CRANE, in mechanics, a machine for raising goods into loft, or from vessels to wharfs, or for lowering them from heights, &c. in a safe manner, and by the application of a comparatively small power. Numberless contrivances have been resorted to for these purposes ; the first of them appears to have been the great drum-wheel, in which one or more men, an ass, &c. being made to walk, a rotary motion followed, whereby a rope fastened to the axis of the wheel was wound up, and with it whatever weight the power thus created was capable of lifting. But it was found that various fatal accidents attended this piece of machinery ; for when the man slipped, the wheel would obtain a reverse mo tion, and by its accumulated velocity would dash him to pieces. On the other hand, if the rope suspending the weight gave way, the motion of the wheel, it be ing released from opposition, became so accelerated as to produce the same fatal effects.
An improvement upon this was the inclined plane, (seen in fig. 1, Pl. IV. Miscel.) which having ribs, or battens, all pointing to its centre, is moved by a man walking either nearer to or further from the axis, according to the weight to be raised, his greatest power being near the edge ; the rope x, sustaining the weight o, passing through the pully y, and winding on the axisp, as it revolves in consequence of the man's pressure. To prevent accidents, the man has hold of the arm n, projecting from the post z so that, in case of a slip, he might save himself by bearing thereon.
But though this plan is certainly a great improvement, yet it cannot be con sidered as altogether safe, and it takes up so much space, that it is very rarely in use. The windlass, worked by various different powers, and those variously ap plied, has in general superseded all other practices. It is to be lamented that some are too complex . and expensive, which would otherwise prove highly servicea. ble. We give that in fig. 2, as being cheap and effective. It consists merely
of a barrel, e, to which three concentric iron wheels, a, b, c, all graduated with equidistant teeth, and laying in one plane, secured also by cross bars, are affixed. 'l'he small wheel D, having teeth fitting to those on either wheel, may be brought forward to work in either a, b, or c, at pleasure. Now D be ing one foot in diameter, and a being two feet, the latter will revolve once, while the former revolves twice : this power is suited to light weights. When the resistance is greater, the power must be increased by removing D farther from the centre of the windlass e, and apply. ing it to the second wheel b, which be ing three times the diameter of D, will turn only once while the latter makes three revolutions again, when the weight is very great, D must be made to work upon the largest wheel e, which being four times its diameter, will cause D to turn four times while c turns but Once. The figure is not exactly in the above proportions ; but the explanation will suffice to render that minutia of less importance. D is turned by a crank-han dle, or winch, and is made to fit into three different sockets, where its axis is keyed down, opposite to whichever wheel is to be acted upon. It should have a small ratchet; or pall, to prevent its retroces sion, in case the weight should overpow er the operator.
Some cranes are made to weigh goods as they are raised ; but this can seldom be done with though the general estimate may be correct enough for ordi nary occasions. It is done by allowing the jib, or projection m, (fig. 2.) to play on a joint at n, and by having a movea ble weight at the other end of the beam a, of which the jib forms a part, with an index on the principle of the steel-yard. But this cannot answer where very hea vy articles, such as cannon, Ike. are to be raised, as the joints would soon give way. Whatever may be the construction of cranes, power and safety ought to be the principal objects.