushing is most useful in the following circumstances:

• When material is friable or not too abrasive
• When a closed-circuit system is not desirable to control top size.

Shear

Reducing large feed by a combination of shear, impact and compression. PCC Hercules Single-Roll Crushers are noted for low headroom requirements and large capacity.

Shear is a trimming or cleaving action that is usually combined with other methods: single-roll crushers, for example, use shear, impact and compression methods combined. Shear crushing is usually called for under these circumstances:

• When material is somewhat friable and has relatively low silica content.
• For primary crushing with a reduction ratio of 6 to 1.

Compression

Crushing by compression is done between two surfaces, with the work being done by one or both surfaces. Jaw crushers using the compression method are suitable for reducing extremely hard and abrasive rock. As a mechanical reduction method, compression should be called for in these circumstances:

• If the material is hard and tough
• If the material is abrasive
• If the material is not sticky
• When the material will break cubically

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