CRANES (so called from the resemblance to the long neck of the bird, cf.
Ducks, cranes and other aquatic birds abound in the delta.
Machines used for lifting only are not called cranes, but winches, lifts or hoists, while the term elevator or conveyor is commonly given to appliances which continuously, not in separate loads, move materials like grain or coal in a vertical, horizontal or diagonal direction.
The use of cranes is of great antiquity, but it is only since the great industrial development of the 19th century, and the introduction of other motive powers than hand labour, that the crane has acquired the important and indispensable position it now occupies.
In all places where finished goods are handled, or manufactured goods are made, cranes of various forms are in universal use.
Jib cranes can be subdivided into fixed cranes and portable cranes; in the former the central post or pivot is firmly fixed in a permanent position, while in the latter the whole crane is mounted on wheels, so that it may be transported from place to place.
The different kinds of motive power used to actuate cranes - manual, steam, hydraulic, electric - give a further classification.
Hand cranes are extremely useful where the load is not excessive, and the quantities to be dealt with are not motive powers.
It is at once evident that hand power is only suitable for cranes of moderate power, or in cases where heavy loads have to be lifted only very occasionally.
Steam is an extremely useful motive power for all cranes that are not worked off a central power station.
Where, however, there are a number of cranes all belonging to the same installation, and these are placed so as to be conveniently worked from a central power station, and where the work is rapid, heavy and continuous, as is the case at large ports, docks and railway or other warehouses, experience has shown that it is best to produce the power in a generating station and distribute it to the cranes.
Down to the closing decades of the 19th century hydraulic power was practically the only system available for working cranes from a power station.
Electricity as a motive power for cranes is of more recent introduction.
For dock-side jib cranes the use of electric power is making rapid strides.
For overhead travellers in workshops, and for most of the cranes which fall into our second class, electricity as a motive power has already displaced nearly every other method.
Cranes driven by shafting, or by mechanical power, have been largely superseded by electric cranes, principally on account of the much greater economy of transmission.
Sometimes, especially in the case of overhead travelling cranes for very heavy loads, the chain is a special pitch chain, formed of flat links pinned together, and the barrel is reduced to a wheel provided with teeth, or " sprockets," which engage in the links.
Cranes fitted with rotating hydraulic engines may be considered as coming under the third category.
Barrel cranes are usually fitted with band brakes, consisting of a brake rim with a friction band placed round it, the band being tightened as required.
An excellent brake for very large cranes is Matthew's hydraulic brake, in which water is passed from end to end of cylinders fitted with reciprocating pistons, cooling jackets being provided.
In electric cranes a useful method is to arrange the connexions so that the lifting motor acts as a dynamo, and, driven by the energy of the falling load, generates a current which is converted into heat by being passed through resistances.
In the second, or " braking off " method, the brake is automatically applied by a spring or weight, and is released either mechanically or, in the case of electric cranes, by the pull of a solenoid or magnet which is energized by the current passing through the motor.
The first method is in general use for steam cranes; it allows for a far greater range of power in the brake, but is not automatic, as is the second.
This method of working is very suitable for electric dock-side cranes of capacities up to about 5 or 7 tons, and for overhead travellers where the height of lift is moderate.
In steam cranes it is usual to work all the motions from one double cylinder engine.
In electric cranes the motor is connected to the barrel, either in a similar manner by spur gear or by worm gear.
Much attention has been paid to the improvement of the mechanical details of the lifting and other motions of cranes, and in important installations the gearing is now usually made of cast steel.
In revolving cranes ease of slewing can be greatly increased by the use of a live ring of conical rollers.
Electric motors for barrel cranes are not essentially different from those used for other purposes, but in proportioning the sizes the intermittent output has to be taken into consideration.
In steam cranes much the same principle obtains in proportioning the boiler; e.g.
The motor in most common use for electric cranes is the series wound, continuous current motor, which has many advantages.
The fifth order (the third of the Dasypaedes) is formed by the Grallatores, divided into 2 " series " - (I) Altinares, consisting of 2 " cohorts," Herodii with I family, the herons, and Pelargi with 4 families, spoonbills, ibises, storks, and the umbre (Scopus), with Balaeniceps; (2) Humilinares, also consisting of 2 " cohorts," Limicolae with 2 families, sandpipers and snipes, stilts and avocets, and Cursores with 8 families, including plovers, bustards, cranes, rails, and all the other " waders."
22) to the cranes (No.
Blaauw's Monograph of the Cranes (1897, folio); St G.
Geese and cranes, chicory, mildew, thistles, cleavers, caltrops, darnel and shade are farmer's enemies.
The water cranes or towers which are placed at intervals along the railway to supply the engines with water require similar care in regard to the quality of the water laid on to them, as also to the water troughs, or track tanks as they are called in America, by which engines are able to pick up water without stopping.
In either case the platform is fitted with a crane or cranes for lifting merchandise into and out of the wagons, and doors enable the shed to be used as a lock-up warehouse.
While working at cranes or capstans 3 411 4.
Gruidae, cranes, cosmopolitan, allied Phororhacos, Tertiary of Argentina.
Cranes may be divided into two main classes - revolving and non-revolving.
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.
The limit of speed of lift of hand cranes has already been mentioned; for steam jib cranes average practice is represented by the.