1008

THEORY AND PRACTICE OF ENGINEERING.

Rook II.

gudgeon, on which a plate with four arms and a hole in the centre is dropped :to each of these arms is fixed a guy rope, wdiich is carried to such a distancefrom the shaft, that when the crane turns round, the gib may keep clear of it.

The lower end is thus connected with a pair of pulley blocks, previouslyfastened to a pile driven in the ground, and then drawn tight and secured.

The gib, usually 26 feet long and 10 inches square, is fixed a little higherthan the middle of the shaft : it is supported by a brace from below, and issuspended from above by a wrought-iron bolt, and worked by a commonwheel and pinion.

The pincers or forceps, for holding the stone elevated by the crane, is madeof iron, and the pivots or rivets are of considerable strength : when the ropeis pulled, the points which attach to the stone are drawn or pressed togetherin such a manner, that the heavier the weight the firmer it holds.

The movable crane for lifting the timbers employed in the construction of the cones forthe breakwater at Cherbourg!* was very simple, and w.is brought immediately overits work. The several pieces were pulled up, and lifted into their respective situationsby means of a pulley.

Fig. 1522.

Fig. 1523. MOVABLE CRANE FOR BUILDING CONES AT CHERBOURG!!.

Where the wheel and axle is used in cranes, the action of the power is liable tooccasional intermission, in which case a ratchet wheel is provided, which permits thewheel to turn only in one direction, the catch which falls between the teeth of a fixedwheel preventing its motion in the other direction. The power of the wheel and axlebeing expressed by the number of times the diameter of the axle is contained in that of thewheel, there are but two methods by which that power can be augmented; viz. by in-creasing the diameter of the axle, or lessening that of the wheel. Where great power isrequired, each of these methods presents some difficulty as well ?s inconvenience; for ifthe diameter of the wheel is enlarged, the power will work through an unmanageablespace; and if the other method is adopted, and the thickness of the axle is much reduced,its strength will become insufficient for the purpose of supporting the weight: to combine,therefore, strength with moderate dimensions, is sometimes accomplished by the methodswe have already described of giving different thicknesses to different parts of the axle,carrying the rope which is coiled to the thinner part through a wheel attached to theweight, and coiling it in the opposite direction on the thicker part. In Eckart’s powercapstan, fig. 1494., the weight is equally suspended by two parts of the rope; therefore eachpart is stretched by a force equal to half the weight; consequently the power multipliedby the radius of the wheel, is equal to half the weight multiplied by the difference of theradii of the thicker and thinner parts of the axle; or, the power multiplied by the diameterof the wheel, is equal to the weight multiplied by half the difference of the diameters ofthe thinner and thicker parts of the axle : such a wheel and axle is equivalent to an ordi-nary one, in which the wheel has the same diameter, and whose axle has a diameter equalto half the difference of the diameters of the thicker and thinner parts. The power of sucha contrivance depends upon the proportion the diameter of the wheel bears to half thedifference of these diameters; therefore this power, when the diameter of the wheel isgiven, does not depend on the smallness of the axle, but on the smallness of the differenceof the thinner and thicker parts of it.