The mortar-mill in ordinary use on large works is shown in Fig. 103.
A cast-iron pan, P, about 6 or 7 feet in diameter is made to revolve by machinery under a pair of heavy cast-iron rollers filled with concrete, and weighing from 1 to 3 tons the pair.
Fig. 103. Mortar-Mill.
The ingredients of the mortar are thrown into the pan while it is revolving; plates of iron, marked h in the figure, are fixed in suitable positions to guide the material, so that it may all come under the rollers.
The pan has a loose bottom of cast iron, formed in segments, which can be removed and replaced as they wear out.
Machines of this description are generally driven by a small portable engine, a 4-horse power engine being required for a 6-feet pan, and in proportion for other sizes. The band from the engine is passed over the driving-wheel D, and thus turns the spur-gearing which moves the pan.
These mills are made in different sizes, the pans varying in diameter from 5 to 10 feet; the rollers from 2 feet 8 inches to 3 feet 6 inches.
A mill with a 7-feet pan will turn out about l1/2 cubic yard of ordinary lime and sand mortar per hour; if, however, the mortar is made with burnt ballast, or brick rubbish, which requires grinding as well as mixing, only about 1 cubic yard per hour will be turned out.
For smaller works, and those which are scattered - as, for instance, along a line of railway - a portable mortar-mill may be used (see Fig. 104).
This machine somewhat resembles the one last described, but is mounted on wheels, and carries a small three horse-power engine with it.
The pan of this machine is sometimes 5 feet, sometimes 6 feet in diameter; the rollers 2 feet 8 inches or 3 feet in diameter.
Fig. 104. Portable Mortar-Mill.
Such a machine will mix enough mortar to keep ten or twelve bricklayers at work.
Fig. 105. Horse Mortar-Mill.
Horse Mortar-Mill- - A special mill, made by Messrs. Huxham and Brown of Exeter, to be worked by horse-power, is shown on Fig. 105.
For still smaller works hand mortar-mills may be used of the forms shown in Fig. 106.
The ingredients of the mortar are poured into the hopper H, and find their way into the cylinder C, which contains a series of blades fixed on a central shaft, and made to revolve by means of the handle.
Fig. 106. Hand Mortar-Mill.
It is stated that by the aid of this machine one boy can keep eight men at work, and that one man using it can keep twenty men at work.
Concrete can be thoroughly well mixed by hand in small quantities; but when large quantities have to be dealt with, it is difficult, without good organisation, discipline, and very close superintendence, to ensure the thorough incorporation upon which the quality of the material so much depends.
In most cases the use of machinery is a cheap as well as an efficient way of mixing large quantities.
Several arrangements have been devised at different times, suited to the peculiar circumstances of particular works, but it is proposed to describe in these Notes only two or three forms that are commonly used, and one or other of which would be applicable in ordinary cases.
Fig. 107. Inclined Cylinder Concrete-Mixer.
A simple form of concrete-mixer consists in an inclined hollow iron cylinder mounted as shown in Fig. 107.
The eccentric motion of the cylinder causes its contents to be rolled over and over, thrown from side to side, and end to end of the cylinder, and thus thoroughly mixed.
A modification of this machine was used at the Dover Harbour works.
This machine is made in four sizes, containing respectively 1/3, 1/2, 3/4, and 1 cubic yard.
Fig. 107 is taken from the circular of a manufacturer, Mr. H. Sykes, of 66 Bankside, London.