Secondary crushers were not used until the large open-side settings in the huge primary crushers built early in the 20th century made them necessary. The first secondary crushers were scaled-down standard gyratory crushers, but they had a tendency to “choke” and their productivity was low, making them inefficient for fine crushing. The growing popularity of reinforced concrete created an increasing demand for smaller sizes of crushed stone and gravel, and attempts to meet this demand with existing crushers highlighted the need for a better fine crusher. The crushing system in a stone plant built in Thornton, Illinois, in 1913 was typical of the new development. It comprised one 1.2-m primary crusher, four secondary crushers with 178-mm feed openings, and 16 finishing crushers—all gyratories. Two sets of roll crushers were added later to augment the production of small stone.
Attempts were made before World War I to adapt the standard gyratory to fine — crushing duties by making a “shorthead” model, which consisted simply of an abbreviated crushing head installed in a standard machine with concaves to match. Crusher liners were known as “concaves,” because they were attached to the inner surface of the crusher forming the concave crushing zone. This model did not prove to be very successful; crushing stresses were concentrated at a point where the top shell was ill-equipped to withstand them. In addition, the throw at the point of discharge was too small to take full advantage of the increased diameter of the discharge opening.
By the early 1920s, several engineering companies were changing the design of the crushing chamber and the concaves in order to build efficient fine crushers. The Superior — McCully fine-reduction crusher was the first to show a significant improvement over other models when the crushing chamber was modified to include a cylindrically bored shell, vertical concaves, and a flared crushing head. The eccentric speeds were increased, and throws were adjusted for effective operation at fine settings. Then the concaves were tapered at both ends to distribute the wear better, and eventually these “nonchoking” concaves became standard. This development of nonchoking concaves, using the principle of curved-profile crushing chambers, was the most important and far-reaching improvement in crusher design that had been made for many years—possibly the greatest since the inception of the gyratory crusher. Allis-Chalmers designed nonchoking concaves that improved the productivity of crushers and had the advantage that they could be fitted to hundreds of crushers existing at the time. Figure 5.16 shows the differences between straight-faced concaves and nonchoking concaves.
The curved nonchoking concaves crushed rocks more effectively before they entered the final crushing region, and they controlled the flow of pebbles through the crusher better. Allis-Chalmers’ strategy was to maintain its position as a leading manufacturer of size-reduction equipment by innovative engineering and design work, and by the occasional purchase of competitors such as the manufacturer of the Superior-McCully fine crusher and its line of primary gyratory crushers known as the McCully crusher. Soon after this purchase, Allis-Chalmers was offered the patents for a reduction crusher developed by Will Symons, a Chicago engineer. Allis-Chalmers declined, however, no doubt because two new and competing crushers within a short time would have been difficult for the company to digest. Instead, the Nordberg Company of Milwaukee purchased the patents for the Symons high-speed secondary crusher. In time the Symons cone crusher virtually killed the sales of the McCully fine-reduction crusher and seriously reduced the sales of the smaller Allis-Chalmers crushers.