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.
It is only with the forms of crane suitable to the use of builders in general that we propose to deal. The whole subject of cranes in general is of far larger scope than could be dealt with in these pages, nor would it be necessary, as many of the types could not in any way be considered as builder's plant. A form much used is shown at Fig. 261, and is really no more than a combination of the crab and derrick, except that in this case the derrick, or jib as it is now called, is not held rigidly in one position by guy-ropes, but is arranged to swing from a pivot at its lower end, the pulley block at the top of the derrick being replaced by a grooved wheel built into the top of the jib. This simple crane is usually worked by hand, but in some cases is fitted with a belt-driven friction hoist or a motor-driven crab. An upright post is fixed as the main support for the jib, and the guy-ropes are replaced by two timber ties set at an angle of 120 degrees behind the jib. The jib itself can be raised and lowered through a large vertical angle by means of a tie-rope, usually of steel wire, and pulley wheel as shown, the radius through which it can work being thus increased or diminished as may be necessary. A pair of guide-ropes are attached to the jib to pull it round in any direction required.
The next advance on this type of crane is that known as the "Scotsman," or Scotch crane, which is practically the same appliance, worked by steam or electricity, mounted on a high, three-legged scaffold which has been previously described. In this case the two timber ties are anchored down by means of a strong chain, fixed to the base of each and carried down the two smaller legs of the scaffold, and loaded at the base with bricks or any other suitable weights. Care must be taken in arranging the position of the legs of this scaffold so that they do not interfere with the construction of the building, and can be conveniently removed after its completion, or rather the completion of the external walls, the legs necessarily rising through the various floors.
A type of crane which is useful during preliminary clearing of sites or excavations is that illustrated at Fig. 263. This is a self-contained travelling steam crane, which is arranged to lift, turn, and travel along the lines laid for that purpose by steam power. The boiler is placed far back in the design, with the object of counterbalancing the load and preventing any tendency to tip. By means of this appliance earth, etc., can be raised from an excavation, carried to a convenient point, and tipped direct into carts. It will be seen that this crane also consists of a jib and steam-driven crab, the traversing and slewing gear being added. Fig. 262 shows a form of grab which may be used with advantage with this type of crane when the material to be raised is of a soft or sandy nature. It consists of stout steel-plate buckets, having steel tines riveted at intervals on the outside, the plate meeting when the bucket is closed. It works automatically with a single chain, the disengaging head being attached by means of its two chains to the jib head at a convenient height to cause the grab to open and discharge its load.
The only other form of crane with which we are concerned is the overhead traveller, running on a gantry. This may be either actuated by hand, steam, fly-rope, square shaft, or electric motor. In any case it consists primarily of a double-purchase crab, mounted on a carriage which traverses rails, these rails being laid on two coupled girders spanning the gantry (Fig. 264). This form of crane can be made capable of lifting and travelling with any desired load, and with a properly constructed gantry can be used to pick up materials from vans, etc., and carry them to their destination on the work. It will be understood that such a crane could only be installed on very large work, as the cost of erecting the gantry and placing the crane thereon must necessarily be considerable. When, however, the magnitude of the contract warrants its use no more efficient appliance could be found, the time saved alone being a great factor in its favour, as well as the fact that material is much less liable to injury when conveyed direct to its destination than when passed through several hands on its way.