Power anar by the above-mentioned transformation, as effect ed by the mechanism of a windlass or by any given machine, an increase of power is gained, time is lost, and vice versa. If the load moved is one huncired times as great as the power exerted, the duration of the time of transport is also one hundred times as great as it would be if the load and the power were equal. A windlass which with a given power lifts ten times as much of a load as another with the same power, transports the load at one-tenth the speed. This simple proportion between increase of power and loss of speed is, however, modified by the factor consisting of the resistances; as, for example, friction, stiffness of the rope or chain, etc., which arc unavoidable with every mechanical contrivance and which cannot be annulled.
Frictional these frictional resistances did not exist, it would theoretically (independent of the cost of production, convenience of arrangement, erection, easy handling, etc.) be entirely immaterial what form of construction should be employed in a given case. But, these resistances being present, there are, therefore, differences of excellence in the various forms of construction according as to whether their capacity under otherwise equal conditions is more or less decreased by these resist ances. The extent of this detriment or the positive effect actually attained remains, therefore, a criterion of excellence as regards the method of con struction, and, as the resistances, which may be designated by TV, can be considered as an additional load to be overcome or lifted and brought into the calculation, this degree of effect or degree of excellence, if designated by can be determined by the symbol L+ If' to the extent that L in dicates the load to be lifted or overcome were the resistances absent.
Comparative greater this degree of effect proves to be— that is, the more it approaches, according to the above symbol, to the unit—the better is the machine. Considered from this standpoint, the above-described transport machines may be classified as follows: Differ ential tackles, the poorest; ordinary tackles, bad; differential windlasses, good; and the ordinary windlasses, the best, provided the gear-wheels are not too small. This order of succession is, of course, reversed as regards cost of construction. Moreover, it must be considered that in using human power or animal motors in general the development of the power K differs according as the force is applied to drawing a rope, pushing a bar, turning a crank, or pressing a lever (see p. 196); and, finally, in wind
lasses the advantage is not to be underestimated that some gearings can be disengaged, by which, according to requirement or occurrence, a more favorable relation between lifting the load and consumption of time can be reached.
use of tackle presupposes a fixed point for its suspension, such, for example, as is furnished by the ceiling-joists of a room, a scaffold, the mast of a vessel, etc. A support is presented to a windlass by a scaf fold, a tower, a wall, a well-enclosure, etc. A tackle or windlass with its appurtenances and supporting frame is called a "hoist." If, however, there is no support or fixed point for suspension, and a frame of any desired form or a scaffold-like structure is provided, the combination of the windlass or tackle with such a frame is called a "crane." The joist secured in the wall of a building, supported by an oblique stay, and carrying on its outer end a tackle or guide-pulley which is con nected by a rope to a windlass, forms the transition from a hoist to a crane. The load being drawn tip straight, the sphere of action of this transport machine is in a vertical line. By suspending the guide-pulley or tackle from a carriage which can be moved forward and backward upon the joist the sphere of action of such a crane is also in a horizontal plane. This sphere of action can also be obtained by securely fastening the guide-pulley or tackle to an obliquely-set jib which is so pivoted as to admit of its being turned at its foot. The principal features that distinguish the various forms of these scaffold-like structures for hoisting are that a derrick has one leg, a shears two, and a gin three.
are divided into two classes as to their motions— name!), rotary and rectilinear and into four groups as to their source of motive power, as (I) hand, when operated by manual power; (2) power, when driven by power derived from line shafting; (3) steam, hydraulic, or pneumatic., when driven by an engine attached to the crane and operated by steam, water, or air under pressure carried to the crane by pipes from a fixed source; and (4) locomotive, which usually combines both rotary and rectilinear motions, and is operated by steam generated in a boiler on the crane itself. The various types of cranes included under these groups are named as follows: swing, jib, column, pillar, pillar-jib, derrick, walking, locomotive, bridge, tram and travelling cranes, and gantries.