Gravity mixers are the oldest type of concrete mixers. They require no power, the materials being mixed by striking obstructions which throw them together in their descent through the machine. Their construction is very ple. Fig. 122 illustra tea a able gravity mixer. This mixer, as will be seen from the figure, is a steel trough or chute in which are containcd mixing members consisting of pins or blades. The mixer is portable, and quires no skilled labor to ate it. There is nothing to get out of order or cause delays. It is adapted for both large and small jobs. In the former case, it is usually fed by measure, and by this method will produce crete as fast as the materials can be fed to their respective bins and the mixed concrete can be taken from the discharge end of Ihe mixer. On very small jobs, the best way to operate is to measure the batch in layers of stone, sand, and cement respectively, and feed to the mixer by men with shovels.
There are two spray pipes placed on the mixer: for feeding by hand, one spray only would be used; the other spray is intended for use only when operating with the measure and feeder, and a large amount of water is required. These sprays are operated by dles which control two gate-valves and regulate the quantity of water flowing from the spray pipes.
These mixers are made in two styles, sectional and non-sectional. The sectional can be made either 4, 6, or 8 feet long.
The non-sectional are in one length of 6, 8, or 10 feet. Both are constructed of -k-inch steel. To operate this mixer, the materials must be raised to a platform, as shown in Fig. 123.
Rotary mixers, Fig. 124, generally consist of a cubical box made of steel and mounted on a wooden frame. This steel box is supported by a hollow shaft through two diagonally opposite corners, and the water is supplied through openings in the hollow shaft. Materials are dropped in at the side of the mixer, through a hinged door. The machine is then revolved several times, usually about 15 times; the door is opened; and the concrete is dumped out into carts or cars. There are no paddles or blades of any kind inside the box to assist in the mixing. This mixer is not expensive itself, but the erection of the frame and the hoisting of the stone and sand often render it less economical than some of the more expensive devices.
Rotating mixers which contain reflectors or blades, Fig. 125, are usually mounted on a suitable frame by the manufacturers. The rotating of the drum tumbles the material, and it is thrown against the mixing blades, which cut it and throw it from side to side. Many of these machines can be filled and dumped while running, either by tilting or by their chutes. Fig. 125 illustrates the Smith mixer, and Fig. 126 gives a sectional view of the drum, and shows the arrangement of the blades. This mixer is furnished on skids with driving pulley. The concrete is discharged by tilting the drum,which is done by power.
Fig. 127 represents a Ransome mixer, which is a batch mixer.
The concrete is discharged after it is mixed, without tilting the body of the mixer. It revolves continuously even while the concrete is being discharged. Riveted to the inside of the drum are a number of steel scoops or blades. These scoops pick up the material in the bottom of the mixer, and, as the mixer revolves, carry the material upward until it slides out of the scoops, which therefore assist in mixing the materials.
Fig. 128 represents a McKelvey batch mixer. In this mixer, the lever on the drum operates the discharge. The drum is fed and dis charged while in motion, and does not change its direction or its position in either feeding or discharging. The inside of the drum is provided with blades to assist in the mixing of the concrete.
Paddle mixers may be either continuous or of the batch type. Mixing paddles, on two shafts, revolve in opposite directions, and the concrete falls through a trap door in the bottom of the machine. In the continuous type the materials should be put in at the upper end so as to be partially mixed dry. The water is supplied near the middle of the mixer. Fig. 129 represents a type of the paddle mixer.
333. Automatic Measures for Concrete Materials. Mechanical measuring machines for concrete materials have not been very ex tensively developed. One difficulty is that they require the constant attention of an attendant, unless the materials are perfectly uniform. If the machine is adjusted for sand with a certain percentage of moisture, and then is suddenly supplied with sand having greater or less moisture, the adjustment must be changed or the mixture will not be uniform. If the attendant does not watch the condition of the materials very closely, the proportions of the ingredients will vary greatly from what they should.
The Trump measuring device, shown in Fig. 130, consists of a horizontal revolving table on which rests the material to be measured, and a stationary knife set above the table and pivoted on a vertical shaft outside the circumference. The knife can be adjusted to extend a proper distance into the material, and to peel off, at each revolution of the table, a certain amount, which falls into the chute. The material peeled off is replaced from the supply contained in a bottom less storage cylinder somewhat smaller in diameter than the table and revolving with it. The depth of the cut of the knife is adjusted by swinging the knife around on its pivot so that it extends a greater or less distance into the material. The swing is controlled by a screw attached to an arm cast as part of the knife. A micrometer scale, with pointer, indicates the position of the knife. When it is desired to measure off and mix three materials, the machines are made with three tables set one above the other and mounted on the same spindle so that they revolve together. Each table has its own storage cylinder above it, the cylinders being placed one within the other, as shown in Fig. 131.