Fig. 7.Fig. 8.

Another type of fixed crane is the “Fairbairn” crane, shown in fig. 8. Here the jib, superstructure and post are all united in one piece, which revolves in a foundation well, being supported at the bottom by a toe-step and near the ground level by horizontal rollers. This type of crane used to be in great favour, in consequence of the great clearance it gives under the jib, but it is expensive and requires very heavy foundations.

The so-called “hammer-headed” crane (fig. 9) consists of a steel braced tower, on which revolves a large horizontal double cantilever; the forward part of this cantilever or jib carries the lifting crab, and the jib is extended backwards in order to form a support for the machinery and counter-balance. Besides the motions of lifting and revolving, there is provided a so-called “racking” motion, by which the lifting crab, with the load suspended, can be moved in and out along the jib without altering the level of the load. Such horizontal movement of the load is a marked feature of later crane design; it first became prominent in the so-called “Titan” cranes, mentioned below (fig. 14). Hammer-headed cranes are generally constructed in large sizes, up to 200 tons.

Fig. 9.Fig. 10.

Another type of fixed revolving crane is the foundry or smithy crane (fig. 10). It has the horizontal racking motion mentioned above, and revolves either on upper and lower pivots supported by the structure of the workshop, or on a fixed pillar secured to a heavy foundation. The type is often used in foundries, or to serve heavy hammers in a smithy, whence the name.

Portable cranes are of many kinds. Obviously, nearly every kind of crane can be made portable by mounting it on a carriage, fitted with wheels; it is even not unusual to make the Portable cranes. Scottish derrick portable by using three trucks, one under the mast, and the others under the two back legs.

Fig. 11 represents a portable steam jib crane; it contains the same elements as the fixed crane (fig. 4), but the foundation bed is mounted on a truck which is carried on railway or road wheels. With portable cranes means must be provided to ensure the requisite stability against overturning; this is done by weighting the tail of the revolving part with heavy weights, and in steam cranes the 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.

Fig. 11.Fig. 12.
Fig. 13.
Fig. 14.

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.