This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol6", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
A Mortar-Mill is indispensable on work where large quantities of mortar are required. The type illustrated (Fig. 236) is one of the most convenient forms, being combined on one portable frame with its own engine and boiler. The lime, sand, etc., is introduced into the pan, together with the proper proportion of water, and the engine is then started, causing the pan to revolve, and also the rollers, since they bear on the bottom of the pan. This combined action of pan and rollers mixes the ingredients intimately together, scrapers being arranged so as to continuously return the mixture under the rollers.
When combined sufficiently the mortar can be withdrawn either by catching it in a shovel while the mill is revolving or by stopping the mill and shovelling it out into barrows, trucks, or skips as the case may be.
The engine in the type illustrated is arranged to be quite independent of the mortar-mill framing, thus doing away with any chance of being thrown out of line by strains or shocks in the mill itself. It can be disconnected from the mortar mill by means of a clutch and lever. The speed is controlled by a high-speed governor. The bearing brasses and wearing surfaces generally should be provided with complete means of lubrication. The boilers, constructed of steel, are supplied with heavy pattern steam fittings, including a spring-loaded safety valve.
The mortar pan is of the under-driven type, with renewable false bottom, heavy rollers, and adjustable toe piece to centre spindle. The frame consists of two deep steel joists connected by strong cross girders. The road wheels are of cast iron. Both front and back wheels are provided with buffers, so as to reduce vibration when travelling over rough roads.
Concrete Mixers, of which Koppel's, illustrated in Fig. 237, may be considered typical, are usually supplied with an elevator, and the elevator box is sufficiently large for one charge, and acts as a measuring gauge. The sand, stone, or ballast is either tipped direct from side tipping waggons, or is shovelled into the box. The quantity of cement desired is then added, and the attendant, by operating a lever, causes the elevator box to ascend and discharge its contents into the feed hopper. The aggregate is admitted from the feed hopper into the drum, of which sectional drawings are given in Fig. 238, immediately the attendant allows the mixed concrete from the previous charge to fall out, so that no interruption occurs in work. The feed hopper is provided with a patent shaker, which facilitates the entrance of the material into the drum and prevents the opening from choking up. The drum is fast on the driving shaft, and rotates with it. Four mixing paddles are attached to the shaft inside the drum; they are set at a slight angle to the axis of the shaft, and serve, when the drum is rotating, to constantly and vigorously disturb the material, with the result that a perfectly mixed concrete is obtained. In the mixing process the larger pebbles operate somewhat like the balls in a ball mill, inasmuch as they rub the cement and sand intimately together and prevent the moist material adhering to the drum. A scraper is also provided to keep the interior sides and ends of the drum clean.
During the mixing process the next charge is elevated ready to be fed into the hopper.
For regulating the water supply the machine is provided with two automatically filled water-gauge tanks fixed underneath the main water tank. They are connected with the main water tank, with each other, and with the water pipe leading to the drum by a 4-way cock. Both valves are controlled by a common draw rod, and an arrangement is provided whereby the exact quantity of water required for each charge can be easily regulated. The drum opening is fitted with an automatic sliding cover, which is operated by a lever. The discharge is effected without any interruption of the rotation of the drum, the sliding cover being released and arrested for one revolution, thus allowing the discharge of the contents. On the completion of the revolution the sliding cover automatically closes the opening ready for the next charge.
A Sand and Gravel Washer and Screener, also made by Koppel, is shown in Fig. 239, the feed being continuous. By turning off the water supply it can be used, when required, as a screener only. It is suitable for builders and contractors for screening and washing sand and gravel for mortar and concrete; and can be obtained either "Portable" or "Stationary," and for either steam or hand power. The hand machine works very easily, indeed, one man being readily able to turn the handle against two men shovelling in dirty material.
Plant required for Building Work of the Largest Size 133
The size of meshes can be made as desired. The outputs and water consumption vary greatly, according to the size of meshes adopted and the proportion of dirt contained in the material. As will be seen, it consists of two concentric drums, of which the inner, formed of a mesh, revolves within the outer one.