POWER TRANSMISSION. It is rarely convenient to develop power at exactly the point where it is to be utilized, and hence the necessity of devices and systems for conveying it from place to place without loss, or with the minimum of loss. These systems of power transmission separate naturally into two groups: one in which the motion produced by a power is transmitted mechanically; the other where the power generated by one engine is used to operate another engine, the motion developed by the second engine being employed in work. To the first group belong the mechanical de vices known as shafting, belts, chains, rope and cable drives, friction gearing, toothed gear ing, link work and the like. To the second group belong the transmissions of power by compressed air, steam, hydraulic pressure and electricity.
Shafting.— Between the different parts of a given machine, or of a given mill, the trans mission is often effected by means of a rotat ing shaft, which is subjected to a torsional moment. In mill work the shaft is rigid, and is made of solid steel, or of steel tubing; while in small machines (as in dental drills and the like) the shaft is often flexible, so that the point of application of the power can be varied at will, without disturbing the source from which the power is drawn. In mill practice it is essential that the alignment of the shafting shall be as good as possible,— that is, that the centre line of each shaft shall be as nearly straight as it can be made. It is also essential that the ujournals,) or bearings which support the shaft, shall be properly designed and well lubricated. If attention is paid to these pants, transmission by shafting may be quite efficient; but if the alignment is poor, or the journals are poorly designed, or the lubrication is inadequate, a great deal of energy may be absorbed by the shaft, and dissipated in the form of frictional heat. In transmission by shafting, the speed re mains invariable, that is, that of the engine as controlled by its governor, and if (as is usually the case) a change of speed is desired, it must be effected by the use of gearing or belting, or some similar device. The more common practice, however, is to transmit the engine power to one or more jack-shafts where it may be divided as desired, and distributed to the several machines where it is utilized.
Belts are employed more commonly than gearing, for transmitting power from one shaft to another. They are generally of leather, but rubber belts are in considerable use and belts of canvass are common for small farm ma chines. Leather belts are single-ply or double ply. The latter cost about double as much as the former, but are more economical in the long run as they deliver four times the power during life. The double-ply belt, however, is much less pliable, and cannot be used on small pulleys. Rubber belts are made of alternate layers of rubber and cotton duck compressed into a solid band. They are more resistant than leather to heat, cold and moisture but are easily ruined by lubricating oils and greases. If kept strictly clean they will long outlast leather, and are considered to give a better grip on the pulleys. Canvas belts are cheap, and are readily affected by the weather. They are much im
proved by waterproofing paints and dressings. Steel belts have been used with success. They are simply flat bands of tempered steel. For an equal power they are about one-fifth the width of a leather belt, and show only one-tenth the slip. They are run at speeds up to 10,000 feet per minute. The resistance of a belt to slipping is independent of the width of the belt, so long as the total stress upon the belt is con stant. It depends chiefly, in this case, upon the arc of contact between the belt and the pulley; and a belt will slip just as readily upon a pulley four feet in diameter as upon one that is two feet in diameter, if the arc of contact (measured in degrees) is the same in both cases, and the surfaces of the two pulleys are in the same con dition. The laying out of a successful system of belting, for use in mill work, calls for prac tical experience with belts, since it must be ad mitted that the formulae that have been given do not correspond with experience. Ther formances of belts have made all the devised rules seem ridiculous. There are too many variables in the problem. In most cases it is customary to make a computation accord ing to Webber's rule for ordinary leather belt ing of single thickness: A belt one inch wide will safely transmit one horse power, when run at a speed of 600 feet per minute; and the power transmitted will be proportional to the width of the belt, and to the speed at which it is run. This rule gives no information with regard to the proper tension; but it implies that the tension may be safely increased until the belt actually does transmit the computed horse power. For efficient operation, that is, the economical transmission of the power, the driver and driven pulleys should not be too close together. A large percentage of power is transmitted if a distinct sag in the driving side of the belt is arranged for. For narrow belts the pulleys should be at least 10 to 15 feet apart and the sag about 2 inches; and for wide belts from 20 feet upward and the sag up to 5 inches. Where the distances between shafts are necessarily less than these either rope or chain drives may be more effectively employed. The angle at which belting is placed with ref erence to the horizontal should never exceed 45 degrees. With a very stiff belt vertical belt ing is practicable if necessary. In placing belt pulleys on the jack shaft they should be so arranged that the pull alternates in different" directions. Unless absolutely impossible, the motion should be from the top of the driver to i the top of the driven pulley: in this case the sag increases the arc of contact. The pul leys should be as large as practicable to lessen the abruptness of bending in the belt but the speed (except for steel belts) should not exceed 4,800 feet per minute. If the pulleys are faced with leather, the tension of the belt may be reduced, with a larger transmission of power and an increased durability of the belt. The greatest efficiency in belt transmission of power is secured with narrow belts on large pulleys, run with low tension at high speed (4,000 feet per minute).