Hoist. —The ordinary rope windlass placed in the attic story of a building and provided with a driving-wheel instead of with cranks, and with an endless rope reaching to the lower story, furnishes the so-called " hoist " as found in storehouses, factories, mills, etc. By reason of the great length of the rope hanging down in front of the building, the drum must have a comparatively greater length. This disadvantage, which is present wherever the rope is very long, can be obviated by the form of construction called the " friction windlass." Friction Windlass. —The principle of the friction windlass is based upon the frictional resistance of a rope wound several times around a cylinder or drum. This resistance, combined with a slight power acting on the loose end of the rope, holds the load suspended at the other end of the rope, the traction exerted being in equilibrium. The power acting on the loose end of the rope may be the smaller the greater the number of convolutions of the rope around the cylinder. The proportion is such that it can be developed by the hand of a workman or simply by the weight of the loose end of the rope hanging down. As from three to eight turns of the rope suffice for this in ordinary cases, the drum, as also the entire windlass, needs only to be very small if even the rope to be wound up on the oite side and unwound on the other is extremely long. The con struction of this windlass is, however, complicated by the necessity of having two drums, and hence two drum-shafts, with cog-wheels, etc., for with the use of only one drum the rope in winding up would move in the direction of the axis of the drum, and hence the latter could not be made less in width than that of an ordinary windlass. Friction windlasses of small dimensions are employed in factories and store-houses in connec tion with tackles, the former being, as a rule, secured to a post or a col umn, while the tackle is fastened to the ceiling. Of course the load can be lifted only to an inconsiderable height, but the rope, as it must pass through all the blocks of the tackle, has a considerable length.
Friction windlasses of larger dimensions and stronger gearing receive the form shown in Figure 13 (fl. ioS), which illustrates a machine em ployed for hoisting the heavy cast-iron girders in the erection of the London Exhibition buildings in 1S62. The two fluted friction drums are placed outside the frame, to facilitate the adjustment of the rope. The small cog-wheel engaging the two spur-wheels sits upon a shaft and is moved by a counter-shaft, which receives its motion from a belt.
A mechanism similar to that in Figure 13 (p/. IoS) has latterly been adopted in towing ferry-boats on several European rivers. The operating mechanism, known as a toneur (towing apparatus), consists of two friction drums, around which is wound a chain whose disengaged parts lie upon the bottom of the channel. The rotation of the windlass by a powerful steam-engine moves the vessel along the chain, which is raised from the bottom of the river and is held firmly to the drums by the counter-weight and its frictional resistapce. This arrangement was preceded by one in which the friction windlass was stationed on the bank of the river and drew the vessel by a chain, but instead of hand-cranks it was provided with bars, which were worked by a crew. Such windlasses, called " cap
stans," are found in Europe even at the present day at rapids and above bridges whose piers, by obstructing the water, cause strong currents.
Differential so-called "differential windlass" (pl. io8, fig. Jo), a machine of peculiar construction which has been employed by the Chinese for many centuries, has been generally known for a long time. The apparatus is practically the combination on one shaft of two wind lasses or drums of different diameters, which counteract each other, thus becoming operative by the difference between the circumference of the two portions. In a modification of the machine the two drums can be mounted on separate shafts, lying in front of or over each other and con nected by cog-wheels, but in this case they have the same diameter, the cog-wheels being of different size. In either case, however, a movable pulley (A) is required, which guides the rope to be wound up and from whose block the load is suspended. The principle of its action is as follows: Each of the two ends of the pendent rope has to carry one-half the load, L, suspended to the pulley-hook, hence Therefore on the large drum, of radius R, there acts the load momentum Y, Lr. The momen tum of the power acting on the cranks of the length / is, on the con trary, f7, when by K is understood the combined force of the workmen. It follows, therefore, from the static equation A7 + I Lr = LR, that the r force required for L, and the load value - Hence it 21 will be seen that with a certain relatively small power—for example, the force of human arms—an enormous load can be lifted, provided the differ ence (I? — r) of the drum radius can be made small enough. If this dif ference becomes naught—that is, if the diameter of both drums is the saine—an unlimited load (in a mathematical sense) can be raised; but the height to which it is lifted, being, of course, also equal to naught, the load, notwithstanding the rotation of the drum, remains stationary. The advantage of the simple and inexpensive construction of this windlass as compared with the ordinary chain-jacks, requiring a considerable number of gearings (shafts, wheels, etc.) which render these constructions expen sive, is reduced by the fact that to raise loads to any considerable height long ropes and long drums are necessary, which considerably increase the cost of such machines.
Mechanical gins, tackles, and windlasses above described can be combined in various ways, and thus frequently have the appearance of entirely different machines. Though practically good for special purposes, they are frequently commended as improvements without a (Inc consideration of their mode of action and their degree of effect. Only novices imagine that power is produced by a combination of levers, screws, or wheels. Such a combination, in fact,.effects only a transforma tion; that which remains constant is what. is called " mechanical effect " or mechanical performance, and consists of a product of power and lifting height, or power and means, or power and speed.