1004

THEORY AND PRACTICE OF ENGINEERING.

Book II.

The Crane is generally employed for raising or lowering heavy weights and large bodies,for removing them from one situation to another, loading and unloading waggons andships, and for hoisting goods from the quay to the different warehouse floors : it consistsordinarily of an upright post turning upon pivots ; the gibor arm which extends from the upper part is now generallymade of iron, or supported by a bracket of the samematerial.

The wheels accompanying it are usually set in motionby turning a winch, a fly wheel being applied to the axisto equalise the motion, as well as to prevent the rapiddescent of the load, should any accident occur. The powersof the crane are adapted to its intended use, and theweights to be raised : some have a variety of movementsintroduced, with a train of several wheels, each turned by apinion smaller than itself. If the barrel or drum on whichthe rope winds be 12 inches in diameter, with a cog-wheelhaving 96 teeth attached to the end’of it, it may be turned ^by a pinion of 12 leaves. On the same axis may be placed J

a wheel of 32 teeth, moved by a pinion of 8 situated on a ( N- _^ \

third axis, which carries the fly wheel: a winch of a --—.l

f>.;ot radius applied to the gudgeon of the drum or barrel, lig. lo!2.doubles the power of the labourer, as the circle it describes is double that of thebarrel on which the rope winds. If the winch is fixed on the axis which carries the pinionof 12 and wheel of 32, it will have a power 16 times greater, and if attached to a pinion of8, 64 times. The fly wheel being fixed upon the axis on which the pinion of eight isattached, all danger from accidents is in a great degree prevented, and is much moreeffective than the brake and ratchet-wheel. The spindles of all the wheels slide or pushendwise, so that they may be combined or disengaged at pleasure.

The statical moments of the forces are inversely as the product of the radii of the axis, lothe product of the radii of the wheels in which they work; and as the number of teethwhich the wheels and leaves that the pinions carry must be as the peripheries of the wheelsand axles, their statical moments are also inversely as the product of the leaves of all thepinions to the product of the teeth of all the wheels.

At Ramsgate harbour a crane was made use of, designed by Mr. Peter Kicr, by whichfour men were enabled to hoist as many tons : *the whole was mounted upon a frame ofcast-iron, 9 feet 7 inches square, weighing 2 tons ; this was supported on four cast-ironwheels, one pair of which on a common axle moved round on a centre, fixed to one side ofthe frame : an arm projected from the axle to the opposite side beneath the framework,where, by means of a rack or segment of a wheel fixed to it, a pinion moved by a winchtwisted the wheels round upon their centre, and allowed the whole crane to be moved inany direction required.

The shaft of cast-iron weighing 23 cwt. was placed over the centre of the iron frame,and supported by oak braces, stepped into boxes, cast in the angles of the frame, thusforming a strong perpendicular shaft, round which axis the whole crane traversed: a steelpivot or gudgeon at the top of the shaft sup-ported the weight of the entire framing and•wheel-work. The former consisted of a longbeam, bearing the pulley at one extremity,and the wheel-work or machinery at the other;the whole was further strengthened by bracesand uprights, to which the platform was at-tached, on which the workmen stood. Thiscrane has since been greatly improved, bybeing made to balance itself perfectly whenhoisting a load, and to facilitate its turning onits pivots, additional machinery is sometimesapplied.

At the West India Docks six men workinga crane raised 224,000 lbs. weight 15 feet highin 8 hours, which is equal to 1166*6 raiseda foot high per minute by one man. Thefriction of this crane was equal on an averageto one-twelfth of the absolute weight.

Wharf Cranes. — "Where cranes cannot con-veniently be held by an upright post, theymay be supported by a piece of framing, as in this example. On the wharf is laid a sill35 feet in length, and 12 inches square, under which at right angles is another piece of the

Fig. 1513.