Mechanical Power Transmission

• Pitch chains are closely allied to gearing; examples are offered by many automobile engines, e.g., the pitch chain often found driving the large shaft, etc. Pitch chains are used to a limited ex tent as a substitute for belts, and the teeth of the chains and the teeth of the wheels with which they work are shaped on the same principles as those governing the design of the teeth of wheels.

If a pair of wheels is required to transmit a certain maximum horse power, the angular velocity of the shaft being w, the pres sure P which the teeth must be designed to sustain at the pitch circle is 55o H.P./coR, where R is the radius of the pitch circle of the wheel, whose angular velocity is w.

In the case of transmission either by belts, ropes, shafts or gearing, the operating principle is that the rate of working is constant, assuming that the efficiency of the transmission is unity, and that the product Tw is therefore constant, whether the shafts are connected by ropes or gearing. Considering there fore two shafts, Tice' = T2w2 ; that is = i.e., the angu lar velocity ratio is inversely as the torque ratio. Hence the higher the speed at which a shaft runs, the smaller the torque for the transmission of a given horse power, and the smaller the tension on the belts or ropes for the transmission of a given horse power.

Long Distance Transmission of Power.

C. F. Hirn origin ated the transmission of power by means of wire ropes at Colmar in Alsace in 1850. Such a telodynamic transmission consists of a series of wire ropes running on wheels or pulleys supported on piers at spans varying from 30o to 500 ft. between the prime mover and the place where the power is utilized. The slack of the ropes is supported in some cases on guide pulleys distributed between the main piers. In this way 30o h.p. was transmitted over a distance of 6,500 ft. at Freiberg by means of a series of wire ropes running at 62 ft. per second on pulleys 177 in. diameter. The individual ropes of the series, each transmitting 30o h.p., were each 1.08 in. diameter and contained f o strands of 9 wires per strand, the wires being each 0.072 in. diameter. Similar in stallations existed at Schaffhausen, Oberursal, Bellegarde, Tortona and Ziirich. For particulars of these transmissions with full de tails see W. C. Unwin's Howard Lectures on the "Development

and Transmission of Power from Central Stations" (bourn. Soc. Arts, 1893, published in book form 1894). The system of telo dynamic transmission would no doubt have developed to a much Mechanical power transmission is bound by many assumptions and conditions, and the selection and successful operation of the various methods employed are largely dependent upon : (f ) source and kind of power; (2) character of equipment to be driven; (3) horse-power, speeds ratios; (4) supports and foundations; (5) bearings—their construction, type and spacing; (6) lubrication; (7) shafting—size, quality and material; (8) belting—type, width, thickness and method of joining; (9) manner of loading and un loading the power to be transmitted; (f o) mechanical and atmos pheric conditions involved ; (i 1) relation of distribution to prime mover; (12) alignment; (13) available space; (14) maintenance facilities.

Modern Methods.

Recent years have brought efficient and economical small-powered prime movers and improvements in mechanical power mechanisms. These developments have segre gated the transmitting of mechanical power into certain groups which, when employed in proper relation to other essential oper ating factors, make for economy, efficiency and low maintenance.

Modern mechanical transmission systems can be classified as fol lows: (I) direct motor coupled connection; (2) direct motor belt connection; (3) direct motor chain connection; (4) belt and line shaft group system; (5) gear reduction unit system; (6) open gear connection system; (7) multiple fabric or manila rope sys tems; (8) the variable speed connection unit; (9) the combina tion of one or more of the above systems.

Direct motor coupled connections afford compactness and are most extensively employed for the driving of modern machine tools (q.v.). The connection medium between motor and the power-receiving shaft should be a flexible coupling.

Direct motor belt connections employ a belt as the connection between the motor and power-receiving shaft of the mechanism and are extensively employed for various apparatus and machine tool driving. With this method any ratio up to 6 to 1 is practic able from the motor to machine.