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Introduction to Design Principle of Jaw Crusher

Introduction

The first stage of size reduction of hard and large lumps of run-of-mine (ROM) ore is to crush and reduce their size. Softer ores, like placer deposits of tin, gold, mineral sands etc. do not require such treatment. Large scale crushing operations are generally performed by mechanically operated equipment like jaw crushers, gyratory crusher and roll crushers.

For very large ore pieces that are too big for receiving hoppers of mechanically driven crushers, percussion rock breakers or similar tools are used to break them down to size.

The mechanism of crushing is either by applying impact force, pressure or a combination of both. The jaw crusher is primarily an compression crusher while the others operate primarily by the application of impact.

Design of Jaw Crushers

Jaw Crushers are designed to impact on a rock particle placed between a fixed and a moving plate (jaw). The faces of the plates are made of hardened steel. Both plates could be flat or the fixed plate flat and the moving plate convex. The surfaces of both plates could be plain or corrugated. The moving plate applies the force of impact on the particles held against the stationary plate. Both plates are bolted on to a heavy block. The moving plate is pivoted at the top end (Blate crusher) or at the bottom end (Dodge-type crusher) and connected to an eccentric shaft. In universal crushers the plates are pivoted in the middle so that both the top and the bottom ends can move.

The Blake crushers are single or double toggle drives. The function of the toggle(s) is to move the pivoted jaw. The retrieving action of the jaw from its furthest end of travel is by springs for small crushers or by a pitman for larger crushers. As the reciprocating action removes the moving jaw away from the fixed jaw the broken rock particles slip down, but are again caught at the next movement of the swinging jaw and crushed. This process is repeated until the particles sizes are smaller than the smallest opening between the crusher plates at the bottom of the crusher(the closed set). For a smooth reciprocating action of the moving jaws, heavy flywheels are used in both types of crushers.

Fig. 4.1 shows a sketch of a Blake Crusher operated by double toggles and controlled by a pitman. These are commonly used as primary crushers in the mineral industry. The size of the feed opening is referred to as the gape. The opening at the discharge end of the jaws is referred to as the set.

Fig. 4.2 is a sketch of a Dodge type of crusher. They are comparatively lower in capacity than the Blake crushers and are more commonly used in laboratories.

The factors of importance in designing the size of primary crushers, like jaw crushers, are:

Vertical height of crusher ≈ 2×Gape(4.1)

where the crusher gape is in meters.

These dimensions vary as individual manufactures have their own specifications and their catalogues are a good guide to the geometry and design of individual makes.

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