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The tip-velocity of the slugger teeth in the Fairmount crusher is from 400 to 450 ft per min., or in the neighborhood of 7 ft per sec. The working faces of the teeth are radial in profile, which means that the faces I\re normal to the line of action when a rock is contacted in any part of the crushing chamber.
When a mixed load of quarry-run stone is dumped into the hopper of the single-roll crusher the following stages may be observed: first, there is a pronounced selective segregation; the entire load is subjected to vigorous agitation, causing the smaller pieces of stone to sift down through the mass of material into the crushing chamber, where they are reduced by a succession of sledging impulses from both regular and slugger teeth as the broken and re-broken pieces are worked down the curved face of the anvil to the discharge point. During this stage, if there is any considerable amount of fine material in the load, the large pieces of rock are shoved up and held away from the roll by the mass of smaller pieces. As the crusher clears itself of the small stone the larger blocks come down into the zone of action. Here, too, there is a certain degree of segregation; the crusher seems to have an uncanny faculty of weeding out the small fry, then the middleweights, and finally taking on the heavyweights.
Unless the stone is of a friable nature very little breaking is done on top of the roll, i.e., by pure impact action; therefore these machines are not suited for crushing stone of massive or blocky structure. They are highly effective on stratified rock, where the maximum thickness of ledge is within the dimension which can be nipped between the advancing slugger teeth and the anvil. Working on such stone, the crusher will handle any size piece that will enter the upper part of the crushing chamber. Large slabs which lodge against the anvil at their lower end are fractured by the action of the slugger teeth beneath them, or are up-ended and thrown against the anvil and the back of the hopper, where they are held while the slugger teeth work on them from the bottom.
Naturally, during the action we have described the agitation in and above the crushing zone is quite violent, and therein lies an outstanding feature of this
crusher. The continuous heaving of the material minimizes bridging, ranking the crusher to a large extent self-feeding. Such bridges as do occur can frequently be broken by the simple expedient of throwing in a single piece of stone of sufficient size to raise the bridged material as the stone passes underneath it. Bridges may also be broken by dumping a load of small rock into the crusher; the selective action we have described will generally break the bridge unless the bridged pieces are very firmly wedged.
Blockades caused by single pieces of stone too large to enter the crusher are a more serious matter, and are usually more difficult to break than in the gyratory or jaw crusher, due to the fact that the process requires some care to minimize the element of personal danger.
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