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to draw it into the suction pipe and obtain a satisfactory capacity without the aid of an agitator.

The dredge for this class of service becomes exceedingly simple, consisting principally of the dredging pump with its driving equipment mounted on a scow; the suction being a pipe of sufficient length to reach to the bottom with a piece of flexible suction hose in same to give necessary flexibility so that the suction can be raised and handled as desired.

The material is usually delivered into a flat deck scow with raised sides, so that the sand is retained and the water flows back into the river. Sometimes these dredge boats are self-propelling and provided with hoppers into which the material is pumped.

[graphic]

FIG. 1,680.-Marion gold dredge used in winter gold dredging, Alder Gulch, Montana.

Elevator Dredges.-This type of dredge is constructed with an endless chain of buckets which travel on sprockets attached to suitable booms, the rig being designed for such service as excavating gold bearing material.

The material is deposited near the center of the dredge, where it is dumped into screens which separate the stones and coarser material from the gold bearing sand. The sand then passes over riffles and gold saving tables and, after the gold is collected, passes on to the tailings pile at the rear.

Fig. 1,680 shows Marion gold dredge used for winter gold dredging, Alder Gulch, Montana.

CHAPTER 33

CONCRETE MIXERS

Concrete mixture is cement, sand, and gravel or broken stone, mixed with water. The quality of a concrete mixture is determined by

1. The quality and proportions of the materials composing it, and

2. Its uniformity.

Hence, the best mixture depends not alone on the quality of material but upon the quality of the mixing.

To properly mix concrete, the elements must be successively subdivided and re-arranged in new relations toward each other. To keep the fine pieces from sifting to the bottom, a pouring action is necessary, combined with a horizontal endwise movement, averaging all parts of the batch.

The essential parts of a concrete mixer are:

1. Charging hopper;

2. Mixing drum;

3. Water tank;

4. Discharge device.

The Charging Hopper. This should be not over, 36 to 40 inches high, so as to be convenient for either shovel or wheel barrow charging. It should also be wide enough to receive the discharge of an ordinary concrete barrow, and have high wings to keep the gravel from spilling.

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FIG. 1,687.-Grand charging hopper. In operation, as hoppers B, and C, reciprocate backward and forward, the material that is thrown into hopper B, is forced toward the mixing trough at one move of the hopper, shoving charge I, off into the mixing trough and at the same time "striking off" charge J. At the next movement of the hopper, charge J, is shoved into the mixing trough and charge I, is "struck off." The amount of these charges is determined by the height of steel wire brushes G1 and G2, which are adjustable. The amount of the cement discharged is regulated by a steel slide which passes over the cement feed roll. This slide is drawn out certain distances for different proportions, allowing only the uncovered part of cement roll to fill and discharge. After slide is set for desired proportion it is locked in place by set screw so there is no possible chance for variation in the proportion. By discharging the cement and other materials at the same time we receive a good gravity mix before the material even reaches the mixing trough.

Mixing Drum.-The drum is usually cylindrical and rotates on bearings called trunnion rollers.

The tracks encircle the drum which bear upon the rollers, and support the drum as it turns. At the center is a circular rack to which power is applied through a pinion for driving the drum. At each end of the drum is a head having a central opening, one for charging and one for discharging. The general appearance of a mixing drum as described is shown in fig. 1,691.

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SAND

CEMENT

STONE

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FIGS. 1,688 to 1,690. partment charging hopper. tion, cement enters through the center and sand, crushed rock, or gravel from either or both sides. A constant stream of materials are fed simultaneously into the mixing drum, the mechanism being adjustable so that any proportion may be obtained. The feed is controlled by an individual clutch and may be stopped or started regardless of the rest of the machine. Often for various reasons, the shovelers stop for a minute or two. In such cases the feed is thrown out of gear by a clutch lever. Further if it be desired to empty the drum, the feed may be stopped while the balance of the mixer continues until the materials are all discharged. The materials are conveyed or forced into the mixing drum by three conveyor chains with scraper links. These conveyor chains are driven by three sprocket wheels, mounted on a square steel shaft, which prevents any possibility of their slipping and all are traveling at the same speed, and by the adjustments of the steel brushes and the fact that it can be fed from either or both sides, gives the machine a capacity up to one hundred yards in ten hours. The cam or eccentric proportioning discs which are graduated and provided with a positive locking device. The parts are: A, chain, B, scraper links for sand; C, scraper links for cement; D, idle sprocket; E, idle sprocket for sand chain; F, idle sprocket for cement chain; G, square shaft; H, sprocket feed drive; J. adjustment for thickness of brushes; K, cement slide; L, brushes for sand or stone; M, proportioning cam for sand or gravel; N, proportioning cam for cement; O, proportioning cam for stone or sand; P, slotted slide for holding proportioning cam; Q, lock nut for double locking proportions; R, short section of mixing drum.

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