For use as guys for derricks when heavy loads are to be handled, the wire rope has many points to recommend it. It is also advisable to use this form of rope for blocks and falls when the size of a hemp or manilla rope becomes excessive. Its advantages consist of its lightness and portability combined with great tensile strength. For use with pulley blocks it is necessary to have the rope as flexible as possible, and this is attained by specifying the number of strands, or separate wires, of which the rope is to consist when ordering from the manufacturer; the greater the number and the smaller the diameter of the strands in any given rope, the more flexible it will be. The ropes most commonly used have six strands, each containing twelve wires, and a hemp strand at the centre (Fig. 229), each wire being about 1/50 of an inch in diameter. The number of strands, and of wires in each strand, is, however, arbitrary, and ropes of 8 strands each of 10 wires, of 10 strands each of 9 wires, and various other proportions are adopted.

A comparison between the safe working strength of white manilla rope and one of steel wire shows the great advantage to be obtained by the use of the latter, the greatest permissible load for a manilla rope being 10,600 lbs. per square inch of section, whereas a steel wire may be loaded up to 55,000 lbs. per square inch with perfect safety. The sheaves of the blocks, however, should be of as large a diameter as possible when wire ropes are used, as the constant bending and unbending of the wires composing them is the greatest source of deterioration. This fact is well illustrated by results obtained from careful experiments made during the construction of the Forth Bridge, where a great deal of this kind of lifting tackle was in use. It was here found that when the diameter of the sheave was equal to six times the circtumference of the rope, the rope could be bent over the sheave 5000 times under load before failure commenced, whereas when the diameter of the sheave was 8 times the circumference of the rope the bending could take place 10,000 times before any sign of defect was noticed.

Wire Ropes And Tightening Screws 261

Fig. 229.

The wire rope is never used when hand labour only is available for lifting, as its comparatively small size and smooth surface does not give sufficient "grip." It is essential that the free end from the top, or from the snatch block (as the case may be), should be passed three or four times round the barrel of a crab or winch, thus obtaining a firm hold on the rope and applying the necessary power with the greatest economy of time and labour.

When the wire rope is used as a stay or guy-rope the conditions, of course, are quite different to those obtaining with pulley blocks. In the latter case its advantage consists in being much less liable to stretch under load. In making use of it in this way two of the three guy-ropes are made fast to their anchor posts in the ordinary manner, and the third is connected to its post by an arrangement for taking up the slack mechanically, as this is impossible by hand for reasons before stated. The device is called a tightening-screw, and consists of a wrought-iron frame of rectangular form having a right-hand female screw at one end and a left-hand one at the other. Into these two hooked bars, having right and left-hand male threads respectively, are inserted, and it follows that, when the two hooks are held still and the frame revolved on its own axis, the two hooks are drawn nearer together. By attaching one hook to the anchor post and passing the other through a loop in the end of the guy-rope, already pulled as tight as possible by hand, it is only necessary to turn the centre frame of the tightening screw to obtain the required tension in the guy-rope.

The Hand Pump (Fig. 230) is a simple lift or " bucket " pump having a galvanised sheet-steel barrel, wrought-iron fittings and handle, brass and leather valves, and a leather bucket. It is most generally used for clearing excavations from accumulated water before starting building operations, and is often left in position during the whole job in case a storm should flood the cellars before they can be covered in. If the water has to be raised to a greater height than the length of the pump - usually about 7 feet from the spout downwards - an extra piece of galvanised steel tube, funnel shaped at the top, is added to the lower end of the pump, the joint being made air-tight by filling in the funnel with moist clay or "pug" after the pump has been inserted. These extra pieces are, as a rule, about 8 feet long, and two of them may be added to the pump if required. It is not advisable to attempt to raise water from a greater depth than 15 feet with

Wire Ropes And Tightening Screws 262

Fig. 230.

Plant required for Building Work of Moderate Size 127 this type of pump, but rather to bring into use one or other of those described later.

For lifts of 6 to 10 feet, however, this type will be found to be the cheapest and at the same time the most efficient appliance suited to the work.

Machinery in Yard. A builder of the class with which we are now dealing will require a certain amount of fixed machinery at his works or yard in order to be able to deal with such joinery and stonework as cannot conveniently be purchased ready finished for use. Some of these machines may be hand driven, such as mortising, boring, and tenoning machines, band saws, etc. For others, such as the circular saw, it is absolutely essential to have a prime mover of some kind installed. When possible an electric motor, with current supplied from the local electric light and power station, is no doubt the best form of prime mover, but, failing this, there remains a wide choice between gas engines, oil or petrol engines, and all the various makes of steam engines and boilers. Of all these the gas engine has probably the most points to recommend it. Once started - a matter of a very few minutes with the modern gas engine having magneto-ignition - it requires little or no attention during the whole of the day's run, and when finished with at night the mere act of turning off the gas tap ensures the stoppage of all expense connected with its working until such time as it is required again. In small power plants the convenience of using the ordinary lighting gas of the town outweighs the saving made by installing a suction gas plant and manufacturing, by an extremely simple process, one's own gas. When the plant, however, is large enough to require an attendant more or less constantly the saving effected by the use of the suction gas plant renders it practically a necessity, - the cost of gas of a quality sufficiently good to be used for power purposes manufactured by this process being, generally speaking, about 75 per cent. lower than that of town gas. An engine of from 15 to 20 actual horse-power would be all that would be required by such a firm as we are dealing with, it being an ascertained fact that it is never necessary to run all the machines in any such works simultaneously.