Modern Gasworks Practice

THE MECHANICAL HANDLING OF MATERIALS 307 operated by a single rope or chain, and those operated by two ropes. As yet, the first type is most common on gasworks; where, too, a preference is still found for chains rather than wire ropes. Modem practice, however, is gradually inclining towards the substitution of ropes for chains, the chief advantage gained being an appreciable reduction in weight for a given safe load, the ratio amounting to so much. as 8 to 1 in favour of the wire rope, while, more important still, wire rope gives warn-ing of impending failure, whereas chains do not. As it is frequently necessary to estimate on the spot the weight and safe load of various descriptions of crane chains, it may be useful to call attention to a quick and ready method which the author uses. For weight, take the size of the chain in eighths of an inch, square this, and the result is weight in pounds per fathom. For safe load, again take the square of the size in eighths and divide it by 10. E.g. A one-inch. chain = 3 inch.: 82= 64, therefore weight of chain =64 Ib. per fathom. Again, = 6-4, therefore safe load = 6-4 tons per square inch, with, a factor of safety of about 5. It would be impossible to deal here with the general construction of grabs, or to describe the numerous types which are now to be found. The subject is one which demands a volume to itself, as an instance of which. it may be men.tion.ed that during the past fifteen years well over a hundred patents for apparatus of the kind have been granted. The Barnard self-acting grab provides a typical example of the latest developments in these machines, but it is only one example of many of the same kind. The self-acting principle in brief consists of the employment of a single wire rope or chain passing round the grab sheeves. The grab in the open position is lowered by the crane on to the material, when the hoisting chain must be run out until the chitches engage with the pins carried by the crossbeam. Then, on hoisting, the jaws close in on the coal. For discharging, the grab is lowered so as to rest on the coal in the receiving hopper or store heap, and the hoisting chain or wire is slackened a few inches. This motion releases the chitches, and, on han lin g up, the jaws are opened, thus discharging tlieir contents. A great advantage attached to the use of the self-acting grab is that its capacity for a given weight is much greater than the capacity of some of the more common forms. For instance, a Barnard grab having a capacity of 1 ton of coal weighs approximately 1 ton, while many of the older grabs of similar capacity weigh as much as 28 cwt. When this factor is considered in relation to the safe load of the crane it will be seen that with the new type of grab considerably more coal can be dealt with in a given time; or, in other words, a sliip of given capacity can be dis-charged in less time. This faet is of supreme importance at the present day, when so much depends upon quick discharge. At what is probably the largest jetty attached to a gasworks in London the time of discharge has been reduced by one-third by the substitution of self-dumping grabs for those of an older pattern. With the self-acting grab, moreover, wear and tear is reduced owing to the elimination of shock due to a high discharge, while the dust difficulty is more or less surmounted. A practical point to be borne in mind—particularly when dealing with. coke—