Crushing is the critical first step in the production process, as opposed to blasting, which is the critical first step in the "reduction" process. The industry is moving toward larger primary crushers as production capacity objectives increase.

The proper selection of the primary crushers in an aggregate plant is extremely important. After the original reduction of material in the quarry shot, the primary crushing stage is the most economical segment of the aggregate plant in which to make material reduction. While for years the primary was merely looked upon as a preparation for further crushing, today the primary machine is considered for its contribution to the overall production of various sizes as well as the product quality created.

Primary selection can make up to a 30-percent difference in production rate. The amount of secondary crushing required is also affected greatly by the type of primary utilized.

Up to a 30-percent difference in the amount of secondary crushing required is also possible. When these factors are analyzed along with the fact that up to 40-percent difference in the initial equipment cost can be realized, the importance of primary crusher selection is easy to see.

The factors to be considered in the correct selection of a primary crusher are:

1. The maximum feed size the crusher is expected to accept.

2. The product size allowed to be fed to the secondary crushers.

3. The production rate required yielding this correct product size.

4. The abrasive content and general analysis of the aggregate to be crushed.

5. The availability of maintenance for the crusher.

The two basic types of primary crushers are compression-type machines; in this case, a primary jaw crusher or gyratory crusher, and the impact type, such as the andreas style or the single-impeller impact breakers. A jaw crusher being a compression-type crusher, reduces material as it advances downward into the crushing chamber and is compressed and fractured. Each particle or rock is crushed an average of two to two and one-half times as it passes through the jaw with a reduction ratio of approximately six to one. Normally fines are scalped out in the feed, via a grizzly feeder or screen, prior to entering the jaw. With a gyratory crusher we do not remove the fines and allow some material on material reduction in the long primary chamber. Gyratory installations are taller and heavier and not suitable for any type of mobile operation.

In an impact crusher, the incoming feed drops vertically through the chamber, strikes the rotating impeller bars and is then impacted against the breaker plates or bars within the chamber. Due to this severe impacting of material, a single-impeller impact breaker realizes up to a 20 to 1 reduction ratio. Because we are working with the theory of mass and velocity, changing the speed of the rotor will change the output gradation. Faster speed produces finer output.