The loads lifted by these cranes are of all degrees of magnitude, varying from a few hundredweights up to the loads of 100 tons or even more, found in steel foundries, gun-making establishments, boiler or machine shops, and the like.

The height to which such loads have to be lifted are as variable as their weights, and in the heaviest cranes the level of the crane-rail may be as much as 40 to 50 feet above floor-line. In such cases the details of columns supporting such crane-loads become worthy of consideration, and the entire structure requires careful design in view of the important results which would arise from failure either in the columns or roadway.

The loads placed by crane-makers upon the wheels of such structures as overhead travelling cranes, titans for marine construction, heavy locomotive wharf cranes, and the like, considerably exceed anything found in ordinary railway practice or locomotive construction.

This is probably explainable by the fact that the speeds of such structures in locomotion are very slow as compared with the ordinary forms of rolling stock.

Thus, to take an illustration, the total weight of an overhead travelling crane of about 60-feet span driven by shafting, and capable of lifting 35 tons, was found to be 42 tons exclusive of the load lifted. This crane is supported by a cradle at each end fitted with two wheels, making four wheels in all. Thus the load per wheel of the crane only with the crab in the centre was 10.5 tons; with the load lifted in the centre, the load per wheel amounts to 19.25 tons; but with the load lifted at, say, 6 feet from one crane-road, the maximum wheel load at the heavy end runs up to approximately 30 tons per wheel, the exact amount depending on the precise ratio between the fixed and moving parts of the crane, or as between the transverse girders and other equally distributed weights and the crab.

In some titans for breakwater construction wheel loads of about 40 tons per wheel are not uncommon in certain positions of the load.

Such loads as these demand considerable attention to the section of rail used and mode of fastening, as the tendency under such moving loads as these is to curl up the rail even when of fairly heavy section, and tear it from its fastenings - if the foundation be at all of a yielding nature. When, however, a sufficiently good and rigid connection has been made, there is no difficulty in dealing with wheel loads of the amount named.

The structure of the wheel itself is somewhat outside the limits of this work, but in general it may be stated that such wheels are usually made of cast iron in a solid form, or with a few simple round holes in the web of the wheel, with Bessemer or Siemens steel tyres of massive section shrunk on.

In many cases, such as wharf cranes, titans, or goliahs, a wheel is used having one central flange, and two treads, requiring a double rail, or two rails placed side by side to run on, which forms a very satisfactory arrangement, but in the ordinary cases of overhead travellers, the rail is single, and the wheel may be either single or double flanged, usually the latter.

The following table, No. 31, gives the approximate total weight of overhead travelling cranes, exclusive of the load lifted, for spans varying from 30 to 60 feet, and for loads of from 5 to 100 tons.

These weights are approximate only, and may be taken to cover the various types of shaft-driven, rope-driven, hand, or electric cranes. The values given are only sufficiently accurate for preliminary calculations, and in all important cases the actual probable weight of the crane should be obtained from the makers, together with other information hereafter referred to.

In connection with this subject it may be mentioned that a recent example of a steam crab to lift a test load of 50 tons, as applied to a goliah, was found to weigh 32 tons 3 cwts. 1 qr. 19 lbs., including boiler, house over crab (for outdoor work), lifting beam and rods, eye-bolts, snatch-block, and wire-rope, but exclusive of coal and water. The test load was lifted at a speed of 10.3 feet per minute, and was traversed at 33 feet per minute, the whole goliah with its load weighing in all about 137 tons, being travelled . at 120 feet per minute. Pressure of steam in boiler, 80 lbs.

Table No. 31

Total weight of crane, exclusive of load lifted, in tons.

Before the actual maximum wheel loads can be arrived at, and the determination of bending moments due to the rolling load made, it is necessary to subdivide the total weight of crane into the fixed and moving portions relative to the two crane-roads. Thus the fixed portion will consist of the pair of supporting girders spanning the distance or gauge between longitudinal roadways, together with such other portions of the gear, shafting, etc., which may be evenly divided between the two end cradles. The moving portion will consist of the crab for lifting the load, which may take any position between the two crane-roads, the lateral amount of travel being limited by the minimum distance which the crab with its load can assume from the rail, a distance which may be determined either by the details of the crab itself or the dimensions of the load to be lifted.

When these. proportions of load are known, the relative reactions at the cradles or the wheel loads can be easily ascertained, and the bending moments on the girders due to the rolling loads can be determined, the most effective means being by graphic analysis.

For approximate and preliminary calculations, and in the absence of more precise information to be obtained from the manufacturer, the total weight of the crane given in the table may be divided evenly into the fixed and moving portions. For example, the weight of a 35-ton traveller of 55-feet span being 42 tons, 21 tons may be considered as evenly divided between the two cradles, and 21 tons may be taken as the weight of the moving crab with its gearing, which may occupy any position laterally between the crane-roads, giving rise to proportionate reactions or wheel loads, as the position may determine, in accordance with the principles of the lever.