The name grizzly screen is often given to a screen surface for separating particles at a very coarse size e.g. >25mm and often 200 to 250mm. As a consequence, the screen surface must be extremely robust and often comprises a series of parallel or slightly divergent alloy steel bars or rails. Indeed, simple fixed or static grizzles may be constructed from rail section or I-section girders. If fixed, the grizzly screen surface must be steeply inclined (40-60 ~ ) to promote material flow. It may also be incorporated in a vibrating frame at a lesser inclination to act as both a sizing device and a regulated feeder device. 'Live' grizzly screens are also constructed using a series of rotating cylindrical or elliptical rolls and travelling link assemblies. These devices are less prone to blockage, provide a very positive control upon feed rate to the crusher and can be mounted horizontally to save headroom. The aperture of the grizzly may also be varied if desired. Common applications are described below.
(1) To prevent occasional oversize particles from entering a primary crusher or a materials transport system such as a field conveyor. The oversize may be broken separately by drop-ball machine, hydraulic hammer etc or simply rejected to waste.
(2) To remove undersize material from the feed to primary crusher either to by-pass this stage of comminution or to form a distinct quarry product often known as 'scalpings'. This product may be sold or processed separately