Devices for transmitting power and motion by the use of cords or ropes with pulleys, levers, and other means might be exemplified indefinitely, but those given will serve to illustrate sonic early inventions which, like all other mechanical methods, have been adapted to the wants of the times in which they have been employed. Many and varied applications will be found in the illustrations which accompany other sections of this volume. In Volume V. (p. 235) are described and illustrated modern rope-transmis sion systems which may be considered as modifications or extensions of the Roman method shown in Figure 3 (pi. 119).
When the power of wind and water was impressed into the service of man, it became necessary' to employ some means for transmitting the action of the motor-wheel to the machine. With this need came the invention of gearing, early examples of which will be found illustrated on Plates 3 and to, and its modern application on Plate 12o and on other plates of this volume.
Belt Transmission.—Belts, by reason of their simplicity and adapta bility, are always preferred for the transmission of motion and force, except when the motion is required to be conveyed in exact ratio of the driving and driven speeds. The driving power of belts is due to their frictional adhesion to the faces of the pulley's; under unusual resistances the belts are liable to slip, but their slipping is an advantage, since it prevents injury to the machinery. Flat belts are generally employed; these are mostly of leather and rubber, but many are made of rawhide, canvas, sheet iron, woven wire, and various combinations of these with other materials. Belts for special purposes are round, triangular, trape zoidal, and square, and require specially-grooved wheels.
The plates of this volume illustrate numerous examples of belt-driv ing, both by primary and by secondary transmission, for the most part by open belts direct from one pulley to another, as is shown in Figure TO (pi. 12o), though occasionally by crossed belts, as is shown in Figure Ir. Figures 4 and 5 (pi. 119) exhibit an arrangement for driving pulleys whose shafts are at right angles, but not in the same plane. When shafts are not at right angles the idler pulley C is given a position which will guide the belt properly, keep it in place, and permit motion either way. A single belt may be employed to drive two shafts at right angles, or at any other angle, from the main line of shafting by fixing, in the position shown in Figures 6 and 7, two idler pulleys C Shafts lying at any angle with one another, and with pulleys on them in almost any position, may be driven with belts by judiciously locating proper-sized and prop erly-erected idler pulleys.
The driving-power of belts may properly be considered to be derived from friction where low pressures and comparatively large surfaces are involved, and where friction is augmented by heavy' oils or applied adhesives, as well as by the tension of the belts. Because of the greatly varied circumstances under which belts are used, rules for driving-power are not always reliable, yet when certain attainable conditions are fulfilled, belts will transmit forces commensurate with the area of pulley-contact. The use of belting has amply shown that a good rule for general acceptance is that 5o square feet of belt passing any fixed point per minute is the equivalent of one horse power, or that each inch of width of good leather belting will transmit a force of fifty-five pounds. This may be depended upon for continuous service when the belt is properly surfaced on smooth pulleys running at comparatively high speeds.
Hydraulic Transmission. —Water possesses three properties which emi nently fit it for the transmission of force and motion: (i) incompressibility, which qualifies it for positive transmission of motion; (2) pressure, which acting equally in every direction upon equal areas adapts it for promptness in action; and (3) freedom of movement of its particles under pressure, which property gives a frictionless means of carrying force. The prin ciple of hydraulic transmission is illustrated by the simple mechanism shown in Figure 8 (15/. 119), which consists of two water-tight cylinders with equal pistons whose rods (a, a') receive and impart the force and motion. If one of the pistons be moved, the other will simultaneously be moved to the same extent and with the same force.
One of the advantages of hydraulic transmission is, that the distance between the actuating and the receiving piston may be adapted to any exigency—that is, the pipes (d, d) may be of any length and either straight or curved, and the cylinders may lie in any plane or at any angle with respect to each other. When it is desired to multiply hydraulically a known liquid pressure—as mechanical force is multiplied by means of levers, screws, and gears—a machine (fig. 9) constructed on the principle of the hydraulic press (pi. 9, figs. 6, 9) meets the requirement. The piston a actuates the larger piston c through the medium of the interposed liquid, the pressure on c being as many times greater than the pressure on a as the area of c exceeds that of a. The motion of each piston, however, is the inverse of these areas, because the area of a multiplied by its move ment must be equal to the area of c multiplied by its movement. The effectiveness of the machine is independent of the distance between the two pistons.
Pneumatic Transmission. —The employment of atmospheric air is a feasible and an economical means for telo-dynamic transmission. The engineers of the Mont Cenis tunnel, according to their report of construc tive progress in 1863, were engaged at a distance of nearly 2000 metres (656o feet) from their air-compressors, which operated nine boring-ma chines with a force of two and one-half horse-power each. The air, under a pressure of six atmospheres, was conducted through an 8-inch tube with a velocity of 3 feet per second. At the working point there was no sensible difference in the pressure, either when the machines were in operation or when they were at rest, and with the mechanical devices employed there was no perceptible loss of power. (See pp. 308, 336.) Eicaric Transmission.—A method for the conveyance of power to greater distances than can be effected by either of the preceding systems is presented by the comparatively economical electric transmission, and it is a matter of regret that this important branch of mechanical engineering is not sufficiently developed to admit here of more than a passing notice. The applications of electricity for local power arc numerous even at this early date of its installation. (Sec p. 201; also Vol. V. p. 236.) The cost of transmitting- power by wire rope, belting, shafting, hydraulic and pneumatic methods increases in rapid ratio as the distance increases, hut it does not so increase with electric transmission, as will be seen from the following interesting table prepared by William Gcipel of Edinburgh: