Rock Products

CRUSH 2018

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50 | Crushing & Breaking October 2018 Products Products McLanahan A secondary impact crusher uses mass and velocity to break down feed material. First, the feed material enters the crushing chamber along a controlled feed angle and struck by rotating hammers (blow bars) installed in the rotor assembly. This initial impact fractures the stone sending particles into the ad- justable curtains or aprons, which form multiple crushing chambers. The material continues to rebound into the path of the hammers, and is allowed to exit the crushing chambers as it reduces in size. A reduction ratio of 10:1 is very common. Impact crushers also tend to be used where shape is a critical requirement and the feed ma- terial is not very abrasive. The crushing action of an impact crusher breaks a rock along natural cleavage planes giving rise to better product quality in terms of shape and soundness. The quality of these products makes them ideal for a multitude of construction projects and mineral processing operations. • Secondary impactors feature disc-type rotor assemblies de- signed to secure reversible hammers, and provide significant recesses between the discs as tramp iron relief, and additional access for hammer changes as a means to reduce mainte- nance time and costs. • Impactors in a secondary application are known to draw 1 hp per short ton of material under load. A crusher charged with 100 short tph will need 100 hp to accomplish the 10:1 reduction in a limestone application. The friability and bulk density of the material are also major factors that affect power requirements. • Unlike compressive crushers, secondary impact crushers are rather forgiving with regard to the characteristics of the feed ma- terial. While all crushers benefit from controlled feed rate and consistency, impactors tend to tolerate larger feed sizes, higher moisture and clay content, elongated particles, and softer ma- terials that may cause compaction or pancaking in compressive crushers. • Impactors have advanced to include a broad scope of replace- ment parts designed to provide extended life and prolonged productivity. Cast hammers are available in a wide range of al- loys to best complement the application. Easy access to the hammers allows for flexibility when moving from one location to another. • Hammer profiles offer a sharp edge to optimize the impact force upon the stone, but subject to wear. A protruding leading edge, provides a means to maintain the output gradation throughout the life of the hammer. • Rotor speed is the most critical adjustment for an impact crush- er. The RPM of the rotor will affect the output with significant variance's in the gradation. In closed circuit operations, the RPM can be changed with sheave selection to keep the recirculating load under control. • Curtain adjustments are initially set to help control the output in material with particular characteristics. Often the optimum set- ting of the bottom curtain may be a gap greater or less than the desired top size of the product. It depends on the friability of the stone. Curtain adjustments also provide a means to maintain the optimum gap setting by adjusting for wear on the hammers. • The efficiency of a secondary or tertiary HSI decreases with the mass and size of the material. The smaller the size, the less effective the crushing. Most HSI crushers offer two curtain or aprons, a crusher with two chambers. These impactors are known for generating outputs ranging from 3- to 3/4-in. with peak efficiency. • Some HSI crushers offer an adjustable third chamber that pro- vides additional reduction through attrition. Coupled with higher RPM, a third curtain may increase the volume of chip stone and provide more control over the recirculating load. McLanahan's Universal NGS Impact Crusher is a secondary impac- tor designed to bring more profitability to an operation by providing a highly cubical product with superior soundness and reducing over- all operating expense. This secondary impactor offers reduced downtime for adjustments, fewer wear parts to inventory and the opportunity for field modifi- cations to accommodate changes in material requirements. Uni- versal NGS Impact Crushers feature hydraulic-shim adjustment as standard on each impact curtain. These independent gravity-hung impact curtains provide more tramp iron relief with significant re- traction capability. This coupled with the ability to hold the gap better under heavy surge conditions by using gravity, a free and reliable force. The NGS Impact Crusher is truly a next generation secondary impact crusher. Offering large inspection doors for easier access, wrench-free cur- tain adjustments, interchangeable wear parts, S-shaped hammers with a protruding leading edge, and optional features that include a cost-effective means for "adjust-on-the-fly," or even full PLC auto- mation with a patented "hands-free" calibration and the Unimation program package.

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