Figure 6-10.-Conveyer.rubber stripping of the wipers adjusted to just makecontact with the face of the belt. This adjustmentremoves rocks from the belt, thus preventing rocks fromentering between the pulley face and inside face of thebelt.Three factors affect the efficiency of conveyeroperation. These factors are as follows: speed, loading,and incline.1. Speed. Most conveyers operate at a speed ofapproximately 300 feet per minute and have a capacityof approximately 300 tons of material per hour. Areduction in speed obviously reduces the conveyercapacity, and an increase in speed theoretically increasesthe capacity. An increase in conveyer speed may alsoincrease wear on the conveyer belt due to increasedslippage of the material at the loading point. An increasein speed also increases the throw of the material at thedischarge end of the conveyer. In some cases, it may benecessary to fit the end of the conveyer with a box orbangboard so the material from the belt falls properly.2. Loading. Proper loading of a belt conveyer ismandatory for efficent operation. This includes placingthe load in a position centered on the conveyer belt. Agood practice is to load a conveyer in such a mannerthat allows the material to strike the belt in the directionof travel. When material is to be delivered from a spoutor belt to another belt from one side, a transfer box orbangboard should be provided to facilitate properdelivery on the belt. Loading a conveyer belt on one sidecauses it to run to the opposite side of the support rollerswhich results in excessive belt wear.3. Incline. Portable conveyers can be adjusted tooperate at the various inclines required to meet jobconditions. The maximum incline is determined by thematerial carried on the conveyer and varies from 12degrees for washed gravel to 20 degrees for loose earth.When the maximum angle is exceeded, slippage mayoccur.WASH PLANTIn many types of construction operations, it is notnecessary to have washed clean aggregates in thefinished product; however, some types of operationsrequire clean aggregates free of objectionable material.The wash and screen plant (fig. 6-11) consists of ascrubber, a vibrating screen, and a screw (sand)dehydrator (fig. 6-12). In operation the raw material isfed into a scrubber designed to break loose alldeleterious material. The scrubbed aggregate and washwater are cast upon a triple deck vibrating screen for sizeseparation. The materials retained on the decks of thescreen are sent by individual chutes to their respectiveconveyers for further disposition. The wash water andsand are carried through the third deck into the well ofthe sand dehydrator, where the undesirable products areremoved from the sand by the abrasive washing actionof the spiral conveyer and carried out into a flume bymeans of the overflow water. The clean sand is carriedup and out of the water and discharged into a stockpilingconveyer.The capacity of the washing and screening plant isbased on the percent of sand in the deposit; for example,each single screw washer can handle a certain amountof material. The screen is also a factor to consider whenfiguring the capacity of this plant. It is necessary to haveenough screening area to handle each gradation ofmaterial desired.6-11
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