The transmission mechanism of a steam en gine consists of the piston rod, the crosshead, and the connecting rod. The duty of the piston roil is to convey the energy developed by the piston outside of the cylinder so that it may be transmitted to the crank shaft or fly wheel. At its inner end the piston rod is attached rigidly to the piston at its centre and at its outer end it is rigidly attached to the crosshead. The rod passes out of the cylinder through an orifice in its front end, this orifice being so constructed that it is steam-tight. It is due largely to the fact that a circular orifice is more easily made steam-tight than any other form that the piston rod is universally cylindrical in form. The cross head is the connecting link which permits the rectilinear motion of the piston rod to be trans formed into the swaying motion of the connect ing rod. The manner in which it accomplishes this is clearly shown by Fig. 9, which is a section through a familiar make of crosshead. The pis ton rod coming from the right is rigidly at tached to the crosshead by a threaded connec tion, while the connecting rod is connected by means of a horizontal pin, so that it can swing up and down. The top and bottom of the cross head are planed smooth and fit a groove or track in the upper and lower guides. These guides are a part of the engine frame, and serve to prevent any vibration of the outer end of the piston rod due to the swaying motion of the connecting rod. The connecting rod is com monly a rectangular or cylindrical bar having at each end a circular bearing, one to embrace the crosshead pin and the other to embrace the crank pin. Fig. 10 shows the construction de scribed. The articles CRANK and FLY WHEEL de scribe the construction and functions of these parts of the engine's mechanism, and they will not he mentioned further. In concluding this section reference may be made to Fig. 11 as showing in a very plain manner the character and relation of the several structural details which have been described individually. In this engraving the different parts are designated by letters as follows: The engine frame, A; the cylinder, B; the piston rod, C; the crosshead, D; the connecting rod, E; the crank, F; the fly wheel. G; and the governor, H.
The first practical use of importance to which steam engines were put was the pumping of water, and the pumping engine still remains one of the principal forms of the steam engine. The various types of pumping engines are de scribed in the article on PUMPS AND PUMPING MACHINERY. The next important use of the stationary steam engine was for driving the machinery of factories, mills, and workshops, and such establishments still consume an enor mous aggregate of steam-engine power. The mill engines are not infrequent. A horizontal simple engine of comparatively small size which can be used for dynamo driving UT other high-speed work, is shown by Fig. 11. A third type of stationary engine is the hoisting engine, which in its smaller sizes combines a vertical steam boiler and a duplex hori zontal or vertical en gine in one machine. Such engines do not have a fly wheel, but connect directly with a crank shaft which drives the drum upon which the hoist ing rope is wound. Hoisting engines of larger size have sep arate boilers and often operate as many as eight separate drums.
The largest sizes of hoisting engines are those used in raising ore from deep mine shafts. These mine hoists have capacities of from 2000 to 5000 horse power. Like the smaller sizes, they are either duplex vertical or duplex horizontal engines. A duplex engine consists of a right-hand and a left-hand engine, both of which couple to the same crank shaft. They are to be distinguished from cross-eompound engines. which have a modern form of mill engine is the horizontal di rect-acting By wheel engine, in which the power is taken from the fly wheel and transmitted to shafting by means of belts. (See BELTS and POWER, TRANSMISSION OF.) For mill engines of large size present practice favors compound en gines; simple engines are used when the unit of power .which is required is small: Generally tandem compound and cross compound engines are preferred to engines using steam with three or four expansions, although multiple-expansion similar appearance structurally, by the impor tant fast that each half of the machine is dis tinct from the other half so far as the use of the steam is concerned. In direct-acting mine hoists the drum or drums are mounted directly on the crank shaft; in general hoists the crank shaft drives a separate drum shaft by means of gear ing. A fourth form of stationary engine is the rolling-mill engine, used for driving the trains of rolls in rolling mills, (See ROLLING MILL.) This is usually a horizontal simple engine of large size and especially sturdy construction. The largest stationary steam engines now used in any form are those employed for driving the generators of electric power plants. These large machines are almost universally of the inverted vertical direct-acting type. illustrated in the ac compan3ing plate. A sixth important form of steam engine is the steam-driven air compressor described in the article AIR CoMPREsSORS.
Engines other than stationary fall into one of two great classes, viz., locomotive engines for railways and marine engines for ship propulsion. The traction engine is essentially a locomotive engine designed to run on common roads, and the portable engine is practically a stationary en gine and boiler plant of small size mounted out wheels so that it may be hauled from place to place. The growth and construction of the locomotive engine are described in the article LocomontE. Marine engines fall into two sepa rate classes. For paddle-wheel boats the beam engine and the inclined engine are universally employed. For screw-propelled vessels the in verted vertical direct-acting engine is almost uni versal. (See STEAM NAvEGATIoN.) For a dis cussion of the theory of steam engines and -heat engines in general, see STEAM and THERMODY NAMICS. For descriptions of special applications of steam engines, see AUTOMOBILE; FlRE-EN GINE ; BLOWING-MACHINES.
BIBLIOGRAPHY. For an account of the deBibliography. For an account of the de- velopment of the steam engine, see Thurston, Growth of the Steam Engine (New York, 1879). Among the best theoretical and descriptive works are: Clark, The Steam Engine (London, 1890) ; Thurston, Manual of the Steam Engine (New York, 1892) ; Hutton, The Mechanical Engineer ing of Power Plants (ib., 1897) ; Seaton, A Man ual of Marine Engineering (ib., 1805) Peabody, Faire Gears for Steam. Engines (ib., 1892).