on the screwed ends, there is no risk of having to do the work over again.
As regards the welding, no actual difficulty occurs in practice, provided a sufficiently mild quality of steel be used. Very rarely have welds been known to fail. There is no special art in making them sound. Scarf joints of about 6 inches in length, sufficiently upset, a suitable heat, the use of sand, and the services of three or four strikers at the critical moment,—these have been found sufficient.
In block-setting Titans the load is generally lifted and traversed by a jenny, or block carriage, running on steel wheels, or steel-tired wheels. In one machine, two jennys have been used, to be coupled together for lifting a heavy concrete block, or to be used separately when lifting a truck of rubble, the truck being supported at one end from one jenny, while the other end is lowered for tipping by the other jenny.
The engines do not, of course, like those of most steam crabs, travel with the block carriage. They are fixed at the back end of the machine behind the rear of the truck. Steam power is almost invariably employed, as being, on the whole, the most convenient. The location of the engines at the rear diminishes the amount of dead load to be traversed; there is the further advantage that, with the gearing and boiler, they assist very materially in counter-balancing the weight due to the overhang of the jib and its load. The aim in modern types is to so arrange parts as to throw all the weight possible behind the truck, the boiler hindermost of all.
The radial traverse of the jenny has been effected in different ways In some of the earliest machines the driving took place from the engines through a square gantry-shaft and bevel gearing to a crab, the gantry-shaft running alongside, and above the jib, similarly to those of ordinary square shaft traveling cranes and some goliaths. A sleeve bevel wheel then, which formed a portion of the crab, and sliding along the shaft, imparted motion through another bevel to the crab and its gearing. Tumbler brackets supported the shaft at intervals.
That practice is not followed now, but a jenny is used, and its radial or racking movement is accomplished through a wire rope and a special drum. The hoisting and lowering are done through another wire rope on its drum, while the slewing is effected through a set of gearing suitably arranged. Particular arrangements of gearing will, of course, differ in different machines, but the foregoing methods apply generally in most cases. Brakes are applied to each motion. A safety brake is necessary on the slewing gear of a Titan, not only for the purpose of arresting the momentum of the heavy superstructure at any required point without straining the toothed gearing unduly, but also to prevent rotation due to strong wind pressure. The various motions are put in and out by means of clutches, which are either of the cone or of the claw type.
There is nothing special to note about the bearings or gears, except that the latter are properly half-shrouded. The largest spur wheels are those used for hoisting. These are keyed directly upon the drum, so relieving the shafts of much torsional stress, and making a stiffer job. In a few cases a single spur wheel has been used; in most instances one is keyed on each end of the drum, the idea being to equalise the work and conduce to smoothness of running. Three keys are properly used, and the keying is done in such a way that the teeth of the pair of wheels come alternately, or "hit and miss."
The winding drums must be large in diameter,—from 3 to 6 feet,—to take a considerable length of winding rope for radial travel of the jenny, and for depth of deposition. They are invariably made with grooves for the rope to lie in as it winds up. There are two sets of spirals, right and left handed, so that the rope leads off from two points, winding up towards the centre of the drum. The grooves are either cast or cut.
In all the early machines chains were
