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 Portable Gravity Mixer, such as Owen's (Fig. 240), is specially suitable for foundation and trench work, and for mixing concrete in small quantities.
It consists of a steel shoot 7 feet long, containing three sinuous mild steel angle bars extending in one length from the top to the bottom. They are fixed rigidly at the top and are held loosely at the bottom by a 1/2-inch pin. In addition there are eight large and six small steel baffle-plates fixed to the sides of the shoot. An inspection door to facilitate cleaning is provided at one side. The water supply pipe is fixed behind the lower edge of the top baffle-plate, and is perforated both back and front with a row of 1/8-inch holes, and provided with a brass regulation cock. By this arrangement the whole of the baffle-plates and sinuous bars receive a spray of water, and thus provide awet surface throughout the mixer to meet the falling aggregate. The ballast or stone and cement are spread evenly over a platform, which is placed immediately in front of the mouth of the mixer. The material is then shovelled into the mixer, and the sinuous bars and baffle-plates arrest its progress and deflect it in various directions, causing a constant splitting up of the mixture into two columns, which meet and separate alternately, while falling the full length of the mixer. The clearance between the sinuous bars can be increased or reduced by fitting bars of a suitable width for each requirement according to the size of the aggregate to be mixed.
The Friction Hoist (Fig. 241) will be seen to be in its main features identical with the single-purchase crab - a pair of friction drums taking the place of the toothed wheels, and a pulley that of the winch handle. It is only used for the rapid hoisting of comparatively light loads, but for this purpose it is very efficient. The power is applied by means of a belt driven by the mortar-mill engine or other source of power.
The machine is of such design and construction that, while amply strong for its work, it is at the same time light enough to be conveniently handled.
The raising and lowering of the load are under the control of one man, who may be stationed at any convenient place, the only communication between him and the machine being by means of a cord attached to the lever, and passed over suitable pulleys. In its normal position the hoist drum is firmly pressed on the brake block. By pulling the cord the drum is raised from the brake, allowing the chain to run out and lowering the load. Pulling still more raises the drum higher, pressing it against the friction pulley, which is revolving the whole time and thus hoists the load.
Pulsometer Pumps are extremely handy for contractor's use, for keeping excavations clear of water, and similar work (see Fig. 242).
Their chief advantages are as follow: -
1. They require no fixing, but will work well hung on a chain.
2. They have no pistons, buckets, or other frictional wearing parts; consequently a little sand or grit in the water, which would soon cut an ordinary pump to pieces, has no effect upon a pulsometer.
3. No exhaust pipe is used; the exhaust steam is condensed inside the pump. This saves complication and extra cost, and does not heat the air in a pit or sump so much as when an exhaust as well as a steam pipe is employed.
The following description will make clear the action of the pump, which it is as well to understand before putting it to work. Once started it will work without attention as long as the steam supply - which may be obtained from any boiler in use on the work - is kept up. A flexible steam hose may be used in cases where it is necessary to lower the pump as the water level in the sump or foundation sinks.
The pulsometer consists, as may be seen by the section given in Fig. 243, of a single casting called the body, which is composed of two chambers (AA) joined side by side, with tapering necks bent towards each other, and surmounted by another casting called the neck (J) accurately fitted and bolted to it, in which the two passages terminate in a common steam chamber, wherein the ball valve (I) is fitted so as to be capable of oscillation between seats formed in the junction. Downwards the chambers (AA) are connected with the suction passage (C), wherein the inlet or suction valves (EE) are arranged. A discharge chamber, common to both chambers, and leading to the discharge pipe, is also provided, and this also contains one or two valves (FF), according to the purpose to be fulfilled by the pump. The air chamber (B) communicates with the suction. The suction and discharge chambers are closed by hinged covers (HH) accurately fitted to the outlets by planed joints, and readily removed when access to the valves is required; in the larger sizes hand holes (LL) are provided in these covers. GG are guards which control the amount of opening of the valves (EE). Small air cocks are screwed into the cylinders and air chamber.
The pump being filled with water, either by pouring water through the plug hole in the chamber, or by drawing the charge, as can readily be done by attention to the printed directions, is ready for work. Steam being admitted through the steam pipe (K) (by opening the stop valve to a small extent) passes down that side of the steam neck which is left open to it by the position of the steam ball, and presses upon the small surface of water in the chamber which is exposed to it, depressing it without any agitation, and consequently with but very slight condensation, and driving it through the discharge opening and valve into the rising main.
The moment that the level of the water is as low as the horizontal orifice which leads to the discharge the steam blows through with a certain amount of violence, and, being brought into intimate contact with the water in the pipes leading to the discharge chamber, an instantaneous condensation takes place, and a vacuum is in consequence so rapidly formed in the just emptied chamber that the steam ball is pulled over into the seat opposite to that which it had occupied during the emptying of the chamber, closing its upper orifice and preventing the further admission of steam, allowing the vacuum to be completed. Water rushes in immediately through the suction pipe, lifting the inlet valve (E), and rapidly fills the chamber (A) again. Matters are now in exactly the same state in the second chamber as they were in the first chamber when our description commenced, and the same results ensue. The change is so rapid that, even without an air vessel on the delivery, but little pause is visible in the flow of water, and the stream is, under favourable circumstances, very nearly continuous. The air cocks are introduced to prevent the too rapid filling of the chambers on low lifts and for other purposes, and a very little practice will enable any unskilled workman or boy so to set them by the small nut that the best effect may be produced. The action of the steam ball is certain, and no matter how long the pump may have been standing it will start as soon as dry steam is admitted.
The steam ball, if once made true, wears itself and its seats true, as it turns in its bed at every stroke, so that no part of its surface falls twice in succession upon the seat. If properly constructed, a spherical steam valve working in a true seat has proved itself the best of all the forms of distributing valve which have been invented.
Centrifugal Pumps, as shown in Fig. 244, will generally be found the most suitable appliance for raising large quantities of water to moderate heights. The foundations necessary for fixing them are not expensive, and owing to the simplicity of their construction it is possible to use them satisfactorily in cases where the water is charged with mud or sediment, which would practically prevent the use of pumps of the ordinary type.
The length of suction, measuring from top of pump down to water level, should not exceed 26 feet. If the water is to be raised a greater height than this the extra piping should be fitted above the pump as delivery and not below it as suction. This means that if the well or sump-hole is more than from 22 to 24 feet deep down to the water level the pump must be fixed on a stage down the well and not on the ground level. If there is any difficulty in taking the driving belt from engine to pump under these circumstances a small intermediate or counter shaft and pulleys can be fixed above the pump to drive the latter by means of a second belt.
A Steam Winch, the principal use of which consists in raising considerable loads rapidly, is really, as can be seen from the illustration (Fig. 245), a double-purchase crab driven by two small steam engines built on its own framework. These engines are reversible, but the load can be lowered on the brake without running the engines round, this being accomplished by throwing the toothed wheels out of gear and so setting the engines free. The crank pins of the engines are set at an angle of 90 degrees from each other in order to avoid the possibility of a " dead centre," the result being that the winch can always be started by turning on the steam, no matter what position the crank pins may be in.