The Mckelvey Mixers2 are made in two styles, continuous and batch. Both styles are cylinders revolving on friction rollers, and having, on the interior, deflecting blades and a patent "gravity shovel" which lies against the rising side of the drum and casts the materials downward when the cylinder has revolved far enough to overturn the blade. The batch mixer has a shorter cylinder and can be discharged at will. These mixers may be fed by shovels, or they may be provided with a hopper into which the materials may be dumped from carts or barrows. They discharge directly into wheelbarrows. The mixer, and an engine and boiler to run it, are mounted compactly on a truck, or the mixer is furnished on a steel frame without an engine.
331. The pan mixer3 consists of a large shallow pan into which may be lowered a framework carrying a series of plows. The materials are spread in the pan in layers, the plows are lowered into it, and the pan is revolved about its vertical axis, the plows remaining stationary. The plows are so arranged as to move the materials radially toward and away from the center of the pan. The water may be added from a rose nozzle. For dumping, an opening is made in the bottom of the pan by withdrawing a slide. Were the plows made to revolve in a stationary pan, the concrete would be more conveniently dumped in a pile, or in a car, instead of being scattered about under the pan.
1 Contractors' Supply Co., 232 Fifth Ave., Chicago.
2 McKelvey Concrete Machinery Co., N. Y. Life Bldg., Chicago.
3 Clyde Iron Works, Duluth, Minn.
The Cockburn,1 a continuous mixer, is in the form of a long box square in cross-section, surrounded at either end by circular rings supported on friction rollers. By suitable gearing the mixer is revolved about its longest axis, which has a slight inclination toward the discharge end. The materials are added through a hopper at one end, and fall from one side of the box to the adjacent side as the machine revolves, working gradually toward the delivery end, which is open. The water is added through a pipe at about one-third of the length of the box from the feed end. While this machine has no complicated system of blades to become clogged, the mortar has a tendency to stick in the corners of the mixer, making the interior cylindrical, and thus much less effective in mixing. Striking the sides of the box with a heavy hammer will detach the mortar, and this requires occasional attention.
333. A common form of continuous mixer consists of a screw working in a cylinder. The materials are fed to the cylinder near one end and are mixed while being gradually worked toward the other end by the screw. The water is added through a fixed perforated pipe at a point about one-third of the distance from the feed end of the cylinder, and the mixed concrete falls from the outlet at the other end. This style is frequently made in a light form and mounted on wheels, and is then convenient in the laying of concrete for pavements.
A modification of the screw mixer consists of a semi-cylindrical trough, in which revolves a shaft carrying blades set at right angles to the shaft and to each other. The trough is sometimes given a slight inclination to the horizontal, and the blades are so shaped as to assist in working the materials toward the delivery end.
The Drake Mixer2 is of the general form just described. One of the machines made by this company is a semi-cylindrical trough in which revolve in opposite directions two shafts, each carrying some thirty blades. Most of the blades are straight, but some of them are curved to work the material toward the delivery end.
1 Cockburn Barrow and Machine Co., Jersey City, N. J.
2 Drake Standard Machine Works, 298-302 W. Jackson Boul., Chicago.
An appliance recently devised, which is called a concrete mixer, consists of a steel trough provided with staggered pins and deflecting plates. The trough is supported in an inclined position and has a hopper at its upper end. water is supplied through spray pipes at the side of the trough. The materials, stone, sand and cement, are spread in layers on the mixing platform, with the stone at the bottom. The materials are then thrown into the hopper; they are mixed as they descend through the pins, and the product is caught in barrows or carts at the bottom.
336. In a very able article on concrete mixers,1 Mr. Clarence Coleman, M. Am. Soc. C. E., makes an analytical discussion of the relative efficiencies of the several forms. In this analysis . he gives the following weights to the several requirements for a perfect mixer. That the entire mass of concrete shall be so commingled that the cement shall be uniformly distributed throughout the batch is given a weight of forty; that the amount of water shall be subject to control is given a weight of twenty-five; perfect dry mixing and relative time of mixing, each ten; and receiving materials, discharging concrete and self-cleaning, are each given a weight of five.
The first three requirements, with a combined weight of seventy-five, relate to the production of good concrete, while the remaining requirements, with a combined weight of twenty-five, pertain to economy in use. In short, the first requisite is that a machine shall be capable of producing a perfect mixture; then the machine that accomplishes this result at the lowest Cost per cubic yard is the best. The choice of a machine will depend frequently on the character of the work to be done, as some machines can only be used economically where large quantities of concrete are to be used in a restricted area, while others are particularly adapted for portable plants.