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Generally speaking, jaw crushers should not be fed from quick-dumping cars unless the receiving opening is large enough to virtually eliminate any danger of bridging. Because the receiving opening of this type of crusher is relatively narrow, it is almost always necessary to use a flared hopper above it to receive the feed from the car or truck-the hopper may be two or three times as long as the receiving opening is wide. This means that the material must converge as it flows or falls toward the receiving opening, and this confluence is a natural invitation to bridging if a large amount of rock is dumped quickly into the hopper.
Controlled-dumping, used in conjunction with side-hinged cars and incorporated in all modern quarry trucks, permits the use of larger equipment. While it does not entirely eliminate bridging, it materially lessens the likelihood of its occurrence by the simple method of holding back part of the load until the crusher has a chance to clear itself of the other part. Controlled-dumping works out well with any type of primary crusher. While it does not provide absolute control of- feed regulation, it is, next to the mechanical feeder, the best means of regulation for coarse material such as quarry-run or mine-run rock-certainly far superior to the quick-dumping car in this respect. It has one disadvantage: the time required to dump a load in two or more portions ties up the transportation equipment while the crusher is disposing of a considerable part of the original load. This tie-up is of no particular moment if the equipment consists of cars that are handled in trains of several units; for such operations the time-difference between quick- and controlled-dumping is negligible. On the other hand, if trucks are used, the aggregate idle time over an entire shift may add up to as much in the crusher house as it does at the shovel. Gyratory crushers, ~ because of the greater volumes of ~ their crushing chambers and their higher capacities, will usually function with les s time-loss on this score than will comparable sizes of jaw crusher’s.
Car, skip, or truck bodies used for dumping directly into crushers of the jaw or single sledging roll types should be as short as is consistent with required capacity because of the relatively narrow receiving openings of these types. This will help to cut down congestion of material in the hopper above the crushing chamber, a tendency which we have mentioned in a preceding paragraph. Another point in favor of the short body is the saving in headroom between receiving opening and dump level.
Somewhat longer bodies are permissible for gyratory crushers of comparable sizes because of the greater area and length of the receiving openings.
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