Page:EB1911 - Volume 07.djvu/391

Rh boiler is so placed as also to form part of the counterbalance. Where the rail-gauge is narrow and great weight is not desired, blocking girders are provided across the under side of the truck; these are arranged so that, by means of wedges or screws, they can be made to increase the base. In connexion with the stability of portable cranes, it may be mentioned that accidents more often arise from overturning backwards than forwards. In the latter case the overturning tendency begins as soon as the load leaves the ground, but ceases as soon as the load again touches the ground and thus relieves the crane of the extra weight, whereas overturning backwards is caused either by the reaction of a chain breaking or by excessive counterweight. When portable cranes are fitted with springs and axle-boxes, drawgear and buffers, so that they can be coupled to an ordinary railway train, they are called “breakdown” or “wrecking” cranes.

Dock-side jib cranes for working general cargo are almost always made portable, in order to enable them to be placed in correct position in regard to the hatchways of the vessels which they serve. Fig. 12 shows an ordinary hydraulic dock-side jib crane. This type is usually fitted with a very high jib, so as to lift goods in and out of high-sided vessels. The hydraulic lifting cylinders are placed inside the revolving steel mast or post, and the cabin for the driver is arranged high up in the front of the post, so as to give a good view of the work. The pressure is conveyed to the crane by means of jointed “walking” pipes, or flexible hose, connected to hydrants placed at regular intervals along the quay. It is often very desirable to have the quay space as little obstructed by the cranes as possible, so as not to interfere with railway traffic; this has led to the introduction of cranes mounted on high trucks or gantries, sometimes also called “portal” cranes. Where warehouses or station buildings run parallel to the quay line, the high truck is often extended, so as to span the whole quay; on one side the “long leg” runs on a rail at the quay edge, and on the other the “short leg” runs on a runway placed on the building. Cranes of this type are called “half-portal” cranes. Fig. 13 shows an electric crane of this class. They give the minimum of interference with quay space and have rapidly come into favour. Where the face of the warehouse is sufficiently close to the water to permit of the crane rope plumbing the hatches without requiring a jib of excessive radius, it is a very convenient plan to place the whole crane on the warehouse roof.

A special form of jib crane, designed to meet a particular purpose, is the “Titan” (fig. 14) largely used in the construction of piers and breakwaters. It contains all the essential elements of the hammer-headed crane, of which it may be considered to be the parent; in fact, the only essential difference is that the Titan is portable and the hammer-head crane fixed. The Titan was the first type of large portable crane in which full use was made of a truly horizontal movement of the load; for the purpose for which the type is designed, viz. setting concrete blocks in courses, this motion is almost a necessity.

As types of non-revolving cranes, fig. 15 shows an overhead traveller worked by hand, and fig. 16 a somewhat similar machine worked by electric power. The principal component parts of a traveller are the main cross girders forming the

bridge, the two end carriages on which the bridge rests, the running wheels which enable the end carriages to travel on the longitudinal gantry girders or runway, and the crab or jenny, which carries the hoisting mechanism, and moves across the span on rails placed on the bridge girders. There are numerous and important variations of these two types, but the above contain the elements out of which most cranes of the class are built.

One variation is illustrated in fig. 17, and is called a “Goliath” or “Wellington.” It is practically a traveller mounted on high legs, so as to permit of its being travelled on rails placed on the ground level, instead of on an elevated gantry. Of other variations and combinations of types, fig. 18 shows a modern design of crane intended to command the maximum of yard space, and having some of the characteristics both of the Goliath and of the revolving jib crane, and fig. 19 depicts a combination of a traveller and a hanging jib crane.

When the cross traverse motion of a traveller crab is suppressed, and the longitudinal travelling motion is increased in importance we come to a type of crane, the use of which is rapidly increasing; it goes by the name of “transporter.” Transporters can only move the load to any point on a vertical

surface (generally a plane surface); they have a lifting motion and a movement of translation. They are of two kinds: (1) those in which the motive power and lifting gear are self-contained on the crab; and (2) those in which the motive power is placed in a fixed position. A transporter of the first class is shown in fig. 20. From the lower flange of a suspended runway, made of a single I section, run wheels, from the axles of which the transporter is suspended. The latter consists of a framework carrying the hoisting barrel, with its driving motor and gearing, and a travelling motor, which is geared to the running wheels in such a manner as to be able to propel the whole machine; a seat is provided for the driver who manipulates the controllers. A transporter of this kind, when fitted with a grab, is a very efficient machine for taking coal from barges and depositing it in a coal store.

In the other class of transporter the load is not usually moved