CONCRETE MIXERS Concrete mixers comprise two general classes —continuous mixers and batch mixers. With the "continuous" type, the feeding of the materials, the mixing, and the discharge of the concrete form one continuous process which may be car ried on indefinitely. With "batch" mixers, on the other hand, only certain measured quantities of cement, sand, etc., are fed to the machine, and only a definite quantity or "batch" of concrete is made and discharged at a single operation, the process being repeated as often as may be neces sary. Some mixers are designed so that they can be adjusted to do work either continuously or in batches, and thus combine features of both classes of machines.
Plates 6 to 10 show various mixers of the batch type; and in Plates 11 and 12 are shown examples of the continuous and combined types.
In continuous mixers, the general mechanical principle is that of a long screw or pug-mill, the mixing being done in a long drum by means of blades or paddles mounted on revolving shafts, which, somewhat like the thread of a screw or the blades of a propeller, agitate the materials and at the same time shove them along toward the discharge end of the drum. In the combined type, the discharge opening in the drum may be closed, the feed cut off, and only a batch mixed at a time.
Concrete mixers are further distinguished ac cording to their mechanical construction and operation, into three classes—namely, gravity mixers, revolving or rotary mixers, and paddle mixers.
In the gravity type, as the name indicates, the operating force is that of gravity. The mate rials are mixed by simply being thrown down an inclined chute. On the interior surface of this chute are fastened projecting members in the shape of pins or blades, which are successively struck by the materials in their descent. These mixers are usually constructed of sheet steel, and may be had in sectional form adjustable to the greater or less height of the raised platform from which the materials are dumped. Mixers of the gravity type were the earliest mechanical mixers used.
Rotary mixers comprise a great variety of commercial types. In general, they consist of a hollow, revolving drum or box usually power driven, but sometimes operated by hand. In some eases the shape of the box or the form of the plates composing the drum, combined with the manner of mounting, is depended upon to do the mixing as the machine revolves, without the assistance of any interior paddles or blades. In other cases, blades or reflectors are mounted in the interior of the drum in positions calculated to intercept the material, cut through it, and throw it about from side to side, thus mixing it thoroughly. Many machines of this type can be filled and discharged while running.
The "Smith" Mixer, illustrated in Plate 6, is a batch mixer of the rotating drum type. The double conical mixing drum rests in a steel, U shaped, swinging cradle. Two cast-iron arms, or "trunnions," are firmly attached to the upper extremities of the IT-frame. These trunnions project into upright supporting castings, often called "pedestals." The cradle, supported by and between the pedestals, swings freely on the trunnions.
To discharge the contents of the drum, the supporting cradle is swung upon the trunnions, and the drum tilted to a steep angle. The tilting process in no way interferes with the continuous revolution of the drum, as the main shaft of the mixer passes through the trunnions, at the center of the circle in which the cradle swings. The mixer tilts with ease, owing to the fact that the center of the tilting arc is considerably below the center of the drum. Thus, the batch is swung on, or almost on, its own center. The smaller-sized machines are tilted by means of a hand-lever; larger sizes, by means of a worm and worm-wheel tilting apparatus or a special power tilting device.
The drum consists of two steel cones riveted to a central iron ring. The cones have removable linings to reinforce the shell at the points where the falling materials cause the greatest wear upon the drum.