Lillingjacks.—With the simple transport machines must also be classed the various forms of lifting-jacks, which include the hand-screw and related implements, for lifting wagons requiring repairs or for uprooting tree stumps. In a certain sense, these jacks take the place of the ordinary lever. Their mechanism consists of a rack actuated by a cog-wheel and a crank, or of a spindle which receives its motion by means of a screw in a threaded socket and is rotated by a lever (pl. los, fig. 2), or, finally, of a mechanism consisting of a ratchet-wheel, click, and lever (fig. 3). By the apparatus shown in Figure 2 the load lifted by the vertical screw can be moved laterally by the horizontal screw; hence the machine is called a "traversing jack." To this class of machines belongs the hydran lie jack (fig. 7). The iron tubular casing encloses an immovable piston or ram, a small plunger-pump, and a reservoir for liquids (water, oil), which are introduced to the reservoir through a screw-fitted opening at the top of the machine. The raising of the pump-lever on the outside permits the water to flow into the pump-cylinder, and the downward stroke of the pump-piston forces the water through a suitable valve into a lower cylin der, where the pressure it exerts on the head of the ram causes the upper or movable body to raise the weight placed either on its head or on its pro jecting foot. The lowering of the load is effected by allowing- the fluid to flow back into the reservoir by means of a screw-valve between the upper and the lower chamber. Hydraulic jacks, of which there are several forms, produce the same effect as the hydrostatic press (Al. 9, fig. 9), whose con struction differs from that of the former in that the pump is separated from the part containing the press-piston. There is, moreover, ill the latter a press-frame, which so holds the mass that it may be compressed or from it may be expressed water, juices, etc. These hydrostatic presses, which must properly be classed with working machines, may, in fact, be used for lifting large bodies, and hence may also be classed as transport machines. Fur example, all the tubes for Stephenson's railroad-bridge over the Menai Strait, each weighing over eighteen hundred tons, and more recently entire blocks in Chicago, have been lifted by means of such machines, the pumps operating them being worked by steam.
Tackle's. —The simple pulley, which is secured to a fixed point and over which a rope or chain is passed, is not a lifting-machinc, as it serves not so much for lifting as for changing the direction of the lifting-rope. When, however, it becomes a so-called "loose " or movable pulley, mov ing- with the load to be raised, it becomes a mechanism, though one of a very simple construction. With its use double the load can be lifted with the same power, requiring, however, double the time. The combination of a movable and a fixed pulley or of several such pulleys is called a "tackle." increasing the number of pulleys in a block any desired weight could be lifted by a given power were it not for the limit set by frictional resistance, together with the stiffness of the rope or chain; and this limit is so soon reached that the advantages of cheapness and sim plicity are counterbalanced by the disadvantage of very slight efficiency.
In the older forms the pulley-blocks were made of wood (p1. 108, fig. 5), which in late productions has been largely superseded by iron (figs. 4, 6). The block farthest to the right in Figure 4 is so constructed as to admit of adjusting the rope to the wheel with greater facility than with the ordinary blocks.
The Drycrential Palley (fig. 8) is a form of tackle which may be employed with great practical advantage. Its mode of operation corre sponds with that of the differential windlass (fig. io), which is known as the "Chinese windlass." On the same shaft are fixed two drums of dif ferent diameters, to which the two ends of a rope are fastened. The rope winds over the two drums, winding on one as it unwinds from the other; the effect thlis gained is as the difference between the two drums—the smaller the difference, the greater the power and the less the speed. Upon this principle is constructed the differential pulley (fig. 8). A pair of pulleys of different diameters form the upper fixed block, and the lower pulley the movable block. To prevent the chain from slipping, the upper pulleys are provided with sprockets. The advantages of this form of tackle over other forms are that it can lift a comparatively large load without requiring much space and that it is inexpensive. It has,
however, the disadvantage of opposing a greater frictional resistance to the lifting, thus making the raising of the load either more laborious or more time-consuming; but this disadvantage is partially compensated, inasmuch as the increased resistance of friction prevents the load from sinking when the chain is let go. To lift 2200 pounds with a traction of 22 pounds one hundred pulleys are required with an ordinary tackle, but only three pulleys are necessary with a differential tackle. This example, which is greatly in favor of the differential tackle, is, however, only theo retically correct—that is, it ignores the resistance of friction; but it is on account of the latter that its superiority is reduced. Figure 9 shows a form of "geared" differential pulley. With this block one man can lift 2000 to 5000 pounds. It will hold the load at any point, and cannot run down.
Winekes.—Figures i and 2 (fit. tog) exhibit derrick winches as applied to single and double poles. They are provided with a safety brake, which prevents the handles from flying back when "let go," and in this case the automatic action of the brake holds the load suspended. The load can only be lowered by turning the crank-handles backward.
Derricks are largely employed for outdoor work, such as building, shipping, etc. They are simple in form and construction, but their con venience and efficiency are less than that of a crane. Derricks arc arranged for operation either by hand or by power, the more common form being simply provided with winches (figs. 1, 2), as illustrated by the boom der rick in Figure 6.
Iliimilasses. —The simplest form of construction of the windlass type of transport machines consists of a roller or drum resting in a frame and rotated on pivots by a crank. Around the drum is wound the rope carry ing the load or the chain catching the object to be hoisted. A construction of this kind with a vertical drum revolved by hand-spikes is called a "cap stan." Crab.—In its adaptation to cranes and derricks the windlass has many modifications. Increased power is obtained by placing a spur-wheel upon the shaft of the drum and by providing a crank-shaft carrying a spur-pin ion engaging with the spur-wheel. A construction of this kind is called a "single-purchase" crab (p. to8,fig. 12), which is completed by a brake plate with brake-lever, a brake-band, which in lowering the load is applied to moderate or to interrupt the motion, and an arrangement for disengag ing the crank-shaft. As used upon vessels, it is generally further provided with a cone set externally on an extension of the drum and serving as a friction-clutch in lowering the anchor. For lifting larger loads than is possible with the above apparatus several hand-cranks are applied, and, as a rope of too great thickness would be required, a chain is substituted. This form of machine, whose separate parts can be arranged in various ways, is called a " chain jack." —By the application of a number of hand-cranks a consid erable load can be lifted by human power alone, though by employing the power of steam this object can be more effectively attained without an increase of cranks. This form of hoisting-apparatus is shown in Figure rt, in which the two small oscillating engines impart a rotary motion to the winding-shaft from the piston-rods, and is called a "steam-winch " or steam-crab. It is frequently employed for hoisting or lowering the freight of transports. For this purpose two ropes winding in opposite directions are generally placed upon the drum, or two drums are used with ropes winding in the same manner, so that an empty vessel descends while the filled one is lifted; thus there is not only a gain of time, but also a gain of power, as the weight of the descending empty vessel assists the lifting force. This apparatus is specially adapted for continuous lifting of mate rials, as, for example, in mining. For the latter purpose the small oscil lating engines are replaced either by horizontal or by beam engines of greater power, the chains by wire-ropes, and the drums by the so-called "rope-rolls," which have relatively a large diameter, to prevent the too severe bending and wearing of the wire-ropes. Constructions of this kind are sometimes called " whimseys" (whims, or winding-engines), though this term is more particularly applied to the old atmospheric engines for hoisting coal.