Rope and Cable Drive.— Rope drives are used where the shaft centres are too close together or too far apart for the economical use of belting, and generally where very large power is to be transmitted. The faces of the pulleys are scored with deep grooves narrow at the bottom, so that the rope will be gripped by the wedging of the sides of the groove. The width of the pulleys is governed by the power to be conveyed, ranks of 30 ropes being not uncommon for large powers. There are two methods of fitting out such a drive: the Eng lish and the American. By the former method each complete wind of the rope is individual, and is spliced in its place. In the American system the entire drive consists of one continu ous rope, with but one splice. The slack of so long a rope is large, but is taken up by a tension carriage around which the rope makes a single turn. After a new drive has run a few minutes all the slack has been gathered upon this carriage, and the tension of all the coils of the drive is uniform, and is kept so by the tension regulator. In the English drives, on the contrary, it is rarely that any two coils are of the same tension, and a lack of efficiency results. The speeds at which rope drives are run vary from 2,000 to 4,000 feet per minute. They are most effective when run under full load. The rope used in such drives is prefer ably a four-strand rope of long-fibre Zebu manila twisted upon a core. This rope is not only soft and pliable, but very durable. Cotton ropes have made remarkable records in some cases. In a certain Scottish cotton mill a rope drive of 24 cotton ropes one and three-fourths inches in diameter ran continuously at a speed of 4,400 feet per minute for 26 years without repair. This drive transmitted 826 horse power from a driving pulley 28 feet in diameter.
Wire ropes or cables are useful in very long horizontal transmission. They work acceptably up to 6,000 feet, if relay pulleys are provided at least every 500 feet, transmitting about 80 per cent of the power. They are generally woven of six strands upon a cotton core, each strand being a cable of from 7 to 19 wires of char coal iron or low steel: the 7-wire cables make a stronger rope, but less pliable. On account of its great liability to rust a wire cable used in power transmission is usually treated with boiled linseed oil, which has also the effect of improving its grip on the pulleys. The pulleys for wire cables should have a leather bed in the bottom of the groove to save wear of the cable.
Chain Drive.— For speeds of less than 1,000 feet per minute the chain and sprocket drive is highly efficient. The disadvantage with it is that the teeth of the sprocket wheel become worn so that the pull is not evenly distributed upon the wheeL In extreme cases the entire pull may fall upon a single tooth. The sprocket wheel should never have less than 12 teeth. The usual top limit is 60 teeth. This form of drive is particularly adapted to situations where the driving shaft and the driven shaft are neces sarily close together. For very heavy powers the gear-chain is effective. It is so constructed
that the ends of the links form a series of cogs on the inner side of the chain-belt, to en gage with the cogs of a broad spur-geared wheels. Because of their noiseless running these chains are sometimes called chains.* Friction Gearing.— In some circumstances friction gearing is desirable, but as it depends for its efficiency upon strong pressure, with the excessive strains which accompany it, this form of power transmission is generally avoided. Where used the driving wheel is usually of iron and the driven wheel of leather, or faced with leather. It has been found, however, that the friction of aluminum on leather is 40 per cent greater than that of iron on leather.
Wheel gearing is discussed at length in a special article under that title, to which the reader is referred. This form of power trans mission is effective in overcoming resistances in which belts, ropes and chains would be insuffi cient. The efficiency of toothed gears ranges up to 98 per cent when the teeth are compara tively small and the speed low, or very fast, medium speeds showing a lower efficiency. The breaking pressure on the teeth of toothed gears increases up to a certain speed, and above that speed decreases.
Link work includes mechanical devices known as cams, eccentrics, cranks, connecting rods, levers and the like. They serve to trans• mit motion from one part of a machine to some other part.
The second group of power transmissions depends upon the elasticity of fluids. Air, steam or water are put under pressure, and this pressure is available for work at the farther end of the pipe-line which confines and conveys the fluid. In the case of electricity the con dition is somewhat different, but is comparable to a hydraulic current flowing in a pipe.
Compressed air (q.v.) is employed in the transmission of power in many cases, especially in the operation of rock drills, and in mining generally, the air serving, in such cases, not only to operate the machinery, but also to aid in the ventilation of the mine. While the effi ciency of the mains is 98 per cent that of the motors varies from 40 to 67 per cent. The compressed air may be delivered to the point at which the power is wanted by means of pipes directly from the compressors, or it may be stored at high pressure in tanks, from which i it is drawn as wanted. For example, in mining locomotives are used which derive their power from large tanks of compressed air that take the place of the ordinary steam boilers of other locomotives; these tanks being charged at convenient points along the line of travel. Steam is used in the same way but with much less efficiency as enormous amounts of heat are lost from naked steampipes and they are very rarely insulated. Power is also transmitted hy draulically, when, as in cranes, presses and ele vators, it is desired to exert great force in some slow-movingpiece of machinery. In London and other European cities there is a regular public service of water under high pres sure for use in small shops as a cheap source of power.