crane-driving, and it is probable that improvements in single and
two-phase motors will eventually largely increase their use for this
class of work.
Tests of the comparative efficiencies of hydraulic and electric cranes tend to show that, although they do not vary to any very considerable extent with full load, yet the efficiency of the hydraulic crane falls away very much more rapidly than that of the electric crane when working on smaller loads. This drawback can be corrected to a slight extent by furnishing the hydraulic crane with more than one cylinder, and thus compounding it, but the arrangement does not give the same economical range of load as in an electric crane. In first cost the hydraulic crane has the advantage, but the power mains are much less expensive and more convenient to arrange in the electric crane.
The limit of speed of lift of hand cranes has already been mentioned; for steam jib cranes average practice is represented by the formula V = 30 + 200/T, where V is the speed of lift in feet per minute, and T the load in tons. Where electric Speed. or hydraulic cranes are worked from a central station the speed is greater, and may be roughly represented by V = 5 + 300/T; e.g. a 30–cwt. crane would lift with a speed of about 200 ft. per minute, and 100–ton crane with a speed of about 8 ft. per minute, but these speeds vary with local circumstances. The lifting speed of electric travellers is generally less, because the lift is generally much shorter, and may in ordinary cases be taken as V = 3 + 85/T. The cross-traversing speed of travellers varies from 60 to 120 ft. per minute, and the longitudinal from 100 to 300 ft. per minute. The speed of these two motions depends much on the length of the span and of the longitudinal run, and on the nature of the work to be done; in certain cases, e.g. foundries, it is desirable to be able to lift, on occasions, at an extremely slow speed. In addition to the brakes on the lifting gear of cranes it is found necessary, especially in quick-running electric cranes, to provide a brake on the subsidiary motions, and also devices to stop the motor at the end of the lift or travel, so as to prevent over-running.
There are many other important points of crane construction too numerous to mention here, but it may be said generally that the advent of electricity has tended to increase speeds, and in consequence great attention is paid to all details that reduce friction and wear, such as roller and ball bearings and improved methods of lubrication; and, as in all other quick-running machinery, great stress has to be laid on accuracy of workmanship. The machinery, thus being of a higher class, requires more protection, and cranes that work in the open are now fitted with elaborate crane-houses or cabins, furnished with weather-tight doors and windows, and more care is taken to provide proper platforms, hand-rails and ladders of access, and also guards for the revolving parts of gearing.
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| Fig. 4. | Fig. 5. |
Typical Forms of Cranes.—Fig. 4 is a diagram of a fixed hand revolving jib crane, of moderate size, as used in railway goods yards and similar places. It consists of a heavy base, which is securely bolted to the foundation, and which carries the Fixed Cranes. strong crane-post, or pillar, around which the crane revolves. The revolving part is made with two side frames of cast iron or steel plates, and to these the lifting gear is attached. The load is suspended from the crane jib; this jib is attached at the lower end to the side frames, and the upper end is supported by tie-rods, connected to the framework, the whole revolving together. This simple form of crane thus embodies the essential elements of foundation, post, framework, jib, tie-rods and gearing.
Fig. 5 shows another type of fixed crane, known as a derrick crane. Here the crane-post is extended into a long mast and is furnished with pivots at the top and bottom; the mast is supported by two “back ties,” and these are connected to the socket of the bottom pivot by the “sleepers.” This is a very good and comparatively cheap form of crane, where a long and variable radius is required, but it cannot slew through a complete circle. Derrick cranes are made of all powers, from the timber 1-ton hand derrick to the steel 150–ton derrick used in shipbuilding yards. The derrick crane introduces a problem for which many solutions have been sought, that of preventing the load from being lifted or lowered when the jib is pivoted up or down to alter the radius. To keep the load level, there are various devices for automatically coupling the jib-raising and the load-lowering motions.
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| Fig. 6. |
Somewhat allied to the derrick are the sheer legs (fig. 6). Here the place of the jib is taken by two inclined legs joined together at the top and pivoted at the bottom; a third back-leg is connected at the top to the other two, and at the bottom is coupled to a nut which runs on a long horizontal screw. This horizontal movement of the lower end of the back leg allows the whole arrangement to assume the position shown in fig. 7, so that a load can be taken out of a vessel and deposited on a quay wall. The same effect can be produced by shortening the back leg by a screw placed in the direction of its length. Sheer legs are generally built in very large sizes, and their use is practically confined to marine work.
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| 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.
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| 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
